Undergraduate ophthalmology lectures

1. ERRORS OF
REFRACTIO
NMOHAMED ABDELZAHER MD, FRCS

2. Change in the direction of rays as they pass between media of different refractive
index.

3. Magnifying lens Minifying lens
• Diopter is the unit power of lenses
• Diopter = 1 / Focal length in meters
Real Image Virtual Image

4. • Diopter is the unit power of lenses
• Diopter = 1 / Focal length in meters

5. • A cylinder is specified by its
axis
• The axis of a cylindrical
lens is parallel to the
cylinder from which the lens
cut
• The power of a cylinder in
its axis meridian is zero.
• Maximum power is 90
degrees away from the
axis. This is known as the
power meridian.
• The image formed by the
power meridian is a focal
line parallel to the axis.
• There is no line focus
image formed by the axis
meridian, because the axis

6. • Carries both spherical & cylindrical correction

7. • Prism deviates light toward
its base, thus the image is
displaced toward its apex
• Prism Diopter =
Displacement of image for 1
cm at 1 m distance

8. Converging Diverging
Magnifying Minifying
Tow Prisms Base to
Base
Two Prisms Apex to
Apex

9. The instrument used for measurement of lenses & prisms power is the Lensmeter.

10. • Change in the refractive power of the eye to
maintain a clear focus on variable distances.
• For near vision, the ciliary muscle contracts,
reducing the zonular tension & allowing the
elastic capsule of the lens to contract,
causing a decrease in equatorial lens
diameter and an increase in the curvatures
of the anterior and posterior lens surfaces.

11. • Accommodation power
decreases with age (Presbyopia)

12. • Far Point (Punctum Remotum, PR): Position of an object when its image clearly
falls on the retina with no accommodation.
• Near Point (Punctum Proximum, PP): The nearest point clearly seen with maximum
accommodation
• Range of accommodation: The distance between far & near point
• Amplitude of Accommodation: The difference in Dioptric power between eye at rest
& eye at maximum accommodation

13. • Far Point (Punctum Remotum):
Emmetropic eye; at infinity
Myopic eye; anterior to the eye
Hyperopic eye; virtual point behind the
eye

14. • Near Point (Punctum Proximum):
Presbyopia is physiological recession of near point of accommodation
N’ Normal near point
N Near point in Presbyopia

15. • The amount of convergence measured in prism diopters per unit (diopter) change in
accommodation
• Normal Value: 3- 5 Prism Diopter / 1 D.
• Abnormalities in AC/A ratio causes strabismus

16. • Visual Axis: Line passing between Object of fixation & Fovea.
• Optic Axis: Line passing through the centre of refractive media of the eye.
• Angle Alpha : Angle between visual axis & optic axis (Normally +5 degrees)
• Nodal point of the eye: Point through which light passing undeviated

17. • Angle between visual axis & optic
axis (Normally +5 degrees)
• Normally, Visual axis cuts cornea
NASAL to the optic axis

18. • Parallel light rays
come into a focus
on the retina,
when
accommodation
is relaxed.
• No refractive
error is present.

19. • Parallel light rays come
into a focus anterior to the
retina, when
accommodation is
completely relaxed.
• AKA Near sightedness
• PR is in front of the eye,
(MR & M’ are conjugate
points)

20. A. Axial: Long axial length (Average AXL is 22 - 23 mm)
B. Refractive:
1. Index; Increased RI of lens nucleus e.g. nuclear cataract or decrease RI of lens
cortex e.g. DM
2. Curvature; increase curvature of cornea (KCN) or lens (Subluxation or
lenticonus)

21. 1. Simple myopia
2. Degenerative (Progressive) myopia
3. Nocturnal myopia: Insufficient contrast, because of
dim illumination for an adequate accommodative
stimulus, the eye assumes the intermediate dark focus
accommodative position rather than focusing for infinity
4. Pseudo-myopia: Increase in ocular refractive power
due to overstimulation of the eye's accommodative
mechanism e.g. during exams or ciliary spasm
5. Induced (acquired) myopia: Result from exposure
to various pharmaceutical agents e.g. Phenothiazine,
active ocular inflammation (uveitis), variation in blood
sugar levels, nuclear sclerosis of the crystalline lens, or
other anomalous conditions. This myopia is often
temporary and reversible.

