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Spectacle Prescription

Dr Harbansh Lal

A PRACTICAL GUIDE

Spectacle Prescription

A Practical Guide

Dr Harbansh LalDr Ikeda Lal Dr Tinku BaliDr Divya Saxena Dr Madhura Ukalkar

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Spectacle Prescription - A Practical Guide

ii | Spectacle Prescription - A Practical Guide

Foreword

Prof. (Dr.) Namrata SharmaHony. General Secretary, DOS

Prof. (Dr.) Namrata SharmaHony. General Secretary

Respected Seniors and Dear Friends

It gives me an immense pleasure to write a foreword to Spectacle Prescription – A Practical Guide authored by Dr Harbansh Lal. This textbook will be very useful for Optometrists, postgraduate students and Ophthalmologists in general in practice.

The process of clinical refraction is the most common and one of the first skill that we teach our students and it is the procedure that they are required to repeat more than any other procedure during their training. Best corrected visual acuity obtained via skilfully performed retinoscopy and subjective refraction, is the primary measure that guides much of our treatment and surgery. The practical guidelines and tricks for prescribing spectacles are taught neither frequently enough nor well enough. Most of the students are often left to learn these practical points by trial and error, with not enough time, or not enough interest, to engage senior colleagues to learn from their wisdom and experience.

This textbook provides simple and elegant exposure to the practical points of refraction. It begins with basic terminology and then includes such important topics like clinical refraction, retinoscopy and spectacles prescription guidelines. The latter half of the textbook teaches subjective refraction by case examples, an entertaining method of embellishing upon, and adding to, the principles previously presented, in real world patient situation. This practical guide is a concise account, yet also comprehensive and will prove to be a valuable guide for spectacle prescription.

Spectacle Prescription - A Practical Guide | iii

Index

I. INTRODUCTION 1

II. CLINICAL OPTICS 2 1. Snellen’s Visual Acuity 2 2. Pinhole Occluder 2 3. Refractive Error 2 • Emmetropia • Ammetropia a. Myopia b. Hypermetropia c. Astigmatism Circle of Least Diffusion/Confusion Spherical Equivalent 4. Accommodation 5 5. Presbyopia 5 6. Anisometropia And Anisekonia 6 7. Aphakia 6 8. Pseudophakia 6

III. RETINOSCOPY 7 1. Retinal Reflex 7 2 Correcting Lens 8 3. Finding Cylindrical Axis And Power 8 4. Practical Aspects 9 5. How To Plot Retinoscopy 9 • Oblique Axis • Range Of Neutralization. 6. Autorefractometry 11

iv | Spectacle Prescription - A Practical Guide

IV. SPECTACLE PRESCRIPTION 12 1. Autorefractometry 12 • Correct AR • Variable AR readings • Variable Spectacle Power • Cycloplegic AR 2. Subjective Testing (ST): Basics 14 a. At Different Distances • Distance ST • Near ST • Intermediate ST b. Transposition c. Astigmatism • Range of Axis • Matching of Axis • Use of Jackson Cross-Cylinder d. Fogging e. Bilateral Testing f. Equalizing the Accommodative Effort g. Compare with Old Spectacles 3. Subjective Testing: Case Scenarios 24 a. Children/Pediatric Age • Spasm Of Accommodation b. Teenager & Young Adults (13-45 years) • Computer Vision Syndrome • Therapeutic Trial of Glasses • Over-Refraction c. Peripresbyopic age (35-50 years) d. Middle Age (50-70 years) • Role of Vertex Distance e. Old Age (> 70 years) Spherical Equivalent V. CONCLUDING REMARKS 43

Spectacle Prescription - A Practical Guide | 1

Amidst the vastly expanding knowledge of various spheres of ophthalmology, post-graduate students are not able to devote time to the management of refractive errors. Even during fellowship or senior residency, their focus is on mastering the sub-specialty, and spectacle prescription takes a backseat. When they come into the clinical practice, they depend upon optometrists more than their own skills. The optometrist training is very variable and not up to the mark in India. Nowadays, with the availability of international brands and large scale acceptance of progressive addition lenses, the cost of purchasing spectacles has gone up significantly. If the glass prescription going out from your clinic is not correct, it may lead to a significant percentage of unsatisfied patients. The art of prescribing glasses is a basic science that must be mastered by every ophthalmologist. I will be dividing this subject into two parts: first, clinical optics, and second, spectacle prescription basics and clinical scenarios, depending upon the age of the patient.

Introduction 1

2 | Spectacle Prescription - A Practical Guide

1. Snellen Visual AcuityThis test is based on the theory that the smallest object which can be resolved by the eye subtends the same visual angle at the nodal point of the eye as a cone photoreceptor, i.e., one minute of an arc. The test employs a chart with rows of letters of diminishing size. Each row is accorded a number indicating the distance in meters at which a person with normal visual acuity should correctly identify the letters. The bars and the spaces of each letter subtend an angle of one minute of a degree. The test chart is normally read from 6 m (20 feet). Thus, a subject who identifies the letters on the ‘12’ line from 6 m has 6/12 vision (20/40) – the numerator indicates the viewing distance. ‘Normal’ visual acuity is 6/6 (20/20), although young adults often achieve 6/4 acuity.

2. Pinhole OccluderThe use of the ideal pinhole leads to the formation of a clear retinal image irrespective of the refractive state of the eye. However, in high degrees of ametropia, although the pinhole helps, the retinal image is still too diffuse to achieve the improvement that is found in a case of low refractive error. Thus, errors outside the range +5 D to –5 D sphere are not correctable to 6/6 with a pinhole. The most useful pinhole diameter for general clinical purposes is 1.2mm. The best use of the pinhole is to find out whether or not the patient’s vision is going to improve with spectacles.

3. Refractive ErrorEmmetropia: Light rays coming from infinity will focus on the retina, without accommodative effort.

Clinical Optics2


Ametropia: They are anomalies of the optical state of the eye that cause imperfect focus on the retina, leading to a poor quality of the final image.

a. Myopia: In a myopic eye, rays of light coming from infinity are focused in front of the retina. This may be because of two reasons. Firstly, if the eye is abnormally long. This is called axial myopia and includes high myopia in which there may be a posterior staphyloma. Alternatively, the eye may be of normal length, but the dioptric power may be increased. This is referred to as refractive or index myopia. Examples of index myopia include; keratoconus, where the corneal refractive power is increased due to the steepness of the cornea, and nuclear sclerosis, which leads to a myopic shift due to an increased density of the nucleus. Myopia is usually corrected by prescribing minus or concave lenses.

b. Hypermetropia: In a hypermetropic eye, rays of light coming from infinity are focused behind the retina. Hypermetropia is classified into manifest and latent hypermetropia. Manifest hypermetropia is defined as the strongest convex lens correction accepted by the patient for clear distance vision. Latent hypermetropia is the remainder of the hypermetropia which is masked by the ciliary tone and involuntary accommodation. This may account for several dioptres, especially in children, for whom cycloplegic refraction is necessary to ascertain the full magnitude of the refractive error. If the eye is short, then axial hypermetropia results. Alternatively, if the refractive power of the eye is inadequate, then refractive hypermetropia results. Aphakia is an extreme example of refractive hypermetropia. Hypermetropia is usually corrected by prescribing plus or convex glasses.

c. Astigmatism: The refractive power of the astigmatic eye varies in different meridians. If the principal meridians are at 90° to each other, it is a regular astigmatism. If the principal meridians are at 90° to each other but lie beyond a 30°variation from 90° and 180°, the term oblique astigmatism is used. If the principal

Clinical Optics


meridians are not at 90° to each other, this is called irregular astigmatism and cannot be corrected by spectacles.

Circle of least diffusion: Refraction through a surface with two radii of curvatures, results in image formation at two different foci. This intervening area where the two images are focused is called the interval of strum and the conoid formed is called the Strum’s Conoid (Figure 1). The circle of least confusion is the circular cross-section of the conoid of Sturm that is halfway between the 2 focal lines—in terms of dioptres, not linearly.

