Dany Najjar, MD
(A). TYPES OF
All have equal
curvatures in all meridians.
(i). Convex, (-F)
lenses or convergent lenses are used for the correction of hyperopia,
presbyopia and aphakia. They make objects look larger in size.
(ii). Concave, (-) lenses
or divergent lenses are used for the correction of myopia. They make
objects look smaller in size.
or toric lenses:
One meridian is curved
more than all the other ones. They are used to correct astigmatism.
A prism is an optical
device composed of 2 refracting surfaces that are inclined toward one
It has an apex and a
base. It refracts light toward its base whereas an object seen through
a prism appears deviated toward the apex of the prism.
It does not change the
size of an object.
Prisms are used to
- Visible spectrum
of light : 330am (violet) — 760nm (red)
- Velocity of
light in vacuum: 101m!sec
- Index of
refraction of a medium: velocity of light in vacuum! that in
- The index of
refraction of a medium varies with the wavelength of light
traveling through it. Blue light (short wavelength) is refracted more
than red light because of its higher frequency. (v = f x
- Refraction of
light: Snell‘s Law: n1 siná1 = n2
- Power of a lens in diopters
1/focal length in meters.
Refractive errors are
the most common cause of poor vision. They are the easiest to treat.
Refraction is a term applied to the various testing procedures
employed to measure the refractive error of the eye in order to
provide the proper correction.
Cycloplegic refraction is
done by applying a cycloplegic agent to the eye (atropine,
cyclopentolate or tropicamide) to paralyze the ciliary muscle so that
the absolute refractive error can be measured. it is helpful to detect
latent hyperopia in children compensating their hyperopia by
EYE AS AN OPTICAL SYSTEM:
1. THE CORNEA:
contributes to approximately 2/3 of the refracting power of the
eye along with the tear film. It contributes ±43 diopters.
2. THE LENS.
contributes to 1/3 of the refractive power of the eye (± 20
diopters). By itself it is more powerful than the cornea as a
convergent surface but because of less difference in indices of
refraction between the aqueous versus the cornea, less convergence
exists at the level of the lens.
The total convergence
power of the eye is 58.7d and not 43±20=63d due to the
distance between the cornea and the lens (depth of the anterior
chamber) that will subtract approximately 4d.
3. THE PUPIL.
The pupil reduces
the amount of light that enters the eye.
It decreases the
It increases the
depth of focus when constricting.
It is the process by
which the eye changes its refractive power to focus on near objects.
It results from increased curvature of the lens due to contraction of
the ciliary muscle. The stimulus to accomodation is a blurred retinal
it is part of the Near
Reaction that involves accomodation along with convergence and
eye. No refractive error.
refractive error is present.
I. HYPEROPJA (Hypermetropia,
The focused image forms behind
the retina. Most children are born with some hyperopia (maximum up
to ±3d. ) but this usually resolves by 12 years of age.
a. Structural or axial
- It is the most common cause of hyperopia
- AP diameter of the eye is shorter than normal.
- These eyes are more prone to angle closure glaucoma because of
anterior segment with crowding of the angle structures.
- The optic nerve is also smaller
- It may be associated with pseudopapilledema:
- usually occurs with more than +4d.
- swollen discs but no other signs of true papilledema such as
of the disc margins, hyperemia of the disc, hemorrhages...
When either the lens
or cornea has a weaker than normal curvature, lower refractive power
or convergence occurs.
c. Index of
Occurs due to a
decrease in the index of refraction (and density) in any part of the
optical system of the eye.
Latent hyperopia: is
that part of the refractive error completely corrected by accomodation.
It can only be measured by cycloplegic refraction and not manifest
Manifest or absolute
hyperopia: is the portion of
the hyperopia not corrected by accomodation.
With aging, the
accomodative power of the eye decreases. This will shift a hyperopic
patient from latent hyperopia to greater degrees of absolute hyperopia.
1. Blurred vision
2. Frontal headache
aggravated by prolonged use of near vision.
fatigue, burning eye sensation and periorbital pain when fixing at
an object for prolonged periods of time.
4. Light sensitivity
5. Decrease in near
visual acuity at a younger age than in emmetropic eyes.
Treatment of hyperopia:
The focused image if
formed in front of the retina.
a. Structural or
The AP diameter is
longer than normal. Patients may have pseudoproptosis due
to the larger globe.
The eye has a
normal AP diameter but at the corneal level the curvature may be
steeper than normal ex: congenital, or keratoconus.
