A lecture from Penang Medical College on Refractive error

1. REFRACTIVE ERRORS
Dr. LEOW THYE YNG
BSc (Hons), MB BCh BAO (NUI),
Dip.Opt., MCOptom, MRCOphth,
FRCS (Glasg).

2. Lenses
POSITIVE or CONVERGING
NEGATIVE or DIVERGING
positive negative

3. The POWER of a lens , P
The “ DIOPTRE (D ) “
A positive lens of one dioptre (+1.00D)
converge parallel light rays to a ‘real’ focal
point one metre from (after) the lens.
A negative lens of one dioptre (-
1.00D) diverge parallel light rays as if
they are coming from a ‘virtual’ point one
metre infront of the lens.

4. ONE DIOPTRE LENS
P = +1.00 D
f = +1.0 m or
+100 cm
P = - 1.00 D
f = -1.0 m or
-100 cm

5. The DIOPTRE (D)
1
Power, P (D) = ---------------------------
Focal Length, f (metre)
eg. If P = +2.00 D, f = +0.5 m or +50 cm
eg. If P = +4.00 D, f = +0.25m or +25 cm
eg. If P = -2.00 D, f = -0.5 m or -50 cm
eg. If P = -4.00 D, f = -0.25m or -25 cm

7. Refractive Index (n)
Velocity of light in vacuum
n =
---------------------------------------
Velocity of light in the medium
eg. Air n = 1
eg. Water n = 1.33
eg. Cornea n = 1.376
eg. Crystalline lens n = 1.38 to 1.42
eg. Crown glass n = 1.52

8. Refractive Power of a
curved surface ( P )
n2 - n1
P (dioptre)=
---------------------
r (metre)
where r = radius
of curvature of the
refractive surface in metres

9. Refractive Power of the
anterior corneal surface
n1 = 1.0 (air) , n2 = 1.376 (cornea) , r
= 8mm or 0.008m (radius of curvature
of cornea)
P = (1.376 - 1) / 0.008 = 47 D

10. Refractive Power of the
Cornea
The total refractive power of the cornea is
approx. +40 D (ie. less than +47D for the
anterior surface as this is reduced by the
negative power of the posterior surface)

11. Refractive Power of the
Eye and its axial length
Power of cornea ~ 40 D
Power of the crystalline lens ~ 20D
Refractive Power of the ave. eye ~ 60D
Assuming n = 1.33 for the eye,
ave. length = n / power = 22.22mm
The axial length of most eyes fall between
22 to 24mm (ultrasound scan).

12. Key Words
EMMETROPIA
AMETROPIA
- Myopia or ‘Short-sightedness’
- Hypermetropia (Hyperopia)
or ‘Long-
sightedness’ - Astigmatism
ACCOMMODATION
PRESBYOPIA
Anisometropia , Amblyopia

13. EMMETROPIA
Light rays from distant objects (parallel
rays) are focused onto the retina in a fully
relaxed eye

14. MYOPIA
Light rays from distant objects are focused
infront of the retina in a fully relaxed eye
Usually too long eyeball length, or sometimes
too high refractive power

15. Myopia - Far Point
A myopic person can see objects placed
at the far point or nearer.

16. HYPERMETROPIA
(HYPEROPIA)
Light rays from distant objects are focused
behind the retina in a fully relaxed eye
Eye too short, or refractive power is too low
(eg. Aphakia where there is no crystalline
lens)

17. ASTIGMATISM
In many people, the corneal surface is not
perfectly spherical (radius of curvature the
same in all meridians) like a soccer ball
surface.
Many corneas have different curvature
(hence different power) in
different meridian, like a rugby
ball surface.

18. Astigmatic eye
Any combination of positions of focal points
in relation to the retina is possible -
myopic, hyperopic or mixed astigmatism

19. Astigmatism - circle of
least confusion

20. ACCOMMODATION
Contraction of the ciliary muscles in the eye
allow the crystalline lens to increase its
power. This increases the power of the eye
so that it can focus at near objects.
It also allows young hyperopes to overcome
the hypermetropia if this degree is not too
high.

21. AMPLITUDE OF
ACCOMMODATION
The range of accommodation decreases
with age as the crystalline lens and, to a
lesser extent, the ciliary muscles become
less elastic.

22. Amplitude of accommodation
with age

23. PRESBYOPIA
By 40 to 45 years of age onwards, the
amplitude of accommodation may not be
sufficient to allow a person to read at
near.
This is PRESBYOPIA.
Additional plus lens power is usually
required.

24. Anisometropia
Difference in the refractive errors of the
two eyes.
If sufficiently different in both eyes,
amblyopia (“lazy eye”) will occur in the
eye with the more blurred image.
Importance of early detection in children
as correction before 8 to 9 years of age
can prevent amblyopia.