Circle of Least Confusion

Fig 1. Sturm’s Conoid

In This figure, V stands for vertical rays and H, for horizontal rays. POINT A: The vertical rays are converging and have not reached the point of focus. The horizontal rays are crossing the point of focus and have started diverging. So, the gap between the blue line is less as compared to the red line, that means the diffusion of blue rays is less than that of the red rays. Hence, the shape of cross-section is vertically oval. POINT B: The convergence of vertical rays is exactly equal to the divergence of the horizontal rays from the axis. So, here the section is a circle which is known as the Circle of Least Diffusion or Circle of Least Confusion. POINT C: The divergence of horizontal rays is more than the degree of convergence of vertical rays. Hence the shape of cross-section is horizontally oval.


Spherical equivalent: The spherical equivalent of a refractive state is defined as the algebraic sum of the spherical component and half of the astigmatic component. Whenever, we are not able to prescribe full cylindrical power, and prescribe spherical equivalent, then the image will form a circle of least confusion which gets focused on the retina.

4. AccommodationAccommodation is a complex mechanism involving the sensory and neuromuscular phenomena. The human eye, through contraction of the ciliary muscle, changes the optical power of the lens to assist the convergence of the image to the retina, adjusting the focus to different distances between the object of regard and the eye. If the lens becomes stiffer and unable to alter its shape, physical accommodation is impaired even with the strength of the preserved ciliary muscle. A similar process may occur with physiological accommodation if the weakness of the ciliary muscle exists.

5. PresbyopiaPresbyopia is the gradual loss of accommodative response resulting from reduced elasticity of the crystalline lens. Accommodative amplitude diminishes with age. It becomes a clinical problem when the remaining accommodative amplitude is insufficient for the patient to read and carry out near-vision tasks comfortably. Fortunately, appropriate convex lenses can compensate for the waning of accommodative power. The symptoms of presbyopia usually begin to appear in patients after 40 years of age. The age of onset depends on the pre-existing refractive error, the depth of focus (pupil size), the patient’s visual tasks, and other variables. The complete accommodative reserve is exhausted by the age of about 60 years. Over-accommodation in moderate hyperopes may result in accommodative fatigue and premature presbyopia. It is also interesting to note that in moderate myopes (–3 D to –4 D), where the far point equals to about 25 cm to 33 cm, simple removal of the distance refractive correction results in the patient being able to see well for near. Such patients may not require near vision glasses. In


pseudo-phakic patients, accommodation is lost irrespective of age, and these patients require presbyopic correction.

6. Anisometropia When the refraction of the two eyes is different, the condition is known as anisometropia. A small degree of anisometropia is common place. Larger degrees are a significant cause of amblyopia. A disparity of more than 1 D in a hypermetropic patient is enough to cause amblyopia of the more hypermetropic eye because accommodation is a binocular function, i.e., the individual eyes cannot accommodate by different amounts. The more hypermetropic eye, therefore, remains out of focus. The myopic patient with anisometropia is less likely to develop amblyopia because both the eyes have clear near vision. However, when one eye is highly myopic, it usually becomes amblyopic.

7. AnisekoniaWhen a difference of more than 3 dioptres is present between both the eyes, and the eyes are corrected with spectacles, the difference in the image size (aniseikonia) that is produced, can lead to difficulties with fusion and even suppression of one of the images. The children withstand aniseikonia to a greater extent than adults.

8. AphakiaAphakia is the absence of a lens in the eye, due to multiple causes. It causes a high degree of hyperopia, loss of accommodation, and a deep anterior chamber.

9. PseudophakiaPseudophakia is the condition obtained by implanting an artificial lens in the eye. Intraocular lens implants are used in both refractive lens exchange and cataract surgery to replace the natural lens of the eyes and correct the refractive error.


Retinoscopy 3Retinoscopy is the technique to obtain an objective measurement of the sphero-cylindrical refractive error of the eye. This is the gold standard test for the calculation of refractive error. It is especially useful for infants, children, and adults who are uncooperative, for examination under general anesthesia, and forbedridden patients.

Most retinoscopes in current use employ the streak projection system. The illumination of the retinoscope is provided by a bulb with a straight filament that forms a streak in its projection.

In the “sleeve up” position, the retinoscope emits diverging light (plane mirror setting), and in “sleeve down”, it emits converging light (concave mirror setting) but it is important to check your streak retinoscope before performing retinoscopy.

Retinoscopy is usually performed using the plane mirror setting so that light is parallel (or slightly divergent) as it enters the pupil of the patient’s eye. It should be performed with the patient’s accommodation relaxed. The patient should fixate on a distant object or better still, look straight ahead as if he or she is looking at infinity.

1. Retinal Reflex The projected streak illuminates an area of the patient’s retina, and this light returns to the examiner. By observing the characteristics of this reflex, the examiner determines the refractive status of the eye.

Through the peephole in the retinoscope, the emerging light is seen as a red reflex in the patient’s pupil. If the examiner is at the patient’s far point, all the light leaving the patient’s pupil enters the peephole and the illumination is uniform.

Myopes display an “against” reflex, which means that the direction of the movement of the light observed from the retina


is in a different direction to that in which the light beam is swept. Hyperopes, on the other hand, display a “with” movement, which means that the direction of the movement of the light observed from the retina is same as that in which the light beam is swept. The state in which the light fills the pupil and does not move is known as neutrality.

2. The correcting lensThe examiner uses the appropriate correcting lenses to neutralize the retinoscopy reflex.

The power of the correcting lens neutralizing the reflex is determined by the refractive error of the eye and the distance of the examiner from the eye. The dioptric equivalent of the examining distance must be subtracted from the power of the correcting lens to determine the actual refractive error of the patient’s eye. The common working distances are 67 cm (1.50 D) and 50 cm (2.00 D). So, we need to subtract this from the retinoscopy values.

If with movement is observed, add plus power; if against movement is observed, add minus power.

3. Finding the axis of the cylinderBefore the powers in each of the principal meridians can be finalised, the axis of the meridians must be determined. The characteristics of the streak reflex aid in this determination:

1. Break- A break is observed when the streak is not oriented parallel to 1 of the principal meridians. The reflex streak in the pupil is not aligned with the streak projected on the iris and the surface of the eye, and the line appears broken. The break disappears (i.e., the line appears continuous) when the projected streak is rotated to the correct axis.

2. Width- The width of the reflex in the pupil varies as it is rotated around the correct axis. The reflex appears narrowest when the streak, or intercept, aligns with the axis.


3. Intensity- The intensity of the line is brighter when the streak is on the correct axis.

3. Finding the power of the cylinderAfter the 2 principal meridians are identified, the previously explained technique is applied to each axis:

1. With 2 spheres: Neutralize one axis with a spherical lens; then neutralize the axis 90° away. The difference between these readings is the cylindrical power. For example, if 90° axis is neutralized with a +1.50 dioptre sphere and 180° axis is neutralized with a +2.25 dioptre sphere, the gross retinoscopy is +1.50 DS +0.75 DC × 180°. The examiner’s working distance (i.e., +1.50) is subtracted from the sphere to obtain the final refractive correction: 0.0 DS +0.75 DC × 180°.

2. With a sphere and cylinder: We should ideally use a trial frame. First, neutralize one axis with a spherical lens. Then, with this spherical lens in place, neutralize the axis 90° away by adding a cylindrical lens in the trial frame. The sphero-cylindrical gross retinoscopy is read directly from the trial lens apparatus.

4. Retinoscopy: Practical aspectsWe normally ask the patients to look at a distance or infinity. This is done to relax his/her accommodation. If the patient cannot accommodate, then you can ask the patient to look into the light itself. This will ensure the retinoscopy of the foveal area and will be more accurate. Pseudophakia, atropinized eyes, and very old patients are ideal candidates for this technique. If the patient is moving his/her eyes too much, then this technique can be used with or without cycloplegia.

