Date post: | 16-Jul-2015 |
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Surgical induced Astigmatism
Dr. Namrata Gupta
Introduction
With continuous advances in cataract surgery, patients’ have higher expectations of surgical and visual outcomes
Astigmatism has considerable impact on quality of vision and is affected by surgical technique, the type and size of incision
Prevalence of Astigmatism
95% of eyes have some degrees of detectable naturally occurring astigmatic error, 60% needs correction
Incidence of post cataract surgery astigmatism – 7.5% -75%
Clinically significant astigmatism >2 D as high as 25 –30%
Astigmatism
Definition: It is a state of refraction where in the refractive power varies in the different meridia such that the rays of light entering in the eye cannot converge to a point focus but form focal lines
The word is derived from Greek α – without and stigma-Spot
A toric surface resembles a section of the surface of an doughnut where there are two regular radii, one smaller than the other one
• Astigmatic eyes : Two principle corneal meridians
a meridian of greatest corneal power
a meridian of least corneal power
Sturm’s conoid
• Geometric configuration of light rays emanating from a single point source and refracted by a toricsurface
Based on axis of the principal meridians
Regular astigmatism – principal meridians are perpendicular
• With-the-rule astigmatism
• Against-the-rule astigmatism
• Oblique astigmatism
Irregular astigmatism- Principle meridians are not perpendicular
Regular astigmatism
The refractive power changes uniformly from one meridian to another
• Etiology:
1. Corneal – abnormalities of curvature (Common)
2. Lenticular (rare)
• Curvatural – lenticonus
• Positional – tilting or oblique placement of lens, subluxation
3. Retinal – oblique placement of macula (rare), posterior staphyloma, scleral buckle
Irregular astigmatism
When the two principal meridians are not perpendicular to each other
Curvature of any one meridian is not uniform
Associated with trauma, disease or degeneration
Corneal- scars, keratoconus, marginal degeneration
Lenticular- cataract maturation
With-The-Rule Astigmatism
When the greatest refractive power is within 30° of the vertical meridian (between 60 ° and 120 °meridians
Correction with concave cylinder at horizontal axis (180 ± 20°) or convex at 90 ± 20°
Most common type
Against-The-Rule Astigmatism
When the greatest refractive power is within 30° of horizontal meridian (between 30° and 150 ° meridians)
Correction with concave cylinder at vertical axis (90 ° ± 20 °) or convex cyl at 180° ± 20 °
Oblique Astigmatism
When the greatest refractive power is within 30 ° of the oblique meridians (between 30 ° and 60 ° or 120° and 150 °)
Based on focus of principal meridians
Clinical features
Asymptomatic: small error
Blurring of objects
Circles become elongated into oval
A point of light appears tailed off
Asthenopic symptoms: headache, burning
Tilting of head, Squinting
Investigations
Retinoscopy
Keratometry
Computerized corneal tomography
Retinoscopy
Power is found to be dissimilar in different meridia
With the movement of retinoscope, the shadow appears to swirl around (oblique)
Finding the axis
1. Break in the alignment between the reflex in the pupil and the band outside it
2. Intensity of the reflex-Bright when aligned
3. Width of streak - Narrow when aligned
Finding the cylinder power
When one axis is neutralised with
spherical lens, movement is still
noticed in the second axis
+2 DS +1 DC at 90o
+3 DS
+2 DS
Keratometry
A keratometer measures the radius of curvature of a small portion of the central cornea (3mm)
Principle
The anterior corneal surface is treated as a specular reflector which forms a virtual image of a ring placed in front of cornea in form of mires
The radius of curvature of the image is converted in corneal power in different meridians
r = 2u (I / O)
u- distance from object to cornea
I- image size
O- object size
Principle
Measurement of radius of curvature ,r (meters) is converted to power , P (diopters) using formula:
P= (n2 – n1 )/r
n1 – refractive index of first medium
n2 – refractive index of second medium
Total power of cornea of anterior radius of 7.5 and RI-1.337=44.44 D
Keratometric reading
• After the mires are aligned, each of the vertical and horizontal drums yields a meridional reading in mm and Diopters (36-52 D)
Corneal topography
• Method of measuring and quantifying the shape and curvature of the corneal surface
Surgical induced astigmatism
Usually following cataract surgery
Usually induced by incision or suture