22. Nocturnal myopia
Causes of Night Myopia:
•Spherical aberration (The optical effect of the larger pupil decreases the depth
of focus)
•In dark adaptation the eye becomes more sensitive to
shorter wave lengths (Blue)
•Chromatic aberration: Shorter wavelengths come into focus in front of the
retina
•Ciliary spasm: the eye assumes the intermediate dark focus accommodative
position rather than focusing for infinity

23. 1. Low Myopia: < 3D
2. Medium myopia: 3 -6
D
3. High myopia: > 6D
1. Congenital: Present since birth
2. Youth onset: < 20 years
3. Adult onset: > 20 years

24. Simple Myopia Progressive Myopia
Common Less Common
Starts at Puberty Starts at younger age
Usually less than 6 D Up to 25 D or more
Normal fundus Degenerative retinal changes

25. • Blurred far vision
• Good near vision
• Young age
• Frowning in trial to improve vision
• Defective night vision (Dilated pupil increases effect of aberrations)
• Retinal symptoms: musca, photopsia (In high myopia > 6 D)

26. • Peri-papillary myopic annulus or
temporal crescent
Of High Myopia
(Chorio-retinal degeneration)

27. • Tigroid (Tessellated) Fundus
• Caused by atrophy of the retinal
pigment epithelium
• Underlying streaks of normal
choroidal pigmentation are
visible

28. • Lacquer Cracks
Cracks in Bruchs
membrane

29. • Fuchs spots
Cracks in Bruchs
membrane allows
neovessels to proliferate
under the retina & causes
recurrent retinal
haemorrhage

30. • Posterior staphyloma
• Posterior protrusion of week
sclera lined with choroid

31. • Vitreous floaters
• Caused by vitreous
degeneration or posterior
vitreous detachment

32. • Peripheral Retinal Degeneration
• e.g. Lattice degeneration
• Predisopse to rhegmatogenous RD

33. The myopic retina
Retina

34. • Nuclear
cataract
Of High Myopia
• Open Angle
Glaucoma

35. • Rhegmatogenous Retinal Detachment,
caused by peripheral retinal degeneration
Of High Myopia
• Macular
Hole

36. • Squin
t
Of High Myopia
In high myopia, the fovea lies NASAL to the optic axis. So, the visual axis
crosses temporal to the optic axis at the cornea. (-ve angle alpha)

37. • Squin
t
Of High Myopia
Right exotropia when fixating at a 6 metres
objection (A and B). Eyes straight with
myopic
correction (C). Exotropia disappeared when
the eyes looked at a near object (D). Right
exotropia
worsened when the eyes looked at an
object further than 6 metres (E).

38. Concave
Lenses
Concave
Lenses
Indications:
A. Cosmetic
B. Anisometropia
Advantages:
A. Less minification of image, 10%
(nearer to the nodal point of the eye)
B. Less aberration of lens periphery
(wider field)
Prescribe
the
lowest
correctio
n that
gives the
best
vision

39. Indications:
A. Cosmetic
B. Anisometropia
• A corneal flap is fashioned by an
automated keratome
• Excimer laser is used to ablate the central
corneal stromal bed, thus reducing the
curvature of the central part of the cornea
• The flap is re-positioned
• Can correct up to - 12 D according to
corneal thickness & curvature

40. • A corneal flap is
fashioned by an Femto
laser
• Excimer laser is used to
ablate the corneal
stromal bed, thus
reducing the curvature
of the central part of the
cornea
• The flap is re-
positioned
LASIK

41. • A lenticule of corneal storm
is fashioned using the femto
laser
• The lenticule is extracted
through a curvilinear small
inferior corneal incision
SMILE

42. Indications:
A. Thin corneas
B. Low refractive errors
• Corneal epithelium is removed
either mechanically or with alcohol
• Excimer laser is used to ablate the
corneal stromal bed, thus reducing
the curvature of the central part of
the cornea
• No flap
• Can correct up to - 4 D according to
corneal thickness & curvature

43. Indications:
A. Thin corneas
B. High refractive errors

44. Indications:
A. Thin corneas
B. High refractive errors
C. Old age > 40 years
Disadvantages:
A. Loss of accommodation
B. High incidence of post-operative
complications e.g. RD

45. • Radial Partial thickness cuts in
corneal stroma
• Increases the curvature of corneal
periphery
• Obsolete

46. • Parallel light rays come
into a focus posterior to
the retina, when
accommodation is
completely relaxed.
• AKA Far sightedness
• PR is a virtual point
behind the eye, (MR & M’
are conjugate points)

47. A. Axial: Short axial length (Average AXL is 22 - 23 mm)
B. Refractive:
1. Index; Decreased RI of lens e.g. cortical cataract
2. Curvature; decrease curvature of cornea (Cornea Plana)
3. Aphakia
4. Posterior Dislocation of the lens

