3. ASTIGMATISM
Greek; A-without, Stigmos-
a point/a spot (Coined by
William Whewell;1849)
Definition
It is that condition of
refraction wherein a point
focus of light cannot be
formed upon the retina.
It occurs due to unequal
refraction of light in
different meridians.
4. Etiology of astigmatism
Corneal Astigmatism
Congenital
-occurrence is almost invariable in small degrees
-most commonly the vertical curve is greater than
the horizontal curve( about 0.5D). This is called
direct astigmatism.
-physiological
5. Acquired astigmatism
• Trauma to the cornea
including surgeries
• Pressure of swellings
on the lid( chalazion or
tumor)
• Pterygium
9. Retinal astigmatism
• The posterior pole of the eye may be placed
obliquely as when it bulges backwards in a
staphyloma in high myopia.
10. Types of astigmatism
Regular astigmatism
• The two principle meridians are at right angles
and are therefore susceptible to correction
– With the rule astigmatism
– Against the rule astigmatism
– Oblique astigmatism
Irregular astigmatism
• There are irregularities in the curvature of the
meridians so that no geometric figure is adhered
to
• Cannot be corrected adequately by spectacles
12. With-The-Rule (WTR) Astigmatism
• The two principal meridians
are at right angle to each
other with the vertical
meridian being more curved
than the horizontal i.e. the
greatest refractive power is
at 90˚(+/-20˚)
• Corrected by convex
cylinder at 90˚(+/-20˚) or
concave cylinder at 180 (+/-
20˚)
13. Against-The-Rule (ATR) Astigmatism
• The two principal meridians
are at right angle to each
other with the horizontal
meridian being more
curved than the vertical i.e.
the greatest refractive
power is at 180˚(+/-20˚)
• Corrected by convex
cylinder at 180 ˚(+/-20˚) or
concave cylinder at 90 ˚(+/-
20˚)
14. Oblique (OBL) Astigmatism
• The two principal
meridia are not
horizontal or vertical
though they are at
right angle to each
other.
15. Bi-oblique Astigmatism
• The two priciple
meridians are not at
right angle to each
other but are
crossed obliquely.
30˚
100˚
16. Regular astigmatism can further be
classified as
• Simple astigmatism
• Compound astigmatism
• Mixed astigmatism
17. Simple astigmatism.
• One of the foci falls
upon the retina
while the other falls
infront of or behind
the retina
Simple myopic astigmatism
Simple hyperemetropic astigmatism
18. Compound astigmatism
• Neither of the foci
fall upon the retina
but both are placed
in-front of or behind
the retina Compound myopic astigmatism
Compound hypermetropic astigmatism
19. Mixed astigmatism
• One focus is infront
of the retina while
the other is behind
so that the refraction
is hypermetropic in
one direction and
myopic in the other
direction
Mixed astigmatism
20. Irregular astigmatism
• The refraction in different
meridians are different.
• Small degree occurs
physiologically owing to
minute difference in the
refractive index of the
lens.(accentuated by
cataract)
• Marked degree occurs in
pathological conditions of
cornea
– Irregular healing of cornea
after trauma or
inflammations
– Conical cornea(keratoconus)
21. Symptoms of astigmatism
• Reduced Visual acuity
• Peculiar vision
• Asthenopia( more with hypermetropic
astigmatism)
• Head tilt in oblique astigmatism in children
• Half closure of the eyelids
22. Clinical Tests
• Visual acuity tests – distance and near
• Autorefraction
• Keratometry
• Retinoscopy
– Most reliable source of information for cylinder power
and axis
• Monocular subjective refraction
– Astigmatic fan test
– Jackson cross cylinder
24. Retinoscopy
• Shows different power in different
meridians.
• If the streak passes exactly through
the axis of the astigmatism, a sharply
defined reflex band is seen which
moves parallel to the band of the
light outside the pupil.
• If it doesn’t pass exactly through the
astigmatic axis, the reflex becomes
poorly defined and tends to remain
fixed in the axis producing a break in
the alignment between the reflex in
the pupil and the band outside it
tending to lie intermediate between
the latter and the true astigmatic axis
25. Cross-cylinders
• Used to refine the axis and
the power of the cylinder.
• Done after the cylindrical
correction has been made.
