National Ocular Biometry Course 2015

An echoing presentation

Prepared by:Anis Suzanna Binti MohamadRohaila Binti AriffinPegawai Optometri U41Hospital Sultanah Bahiyah, Alor Setar

COURSE OBJECTIVES:

Understand the basic principal of biometry Perform, define the discuss the pathology that would require the

biometry test Interpret the result of biometry test Measure axial length Name four ocular structures that reflect ultrasound echoes Able to perform biometry test Able to perform topography, discuss corneal contours of normal,

pathological and post-surgical cornea Take pre-operative testing and intraocular lens power calculation

method to the next level of accuracy and efficiency Utilize standardize diagnostic ultrasonography as a valuable

adjuvant to current clinical evaluation.

Cataract surgery

• What is cataract surgery?

– Precise biometry is essential for accurate outcomes in cataract and refractive surgeries.

– The measurement of axial length by ultrasound was the gold standard for many years.

– With the introduction of optical biometry in the US in the year 2000, this technology has become more & more popular & is now the most common method for the measurement of axial length.

– Optical biometry uses a partially coherent wave that has approx. 9x the resolution of a 10 MHz sound wave, making the measurement of axial length very precise.

– It helps to avoid operator variations in measurement.– Also, to increase the accuracy as contact with cornea is not needed

(eliminates compression to cornea).– Since Optical biometry measures to the centre of macula, it gives

the refractive AXL vs the anatomical AXL achieved with ulrasound.

Pre-op assessment

1. Keratometry2. A-Scan Biometry3. IOL Formula

1. K-reading

Keratometry

Manual K Topography

Placido disc Scheimflugtopography

Auto-KOptical

biometry

Manual Keratometry Topography (Placido Disc)

K-reading in our center

Auto-K Optical Biometry (Lenstar)

K-reading in our center

Why we choose Lenstar in our center?

2. A-Scan Biometry

• Measurement of Axial Eye Length by Ultrasound

• Average Axial Length of Normal Eye 23.06 mm (Majority : 22.0 to 24.5 mm)

• Accuracy of AL measurement using A-scan ultrasound is +0.1 mm

• Difference in AL measurement between both eyes + 0.3 mm

• Values are 0.14 to 0.36 mm longer with immersion technique than with contact method

Examination Procedure1. History Taking2. Patient Preparation3. Biometry Technique

Biometry Technique

Ultrasound

Applanation Immersion

PCI

IOL Master Lenstar

Ultrasound biometry

Ultrasound is produced in the ultrasonographic

probe by the oscillation of a

pizoelectriccrystal

Converting electrical energy into mechanical

energy

The probe emits and receives pulsed signal

Reflectivity vstime is displayed

for the single direction in which

the probe is pointing

This value then be converted to

milimitres

Contact A-scan

Immersion A-scan

Applanation (NIDEK)

Immersion

Scleral shell (Prager)

Scleral shell

(Prager) (Ossoining) (Kohn)

Potential Sources of Errorwith Contact Method

1. Corneal Compression2. Fluid Excess3. Misalignment of Sound Beam4. Inappropriate Eye type

Error caused by 1 mm Corneal Compression:

Average eye 2.5 DLong eye 1.75 DShort eye 3.75 D

Corneal compression

Fluid excess

Potential Sources of Errorwith Immersion Method

1. Air bubbles within fluid

2. Inappropriate eye type

2. Inappropriate eye type

Instrument Setting

1. Eye to measure (OD/OS)

2. Eye type

3. Measurement mode

4. Technique

5. Gain

Eye Type( Sound Velocity )

1. Phakic

2. Aphakic

3. Pseudophakic

C A P + R S

Measurement Mode

Automatic Semiautomatic Manual

Gain Setting• Initially high gain setting should be used to assess the overall appearance of the echogram , then gain should be reduced to a medium level to improve resolution of spikes .

• Error can occur when the gain is set too high or too low .

Very high gain : short readingVery low gain : long reading

Use of average sound velocity ,althoughsufficient in normal phakic eye , may result in slight error when the lens is inordinately thin or thick or when theeye is very short or very long .

• The use of individual sound velocity may provide more consistent and accurate AL reading .

• Manual measurement mode is better to help ensure alignment of sound beam .

Aphakia & Pseudophakia

Posterior Staphyloma

• Posterior staphylomas often causes an irregular shape of the ocular wall resulting in an inability to display a distinct , high retinal spike , leading to a significant error in A-scan measurement .

• Deepest portion of the staphyloma may be located eccentric to macula thus the measurement may be longer than true AL along the visual axis .

• B-scan can be used to demonstrate the shape of posterior ocular wall and the relationship of macula to the staphyloma .

• Probes with fixation light are preferable

High Hyperopia

• Immersion technique is preferable .

Edema DMS Tumor

Macular Lesions

RD

The presence of an elevated macular lesion may prevent the display of a distinct retinal spike and often causes a shortened AL measurement .

Vitreous Lesions

Asteroid Hyalosis Vitreous Hemorrhage Gas Bubble

Dense Cataract

• Strong sound attenuation produced by a very dense cataract can significantly impair the ability to display spikes from the various interfaces along the visual axis .

• Maximum gain setting may be required to obtain spikes of sufficient height from the posterior lens capsule and retina .

• Semiautomatic mode should be used in eyes with dense cataract.