When you are not getting astigmatism right, you can ask the patient to look into the retinoscope light itself. When a patient is looking at this light, the astigmatism assessment is very accurate. After assessment of the astigmatism, you can ask the patient to look at a distance for assessment of the spherical power.


5. How to plot retinoscopy Right Eye Left Eye 90° 90°

180° 180°

Now, when the retinoscope is moved side to side (streak vertical), suppose the neutralization is at + 4.0 DS. When the retinoscope is moved up and down (streak horizontal), it neutralizes at + 6.0 DS. Then, the plot is shown as below: +6.0DS

+4.0 DS

Acceptance: If we perform retinoscopy at 67 cm, then we need to reduce the power by 1.50 Diopters (100/67).

This patient (in the above example) has a cylindrical power of + 2.0 DC (+6-4) at 180° and spherical power {+4DS -1.5DS (retinoscopy distance)} of + 2.50 DS.

Therefore, the final prescription becomes: + 2.50 DS /+ 2.0 DC at 180°.

Oblique AxisIf this reflex is oblique after making a cross, plot a dotted line and write the axis.

For example: If the patient’s one axis lies at 70°, another axis will be at 160°.When the streak is parallel to 70°, reading is + 4 DS and when the streak is perpendicular to 70°, then the reading is + 6 DS. This should be plotted as below:

160°

70°

+6DS

+4 DS


Range Of NeutralizationIf you are unsure, you can write the range of neutralization for power and axis both (like in case of media opacities or uncooperative patients or young children).

For Example: +6.0 DS +6.0DS to +6.5DS +4.0 DS +4.0DS to + 4.5 DS

60° to 70°

6. Auto-refractometry Refractometry is the estimation of refractive error with a machine, called a refractometer. Automated Refractometers (AR) are designed to objectively determine the refractive error and are of various types depending upon the underlying principle they are based on. AR comprises an infrared source (around 800-900 nm) which is invisible and helps to overcome instrument accommodation to a certain extent} and a fixation target. All refractometers use the anti-fogging technique to relax accommodation before objective refraction.


4 Spectacle Prescription

Autorefractometry has made the job of prescribing glasses much faster, but one cannot prescribe solely based on the AR readings. There are three important steps. The first step is to get the correct AR readings. The second step is accurate subjective testing, and the third and most important is prescribing according to the need of the patient, which largely depends upon the age and profession of the patient. We will divide this section into three major parts:

1. Autorefractometry [AR]2. Subjective Testing: Basics3. Subjective Testing: Case Scenarios

1. Autorefractometry (AR)Our aim is to get correct AR Readings and then do a proper subjective testing.

a. Correct AR reading: We need to look at the crowding of readings rather than average. Take a printout of ‘5’ readings and then analyze them. For Example: -1.25/-2.50 at 32° -1.00/-2.00 at 10° - 0.75/-2.25 at 25° -1.50/ -2.50 at 29° -0.75/ -3.50 at 34°

There are three variables i. Spherical power ii. Cylindrical power iii. Cylindrical axis.

This data gives us the range for all of them. This means i. Spherical power is between -0.75 and -1.50 ii. Cylindrical power is between -2.00 and -3.50 iii. The axis of astigmatism is between 10° and 34°


Any one of the five readings may be hitting the bull’s eye. If we take an average of these readings, any one outlier will change all the parameters. But by looking at this reading, most of the spherical power crowding is around -0.75 and -1.00, cylindrical power around -2.25, and the axis is around 30°. While doing subjective testing, we should start with -1.00DS/-2.25DC at 30°.

b. Variable readings: The common reasons for getting inconsistent readings are media

opacities, lid abnormalities, watery and dry eye. i) Media Opacity: • Corneal opacity • Cataract • Vitreous opacities ii) Lid Socket And Spinal Abnormalities iii) Watery Eyes iv) Dry Eye: This is one of the commonest causes of error in AR

readings, particularly in an elderly population. First, ask the patient to close the eye for a few seconds and then ask the patient to open the eye and take the readings immediately. Repeat the procedure a few times to get more readings. Secondly, instill lubricant eye drops and take the readings.

c. Variable spherical power: The variability can be due to accommodation. The younger the

patient, the more he accommodates. Usually, the least minus power and the highest plus power with which the patient is comfortable, should be prescribed. The best is to use cycloplegic AR to get the ideal spherical power.

Ask the patient to look at a distant object in the auto-refractometer which may be an airplane, parachute, or any other target. This ensures an accurate reading. But if a patient is not able to see the fixation target due to poor vision, lack of understanding, mental health issues, nystagmus, or unstable body, the readings are going to be inaccurate. Retinoscopy in such cases will give a better idea of the refraction.


d. Cycloplegic autorefractometry (CAR): We must take non cycloplegic and cycloplegic readings.

• Non- cycloplegic readings: The assessment of cylindrical power of the central cornea is more accurate with non-cycloplegic refraction. This is the physiological state of the pupil.

• Cycloplegic AR: The assessment of spherical refractive error is better after cycloplegia as the accommodation has been relaxed completely or partially depending upon the cycloplegic agent used.

TIP: USE CYLINDRICAL POWER OF UNDILATED READINGS AND SPHERICAL POWER OF MYDRIATIC READINGS FOR SUBJECTIVE TESTING.

2. Subjective Testing: Basicsa. At Different Distances:

Distance subjective testing: y Subjective testing is done for distance, near, and intermediate

distances. Distance vision is initially corrected and then we add plus number lenses, for intermediate or near vision.

y The trial frame should be of good quality and should be positioned in such a way that the center of the trial lens gets aligned with the visual axis or the pupillary center. If a patient is not seeing through the center of the trial lens, a prismatic effect can be induced. He may always be confused in his response because none of the lenses would give him a clear vision. This becomes even more critical while testing a higher power or astigmatic error.

Steps: y Do not start with under correction of spherical or cylindrical

power. y First give full spherical and cylindrical correction. Only once


you get vision 6/6 or the best possible vision, then make fine adjustments.

y Ask the patient to concentrate upon the margins and clarity of one letter at a time. This letter could be from the last line or one line above that. Concentrating on one letter is helpful because, when the patient looks at full line or the whole chart, he might be unable to appreciate subtle changes.

y Repeat the process in the left eye. y Usually patient accommodates more while seeing with one

eye than with both eyes open. They usually accept 0.25DS less for distance in myopes and 0.25 DS more in hyperopes, when examined with both eyes open.

Near Vision ST: y Near addition will depend upon the age, working distance,

and visual acuity of the patient. y Add plus lenses on top of the distance correction in the trial

frame. y The near vision is usually assessed for both eyes together. y Always check for a range of near vision. y Myopes have poor accommodation because they have not

been using their accommodation as much as hypermetropes so they usually need more addition than hyperopes.

y We can prescribe slightly higher plus addition for patients using progressive addition lenses (PAL) than bifocals as they cover all distances.

y If a patient has got normal distance vision for reading in good light, at 33 cm distance, his addition according to his age will be: y 35 years: +0.5 DS y 40 years: + 1.0 DS y 45 years: + 1.50 DS y 50 years: + 2.0 DS y 55 years: + 2.5 DS y 60 years and above: + 3.0 DS


Usually, we do not prescribe + 2.50 DS or more these days as the reading distance is mostly 40 cm and not 33 cm (due to use of laptops, mobiles, and tablets).

y If the patient reads at 40 or 50 cm distance, his near addition will be less by half a diopter.

y If his vision is poor for distance, he will need more near addition.

y Myopes need more addition than hyperopes. y If hyperopes accept less distance, then they need more power

for near addition. y Progressive addition lenses need more addition than bifocals

or separate reading glasses. Intermediate ST:

y The vision between 50 cm and 100 cm distance can be considered as Intermediate vision.

y Because of the decreasing accommodative response as we age, the power needed for this range is different from a near distance.

y Now let us consider that a patient has zero accommodation and zero power for distance (for example, pseudophakia) so: y Infinity: 0 y 100 cm: + 1.0 DS y 50 cm: + 2.0 DS y 40 cm: + 2.50 DS y 33 cm: + 3.0 DS

This means that for near vision, he needs somewhere around + 2.50 DS to + 3.0 DS and for the intermediate between + 1.0 DS to + 2.0 DS. So reducing the near addition by 0.5 DS to 1 DS, will give a comfortable intermediate range of vision.

y There are apps also available which can be used for intermediate vision testing. But by keeping the near vision chart at the working distance, which is usually between 50 cm to 100 cm, one can test the intermediate vision. Here, we are not looking at N5 or N6 vision but N8 or N10 clarity is a good


enough guide depending upon the distance. More importantly, just asking the patient how he is seeing or how comfortable he is feeling, works very well.

b. Transposition y The conversion of the prescription from plus cylinder form to

minus cylinder form or vice versa, and changing the axis by 90 degrees is called transposition.

