Astigmatism

1. Astigmatism, Aniseikonia
& Anisometropia
Presenter : Dr. Om Patel
Moderator: Dr. Minal Kaur

2. REFRACTION
 Refraction of light occurs when light passes from one
medium to another of different refractive index.

3. REFRACTIVE PHYSIOLOGY
 Light rays are focused on the retina because they are refracted
passing through the cornea and lens (Snell’s Law)
 Corneal refractive power is constant
 Lens refractive power is modifiable with accommodation

4. EMMETROPIA
 Adequate correlation between axial length and refractive power
 Parallel light rays fall on the retina (relaxed accommodation)
Fovea
Light rays

5. ASTIGMATISM
• Type of refractive error wherein the light fails to
come to a single focus on the retina
• Unequal refraction of light in different meridia
• Resulting in blurred vision

6. • Based on axis of the principal
meridia:
• Regular astigmatism – principal
meridia are perpendicular
• Irregular astigmatism - principal
meridia cannot be defined
E.g. Keratoconus, Corneal scars
TYPES OF ASTIGMATISM

7. Regular astigmatism
The refractive power changes regularly from one meridian
to the other
Aetiology
Corneal – curvatural abnormalities
Lenticular – rarer
- Curvatural: congenital abnormalities in curvature
- Positional: d/t tilting or oblique placement of the lens
- Index : DM and cataract

8. Types of Regular astigmatism
• With-the-rule astigmatism
• Against-the-rule astigmatism
• Oblique astigmatism
• Bi-oblique astigmatism

9. With the rule astigmatism
• Vertical meridian is steeper than horizontal meridian
• Eyes see vertical lines more sharply than horizontal
lines
• Requires concave cylinders at 180 ͦ+ /- 20 ͦ
• Or convex cylinders at 90 ͦ +/- 20 ͦ

11. Against the rule astigmatism
• Horizontal meridian is steeper than vertical meridian
• Eyes see horizontal lines more sharply than vertical
lines
• Requires concave cylinders at 90 ͦ +/-20 ͦ
• Or convex cylinders at 180 ͦ+/-20 ͦ

12. Oblique astigmatism
• Two principle meridians lies somewhere
between the axis defining either with-the-rule
or against-the-rule astigmatism
• Complementary (45 ͦin one meridian,135 ͦ in
other meridian)

13. Bi-oblique astigmatism
• Principle meridia are not at right angle
• One at 30 ͦ& other at 100 ͦ
• Rarely seen

14. Refractive types of regular
astigmatism
• Simple
Rays are focused on retina in one meridian or either in
front or behind in the other.

15. • Compound
Rays of light in both meridia are focused either in front or
behind the retina.

16. • Mixed
Light rays in one meridian are focused in front and in the
other meridian behind the retina.

17. EXERCISE
1. -1.00 DC * 180
- SIMPLE WTR
2. -1.00 DS / -2.00 DC * 170
- COMPOUND WTR
3. +1.00 DS / -3.00 DC *180
- MIXED WTR
4. +1.00 DS *165
- SIMPLE ATR
5. +1.00 DS / +2.00 DC *180
- COMPOUND ATR
6. -1.00 DS / -2.00 DC * 135
- OBLIQUE

18. Optics of regular astigmatism
• Parallel rays of light are not brought to focus on a point
but form focal lines
• Configuration of rays refracted from an astigmatic
surface– STURM’S CONOID
• Distance between to focal lines– focal interval of Sturm
(measures the degree of astigmatism)

19. Symptoms
Blurring of vision
• Transient blurring of vision
• Relieved by rubbing or closing eyes
• Tries to focus only on one meridian, meridian near to
emmetropia (mostly vertical) is chosen
• Point of light appears tailed off
• A line appears as a succession of strokes fused into
blurred image

