Download ERRORS OF REFRACTION

Refractive
Errors
OPTICS
When light rays passes through space
3 things can happen
1-If it strikes on opaque medium e.g.
black object- absorbs light
2-If it strikes a mirror, it is reflected
3-If it strikes a transparent medium, it
changes its path.
This is called refraction
OPTICS
THIS BENDING OF LIGHT DEPENDS ON 3
FACTORS:
1-The difference of refractive indices of the
two media
2-The angle at which the light rays strikes
the medium
3-The wavelength of light
(blue light bends more as compared to red
light)
REFRACTION
EMMETROPIA
AMMETROPIA
EMMETROPIA
When incident parallel rays of light from
infinity come to focus on the retina (fovea
centralis) With accommodation at rest,
the eye is considered to be emmetropic.
It is the normal optical condition of eye
with no errors of refraction.
AMMETROPIA
►Myopia
►Hypermetropia
►Astigmatism
ERRORS OF REFRATION
AMETROPIA
When incident parallel rays of light
from infinity do not focus on the
retina with accommodation at
rest, the condition is called
ametropia
AMETROPIA
1-AXIAL AMETROPIA: abnormal length of eyeball.
2-CURVATURE AMETROPIA: abnormal curvature of
refracting surface of cornea and lens.
3-INDEX AMETROPIA: abnormal refractive index of
media
4-ABNORMAL POSITION OF LENS: forward or backward
displacement of lens.
MYOPIA
When the incident parallel rays of
light coming from infinity focus at
a point in front of retina ,with the
accommodation at rest ,eye is
said to be myopic.
(Refractive power is more than
required for the available axial
length)
MYOPIA
ETIOLOGY:
Axial: increase length of eyeball
Curvature: increase curvature of
cornea of lens
Index: increase refractive index of
lens nucleus
Forward displacement of lens
MYOPIA
TYPES:
1-Simple
2-Pathological
SIMPLE MYOPIA:
 No degenerative changes in fundus
 Doesn't progress after adolescence
 Doesn't progress more than -5 or -6 D
PATHALOGICAL MYOPIA:
 Degenerative and progressive
 Begins at the age of 5-10 years
 May attain a degree of -15 to -20 D
 It is strongly hereditary
MYOPIA
SIGNS:
 Prominent eyes, large pupil,
deep Ant. Chamber
 Apparent divergent squint
may be present
MYOPIA
SIGNS:
Fundus:
1-optic disc: temporal crescent
posterior staphyloma
2-macula:
chorioretinal degeneration
foster fush’s flecks
3-periphery: holes and tears
cystoid degeneration of ora serrata
tesselated fundus
MYOPIA
•
COMPLICATIONS:
Vitreous degenerations, opacities,
detachment
Tears and hemorrhage
Retinal detachment
Chronic simple glaucoma
MYOPIA
TREATMENT:
Spectacles: suitable spherical concave
lenses
Operative: Radial keratotomy
Excimer laser
Epikeratophakia
Keratomileusis
LASIK
HYPERMETROPIA
When the incident parallel rays of light
coming from infinity focus at a point
behind the retina, with the
accommodation at rest, eye is said to
be hypermetropic.
(Refractive power is less than required for the
given Axial Length)
All new borns are hyperopic (average 2.5 D)
HYPERMETROPIA
ETIOLOGY:
Axial: shorter length of eyeball
Curvature: flat cornea curvature
Index: Decreased refractive index of
lens
Backward displacement of lens
Aphakia
HYPERMETROPIA
1- LATENT HYPEROPIA:
Overcome by normal tone of ciliay muscle
Detected by using atropine to paralyze
ciliary's muscle (Cycloplegia)
2-MANIFEST HYPEROPIC:
Detected without ciliary's muscle
paralysis
HYPERMETROPIA
SYMPTOMS:
 Blurring of vision
 Frontal headache and eye strain
 In adults, presbyopia commences earlier
SIGNS:
 Shallow Ant. Chamber, prone to close angle
glaucoma
 Accommodative convergent squint
 Fundus may be normal or pseudopapillitis may
be present
TREATMENT:
ASTIGMATISM
It is a condition in which a point of light cannot
be made to produce a punctate image upon
the retina by any spherical correcting lens
ETIOLOGY:
Unequal curvature of cornea in different
meridians
Decentring of lens
ASTIGMATISM
TYPES:
1-Regular
a) simple
myopic
hypermetropic
b)compound
myopic
hypermetropic
c)mixed
2-Irregular
ASTIGMATISM
1-REGULAR ASTIGMATISM:
It is present when two principle meridians are at right
angle to each other, it can be corrected by lenses
WITH THE RULE ASTIGMATISM: vertical meridian is
more curved ,as in normal cornea
AGAINST THE RULE ASTIGMATISM: horizontal meridian
is more curved
ASTIGMATISM
1-REGULAR ASTIGMATISM:
a- simple: one focus is on the retina
other is in front of retina in simple myopic
behind the retina in simple hyperopic
b-compound: both focus are
in front of retina in compound myopic
behind the retina in compound hypermetropic
c-mixed: one focus is in front of retina
Other behind the retina
ASTIGMATISM
2-IRREGUALR ASTIGMATISM:
It is present when corneal
surface is irregular, it can not
be adequately corrected by
lenses
ASTIGMATISM
SYMPTOMS:
 Diminished visual acuity
 Eye strain and headache after near work
 Letters in the book appear to be running together
TREATMENT:
 Suitable cylindrical lenses for regular astigmatism
 For irregular astigmatism
hard contact lens
keratoplasty
APHAKIA
• Absence of lens is called aphakia
OPTICAL DEFECTS:
• Eye is highly hypermetropic
• Complete loss of accommodation
• Presence of astigmatism
SYMPTOMS:
• Gross dimness of vision due to acquired high hyperopia
SIGNS:
•
•
•
•
Deep anterior chamber
Iridodonesis
Jet black pupil
Corno-scleral scar mark at upper limbus in case of surgical aphakia
APHAKIA
TREATMENT:
• Correction by spectacles: by prescribing
suitable spherical convex lens (about +10 D)
• Contact lens
• Intraocular lens implantation: after calculating
the power, standard power is +20 D, it can be
implanted in anterior or posterior chamber
PRESBYOPIA
It’s the insufficiency of accommodation
due to advancing age, usually 40 years. It
is not an error of refraction.
CAUSE
 Physiological failure of the lens to
accommodation due to
hardening of lens with age
weakness of ciliary’s muscle and suspensory
ligament
PRESBYOPIA
SYMPTOMS:
 blurring of vision on near work
 People involve in close work appreciate the symptom earlier
 Depends on existing error of refraction i.e. early onset of
hypermetropia, delayed onset in myopia.
SIGNS:
 Impaired near vision, tested by near chart
TREATMENT:
 By prescribing suitable convex spherical lens for near work
This correction is added to the correction for distant vision