At the lens level:
lens curvature is increased ex in intumescent cataract.
c. Increased Index
nuclear sclerosis making the eye myopic
displacement of the
trauma or after glaucoma surgery.
vision for distance
(due to blepharospasm-like action to act as a pinhole)
Myopia is usually
detected at the age of 9-10 years and keeps increasing till
mid-teens when it stabilizes at -5d. or less
- rare form of
- may increase at
a rate of up to -4d. rare form of myopia
- may increase at
a rate of up to -4d. per year
- is associated
with chorioretinal degeneration and vitreous floaters and
stabilizes at the age of 20 years but can progress until mid 30’s
- may reach up to
—10 or —20d.
- high myopes
(more than —7d) are predisposed to retinal detachment and POAG.
- more than
—10d. in infants
- generally not
- should be
corrected as soon as detected.
Always give full
correction with (-) lenses.
The curvature of
the optical system varies in different meridians thus refracting the
incident light differently in those meridians.
With—the—rule astigmatism: the
vertical meridian is steeper
Against-the-rule astigmatism: the horizontal meridian is
Regular astigmatism: Principle meridians are 90 degrees apart
Principle meridians are not 90 degrees apart. This type of astigmatism
cannot be completely corrected by spectacles and may need contact
lenses ex: corneal scarring , keratoconus.
- blurred vision
for far and near
2- squint (for
5- tilting of the
head (for oblique astigmatism)
Treatment is with
It’s the physiologic
decrease in the amplitude of accomodation associated with aging.
There is less bulging
of the lens with accomodation due to a change in the crystallins of
the lens that result
in decrease in the elasticity of the lens fibers or hardening of the
- larger reading
- inability to focus
on close work.
illumination required for close work.
positive lenses to far correction according to age.
It is decreased visual
acuity of one eye (uncorrectable with lenses) in the absence of
organic eye disease
insufficient enough to explain the level of vision.
It is caused by visual
deprivation due to any cause (congenital or acquired) during the
critical period of
development (up to age 8-9 yrs) that prevents the establishment of
normal vision in the
- strabismus (most
- high hyperopia
- opacities: corneal
- optic nerve
- retinal disease
- Sphere, cylinder X
To transpose from (-) cylinder to (±) cylinder
a. add the cylindrical power to the spherical power
b. reverse the sign of the cylinder
c. add 90 degrees to the axis
(E) IOL POWER
The power of the
intraocular lens to be replaced after cataract surgery is usually
calculated according to the SRK ( Sanders-Retzlaff-Kraff)
IOL power =
where A is a constant
particular to the lens
L is the A-P diameter
of the eye in mm (obtained from A scan)
K is the measurement
of the curvature of the cornea (obtained from a keratometer)
LASERS IN OPHTHALMOLOGY:
It has a thermal
effect and is used for.
a. photocoagulation of
the retina in diabetic retinopathy
degeneration (if neovascularization is present)
c. sealing of retinal
holes and tears
d. trabeculoplasty in
open angle glaucoma
e. suture lysis
It causes photodisruption
of tissues and is used for:
a. iridotomy in angle
cutting a hole in the posterior capsule of the lens which
remains after ECCE and which thickens
vitreous bands and opacities
it causes photoablation
of tissue and is used for:
surgery to change the surface of the cornea in order to correct errors
1. A new patient
presenting for visual check-up is found to have a vision of 20/40 OU.
What test will you do to predict that his vision will improve with
2. A 68 y. old male
patient is known to have cataract for the last 3 yrs. His vision 3
yrs ago was 20/50 with +3.50d eyeglasses. He is now seeing
better without his correction. How do you explain this?
3. After dilatation
and full cycloplegia of a 20 yr old lady whose vision was 20/20, she
worsened to 20/50. Does she have an error of refraction? Would you
prescribe eyeglasses for her?
4. What would be the
near correction for a 52 y.old man whose correction for far is:
1.50 X 120
-1.00 + 2.25 X
5. A 26 y. old
patient is presenting for check-up. You find —3.25 +6.00 X 90.
However the patient is seeing better without any correction. How do
you explain this?
6. A 65 y. old male
patient with ARMD and mature cataract presents with a vision of CF 1
m. What test would you do to predict what would be his best
corrected vision after cataract surgery?
7. For each of the
following patients presenting with low visual acuity 20/100, select
the most likely
(a) 6 y old girl
operated several times for strabismus
(b) 68 y old lady
case of RA on chronic intake of steroids
(c) 8 y old girl
with the following error of refraction:
+5.00 + 3.50 X 100 20/i
(d) 78 y old male
patient with hard drusen in the macula
(e) 12 y old boy
with history of recurrent attacks of herpes simplex resulting in a
disciform central scar.
(f) 15 y old
male patient with difficulty reading the blackboard.