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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