Astigmatism

ASTIGMATISM
Dr. Sameeksha Agrawal

Astigmatism is refractive error of the eye in which there is
a difference in degree of refraction in different meridians
(i.e the eye has different focal points in different planes)
The image may be clearly focussed on the retina in the
horizontal (saggital) plane, but not on the retina in the
vertical (tangential) plane

Most Astigmatic corneas have two curves, a steeper curve and
a flatter curve. This causes light to focus on more than one
point in the eye, resulting in blurred vision.

Optics of Astigmatism
• In astigmatism, the rays of light from one sector fall on one
point and rays from another sector fall on another point
• A point focus of light can not be formed upon the retina
• The configuration of rays refracted through the astigmatic
surface (Toric surface) is called sturms conoid

• There are 2 focal points seperated from each other by a
focal interval, called as Interval of sturm
• The length of this focal interval is the measure of
degree of astigmatism and the correction of the error
can only be accomplished by reducing these two foci
in to one

• About 60% cases of refractive errors have astigmatism which
needs to be corrected.
• Occurs with equal frequency in males and females.
• Approximate distribution according to degree of astigmatism is :
 0.25-0.5D 50%
 0.75-1.0D 25%
 1.00-4.00D 24%
 >4.00D 1.0%
Incidence

• The most common type is compound myopic followed by
compound hyperopic, mixed, simple myopic and simple
hyperopic.
• One study reports as:
 With the rule 38%
 Against the rule 30%
 Oblique 32%

1. Corneal Astigmatism
• It occurs due to abnormalities of cornea
• Most common cause of astigmatism
• Eg- Keratoconus, Pterygium, Mild corneal opacities, chalazion
Etiology

2. Lenticular Astigmatism
It is comparatively rare
It may be:
• Curvatural  Lenticonus
• Positional  Congenital tilting and traumatic subluxation of lens
• Index  Cataract

Lenticular Subluxation Anterior Lenticonus

The severity of Astigmatism can be classified as follows:
• Mild Astigmatism <1.00 D
• Moderate Astigmatism 1.00 to 2.oo D
• Severe Astigmatism 2.00 to 3.00 D
• Extreme Astigmatism >3.00 D
Severity of Astigmatism

Astigmatism
Regular Irregular
Simple Compound Mixed
Classification

1. Astigmatism – Based on asymmetry of structure
• Corneal astigmatism – Due to irregularly shaped cornea
• Lenticular astigmatism – Due to irregularly shaped lens
Classification

2. Astigmatism- Based on axis of principal meridians
a. Regular Astigmatism
• Against-the-rule astigmatism
• With-the-rule astigmatism
b. Oblique astigmatism
c. Bioblique asigmatism
d. Irregular astigmatism

• The astigmatism is regular when refractive power changes uniformly
from one meridian to another (i.e there are two principal meridia)
• Can be corrected with spectacles
• Normally, horizontal curvature of cornea is flatter than vertical and
this is attributed to the pressure of lids on the corneal surface
So vertical cornea is more curved than horizontal
• On this basis, it has two types:
 With-the-rule astigmatism
 Against-the-rule astigmatism
Regular Astigmatism

• Principle meridia are at right angle to each other
• Vertical curve is more than horizontal
• Concave cylinder is prescribed in horizontal axis (180) and
convex are prescribed in vertical axis (90)
• Normally the vertical meridia is rendered 0.25D more convex
than horizontal by the pressure of fleshy upper eyelid
With-the-rule astigmatism

• Principle meridia are at right angle to each other
• Horizontal curve is more than Vertical
• Convex cylinder are prescribed in horizontal axis (180) and
concave are prescribed in vertical axis (90)
• Usually associated with old age
Against-the-rule astigmatism

• The principle meridia are not horizontal or vertical but are at right
angle to each other (45 and 135)
• 2 types -
 Symmetrical in both eyes (eg.cylindical lens required at 30 in both
eyes)
 Complementary (eg. Cylindrical lens required at 30 in one eye and
150 in other eye)
Oblique astigmatism

The two principle meridia are not at right angle to
each other (eg. One maybe at 30 and other at 100)
Bi-oblique astigmatism

• Characterized by irregular change of refractive power in
different meridia
• There are multiple meridia which admit no geometrical analysis
• Cannot be corrected by spectacles
• It occurs due to corneal scars, during maturation of cataract etc
Irregular astigmatism

• Simple Astigmatism
 Simple hyperopic astigmatism
 Simple myopic astigmatism
• Compound astigmatism
 Compound hyperopic astigmatism
 Compound myopic astigmatism
• Mixed astigmatism
3. Astigmatism – Based on focus of
principle meridians

