A review for medical students.

1. REFRACTIVE
ERRORS
By: Qurat-ul-ain
Ophthalmic Medical
Technologist/ MBA Health &
Hospital management

2. REFRACTIVE
PHYSIOLOGY
Light rays are focused on the retina
after passing through the cornea
and lens (Snell’s Law). Refracting
mediums of eye are:
• Cornea
• Aqueous humor
• Lens
• Vitreous humor

3. REFRACTIVE
ERRORS
In refractive errors vision is
impaired because light rays are not
focusing sharply on the retina
resulting in blurry vision.
Definition:
Refractive error occurs when light
rays do not strike at retina when
accommodation is at rest. There are
three main types of refractive
errors:
• Myopia
• Hypermetropia
• Astigmatism

4. EMMETROPIA V/S AMETROPIA
Emmetropia
• Optically normal eye can be defined
as state of refraction when the
parallel rays of light coming from
infinity are focusing at the sensitive
layer of the retina with the
accommodation being at rest.
Ametropia
• A condition of refractive error, is
defined as a state of refraction,
when the parallel rays of light
coming from infinity, with
accommodation at rest, are focused
either in front or behind the sensory
layer of retina.

5. MYOPIA/NEAR
SIGHTEDNESS
Etiology of myopia
• Axial myopia:
When the anteroposterior length of the eye ball is
more than normal.
• Curvature myopia:
When the curvature of the cornea is more than
normal or is more steep than normal.
• Index myopia:
When the refractive index of different media,
particularly of the lens is more then normal
Lental sclerosis.
• Pseudo myopia/ accommodative spasm:
The patient cannot relax accommodation
looking in the distance.
When parallel rays of light
from infinity come to a focus
in front of the retina causes
myopia.

6. NORMAL
EYE
MYOPIC EYE

7. TYPES OF MYOPIA
Congenital
myopia
Present since birth
Usually the error is about -
8.00 to -10D, which mostly
remains constant.
May be associated with
other eye anomalies e.g.,
cataract, microphthalmos,
megalocornea, aniridia.
Simple myopia
It is a commonest type of
myopia.
The power of glass is usually
increases during the school
year.
progresses very slowly till
adult age.
Usually the error does not
exceed -6 to -8 diopter.
Degenerative
myopia
The myopia rapidly
progresses so that in early
adult life there may be
myopia of -20D or more.
Strongly linked with
heredity.

8. COMPLICATIONS
• Myopia crescent
• Chorio-retinal atrophy
• Atrophic patches
• Vitreous degeneration
• Detachment of the retina
choroidal hemorrhage (Forster
Fuch’s spot)
• Nuclear type of cataract

9. MANAGEMENT OF
MYOPIA
• Corrected by prescribing concave lenses in
the form of glasses or contact lenses
• Radial keratotomy
• LASIK surgery
• Clear lens extraction with IOL implantation
• Orthokeratology

10. HYPERMETROPIA
/ FAR
SIGHTEDNESS
Etiology of hypermetropia
• Axial hypermetropia:
When the anteroposterior length of the eye ball is
smaller than normal.
• Curvature hypermetropia:
Due to the flatter curvature of cornea or lens or
both
• Index myopia:
Results due to the change in refractive index of the
lens.
• Aphakia:
Absence of the lens either congenital or acquired
leads to high hypermetropia.
When parallel rays of light
from infinity come to a focus
in behind the retina causes
hypermetropia.

12. TYPES OF HYPERMETROPIA
Latent
hypermetropia
corrected normally by the
normal tone of ciliary
muscles. It is more in young
children than in elderly
persons, as the tone of
ciliary muscles is much more
in young than in adults.
Manifest
hypermetropia
it is the hypermetropia
which remains uncorrected
in normal circumstances. It
has two types:
1. Facultative
hypermetropia:
it is that part of
hypermetropia which
can be corrected by an
effort of
accommodation.
2. Absolute
hypermetropia:
it is that part of
hypermetropia which
cannot be overcome by
active exertion of
accommodation.

13. COMPLICATIONS
• Dislocated lens
• Retinal detachment
• Choroidal tumor
• Retinal fluid

14. MANAGEMENT OF
HYPERMETROPIA
• Corrected by prescribing convex lenses
in the form of glasses or contact lenses
• Refractive corneal surgery
• Clear lens extraction with IOL
implantation

15. ASTIGMATISM
Etiology of Astigmatism
• Cornea:
the cornea has an unequal curvature on its
surface
• Lens :
the crystalline lens has an unequal curvature on its
surface or in its layers.
• Total astigmatism:
Sum of the corneal and lenticular astigmatism.
When parallel rays of light
enter the eye with
accommodation at rest and
do not come to a single
point focus on or near the
retina.

16. TYPES ACCORDING TO THE MAIN
MERIDIANS
Irregular
astigmatism
When the two principal
meridians are not
perpendicular to each
other.
Curvature of any one
meridian is not uniform.
Associated with trauma,
disease or degeneration
Regular
astigmatism
When the two principal
meridians are
perpendicular to each
other. Most cases of
astigmatism are regular
astigmatism.
The three types are:
With the rule, against the
rule and oblique
astigmatism

17. 1. WTR
astigmatism
When the greatest
refractive power is within
30degree of the vertical
meridian i.e., between 60
& 120 meridians.
Minus cylinder axis
around horizontal
meridian
2. ATR
astigmatism
When the greatest
refractive power is within
30degree of the
horizontal meridian i.e.,
between 30 & 150
meridians
Minus cylinder axis
around vertical meridian.
3. Oblique
astigmatism
When the greatest
refractive power is within
30 & 60 or 120 & 150.

19. TYPES ACCORDING TO THE
REFRACTION:
Simple
astigmatism
When one of the principal
meridian is focused on
the retina and the other is
not focused on the retina
(with accommodation
relaxed)
Compound
astigmatism
When both principal
meridians are focused
either in front or behind
the retina (with
accommodation relaxed).
Mixed
astigmatism
When one of the principal
meridian is focused in
front of the retina and the
other is focused behind
the retina.

20. MANAGEMENT OF
ASTIGMATISM
• Cylindrical lenses and spherocylindrical
lenses in spectacles and contact lenses for
simple and compound astigmatism
respectively.
• Refractive surgery
• Single vision glasses with cylinder
• Toric soft contact lenses
• Toric rigid gas permeable contact lenses
• PRK
• LASIK

21. THANK YOU!