Caused by some degree of flattening of the corneal meridian at right angle to the direction of the incision
Determining variables
Incisional funnel An imaginary pair of curved lines approx. 3mm apart at
limbus that diverge from the limbus
Incisions made within this funnel will be astigmaticallyneutral
Incisions made very anteriorly results in more post-operative astigmatism than posteriorly
Configuration of external incision
Curvilinear incision: wound gape potential causes high against the rule astigmatism
Straight incision: lesser astigmatism than curvilinear type
Frown incision: least amount of astigmatism
Length of external incision
Smaller incisions causes less amount of astigmatism
A 3 mm incision length prevents >0.25 D flattening
Wider internal entry causes higher astigmatism
Orientation of the wound
Cornea flattens along the meridian of the scleral section, incision can be fashioned on the steep meridian
Incision located at the superior limbus will induce with-the-rule astigmatism
Incision located temporally will induce against-the-rule astigmatism
Effect of sutures
Using non-absorbable suture material (nylon) leads to with-the-rule astigmatism
Using absorbable or removable suture (silk) leads to against the rule astigmatism
Mechanism
Wound compression
Wound gape
Wound compression – corneal steeping
factors that appear to increase wound compression are
deeply inserted suture
Wide suture bite
Tightly tied suture
Greater number of suture
Tight sutures cause the peripheral cornea under the suture to be quite flat and bending of central cornea near apex
longer suture cause more steepening than shorter suture
Wound gape: corneal flattening
Loose sutures or suture placed too superficial can also result
It is associated with against-the-wound (ATW) astigmatism – cylinder 90° away from incision
Suture removal Tight sutures can be left intentionally in recognition of a
fairly rapid reduction in WTR astigmatism in initial week after large incision surgery – cylinder regression
Selective suture removing is recommended at 2 months post-op if >2-3 D of WTR astigmatism is present
Selective removal of sutures in axis of steepest curve can be done (axis of plus cylinder or higher keratometry)
Early suture removal(older) may result in progressive ATR astigmatism
IOL Tilt
Significant tilting required to induce clinically significant cylinder
A 20 D IOL must be inclined 10° form the vertical plane to cause 1 D cylinder
Keratometric astigmatism after ECCE
The study aimed to determine the keratometricastigmatism induced by interrupted suture in conventional ECCE with IOL implantation
24 eyes of 24 patients were studied . All patients received conventional ECCE with PCIOL implantation operated by single surgeon
Methods
All patients underwent conventional ECCE under peribulbaranesthesia
A fornix based conjunctival flap and scleral bleeding points cauterized by bipolar thermal cautery
Size of incision at superior limbus ̴ 8-8.5 mm (10-2’o clock)
Can opener Capsulotomy and in-the-bag PCIOL implant
Depth of suture bite ̴ 2/3rd thickness of cornea and sclera, length ̴2-3 mm on either side with Nylon 10-0 interrupted sutures
Keratometry of operated eye was taken on POD1
Surgical induced astigmatism calculator
• Surgically Induced Astigmatism Calculator (SIAC) has been designed to calculate the average amount of surgically induced astigmatism created during the cataract surgical procedure - Warren Hill, MD
SIA- Calculator
Discussion AS et al studied Keratometric Astigmatism after ECCE
in Eastern Nepal-Continuous Vs Interrupted sutures
The post operative astigmatism on Day 1 was 6.8 ± 0.61 D and 5.7 ± 0.18 D respectively in continuous and interrupted sutures
At the and of 6 weeks interrupted sutures induced astigmatism at 1.7 ± 1.35 D, significantly less than continuous sutures at 3.53 ± 2.19 D
The pattern of astigmatism was mostly WTR (60%)
Discussion
• Bansal et al studied ‘selective suture cutting for control of astigmatism following cataract surgery’ at PGIMER, 1992
• Mean keratometric astigmatism at three and six weeks post operative was 5.76 and 5.42 dioptres (D) respectively
• Selective suture cutting along the axis of the plus high cylinder was done after six weeks of surgery
• Mean post suture cutting keratometric astigmatism was 3.3 D and 70% of the eyes had astigmatism below 2 D
Basti et al ‘Extracapsular cataract extraction-surgical techniques’ 1993
Suture 1.5 mm long, equal length on either side, separated by distance equals length
Radially oriented sutures placed at 90% depth with optimal opposition of wound
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