Steps for transposition

y Algebraic addition of the spherical and cylindrical power including sign.

y Change the sign of the cylindrical power. y Either add or subtract 90° from existing degrees.

For example: (i) + 1.0 DS / + 5.0 DC @ 180° Transpose:

y Algebraic sum of the spherical and cylindrical power: y + 1.0 DS + 5.0 DC = + 6.0 DS. y Change the sign of the cylindrical power: - 5.0 DC y Change the axis: 180-90 = 90° y Prescribe: + 6.0 DS /-5.0 DC @ 90°. y (ii) -0.5 DS / + 1.0 DC @ 17°

Transpose:

y Algebraic sum of the spherical and cylindrical power: y -0.5 DS + 1.0 DC = + 0.5DS. y Change the sign of the cylindrical power: - 1.0 DC y Change the axis: 90 + 17 = 107° y Prescribe: + 0.5 DS / -1.0 DC @ 107°

c. Astigmatism Always start with full cylindrical power on AR even if it is very high.


y Range of AxisMost of the patients will have a range of acceptance of the axis which may be 10° to 20°. The higher the magnitude of astigmatism, the lower is the range. So always try to find out range of acceptance for the axis and try to prescribe in the middle of that range.

For example:

i. AR: -1.0 DS/-2.0 DC @ 35° Acceptance: -1.0 DS/-2.0 DC @ 35° - 6/6 Clear Range: -1.0 DS/ -2.0 DC @ 30° - 6/6 Not so Clear @ 35° - 6/6 Clear @ 40° - 6/6 Clear @ 45 ° - 6/6 Clear @ 50° - 6/6 Not so Clear Prescribe: 40° instead of 35°(in the middle of the range).

ii. AR: + 1.0 DS/-5.0 DC @ 27° Acceptance: + 1.0 DS / -5.0 DC @ 27° - 6/6 Clear Range: 25-30° Prescribe: 27°

These days spectacles are made on automated computerized machines, so any axis can be designed.

y Matching the axis in both the eyesMostly, the two eyes are mirror images of each other. So, the axis of astigmatism co-relates. This means that if one eye has a cylindrical power at10°, the chances of the other eye axis being 170° are very high.

For example:

AR: RE -1.0 DS/-0.50 DC @ 135 LE:-1.0 DS/0.0 DC

Acceptance: RE -1.0 DS/-0.50 DC @ 135° - 6/5 LE -1.0 DS/0.0 - 6/6


In the left eye, try adding -0.25 or -0.50 DC @ 45°, the patient may improve to 6/5. This patient should have a left eye better vision as there is no cylinder in that eye but his vision is better in the right eye. This means that the AR machine has not picked up the cylindrical power in the left eye. So using the matching axis principle, the patient’s vision can be improved.

y Use of Jackson’s Cross Cylinders (JCC): A combination of a negative and positive cylinder of equal strength (either 0.25D or 0.50D) is mounted at an angle of 90°.The total spherical equivalent of this lens becomes 0 diopters.

Fig 2. Jackson’s Cross Cylinder

Black Dots = axis of the plusRed Dots = axis of the minus cylinder

Uses:i. To find out whether the patient needs a cylindrical correction.ii. To find out the power of astigmatismiii. To find out the axis of astigmatism.

i. To find out whether the patient needs cylindrical correction: For Example: A 28- year -old patient with

AR Readings: RE: -1.0 DS / 0.00 DC LE: - 1.0 DS/ -0.00 DC


Acceptance: RE: -1.0 DS / 0.00 DC 6/6 LE: -1.0 DS/ -0.00 DC 6/6 (but vision is not very sharp)

If a patient’s vision is not sharp, he or she may have a small cylindrical refractive error which was not picked up by the AR. Now, when we place the cylindrical lens to correct small astigmatism. We will be faced with two difficulties.

First, since we don’t know the axis, we will have to try at different cylindrical axis to assess where the patient’s vision improves.

Second and most importantly, adding a cylindrical lens changes the spherical equivalent. Therefore, we will have to adjust the spherical power accordingly to maintain the same spherical equivalent. This makes our job very difficult. The biggest advantage of JCC is that its spherical equivalent is zero. We can quickly assess astigmatism by keeping the ‘–‘sign at 90, 180, 45,135°. If JCC is placed at 180° and the patient’s vision becomes sharper, then this patient needs minus cylindrical correction around 180°. But when we are finally prescribing the glasses, we should keep the principle of spherical equivalent in mind. So, during subjective testing, try giving -0.25 DC or -0.50 DC at 180° and reduce the spherical power by - 0.25 DS if needed in case of myopes. We can also check for hyperopes in the same way using the ‘+ ‘sign of JCC.

ii. To find out the correct power of the astigmatism The cross-cylinder is placed with its axis parallel to the axis of

the cylinder in the trial frame (first with the same sign, and then the opposite sign). If vision is improving in any position, then the corresponding correction should be made.

For Example: A 28- year- old patient with Acceptance: RE: -1.00 DS / -1.75 DC @ 120°

y Place the minus cylindrical power of the cross-cylinder at 120° .


y Ask the patient if he sees better or worse with a cross-cylinder at 120°. If he sees better, then the cylindrical power needs to be increased. If he sees the worse, it means that we do not have to increase the power.

y Now, place the plus cylindrical power of cross-cylinder at 120°.

y Ask the patient if he sees better or worse. If the patient sees better, then cylindrical power needs to be reduced.

y If the cylindrical power given in the trial frame is correct, then the patient will see worse in both the positions (minus or plus cylinder), because he does not require an additional cylindrical power.

Cylinder Patient Seees Worse Patient Sees Better

Minus No action Increase cylinder

Plus No action Decrease cylinder

iii. To find out the correct axis of the astigmatism JCC can be used to fine tune the axis of the cylindrical

prescription. For Example: A 28- year-old patient with Acceptance: RE: -1.00 DS / -1.75 DC @ 180°

y Place JCC handle at 180° parallel to the axis of the cylindrical power with the minus sign above.

y Now, flip and check with a minus sign below. y If the axis of this cylinder is correct, then the patient will see

the same or worse in both positions. y But if the patient sees better with any one of the positions,

then we need to rotate the cylindrical axis in that direction. y Let us say, in this case, patient sees better when the minus

sign is at 45°. Now rotate the cylindrical lens from 180° to 10° (that is towards 45°) and repeat the test.


y If there is no further improvement on keeping the handle aligned at 10°, then10° is the correct axis.

y If there is improvement in any of the positions, then we should slightly rotate in that direction.

y For the plus lenses, we will have to follow the same principle and the plus lens will be rotated towards the plus sign of the JCC, then it shows improvement in the vision.

d. Fogging

This technique is used to find out the maximum correction, a hyperopic person can be prescribed.

y Patient is made to sit with both the eyes open, after adding nearly full hyperopic cycloplegic correction in the trial frame.

y He is asked to look at a distant object to relax his accommodation and called in after 15-20 minutes. The power is reduced by adding minus lenses till he can see clearly.