• One of the foci falls on retina and other falls in front or behind retina
• One meridia being emmetropic and other being myopic
(one focus on retina and other focus falls in front of retina)
- Simple myopic astigmatism
Simple astigmatism

• One meridia being emmetropic and other being hyperopic
(one focus on retina and other focus falls behind retina)
– Simple hyperopic astigmatism

• Neither of the two foci falls on retina
• If both foci are at the back of retina
– Compound hyperopic astigmatism
Compound Astigmatism

• If both foci are at front of retina
– Compound myopic astigmatism

• One of the two foci lies at back while other at front of the retina
• In one meridian, the eye is myopic and in another hypermetropic
• It can be with the rule or against the rule
• Such patients have comparatively less symptoms as “circle of least
diffusion” is formed on retina.
Mixed Astigmatism

• Largest element of total astigmatism is due to anterior corneal surface
• Other components like
 Posterior corneal surface
 Lens
 Refractive indices
constitute the residual astigmatism
Residual astigmatism = Total – Corneal astigmatism
Residual astigmatism

Type of symptoms produced, depends on type of astigmatism:
1. Blurring of vision-
• Transient blurring of vision in low astigmatism
• Relieved by closing/rubbing eyes
• A point of light appears tailed off
• A line appears as a succession of strokes
fused into a blurred image
Signs and Symptoms

2. Asthenopic symptoms:
• More marked in patients with low astigmatism
(more accomodative effort)
• Severe in hyperopic astigmatism (more accomodative effort)
 Tiredness of eyes
 Headaches
 Nervous disturbances- Dizziness, fatigue, Irritability

3. Half closure of the lids :
• Seen in patients with high astigmatism
• This is done in order to make a Stenopaeic slit, and so by cutting
out the rays in one meridia, the object may appear more distinct

4. Reading material is held too close:
By many patients with high astigmatism in a bid to achieve large but
blurred image (as in myopes)
5. Burning and Itching:
• May be seen in patients with low astigmatism
• Patient develops habit of rubbing of eyes resulting in
 falling of lashes
 hyperemia of lid margin
 recurrent stye
 chalazia

• VA with and without correction monocularly
• Pinhole VA
• Retinoscopy
• Keratometry
• Keratoscopy with Placido’s disc
• Computarized corneal topography/videograph
Diagnosis

• Jackson cross cylinder
• Astigmatic fan and block
• Trial and error technique (axis then power)
• Maddox V
• Stenopaeic slit
Subjective verification

Scissoring Shadows:
 Sometimes seen in
‘Astigmatism’
 Two band reflexes
appear which move
towards & away from
each other like the
blades of a scissor.

Astigmatism test-
Tell patient to close one eye and then the other one,
If he does not see all the lined squares in the same black colour,
or sees one or more squares grey
Then he has an Astigmatism

Colour Scale
Red shifted-steep
& thin
Green reference colour
Blue shifted-flat & thick

The typical spiral pattern of keratoconus progression.
In color coded topographic images-
Red represents steeper corneal curvature
The spectrum of yellow, green and blue represents progressively flatter curvatures

Modes:
1. Spectacles
2. Contact lens
3. Laser
4. Refractive Surgery
5. Keratoplasty
Treatment

• Astigmatism is corrected optically with a cylindrical lens
• A combination of a spherical lens and a cylindrical lens
(Spherocylindrical lens) is used to correct a spherical
error with an astigmatic error.
• Cylindrical has power(Curvature) in one meridian and
no power in other meridian
• The axis of cylinder is lined up with the axis of
astigmatism to correct the astigmatic power difference
Spectacles

Various types of contact lenses are used:
• Soft
• Hard
• Rigid gas permeable
• Hybrid (Hard centre and soft periphery, used in
keratoconus)
Depending upon the degree of astigmatism:
• Spherical
• Toric
• Bitoric
Contact lens

• Photorefractive keratectomy PRK
• Relaxing incisions (Transverse and arcuate keratotomy)
• Wedge resection
• Compression sutures
• Continuous sutures (astigmatism low)
• Interrupted sutures (astigmatism high)
• Orthokeratology (Hard lens)
• Keratoplasty (Keratoconus)
Laser and Refractive Surgeries