• Combination of a negative
and positive cylinder of
equal strength (usually
0.5D) mounted at an angle
of 90 degrees
• In practice combined into
one piece of glass
26. Refinement of the axis
• The cross-cylinder is placed with its
axis at 45˚ to the axis of the cylinder in
the trial frame( first with +0.5 cylinder
and then with -0.5 cylinder or vive-
versa) and the patient is asked to tell
any change in the visual acuity.
• If the patient notices no difference
then the correcting cylinder in the
trial frame is correct.
• If visual improvement is attained by
one or the other the correcting
cylinder is turned slightly in the
direction of the axis of the cylinder of
the same denomination in the cross
cylinder.
• It is repeated until the position of the
trial cylinder is found at which
rotation of the cross cylinder gives no
alteration in distinctness.
27. Refinement of the power of the cylinder
• The cross-cylinder is placed
in the same direction as the
axis of the cylinder in the
trial frame and then
perpendicular to it.
• If the visual acuity is
unimproved in either
position the power of
cylinder in the trial frame is
correct.
• If visual acuity is improved
in any of the positions then
change should be made.
28. Astigmatic fan test(Maddox V test)
• The patient is asked to look
at the figure and if any of
the line is more distinct
than the other then
astigmatism is present.
• Rotating the V slightly to the
direction of the blacker limb
an intermediate position is
reached when the two
limbs of V are equally
distinct. This gives the
direction at right angles to
the exact axis of the
correcting cylinder.
30. Guidelines for optical correction
• Small astigmatism(0.5D or less) should be treated
only if there is visual deterioration or asthenopic
symptoms.
• High astigmatism should be fully corrected
• Change in the axis of the lenses in patients used
to the previous axis should be done cautiously
• New astigmatism correction in adults is not
tolerated. In such cases it is better to undercorrect
and give full correction gradually.
• Bi-oblique astigmatism, mixed and high
astigmatism are better treated by contact lenses
• Spherical component should also be corrected.
31. Management
Refractive Surgery
Incisional correction of
Astigmatism
• Transverse keratotomy
• Arcuate keratotomy incisions
are placed in the cornea at
7mm optical zone) to the
steepest corneal meridian.
• Can correct upto 4-6D of
astigmatism.
33. Conductive keratoplasty
• CK is a nonablative, collagen-shrinking
procedure based on the delivery of
radiofrequency energy through a fine
conducting tip that is inserted into the
peripheral corneal stroma
• As the current flows through the
tissue surrounding the tip, resistance
to the current creates localized heat.
Collagen lamellae in the area
surrounding the tip shrink in a
controlled fashion and form a column
of denatured collagen. The shortening
of the collagen fibrils creates a band of
tightening that increases the curvature
of the central cornea.
• Corrects <0.75D of astigmatism
34. Laser based corneal refractive procedure
• Photo refractive
keratotomy(uses cylindrical
photoablation pattern.)—can
correct upto 3D of astigmatism
• Astigmatic LASIK can correct
upto 5D
• Wavefront-guided and
wavefront-optimised LASIK can
correct
– -10 to -12D of Myopic astigmatism
– +6D of hyperopic astigmatism and
– +5D of mixed astigmatism
36. Treatment of irregular astigmatism
• Spectacles though may not provide full
correction some amount of correction can be
attempted
• Contact lenses
• Surgical correction by keratoplasty
38. Definition (Latin; old man’s eye/aged eye)
Presbyopia refers to the slow, normal,
naturally occurring, age-related, irreversible
reduction in maximal accommodative
amplitude (i.e., recession of the near point)
sufficient to cause symptoms of blur and
ocular discomfort or asthenopia at the
customary near working distance.
39. • first reported clinically between 40 and 45
years of age, with its peak onset between 42
and 44 years
• Its onset may occur any time from 38 to 48
years of age, depending on a variety of
factors.
• From approximately age 52 years on, the
prevalence of presbyopia is considered to be
essentially 100%
40. Risk factors for development of presbyopia
Refractive error
• Hyperopes have their near point considerably
further away than emmetropes(exhibit apparent
relatively reduced accommodative amplitudes)
and thus effectively become presbyopic a few
years earlier than either myopes or emmetropes.
• In the myopes develop presbyopia later in life and
if he has the error of -4D, presbyopia will never
develop
41. Ambient Temperature
• With the eyeball being peripheral to the body
core, it may exhibit considerable surface
temperature variations because of the
influence of ambient temperature.