For Example: Present Prescription: RE: + 1.0 DS, LE: + 1.0 DS Non Cycloplegic AR: RE: + 2.0 DS LE: + 2.0 DS Cycloplegic AR: RE: + 8.0 DS LE: + 8.0 DS

y First, add + 8.0 DS lens in the trial frame in both the eyes and make the patient wait outside with both the eyes, looking at a distant object for 15-20 mins.

y Now, call the patient inside and check his/her vision. y If the patient’s vision is less than 6/18 with both eyes open,

try fogging with +6.0 DS both eyes and keep on reducing the power of the lenses till you get around 6/18.

y Now, with both eyes open keep on adding minus lenses in front of both the eyes, till vision becomes 6/6.

y Usually by reducing the number by +1DS or + 2DS, the patient should be able to read 6/6.


Fogging depends on the following parameters: i. The cycloplegic AR ii. The difference between the cycloplegic and the non-cycloplegic

AR. If this difference is more, the patient is accommodating a lot and we will not be able to fog with full cycloplegic AR.

iii. The difference between cycloplegic AR and the present glass prescription.

iv. The age of the patient. This determines the effort required for a maximum tolerable hyperopic prescription.

E. Bilateral Testing:

After checking each eye separately, now ask the patient to see with both the eyes. If the patient is not comfortable, adjust the frame according to the inter-pupillary distance (IPD) of the patient. Then, ask the patient if he has a strain or the letters seem too sharp and small. The letters should appear normal and natural, not too sharp or small. If they appear too sharp and small or if there is strain, then reduce the minus number.

Most of the times, the patient accommodates more when seeing with one eye, so reduce by - 0.25DS in both eyes in myopes and check again. Most of the patients will read clearly with no strain. In most of the hyperopes, we can keep adding plus lenses as long as he is comfortable and can see clearly to relax his accommodation.

f. Equalizing the Accomodative Effort: For Example: Cycloplegic AR: RE: +2.00DS / +2.00 DC at 180° LE: +4.00DS/ +2.00 DC at 180° Acceptance: {Post Mydriatic Test (PMT) with one eye at a time} RE: +1.00 DS/ +2.00DC at 180° LE: +2.00 DS/ +2.00DC at 180°The spherical power difference between the two eyes in the above example is +2.00 DS in cycloplegic AR reading. This difference should be maintained as far as possible. The eyes cannot accommodate


unequally, that is, +1.00 D in one eye and +2.00 D in the other eye (as seen in PMT Acceptance). Therefore, this difference should be maintained or reduced as far as possible.

So, prescribe the cylinder as per acceptance but change the spherical power: Options: i. RE: +0.00 DS LE: +2.00 DS ii. RE: +0.50 DS LE: +2.50 DS iii. RE: +1.00 DS LE: +3.00 DS

The same principle shall be followed for myopia but probably not so strictly.

g. Compare with Old SpectaclesWe must compare the patient’s old prescription with the new prescription. If the patient appreciates a difference, only then we need to prescribe the new number. If there is not much difference and the patient is happy with old spectacles, then there is no need to change the prescription.

3. Subjective Testing: Case ScenariosGuiding Principle:We treat a patient and his eyes, not the optics. If a patient is comfortable with his vision, we do not have to force him to use the spectacles (irrespective of the refractive error). In the pediatric age group, if the patient has amblyopia or squint, then we need to stress upon the importance and role of spectacles.

In other words, if No symptomsNo amblyopia No spectaclesNo squint


y If any of these is present, then he needs spectacles.We will be discussing this topic, depending upon the age of the patient. A child less than 3 years old, needs special care, usually needs extended wear contact lenses. They are best managed by a pediatric ophthalmologist and most of them need treatment for amblyopia or squint. The rest of the patients can be broadly divided into the following categories:3a. Children (3-12 years)3b. Young (13-35years)3c. Peri-presbyopic (35-50 years)3d. Middle and early old age (50-70 years)3e. Old age (>70 years)

3a. Children/ Pediatric AgeMost of these patients present with complaints of watering, rubbing of eyes, squeezing of lids, and watching TV from a close distance. They might also come for a routine check-up or a diminution of vision detected by screening at the school. The important point to keep in mind in this group is their strong accommodation, and they may also have associated amblyopia and squint.

(i) Case 1: A 4- year-old boy with Cycloplegic AR: +0.5 DS/+1.0 DC at 180° Transposition: + 1.5 DS/ -1.0 DC at 90°. On subjective testing: (a). +1.5 DS/-1.0 DC at 90° vision becomes blurry (b). +0.5 DS/-1.0 DC at 90° vision better

What to prescribe?We can prescribe (b) as young children have strong accommodation. We should review these children every 3 months and sequentially increase the hypermetropic correction. In children, hyperopic correction can be under prescribed but myopic correction should be fully prescribed. Even slightly overprescribing by -0.25 DS to -0.5DS in myopes, will do no harm as they can accommodate well.


(ii) Case 2: A-6-years-old child presents with:

No complaints of strain like watering, headache, or decrease in vision

Vision: RE: 6/6, LE: 6/6 AR reading: RE: +0.5 DS/+1.0 DC at 180° LE: + 0.25 DS/+0.75 DC at 180° Or RE: - 0.5DS /-1.0 DC at 180° LE: - 0.25 DS/0.50 DC at 180°

Prescribe or not to prescribe:It is not imperative to prescribe glasses. We can keep the child under observation every 6 months. The parents will usually ask ‘will the number increase if we do not use spectacles?’. The use of the spectacles has got little or no effect on increase of power. In fact, plus power usually decreases with age while the minus power tends to rise. The child may eventually need spectacles. In other words, as already explained:

No amblyopiaNo symptoms No spectaclesNo squint

Just follow up.

(iii) Case 3: A-7-year-old child with: Best corrected visual acuity (BCVA): 6/9 or less in both the eyes

but no symptoms and no complaints. Prescribe or not to prescribe: We must prescribe glasses, irrespective of the power as he is

suffering from amblyopia. Amblyopia: Traditional teaching has been that amblyopia can

improve up to 12 years of age, but this is not always true. Amblyopia can be treated at any age. The younger the patient, easier it is to treat and faster is the response.


(iv) Case 4: Pediatric age group. RE: -1.0 DS/ -5.0 DC at 90° (high astigmatism) LE: -1.50 DS /-4.50 DC at 90° (high astigmatism) In the pediatric age group, full astigmatic correction should be

given irrespective of the magnitude of astigmatism. There is no need to under correct astigmatism. The adaptability of the children is excellent, and they accept full correction without any strain or discomfort.

(v) Case 5: Age: 6 years, Cycloplegic AR: RE: +1.0DS/+5.0DC at 180° LE: +2.00 DS / +5.0 DC at 180° Transposition: RE: + 6.0 DS/ -5.0 DC at 90° LE: + 7.0 DS/-5.0 DC at 90° The full cylindrical prescription is given but the spherical

prescription can be reduced according to the acceptance. This may range between +2 to +5 diopters usually. Every 3 to 6 months, we can increase the spherical power. In these cases, non-cycloplegic AR gives a particularly good idea of the minimum spherical acceptance by this child.

Non Cycloplegic reading: RE: + 1.0 DS / -5.0 DC at 90° LE: + 2.0 DS / -5.0 DC at 90° The reading taken without cycloplegia is important here. This is

the reading while the child was accommodating, which means, he will accept this spherical power without any problem. We need to increase the spherical component to a maximum from this value.

y Spasm Of Accommodation Inability to relax accommodation is called spasm of

accommodation. This causes pseudo-myopia in children, especially when they watch and play video games on the mobile phone for a long time.