• Points listed here are most applicable to the dynamics encountered in
cataract surgery
1.Normal cornea flattens over any incision
2.Radial corneal incisions flatten the adjacent cornea and the cornea 90*
away.
3.Flattening effect of radial incision increases as incision approach the visual
axis.
4.The cornea flattens directly over any sutured incision.
5.Cornea flattens adjacent to loose limbal sutures ,flattens 180* away and
steepens 90* away
6.Cornea steepens adjacent to tight limbal sutures ,steepens 180* away and
flattens 90* away
Astigmatism associated with cataract surgery

Factors affecting astigmatism after cataract
surgery
Pre-operative factors
1. Native astigmatism
2. Corneal changes
 Undetected keratoconus
 Corneal scarring
 Corneal thinning
 Corneal vascularisation
 Contact lens wear
3. Scleral changes
 Scleral thinning
 Scleral surgery, i.e., buckle

Factors affecting astigmatism after cataract
surgery
Intraoperative factors
1. Incision
 Length
 Location
 Architecture
2. Suture
 Technique
 Material
 Tension
 IOP
3. Cautery
4. Intraocular lens tilt
Postoperative factors
1. Wound healing
 General health, nutrition
 Topical steroids
2. Wound dehiscence
3. Suture slippage, hydrolysis
4. Suture cutting
5. Intraocular pressure

• Burgansky et al. have shown an increase in astigmatism
with an increase in the incision size.
• A 3-mm incision is too small to alter the corneal shape
appreciably. Such an incision maintains the preoperative
cylinder profile.
• Large incisions cause more cylinder regression.
Incision size

• Paul Koch described the “Incisional Funnel” indicating the
astigmatic neutral zone.
• More the distance from the limbus (on sclera), less is the
induced astigmatism although tunnel making and
maneuverability are difficult.
The ideal distance is around 1–2 mm.
Incision location

• The concept of an astigmatic neutral funnel is derived
from two mathematical equations:
1) Corneal-surgically induced astigmatism is directly
proportional to the cube of the length of the incision.
Astigmatism ∞ Length3
2) Corneal-surgically induced astigmatism is inversely
proportional to the distance of the incision from the
centre of the cornea.
Astigmatism ∞ 1/Distance
Astigmatic Neutral Funnel

Incision architecture
 Smile :
• A curvilinear incision which runs parallel to the limbus
• Easy to make
• Increased chance of corneal flattening after surgery in the
vertical meridian with increased induced astigmatism

 Straight :
• Straight incision, as the name suggests, is a straight line
incision about 2 mm away from the limbus
• Moderate flattening and consequently moderate
astigmatism after surgery

 Frown :
• The 15-G blade that is used to make the initial groove has
to be made with one smooth movement of the hand.
• The base of the curve is about 2 mm from the limbus.
• Difficult to make
• There is minimal induced astigmatism

 Blumenthal side cuts :
• A straight incision with oblique cuts placed at its either
ends.
• This increases the space in the tunnel for an easydelivery
of the nucleus
• Minimal induced astigmatism

 Chevron ‘v’ incision :
• The tunnel size in this incision is relatively smaller. Hence,
maneuvering a large nucleus through this would be
difficult.
• Difficult to make
• Least / nil induced astigmatism

• Types of clear corneal incisions:
1) Triplanar
2) Biplanar
3) Uniplanar
4) Hinged
• Valve action & Post –operative Astigmatism:
Hinged > Triplanar > Biplanar > Uniplanar
Clear Corneal Incision

Surgically induced astigmatism according to
incision length
Incision length (mm) SIA (D)
Scleral tunnel incision 2.0-2.5 0.05-0.10
3.0-3.5 0.25-0.50
4.0 0.45-0.70
5.0-5.5 0.50-0.90
6.0-7.0 1.00-2.00
10.0-11.0 2.50-3.00
Clear corneal incision 2.0-2.5 0.05-0.10
3.0-3.5 0.25-0.70
4.0 0.40-0.75
5.0-5.5 0.60-1.25

Incision location
Superior, Temporal or Nasal
• Anders and colleagues noted significantly more astigmatism 8 months
postoperatively with superior incisions than temporal CCI
• In a 12 month study of CCIs placed on the superior meridian, Long and
Monica noted superior incisions produced slightly more astigmatism
(0.9D) than horizontal incisions (0.6D)
• Barequet and colleagues found less induced astigmatism with
temporal (0.74D) than nasal (1.65D) 3.5 mm CCIs
‘Superior CCIs are closer to the corneal centre than lateral incisions’

• As the cataract wound heals, the meridian along which the
wound is centred tends to progressively flatten.
• This would usually occur till the healing of the tissues is
completed and usually would continue for up to 3 weeks
after surgery.
Effects of various incisions

It is suggested that the results are determined by one of the
two mechanism:
– Wound gape
– Wound compression
Suture techniques and materials

Wound gape
• Larger sutures that are removed [7-0 silk] and those that
tend to disintegrate early [7-0 catgut] permit the wound to
gape, thus increasing the circumference of the globe in the
meridian perpendicular to the line of incision this causes
flatening of vertical meridian and hence ATR.
• There is also some increase in curvature of horizontal
meridian .