Silicone Oil

• Sound velocity in silicone oil

1040 m/s 5000 cs

980 m/s 1000cs

• This low sound velocity can result in pronounced sound attenuation and difficulty in identifying the retinal spikes .

• If proper sound velocity are not used , erroneously long AL measurement will be obtained .

• For accurate AL measurement ,various ocular components should be measured separately with appropriate sound velocity .

• The least preferred method is use of average sound velocity. Average sound velocity in eyes with average length (23.5 mm):

1,139 m/s phakic eye

1,052 m/s aphakic eye

• Due to strong sound attenuation AL measurement often can not be obtained from an eye containing emulsified silicone oil .

IOL Master/Lenstar

Zeiss IOL Master

Axial Length ACD Corneal Power IOL Power Calculation

Hoffer-Q , SRK/T ,Holladay 1, Haigis

Keratometry

A second person should confirm measurements prior to A-scan ultrasonography if: The corneal power is less than 40.0 diopters, or greater than 47.0 diopters.

If there has been prior keratorefractive surgery. In this case the corneal power will need to be estimated by either the historical, or the contact lens method.

The average corneal power difference between the two eyes is greater than 1.00 diopter.

The patient cannot fixate, as seen with a mature cataract, or macular hole.

The amount of corneal astigmatism by keratometry, or topography,correlates poorly with the amount of astigmatism on the most recent manifest refraction.

The corneal diameter is less than 11.00 mm.

There is any problem with patient cooperation, or understanding.

Immersion A-scan Ultrasonography

A second person should re-measure both eyes if: The axial length is less than 22.00 mm, or greater than 25.00 mm in either eye.

The axial length is greater than 26.0 mm, and there is a poor retinal spike,or wide variability in the readings. There is a difference in axial length between the two eyes of greater than 0.33 mm that cannot be correlated with the patient's oldest refraction.

Axial length measurements do not correlate with the patient's refractive error. In general, myopes should have eyes longer than 24.0 mm and hyperopes should have eyes shorter than 24.0 mm. Exceptions to this rule involve steep, or flat corneas. Be sure to use the oldest refractive data.

There is difficulty obtaining correctly positioned, high, steeply rising echoes, or wide variability in individual axial length readings for either eye.

Intraocular Lens Power

A second person should repeat the axial length measurements, keratometry readings and re-run the IOL power calculations for both eyes if: The IOL power for emmetropia is greater than 3.00 diopters different than anticipated.

There is a difference in IOL power of greater than 1.00 diopter between the two eyes.

If the patient has had prior keratorefractive surgery and the calculated IOL power for standard phacoemulsification is less than +20.0 D or greater than +23.0 D.

Formula for

IOL Power Calculation

IOL Power Formula

Theoretical Regression Refractive

Theoretical Formulas

These are derived fromgeometrical optics

Regression Formulas

Actual postop refractive resultsof many lens implantations areused to predict IOL power

Theoretical Formula

These formulas contain manyassumptions including values of postop ACD , refractive index of cornea and ocular humors , retinal thickness

Theoretical Formula

These formulas are reliable for average AL , but overestimatesin short eyes and underestimatesin long eyes

Refractive Formulas

IOL power calculation withoutdetermination of axial length

SRK I(Sanders,Retzlaff,Kraff)

P = A – 2.5L – 0.9K

It generally undercorrects short eyesand overcorrects long eyes

SRK II

A1 = A + 3 AL < 20mmA1 = A + 2 AL 20-21A1 = A + 1 AL 21-22A1 = A AL 22-24.5A1 = A – 0.5 AL >24.5

SRK/T

• It is a nonlinear theoretical optical formula empirically optimized for postop ACD, retinal thickness , corneal refractive index . It combines advantages of theoretical and regression formulas .

Generations of IOL Formulas

• 1st GenerationFyodorov , Colenbrander ,Hoffer , SRK I• 2nd GenerationBinkhorst , SRK II• 3rd GenerationHolladay 1 , Hoffer-Q , SRK/T• 4th GenerationHolladay 2 , Haigis, Hoffer-H• 5th Generation (2014)Hoffer H-5

There are currently three IOL constants in use: The SRK/T formula uses an "A-constant."

The Holladay 1 formula uses a "Surgeon Factor."

The Holladay 2 formula, and the Hoffer Q formula, both use an "Anterior Chamber Depth." aka: ACD.

d = the effective lens position, where ...

d = a0 + (a1 * ACD) + (a2 * AL)

Haigis Formula

* The a0 constant basically moves the curve up, or down, in much the same way that the A-constant, Surgeon Factor, or ACD does for the Holladay 1, Holladay 2, Hoffer Q and SRK/T formulas.

* The a1 constant is tied to the measured anterior chamber depth.

* The a2 constant is tied to the measured axial length. The way the a0, a1 and a2 constants are derived is by generating a set of surgeon, and IOL-specific

AL < 19 mm (<0.1%) Holladay 2

AL 19-22 mm (8%) Holladay 2 , Hoffer-Q

AL 22-24.5 mm (72%) SRK II , Hoffer-Q ,Holladay 1

AL 24.5-26 mm (15%)Holladay 1 , Hoffer-Q

AL > 26 mm ( 15%)SRK/T

Formula Choice

Haigis formula may be appropriate for all ranges of axial lengths