(vi) Case 6: A-6-year-old child with

Distance vision: RE: 6/60, LE: 6/60 Near Vision: RE: N6, LE: N6 Non Cycloplegic AR: RE -8.0 DS, LE: -8.0 DS Acceptance: RE: - 8.0 DS 6/6, LE: - 8.0 DS 6/6 With this prescription, he has difficulty in seeing near objects.

He cannot read well as his eye is already accommodated. He is unable to increase his accommodation any further.

AR Under Atropine:(Pharmacological relaxation of accommodation)

RE: + 2.0 DS, LE: + 2.0 DS Depending on the duration and severity of spasm, the spasm of

accommodation can be corrected by the following methods: y Treatment by pharmacological relaxation of accommodation

by instilling atropine/ cyclopentolate/ homatropine/ tropicamide eye drops.

y Progressive or bifocal glasses may be needed, initially for few months.

3b. Teenagers and young adults (13 to 45 years):Most of these patients use their accommodation all the time. They use electronic gadgets for studying, entertainment, and professional work. Most of them have what we call as “electronic vision syndrome” or “computer vision syndrome”. Besides prescribing spectacles, they should be given some additional advice as follows:

y Rule Of 20-20-20: Every 20 minutes, take a break for 20 seconds and look 20 feet away. This practice helps in relaxing accommodation and convergence.

y Resting Interval: Every 30 minutes for 5 mins, or every 60 minutes for 10 mins, relax, get up from the chair, walk around stretch your neck, spine and relax your eyes.

y Palming: Rub your hands together to generate heat and place them on your eyes.


y Lubrication: These maybe prescribed frequently for the management of associated dryness.

y Exercises: Practice ocular muscle exercises in the morning. Keep a pen or the thumb of your hand in front, then move it to the right, left, up, down and also bring it closer to the nose while keeping your eyes fixated on the object. Repeat this morning or evening for 3 to 5 minutes. Cervical and spinal exercises for 10-15 mins twice daily are also helpful.

(i) Case 1: A15-year-old complaints of headache and eye strain: Uncorrected visual acuity (UCVA) RE: 6/6 LE: 6/6

Cycloplegic AR: RE: 0.0 DS /-0.5 DC at 10° LE: -0.25 DS /-0.75 DC at 170°

Acceptance: RE: 0.0 DS /-0.5 DC at 170° LE: -0.25 DS /-0.75 DC at 170°

y Therapeutic trial of glassesWe must prescribe in this case, even if the refractive error is low. We call it as therapeutic trial of glasses. These patients are instructed to use glasses constantly, (only to be removed while sleeping) at least for 1 month. They are asked to observe whether the symptoms have decreased with the use of glasses. If the symptoms have decreased or disappeared, then they can stop wearing the spectacles. If the symptoms persist, they should use glasses. In other words, the spectacles were prescribed to them to alleviate the symptoms not to improve their vision.

(ii) Case 2: A 30-years-old computer professional presents with symptoms suggestive of eye strain

Vision: RE: 6/6, LE: 6/6 Cycloplegic AR: RE: +0.50 DS, LE: + 0.75 DS The distance vision becomes blurry even with +0.25 DS. Here,

check for near or intermediate vision which will not get blurry with correction. We can advise him to use glasses while working on the computer initially.


Prescription: RE: + 0.25 DS, LE: + 0.50 DS

(iii) Case 3: A 21-year-old college student with symptoms of headache and watering of eyes.

Visual acuity: RE: 6/6 LE: 6/6 Cycloplegic AR: RE: +0.25 DS/+0.25 DC at 180° LE: + 0.25 DS /+0.50 DC at 180°Here, prescribe hypermetropic cylindrical correction if it is not blurring the vision. Most of the times, when you put a plus cylinder, the vision becomes blurry. So, try a minus cylinder at 90° and now usually the patient will accept this correction and see better. Now, add a maximum plus spherical power that he can accept. Here, the prescription may be:

Prescribe: RE: +0.25 DS/ -0.25 DC at 90° LE: +0.25 DS/ -0.50 DC at 90°Or RE: 0.0 DS / -0.25 DC at 90° LE: 0.0 DS/ -0.50 DC at 90°

The correction of cylindrical power is more important as low cylindrical powers cause more eye strain than higher cylindrical powers.

(iv) Case 4: A 25-year-old computer professional with perfect vision for near and distance presents with symptoms suggestive of eye strain.

Cycloplegic AR: RE: + 2.00 DS LE: +2.00 DS No acceptance for distance. Prescribe: RE: + 1.0 DS LE: + 1.0 DS Remark: For computer work.


Now, check the acceptance for intermediate distance. This patient can be given +1.0 DS both eyes as a single vision lens. This means that he can use glasses while working on the computer and for near initially. Slowly, over a period, if he starts feeling comfortable, then these glasses can be used for distance as well.

(v) Case 5: A 30-year-old asymptomatic patient comes for a routine check-up.

Present spectacles: RE: -3.00 DS/-1.50 DC at 65° LE: -3.00 DS/ -1.50 DC at 145° Cycloplegic AR: RE: -3.50 DS/-1.50 DC at 65° LE: -3.50 DS/-1.50 DC at 145° Patient feels better with: RE: -3.25 DS/-1.50 DC at 65° LE: -3.25 DS/ -1.50 DC at 145°.

y Over refractionAsk the patient to put on their own spectacles and place - 0.25 DS in both the eyes on top of their own glasses. If the patient feels better, only then you should change the glass prescription. This technique is called over refraction. If the cylindrical power is same, then the spherical correction can be re-confirmed by placing the lenses on top of the patient’s prior glass prescription.

(vi) Case 6: A 32-year-old female, complains of symptoms of strain, and has been using spectacles for the last 15 years.

Vision with glasses: Both eyes 6/5 Present glasses: RE: -3.0 DS / -1.0 DC at 70° LE: -3.0 DS / -1.0 DC at 130° Cycloplegic AR: RE: -3.0 DS/-1.0 DC at 70° LE: -3.0 DS/-1.0 DC at 130° Patient feels better with: RE: -3.25 DS/-1.50 DC at 70° LE: -3.25 DS/ -1.50 DC at 130°.The focus should be on correcting astigmatism. So, increase the cylindrical power without any hesitation.


With advancing age, the tolerance for accommodative effort may decrease. In myopes, the spherical power can be reduced, even if the vision is not very sharp. In hypermetropes, the correction can be increased. The letters should not be unusually sharp, bright, and small and the patient should not feel eye strain.

3c. Peri-presbyopic age (35 to 50 years):In this age group of patients, we need to decrease the accommodative effort as much as possible. The older the patient, more relaxation of accommodative effort is needed. If the patient has good accommodation and has asthenopia, using the accommodation will keep the ciliary muscles stronger than not using it. Myopes usually have poor power of accommodation as most of the time they read without spectacles, and do not use accommodation. Therefore, they may need more addition for reading than the hyperopic patients. On the other hand, hyperopes have a good power of accommodation as they have been using it even for distance since childhood.

If a patient can see with his accommodative effort without symptoms, we do not need to relax his accommodation irrespective of the age of the patient.

In this age group, an important consideration is the profession of the patient which determines the duration of the work and the working distance:

So, they need to be prescribed according to their requirement which may be for near, intermediate, distance, or combination of them or all of them.

Pre-existing refractive error: Mild hyperopes can manage for distance without glasses but may need them for working and reading. Myopes with slight under correction, may manage for a few years with a single lens correction.

(i) Case 1: Primary need for near A 35-year-old young jeweler presents with headache after

working for some time.


Cycloplegic AR: RE: 0.0, LE: 0.0 Jewelers do very intricate work and their working distance

is usually 20 cm or lesser. They require magnification and accommodative support.

Prescribe: +1.00 DS or +1.50 DS both eyes for working. They may need more correction which can be re-checked after a few months.