• There is an inverse relation between ambient
temperature and age of onset of presbyopia
42. Factors contributing to the age related
decrease in accomodation
• Decrease in modulus of elasticity of lens
capsule—cannot effectively mold the
underlying lens substance
• Increase in stiffness of the lens substance—
energy required to deform the lens substance
increases
• Increase in size/volume of lens—decreases
the effectivity of lens capsule function
43. • Anterior shift of the equatorial zonular fibres
due to inrease in the size of the lens
• Decrease in equatorial zonular fibres
• Increase in the number of disulfide bond in the
lens substance—stabilises the collagen
molecules within the capsule and the lens by
the process of cross-linking—lens substance
becomes rigid and capsule becomes less elastic
• Mechanical and anatomical changes in the
ciliary muscle
• Decrease in choroidal elasticity
44. Amplitude of accomodation and age
• The amplitude of accommodation represents the
maximal accommodative level, or closest near
focusing response, that can be produced with
maximal voluntary effort in the fully corrected
eye.
• It is calculated as
Accomodative amplitude(A)= diopteric power
needed to see clearly at near point-diopteric
power needed to see clearly at far point
45. At the age of 10 yrs
Near point=7cm
Far point=∞
Hence,
A= 100/7-1/∞
=14-0
=14D
46. At the age of 40 yrs
Near point=25cm
Far point=∞
Hence,
A=100/25-1/∞
=4-0
=4D
49. Symptoms
• Reduced vision at customary near-work distance.
• Drowsiness after short period of reading or near work
• Reading materials must be held farther away
• Asthenopia related to attempts at excessive
accommodative effort is reported. It may even lead to
an accommodative spasm and pseudomyopia.
• Transient diplopia and variable esophoria may be
experienced as a result of the increased
accommodative response/effort and the consequent
synkinetically overdriven accommodative convergence
that may be difficult to control consistently using
compensatory negative fusional vergence.":"
51. Treatment
• The treatment of presbyopia is to
provide the patient with convex
lenses so that his accomodation
is reinforced and his near point
brought within a useful working
distance.
• To do this
– know the working distance of the
individual
– estimate his refraction
– determine the amplitude of
accomodation
– supplement this by appropriate
strength of lens allowing him
sufficient reserve of accomodation
52. • One-third of the amplitude of accomodation
should be kept for reserve if the patient is to
work comfortably
• The near correction should never be
overcorrected. It should be such that the
patient should be able to read the near vision
chart satisfactory not only at his reading
distance but also some 12-15cm further away.
53. Example
• Emmetropic patient, whose working distance is
25cm has his near point receded to 50cm
• His amplitude of accomodation is 2D
• With 1/3rd kept for reserve, remaining amplitude
is 2/3rd i.e. 1.3D
• To see at the distance of 25cm, he will require
amplitude of 4D
• Hence the prebyopic lens he requires is
4-1.3=2.7D
55. • Contact lenses
– Single vision contact lenses with glasses
– Monovision contact lenses
– Bifocal and multifocal contact lenses
– Modified monovision contact lenses
56. Correcting Presbyopia: Contact Lens
Monovision
Dominant eye:
mainly
corrected
for distance
Non-dominant eye:
mainly corrected
for
near
59-67%
Patients Tolerate
Brain merges two images
to see near and far
without
glasses
57. Refractive surgery for presbyopia
Non-Accomodative treatment of presbyopia
• Monovision
• Conductive keratoplasty
• Multifocal IOL implants
• Custom or Multifocal Ablations
• Corneal Inlays
58. Refractive surgery for presbyopia
Accomodative treatment of Presbyopia
• Scleral surgery
• Accomodating IOLs
59. Monovision by refractive surgery
In this mild myopia is created in the near eye in
the presbyopic or peripresbyopic population.
Also called modified monovision.
The best candidates for modified monovision
are myopic patients over the age of 40
60. Conductive keratoplasty
• conductive keratoplasty (CK) is a
nonablative, collagen-shrinking
procedure approved for the
correction of low levels of
hyperopia(+0.75 to +3.25 D). In
CK, radiofrequency (RF) energy
is delivered through a fine
conducting tip inserted into the
peripheral corneal stroma in a
circular pattern. The application
of RF energy shrinks the
collagen in the periphery, which
steepens the central cornea and
induces a myopic shift.