48. 1. Total (Ht): Amount of hyperopia measured under the
effect of Atropine
2. Manifest (Hm): Amount of hyperopia measured
without Atropine
3. Latent (Hl): Amount of hyperopia caused by CB tone
(1D)
4. Facultative (Hf): Amount of hyperopia corrected by
Accommodation
5. Absolute (Ha): Amount of hyperopia NOT corrected
by accommodation (to be corrected optically)
• Ht = Hm + Hl
• Hm = Hf + Ha
• In young age with stronger accommodation, most, if not
all Hm is Hf
• Hf decreases with age as accommodation gets weaker

49. • None, in young age with small error because of strong accommodation
• Poor near vision
• Better far vision (later becomes poor, virtual PR)
• Ocular asthenia, because of continuous accommodation
• Early presbyopia

50. • Shallow
AC
Small eye

51. • Crowded Optic disc
(Pseudo swelling)
• with tortuous retinal
vessels
Small eye

52. • Narrow angle Glaucoma

53. • Squin
t
In high hyperopia, the fovea lies more Temporal to the optic axis than normal.
So, the visual axis crosses nasal to the optic axis at the cornea. (large +ve
angle alpha, > 5 degrees)

54. • Squin
t
Due to excessive accommodation

55. Convex
Lenses
Convex
Lenses
Indications:
A. Cosmetic
B. Anisometropia
Advantages:
A. Less magnification of image (nearer to
the nodal point of the eye)
B. Less aberration of lens periphery
(wider field)
Prescribe the
highest
correction
that gives
the best
vision

56. Indications:
A. Cosmetic
B. Anisometropia
• A corneal flap is fashioned by an
automated keratome
• Excimer laser is used to ablate the
periphery of corneal stromal bed, thus
increasing the curvature of the central
part of the cornea
• The flap is re-positioned
• Can correct up to + 6 D according to
corneal thickness & curvature

57. • A corneal flap is
fashioned by an Femto
laser
• Excimer laser is used to
ablate the corneal
stromal bed, thus
reducing the curvature
of the central part of the
cornea
• The flap is re-
positioned
LASIK

58. Indications:
A. Thin corneas
B. Low refractive errors
• Corneal epithelium is removed
either mechanically or with alcohol
• Excimer laser is used to ablate the
periphery of corneal stromal bed,
thus increasing the curvature of the
central part of the cornea
• No flap

59. Indications:
A. Thin corneas
B. High refractive errors
Might not be
applicable in some
cases with very
shallow AC & narrow
anterior segment

60. Indications:
A. Thin corneas
B. High refractive errors
C. Old age > 40 years
Disadvantages:
A. Loss of accommodation
Might not be a better
option than Phakic
IOL, especially in
cases with narrow AC
angle

61. Indications:
• Low refractive errors
Principle:
CTK delivers controlled-released
radio-frequency current (within
peripheral corneal stroma, raising the
temperature of the peripheral
collagen lamellae to the 65°C
resulting in controlled shrinkage of
the peripheral collagen lamellae &
subsequent central corneal
steepening.

62. • Parallel light rays do not
come into a focus but
rather a line.

63. A. Cornea:
1. Keratoconus
2. Corneal scars
3. Corneal stitches
4. Pressure over cornea
B. Lenticular:
Incipient cataract
Lenticonus
Lens subluxation
C. Retinal: e.g.
staphyloma in high
myopia, because of
posterior pole obliquity

64. • With (WTR)
• Against (ATR)
• Oblique
• Simple
• Compound
• Mixed

65. The two Principal Meridians (of highest & lowest power) are at right angle to
each others, and change of power from one meridian to the other is gradual &
regular
• Vertical meridian is more curved than
Horizontal meridian
• More common

66. • Vertical meridian is less curved than
Horizontal meridian
• Less common
• The two principal meridians at at right angle
to each other but they are not at 90 & 180
degrees
• Less common

67. • One principal meridian is emmetropic & the
other is ametropic, with accommodation
relaxed
• One principal meridian is Myopic & the
other is emmetropic
• One principal meridian is Hyperopic & the
other is emmetropic

68. • Both principal meridians are ametropic, with
same sign, with accommodation relaxed
• Both Principal meridians are myopic
• Both Principal meridians are hyperopic

69. • Both principal meridians are ametropic, with
opposite sign, with accommodation relaxed
• One Principal meridians myopic, the
other is hyperopic