(ii) Case 2: Primary need for distance with intermediate Symptoms: A38-year- old male, complains of difficulty in night

driving and in seeing the dashboard for the last 6 months to 1 year. He has never used spectacles till now.

Cycloplegic AR: RE: +1.50 DS LE: +1.25 DSUsually, he should be able to accept right eye +0.75DS and left eye +0.50 DS after fogging. If the patient is uncomfortable with this correction, then prescribe right eye +0.50 DS and left eye +0.25 DS. These glasses will help him while driving and spectacles can be re-prescribed after 6 months if needed.

(iii) Case 3: Primary need for intermediate and near A 42-year-old female computer professional can see well at all

distances, but complains of brow ache, irritation, and redness of both the eyes by evening.

Cycloplegic AR: RE: +0.25 DS/ +0.50 DC at 100° LE: +0.50 DS/ +0.25 DC at 80° On transposition: RE: +0.75 DS / -0.50 DC at 10° LE: +0.75 DS /-0.25 DC at 170° Acceptance for distance: RE: +0.25 DS/ -0.50 DC at 10° LE: +0.25 DS /-0.25 DC at 170° With both eyes open, acceptance for distance: RE: +0.50 DS / -0.50 DC at 10° LE: +0.50 DS / -0.25 DC at 170°


Option one: Add + 0.50 DS or + 0.75 DS on distance correction. Prescription: RE: + 0.75 DS / -0.50 DC at 10° LE: + 0.75 DS/-0.25 DC at 170°Instruction: Use glasses for computer and near. Take glasses off while looking at the distance.

y Option two: PAL (progressive addition lens):Doctors, lawyers, bankers, or anyone who deals with people while working need good vision at all distances. As ophthalmologists, we need to look at distance, intermediate and near while working with patients. Progressive addition lenses are ideal for these professions. Here, there is no need to under correct myopia and no need for aggressive fogging. As compared to a single lens near correction, the addition of plus over distance correction will be +0.25 DS to +0.50 DS more. This will help in clear near vision and they will also work well for the next 2-3 years (as these are expensive lenses, patients usually do not like to change them frequently).

Instructions to the patient: We need to convince the patients that PAL are expensive, and they take time to get used to, but this is the ideal solution for long term as they cover all the distances well. But they may produce a little distortion of vision when looking from the periphery of this lens or while looking down by rotating the eye instead of head. This may cause disturbance of vision depending on the quality of PAL. These patients are instructed to use them for watching TV, use computers for a few hours and keep on increasing the time gradually. It may take them 15 days or even more to walk around with them comfortably. “Give yourself enough time to adapt”, is what you can tell them.

Prescription:

For Distance: RE: + 0.25 DS / -0.50 DC At 10° LE: + 0.25 DS / -0.25 DC At 170 For Near: ADD + 1.25 DS BE Remark: BE Progressive Addition Lenses.


(iv) Case 4: A48-years-old male, bank employee by profession: Cycloplegic AR: RE: -1.0 DS/ -0.50 DC at 160° LE: -1.25 DS/ -0.50 DC at 20°

Acceptance for Distance: RE: -1.25 DS/ -0.50 DC at 160° 6/5 LE: -1.50 DS/ -0.50 DC at 20° 6/5

Normally, we would have given addition +1.75 DS at his age, but this patient needs PAL with addition of +2.25 DS for two reasons. First, he is accepting -0.25 DS more for distance than CAR. Progressive glasses should compensate for this -0.25 DS of over-correction. Second, myopes tend to have poor accommodation.

Prescribing more near add is especially important if the person has computer work and public dealing (for example, bank employees), when they need to work on the computer and look up and interact with people also.

(v) Case 5: A50-year-old male wants to shift to progressive glasses.

Cycloplegic AR: RE: +1.50 DS LE: +1.50 DS

Distance acceptance: RE: +0.50 DS LE: +0.50 DS

Here, give near addition: +2.25 DS

At the age of 50 years, we usually a give near addition of + 2.00 DS in both the eyes. But, as his acceptance is less than the cyloplegic AR, so we have increased it to +2.25 DS.

If Distance Acceptance was: RE: +1.50 DS LE: + 1.50 DS

Near addition of + 2.0 DS is prescribed.

3d. Middle age (50-70 years)The people in this age group are usually financially well established and they usually buy the most expensive frame and spectacles. At


the same time, their adaptability to change is limited. So, a careful history taking is particularly important. You need to ask:

y Do they really have any difficulty with their present glasses, or they just want to have a new pair of spectacles (which is quite often the case)? Probably, these patients would have been wearing spectacles with full correction for many years now.

y If they have difficulty with their present prescription, which distance are they uncomfortable for?

y Do they use spectacles regularly or as and when needed? This question may help in deciding whether to convert them from single/Bifocal to PAL.

(i) Case1: A50-year-old male, who has never used spectacles before, presents with complains of difficulty in working on a computer and reading now.

Cycloplegic AR: RE: -0.50 DS/ -0.50 DC at 20° LE: -0.25 DS/ -0.75 DC at 70° Acceptance for distance: RE: -0.50 DS/ -0.50 DC at 20° LE: -0.25 DS/ -0.75 DC at 70 ° Questions we should ask:

y Do you drive or love to drive? y Do you have public dealing?

If the answer is “yes”, then encourage him to go for PAL.

Acceptance for Distance: RE: -0.50 DS/ -0.50 DC at 20° LE: -0.25 DS/ -0.75 DC at 70 ° Near Add: +2.00 DS Remark: BE Progressive Addition LensesIf the answer is “no”, then a single lens which can give good computer vision and reasonable reading comfort, or the other way round, is prescribed.

Single Lens will be given according to need of patient:


(ii) Case 2a: Primary computer user Distance: RE: -0.50 DS / -0.50 DC at 20° LE: -0.25 DS / -0.75 DC at 70 ° Near Add: +1.25 DS instead of + 2.00 DS Prescription: RE: +0.75 DS / -0.50 DC at 20° LE: +1.00 DS / -0.75 DC l at 70° Instruction: Use for computer and reading only, not for distance

(iii) Case 2b: Primary reader If the patient’s primary need is reading newspaper, then add

+1.75 DS over the distance prescription: Distance: RE: -0.50 DS / -0.50 DC at 20° LE: -0.25 DS / -0.75 DC at 70 ° Near Add: +1.75 DS Prescription: RE: + 1.25 DS / -0.50 DC AT 20° LE: + 1.50 DS / -0.75 DC AT 70°(iv) Case 3: A 58-year-old male, Present spectacles: RE: -10.00 DS / -2.50 DC at 140° 6/9 LE: -10.00 DS/ -2.50 DC at 50° 6/9 With a progressive add of +2.50 DS. Cycloplegic AR: RE: -11.00 DS/ -2.50 DC at 140° LE: -11.00 DS/ -2.50 DC at 50° Acceptance: RE: -11.00 DS/ -2.50 DC at 140° LE: -11.00 DS/ 2.50 DC at 50°We should avoid changing his glasses, although he is seeing better with -1.00D increase in both the eyes. Increase the prescription by maximum-0.25 DS or - 0.50 DS as high myopes do not have any accommodative reserve left at this age.

Over refraction y Ask the patient to wear his own spectacles. y Now with both eyes open add -0.25 DS in front of spectacles

(figure 3).


y Check the vision again. y If the letters become too sharp and small, then you are over

prescribing.

VERTEX DISTANCE

CORRECTIVE LENS

CORNEA

Figure 3. The Technique Of Over Refraction

y Role of vertex distance:

Figure4: Vertex Distance


Vertex distance determines the lens effectivity: The effective lens power changes with change is the distance from the cornea. If lens moves away from the eye, a plus or a convex lens become stronger whereas a minus or a concave lens becomes weaker. To summarize:

Increasing the vertex distance, increases the power of a plus lens and decreases the power of a minus lens. The reverse is the case, when the vertex distance is decreased.