20. Asthenopic symptoms
• Includes tiredness of eyes
• Headache(mild frontal ache to violent explosions of
pain)
• Dizziness
• Irritability, fatigue
• Symptoms more common in patients with
Low astigmatism
Hypermetropic astigmatism

21. Symptoms
• Tilting of head- to reduce image distortion.
• Half closure of lids – high astigmatism in an effort to
make a stenopaeic slit
• Rubbing of eyes, hyperaemia of lid margin recurrent stye
and chalazia

22. Investigations-Retinoscopy
• Shows different
power in two
different axis

23. KERATOMETER
Bausch and Lomb keratometer Javal and schiotz keratometer

24. JACKSON’S CROSS CYLINDER
TEST
It is a combination of
two cylinders of
equal strength but
with opposite sign
placed with their axis
at right angles to
each other and
mounted in a handle.

25. COMPUTERISED CORNEAL
TOPOGRAPHY
• Corneal topography
system or
videokeratography
implies computerised
video assisted
technique that
provides detailed
information about
shape of corneal
surface

26. Treatment Options
• Spectacles
• Single vision glasses with cylinder
• Contact lenses
• Toric soft contact lenses
• Rigid gas permeable contact lenses
• Refractive surgery
• Photorefractive keratectomy (PRK)
• Laser in-situ keratomileusis (LASIK)
• Penetrating Keratoplasty (PK)
• Corneal Cross-Linking With Riboflavin and Ultraviolet
Irradiation

27. AAO GUIDELINES
• In children, give the full astigmatic correction
• In adults, try the full astigmatic correction first. Give warning
and encouragement. If problems are anticipated, try a
walking-around trial with trial frames before prescribing
• To minimize distortion, use minus cylinder lenses and
minimize vertex distances
• Spatial distortion from astigmatic spectacle corrections is a
binocular phenomenon. Occlude one eye to verify that this is
indeed the cause of the patient's complaints

28. AAO GUIDELINES
• If necessary, reduce distortion still further by rotating the
cylinder axis toward 180 or 90 (or toward the old axis) and or
by reducing the cylinder power. Balance the resulting blur with
the remaining distortion, using careful adjustment of cylinder
power and sphere.
• Residual astigmatism at any position of the cylinder axis may
be minimized with the Jackson cross cylinder test for cylinder
power.
• If distortion cannot be reduced sufficiently by altering the
astigmatic spectacle correction , consider contact lenses
(which cause no appreciable distortion) or iseikonic
corrections.

30. Irregular astigmatism
• Characterized by an irregular change of refractive power in
different meridia
Etiological types
1. Corneal irregular astigmatism – corneal scars or
keratoconus.
2. Lenticular irregular astigmatism
3. Retinal irregular astigmatism – due to distortion of the
macular area

31. Symptoms:
Defective vision , distortion of objects and polyopia.
Treatment
• Optical – prescribing contact lenses which replace the
ant. Corneal surface.
• Surgical – indicated in extensive corneal scarring and
consists of PK.

32. ANISEIKONIA
• Anomaly of binocular vision
• Ocular images are unequal in size or shape or
both
• Its importance lies in causation of eye strain
which is difficult to assess

34. ETIOLOGY
• Inherent
• Acquired anisometropia of
high degree
Optical
aniseikonia
• Displacement of retinal
elements towards nodal point
in one eye
Retinal
aniseikonia
• Asymmetrical simultaneous
perception in spite of equal
size of two images
Cortical
aniseikonia

35. CLINICAL TYPES
Symmetrical
i. Spherical.
Image may be magnified or minified equally in both meridia
ii. Cylindrical
Image is magnified or minified symmetrical in one meridian

36. A – Spherical B – Cylindrical C – Prismatic D – Pincushion E – Barrel
distortion F – Oblique distortion

37. Asymmetrical
i. Prismatic
Image difference increases progressively in one direction
ii. Pincushion
Image distortion increases progressively in both directions, as
seen with high plus correction in aphakia
iii. Barrel
Image distortion decreases progressively in both direction, as
seen with high minus correction
iv. Oblique
Image size remains the same but there occurs an oblique
distortion of shape