• Treatment of presbyopia inn
hyperopic and emmetropic
individuals
61. Multifocal IOLs
• They are good options
for patients
undergoing cataract
surgery.
ReZoom Multifocal IOL showing 5
concentric refractive zones
ReSTOR multifocal IOL with apodized
diffractive changes in lens surface
62. Custom or multifocal
ablations
• Uses eximer laser to create a
multifocal cornea.
Corneal Inlays
• Placing a biocompatible polymer
lens in the central cornea either
beneath a LASIK flap or via a
stromal tunnel.
• Inlays create near vision through
different methods like change in
corneal curvature, multifocality,
pin-hole effect.
63. Accomodative treatment of
presbyopia
Scleral surgery
• Increase zonular tension
by weakening or altering
the sclera over the ciliary
body to allow for its
passive expansion
• Consists of
– Anterior ciliary
sclerotomy
– Placement of scleral
expansion bands
64. Accomodating IOLs
• During ciliary muscle
contraction, forward
displacement of IOL
led to the increase in
effective IOL power
and increase in near
vision
Crystalens has flexible hinge in the
haptic at the proximal end and a
polyamide footplate at the distal end
67. CAUSES
• Congenital absence of crystalline lens
• Surgical aphakia after cataract extraction
• Post-traumatic absorption of lens
• Traumatic extrusion of lens
• Posterior dislocation of lens
68. Optics of an aphakic eye
• High hypermetropia
• Total power of the eye reduced to +44 D
• Aphakic eye consists of a curved surface, ie.
cornea , separating two media of different
refractive indices, air (1) and aqueous and
vitreous humor (1.33)
• Anterior focal point becomes 23.2 mm in front
of the anterior surface of the cornea
• Posterior focal point lies 31 mm behind the
anterior surface of the cornea
• Total loss of accomodation
69.
70. CLINICAL FEATURES
• Symptoms: defective vision for near and far
• Signs:
- Limbal scar may be seen in case of surgical aphakia
- Deep anterior chamber
- Jet black pupil
- Iridodonesis
- Purkinje image test: 3rd and 4th images are absent
- Fundus examination: small hypermetropic disc
72. Difficulties encountered in correcting
aphakia with the use of spectacles:
- Image magnification of
~30% (cannot be used to
correct unilateral aphakia
as it will result in diplopia)
- Distance misjudgement
leading to accidents
- Spherical aberrations
resulting in “pin-cushion”
effect
73. Jack-in-the-box
phenomenon
• Prismatic deviation
occurring at the periphery
of a strong lens gives rise to
a ring of blindness around
the central field.( about
15˚extending from 50˚-65˚
from central fixation)
• When the eyes move, the
circle of blindness also
moves(in opposite
direction) so that a person
or and object appear and
disappear like Jack in the
box.
74. • Restricted field vision
• Coloured vision due to
chromatic abberation
• Cosmetic blemish
• Cumbersome to use
• Problem of near vision
75.
76. Contact lenses
Advantages:
- Less image magnification
(6-8%)
- Elimination of
aberrations and prismatic
effects of thick glasses
- Better field of vision
- Cosmetically more
acceptable
- Suitable for unilateral
aphakia
77. • Disadvantages of contact lenses:
- Costly
- Cumbersome to use esp in children and
elderly
- Associated corneal complications
78. IOL implantation
• Best available method of
correcting aphakia
• The commonest
modality being
employed nowadays.
79. Refractive surgery for aphakia
• Keratophakia: plus power lens is placed
intrastromally to increase the curvature of
anterior cornea.
– Homoplastic(prepared from donor cornea)
– Alloplastic( prepared from synthetic material)
• Epikeratophakia: a lenticule prepared from the
donor cornea is sutured to the surface of
patient’s cornea after removing the
epithelium
80. References
• Duke-Elder’s Practice of Refraction(10th edition)
• AK Khurana Theory and Practice of Optics and
Refraction
• American Academy of Ophthalmology, Basic and
Clinical Science Course,Clinical Optics(2011-2012)
• American Academy of Ophthalmology, Basic and
Clinical Science Course, Refractive Surgery (2012-
2013)
• Borish’s Clinical Refraction
• Ophthalmology -Myron Yanoff & Jay S.Duker