Vertex Distance in Myopes

• If a patient is wearing a spectacle power of -10.0 DS, bring the spectacle closer to the eyes. The effective spectacle power now increases to – 10.50 DS. Therefore, he may see near objects blurry.

• If this patient moves the spectacle away from the eyes, then his effective power will decrease and become – 9.50 DS. Now, he will be able to see clearly for near, but the distance vision will become blurry.

• Similarly, we take an example of the trial frame. The distance between the back and the front slot of the trial frame is more than 10 mm. The patient may accept -11.0 DS when placed at the front slot, -10.0 DS in the middle and -9.0 DS in the back slot, So the patient’s prescription will depend upon the vertex distance (figure 5). Hence, we should be careful in changing the prescription in high myopes, and here lies the importance of over refraction.

Figure 5: the difference in the front and the back slot

of the trial frame


Vertex Distance in Case Of Hyperopes

y If a + 10.0 DS is placed in the front slot of the trial frame, it will act as + 11.0 DS, but if placed in the back slot, it will act as a + 9.0 DS.

y The plus lenses, on keeping them away from the eye (over the tip of the nose), will improve near vision. For example, in the good golden era, “muneems” use to keep their spectacles on top of the nose for reading and writing on ‘bahi khata’(notebook). The same is true for minus lenses.

3e. Old age (70 years +)At this age, the general fitness level of the patient matters a lot. The decreasing flexibility and muscular strength, coupled with diminishing contrast sensitivity, make this population particularly vulnerable to fall while climbing down the stairs or walking in the dark on rough roads.

Advise them to use separate spectacles for near and distance. Bifocals are usually the most dangerous, while walking or climbing down the stairs. They need to look down but are unable to bend their bodies and neck down adequately. Sometimes, they see from the near segment and sometimes from the distance segment and end up getting confused and may fall.

Progressive Addition Lens: If a patient is already using them and is comfortable, we can ask them to continue. Fresh PAL is to be prescribed if the patient’s mental and physical age is lower than the chronological age. If a 70-year-old person is physically active like playing outdoor games (golf) and alert just like a 50-year-old person, then you can prescribe fresh PAL.

y Spherical equivalent(SE):

There are situations where it may not be possible to prescribe the full cylindrical power to the patient, particularly in old age. In such a case, instead of simply decreasing the cylindrical power and not adjusting the spherical power accordingly, may not give him a good vision.


Spherical Equivalent (SE)=Spherical Power+Cylindrical Power/2

For Example: -2.0 DS / -6.0 DC at 90

SE: -2.0 + (-6.0/2) = -5.0 DS

Prescribe: -3.0 DS with – 4.0 DC (-3.0 - 4.0/2= -5.0 DS) or -4.0 DS with -2.0 DC (-4.0 - 2.0/2 = -5.0 DS)

The advantage of prescribing the spherical equivalent is that the circle of least confusion falls on the retina and improves vision. So, whenever you reduce the cylindrical power for comfort of the patient, add half of it to the spherical power.

(i) Case 1: A 75-year-old female: Present spectacles: RE: +0.50 DS/ -1.50 DC at 70° LE: -0.50 DS / -1.00 DC at 180° Acceptance: RE: +0.50 DS/ -3.00 DC at 70° 6/6 LE: -0.50 DS/ -2.50 DC at 160° 6/6Option 1:

You can prescribe full cylindrical correction at this age, but with certain instructions:

y Spectacles should be well centered. y For 15 days, use them for watching TV, then slowly move around

the house. Once you become comfortable, only then use them for going out.

y Most importantly, if you are not able to adapt, then we might have to reduce the cylindrical power. This will increase the comfort level at the cost of visual acuity.

Option 2:

Increase the cylindrical power partially and prescribe close to the spherical equivalent. For doing this, place the cylindrical power in the frame, then add or subtract the spherical power till she sees clearly.


In this case (above example), prescribe:

RE: 0.00 DS/ -2.00 DC at 70°

LE: -0.75 DS/ -2.00 DC at 160°

Option 3:

Do not increase the cylindrical power. Change the spherical power, particularly in patients who are not highly active mentally or physically.

(ii) Case 2: A80-year-old pseudo phakic patient using -1.50 DC in both the eyes.

AR Reading: RE: -5.00DC at 180° LE: -4.00 DC at 180° Distance Acceptance: RE: 0.0 DS/ -4.50 DC AT 180° 6/6 LE: 0.0 DS / -3.75 DC AT 180° 6/6 The Principle of Spherical Equivalent: Check with: RE: -1.0 DS / -2.50 DC AT 180° 6/9 LE: -0.75 DS / -2.50 DC AT 180° 6/9We keep the cylindrical power constant (-2.50 D) and adjust the spherical power to give him the best vision possible. Though the principle of spherical equivalent is largely applicable, the patient may accept a little less or more than the spherical power. In my experience, they usually accept a little less. He may not have 6/6 vision. We can explain this to the patient and prescribe.

In my experience, cylindrical power beyond 2.50D to 3.00D should not be prescribed at this age, unless they have been using it previously. I recommend prescribing -2.50DC and then calling them back after 6 months to re-assess whether we can increase the cylindrical power further.


Concluding Remarks 5I would like to tell you a little anecdote to highlight the importance of correct spectacle prescription. When I was a junior consultant, a young man approached me for his mother’s cataract surgery. Her previous records showed that she had been seen by a very senior and respected surgeon of this town. The young man shared that he previously got his own refraction done by that surgeon. However, he was not comfortable with the spectacles prescribed by him. He visited me first for refraction and was comfortable with my spectacle prescription. In his opinion, though the previous surgeon was a big name, if he could not prescribe glasses accurately, could he be a good surgeon? A patient who is dissatisfied with your refraction may never come back to you. Therefore, refraction is a crucial and critical part of practice, just as important as performing good surgery.

In the end, I would like to say that refraction is an ongoing learning process for life. I am still learning. My aim is to be so perfect that “no one should be able to change my prescription”.

I take pride in my prescription. It hurts my ego if an optician changes my prescription. I do not delegate this job to others. I always check and sign.

Over a period, I have developed an insight to know, which prescription I need to cross-check and the common mistakes the optometrist is likely to make. I am sure if you are vigilant, prescriptions going out from your center will be appreciated by your patients. You will be respected by your optometrist and the opticians in your town.


Present Post:• Director: Delhi Eye Centre

• Co-Chairman, Department of Ophthalmology,

Sir Ganga Ram Hospital

• Chairman, Department of CME,

Sir Ganga Ram Hospital

Past PositionsAll India Ophthalmological Society (AIOS)

• MemberScientificCommittee:2002–2005

• JointSecretary:2005–2008

• Treasurer:2008–2014

• Dean FAICO (Fellow All India Collegium of

Ophthalmologists):2018–2019

• Co-Chairman,HeadquartersCommittee:2019–2020

• DeanFAICOCommittee:2019–2020

• Chairman,HeadquartersCommittee:2019–2020

Delhi Ophthalmological Society (DOS)• MemberExecutive:2001–2003

• JointSecretary:2003–2005

• Secretary:2005–2007

• LibraryOfficer:2009–2011

• President:2012–2013

IMA Karol Bagh Branch• President:2018-2019

Delhi Medical Association• ChairmanDepartmentofCME:2016–2017

Rotary Club of Delhi, Rajendra Place• President:2005-2006

website: http://drharbanshlal.com

Dr. Harbansh Lal

Prof. (Dr.) Namrata Sharma Secretary, Delhi Ophthalmological Society

DELHI OPHTHALMOLOGICAL SOCIETY Room No. 479,4thFloor,Dr.RajendraPrasadCentreforOphthalmicSciences,

All India InstituteofMedicalSciences,NewDelhi-110029,IndiaTel:+91-11-2086371Email: [email protected] / [email protected] · Website: www.dosonline.org

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