38. Symptoms
• Asthenopic symptoms
Occurs when the differences in image size of the two images is
between 0.75 to 5.0%
Headache , difficulty reading, photophobia, difficulty of fixation,
vertigo, etc
• Disturbances of binocular vision
Diplopia occur only if the difference exceeds 5%
• Disturbances in depth perception and spatial disorientations
• Suppression of one eye

39. Treatment
• Optical aniseikonia
- Unilateral aphakia is best corrected by IOL implantation
- Contact lenses are a better choice than spectacles for
correcting anisometropic aniseikonia
- Refractive corneal surgery
• Retinal aniseikonia
- Corrected by treating the causative disease

40. ISEIKONIC LENSES
• Lenses which cause magnification without introducing
any appreciable refractive power by changing the
direction of the pencils of light passing through them

41. ANISOMETROPIA
• Total refraction of the two eyes is unequal
• Difference of 1D in two eyes cause a 2% difference in the
size of the two retinal images
• 5% size difference / 2.5D - well tolerated
• 2.5-4D – individual sensitivity
• >4D – not tolerated

42. • Alternate vision when one of the two eyes is used at a
time
• Occurs when one eye is emmetropic or moderately
hypermetropic and the other myopic
• Patient uses former eye for distant vision and latter
for near vision
• Anisometropia leads to development of squint

43. UNIOCULAR VISION
• If defect in one eye is higher, it may be excluded from
vision at an early stage in life
• The more ametropic eye becomes amblyopic
• This amblyopia from disuse is known as amblyopia ex
anopsia

44. ETIOLOGY
Congenital and developmental
Acquired
Uniocular aphakia
Wrong power IOL
Trauma
Keratoplasty

45. CLINICAL TYPES
Simple
One eye normal, other myopic / hypermetropic
Compound
Both eyes either myopic / hypermetropic
Mixed
One eye myopic, other eye hypermetropic

46. DIAGNOSIS
Retinoscopy under cycloplegia
State of vision- FRIEND TEST / WORTH’S FOUR DOT TEST
FRIEND Test
F, I, N - GREEN
R,E,D - RED
1. The patient wears red green goggles and is seated at
a distance of 6m from the chart
2. Binocular single vision- will read FRIEND at once
3. Uniocular vision – will read either FIN or RED
4. Alternate vision– will read FIN at one time and RED
at other time

47. WORTH’S FOUR DOT
TEST
Sees all four lights in
absence of manifest squint-
normal binocular vision
ARC- sees four lights in
presence of manifest squint
Sees 2 red lights- LE
suppression
Sees 3 green lights- RE
suppression
Sees 2 red and 3 green
alternately- alternate
suppression
Sees 5 lights(2red, 3
green)- diplopia

48. TREATMENT
Glasses
• In children prescribe full refractive difference regardless
of age, presence of strabismus or not, degree of
anisometropia
• The corrective spectacles can be tolerated up to a
maximum difference of 4D,after that diplopia occurs.
• So in children where best corrected visual acuity is
required in both eyes, contact lenses are preferred

49. • In adults, the more ametropic eye is under corrected
• In adults with alternating vision the condition is usually
left alone
CONTACT LENSES
• Advised for higher degrees of anisometropia and for
children

50. • Anisometropic
spectacles-
In these spectacles
margin of the
stronger lens is made
weaker, thus
minimizing the
annoyance of
peripheral prismatic
effect of
conventional lenses

53. OTHER MODALITIES
• Intraocular lens implantation for uniocular aphakia
• Refractive corneal surgery for unilateral
myopia,astigmatism,hypermetropia
• Phakic refractive lenses(PRL) - for 4 to 10D
• Refractive lens exchange(RLE) - for more than 10D

54. THANK YOU