Surgical induced astigmatism

Surgical induced
Astigmatism
Dr. NamrataGupta

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 toric
surface

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

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 astigmatically
neutral
 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 keratometric
astigmatism 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 peribulbar
anesthesia
 A fornix based conjunctival flap and scleral bleeding points
cauterized by bipolar thermal cautery
 Size of incision at superior limbus̴8-8.5 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

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

Surgical induced astigmatism

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Editor's Notes