25. Correction of refractive
errors
Spectacle lenses
Contact lenses
Intraocular lens implants esp. after
cataract removal
REFRACTIVE SURGERY
- Excimer Laser (“LASIK”or “PRK”)
- Intracorneal ring implants
- Intraocular ‘contact lens’ (“ICL”) or
Phakic Intraocular lens

26. Myopia - correction with a
minus lens

27. Hypermetropia - correction
with a positive lens

28. Astigmatism requires a
spherocylindrical lens
Spherocylindrical lenses have different
powers in different meridians

29. Presbyopia - spectacle
correction
Two pairs of glasses - one distant, one near
Bifocals lenses
Multifocals or Progressive lenses

30. EXCIMER LASER
EXCIMER = “ Excited Dimer “
193 nm (ultraviolet)
Breaks the intramolecular bonds of the
corneal tissue (photoablation )
PRK - “Photorefractive Keratectomy”
LASIK - “Laser in-situ keratomileusis”
Flatten the corneal curvature ie.reduce the
refractive power of the cornea in myopia
may also correct hyperopia and astigmatism

31. PRK – Photorefractive
Keratectomy

32. LASIK – Laser in-situ
keratomileusis

33. Lasik 1

34. Lasik 2

35. Lasik 3

36. Lasik 4

37. Lasik 5

38. Lasik 6

39. Lasik - Complications
Corneal stromal flap complications
Infections
Corneal melting, corneal haze, corneal
ectasia
Dry eyes
Glare - esp. night driving
Loss of visual acuity or constrast sensitivity
Retinal detachment

40. Epi-LASIK
Epithelial Flap instead of Corneal Stromal
Flap i.e. more superficial cut
Excimer laser as in PRK
Said to be safer than LASIK

41. Intraocular Contact Lens /
Phakic Intraocular Lens

42. Phakic IOL

43. Phakic IOL

44. CONDUCTIVE
KERATOPLASTY (CK) for
presbyopia
Radiowaves applied
to corneal periphery
to alter the shape of
the cornea i.e.
steepen the corneal
curvature
Reduce
hypermetropia /
increase myopia

45. Visual Acuity
Minimum angle of resolution of the eye
~ 1 min. of arc (60 sec)
The normal eye can discriminate two
points as separate if they subtend at least
an angle of 1 min. at the eye

46. Snellen Charts

47. The Snellen “ E “
D ( D -
distance m. this letter subtend 5 min.
eg. 60, 36, 24, 18, 12, 9, 6, 5 metres)

48. Snellen Acuity

49. Recording Visual Acuity
(Snellen Acuity)
Test Distance (m.) Snellen
Acuity = ------------------------------
Distance (m.) at which
the smallest visible letter
subtend 5 min. of arc
Test Distance is usually at 6 m.
eg. 6/5, 6/6, 6/9, 6/12, 6/18, 6/24, 6/36,
6/60 ; 5/60, 4/60, 3/60, 2/60, 1/60 ; CF
(Counting fingers), HM (Hand movements),
PL (Perception of light), NPL (No PL)

50. Determination of
Refractive Errors
OBJECTIVE - does not require a response
1) Infants and young children requires
retinoscopy under cycloplegia
(Cyclopentolate 1% or rarely atropine 1%
eyedrops are used to immobilise the ciliary
muscles and hence block accommodation)
2) AUTOREFRACTORS (computerised)
SUBJECTIVE - patient asked to choose
between lenses

51. Importance of vision
checks on young children
In addition to manifest squints, high
degrees of anisometropia, astigmatism,
hyperopia and myopia can cause
amblyopia (lazy vision) due to blurred
image on the fovea of one or both eyes.
A sharp retinal image is essential for
development of a normal visual acuity
Importance of early detection of visual
problems for early treatment

52. Treatment of Amblyopia
Optical correction of refractive errors (with
or without patching of the better eye)
before 8 to 9 years of age is crucial.
The younger the age at commencement
of treatment, the better the results.
Results are generally disappointing after 9
to 10 years old.

53. Change of refractive errors
with age
Low grade hyperopia (ave.~ 2D) at birth
Slight increase in hyperopia during first 7
years
Gradual decrease in hyperopia throughout
primary school
Trend to drift into myopia by end of
primary/early secondary, and increase in
myopia throughout secondary school

54. Change in refractive errors
If hyperopia of about +2.50D at 6 years, tend
to be emmetropic at 14 years; if > +2.50D at
6 yrs., some hyperopia will remain at 14 yrs.
Myopia tend to increase through secondary
school till early 20’s, then level off
Some drift towards hyperopia esp. after 40
yrs., but hardening of the lens nucleus cause
a shift into myopia esp. in the older age.

55. Factors in development of
myopia
Genetic - family, uniovular twins, race
- Japanese, Chinese, Jews,
Germans
Environment - close work
- indoors
?Pre-existing astigmatism
?Lack of exercise, ?food
?Role of parasympathetic system - ?Use of
parasympathetic blocker like atropine