• Finer sutures such as 10-0 monofilament nylon and 9-0 silk
,which are intended to remain in situ ,tend to compress the
wound . This tendency is increased if incision is made more
anteriorly and if sutures are inserted more deeply.
• Wound compression results a shortening of the vertical
meridian of the globe . This increases the curvature in vertical
meridian and hence astigmatism WTR .
• There is also some flattening of the horizontal meridian
Wound compression

• Most surgeons have a compulsion to pull up the sutures
tightest at their termination to ensure an air tight closure.
This tends to cause greatest wound compression near their
final point of anchorage.
• It was also found that closer the incision to the cornea
,greater the induced astigmatism , whether due to wound
gape or wound compression.
Wound compression

Factors that appear to increase wound compression are the
following:
• Fine sutures such as 10-0 nylon and 9-0 silk used to close
relatively anterior incision
• Deeply inserted fine sutures
• Wide suture bites
• Tightly tied sutures
• Greater number of sutures
• Overlapping of the wound.
Wound gape is favored by opposite of these factors.
Suture techniques and materials

Other operating factors affecting astigmatism
• Intraocular pressure : Securing wound in soft eye result in
greater induced post operative astigmatism , because greater
tension is placed on sutures after the IOP increases to normal
range in postoperative period
• Incision distance from cornea : Posterior incision minimizes any
slip the cornea may experience from a more anterior incision
length of suture ; greater vector forces are generated by each
long suture which opposes wider amount of tissue margin.
• Post op corticosteroids : Prolong use of steroids in selected cases
may allow great wound slippage to help treat preexisting WTR
astigmatism

Clinical application of findings :
Control of post op astigmatism
• From variety of suture techniques the surgeon may choose
one which will not exaggerate a pre existing corneal
astigmatism.
• Rule to be remembered is to cut the suture in the axis of
the correcting plus cylinder. This technique cannot be
used in instances of astigmatism caused by wound gape.
• Effect of suture cutting : Cutting the suture in the
meridian of the steepest corneal curvature is an effective
method of reducing high degrees of corneal astigmatism
caused by wound compression

• 2 transverse corneal incision produce the greatest degree of
flattening and radial component is not necessary
• Factors that influence the amount of meridional flattening are
1. Diameter of optical zone in which the incision are made
2 . Length and depth of transverse incision
3. Single vs dual paired incision
4. Age ,gender and IOP are other factors that determine the way the
cornea responds to relaxing incisions
Transverse Astigmatic keratotomy

• It creates corneal relaxation equidistant from the optical
centre along the entire length of the incision whereas the
tangential incision encompasses series of optical zones
that increases toward the end of incision .
• Therefore arcuate incision provide greater relaxation per
chord length than do transverse incision
Arcuate astigmatic keratotomy

• Conventional method dictated the use of relatively deep
incision fixed around 90% of corneal thickness .
• Length of incision was main factor controlling the
astigmatism .
• But a new method uses relatively long incision covering
the full arc of the steep area and controls the degree of
astigmatic correction by varying the incision depth.
Arcuate astigmatic keratotomy

Arcuate astigmatic keratotomy :

• With a diamond blade set at 600microns a single 6mm
incision placed anterior to palisades of vogt corrects upto
1D of astigmatism.
• For 1-2 D ,paired incisions are placed at steep axis and for
2-3 D each incision is extended to 8mm.
• For astigmatism more than 3 D limbal incision may be
combined with corneal relaxing incision.
• These incision prevent overcorrection ,as their effect is
weaker than that of corneal relaxing incision.
Limbal relaxing incisions :

Limbal relaxing incisions :
• In Against-the-rule astigmatism, Clear corneal Incision (CCI)
is placed at the steep meridian and the Limbal relaxing
incisions (LRI) are incorporated in the phaco incision
• In With-the-rule astigmatism, CCI is placed temporally,
placing the LRI at the steep meridian

• A small incision as far away from the centre of the cornea
as possible, with no gape & no suture
• It induces least postoperative astigmatism
Ideal Incision

Broad guidelines for the cataract incision
• Centre incision along steep meridian
• Longer incisions produce more flattening
• Posterior incisions decrease ATR drift
• Straight or frown incisions decrease ATR drift

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