Dany Najjar, MD

INTRODUCTION:

(A). TYPES OF CORRECTIVE LENSES:

 

(a). Spherical lenses:

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.

(b). Cylindrical or toric lenses:

One meridian is curved more than all the other ones. They are used to correct astigmatism.

c .Prisms:

A prism is an optical device composed of 2 refracting surfaces that are inclined toward one another.

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 correct strabismus.

(B). BASIC PRINCIPLES:

- 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 medium

- 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 wavelength)

- Refraction of light: Snell‘s Law: n1 siná1 = n2 siná2.

6. Power of a lens in diopters 1/focal length in meters.

 

(C). REFRACTION:

 

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.

 

(a) Subjective refraction

(b) Automated refraction

(c) Cycloplegic refraction

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 accomodation.

 

 

THE EYE AS AN OPTICAL SYSTEM:

 

(A). REFRACTIVE MEDIA:

 

1. THE CORNEA:

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.

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 aberrations.

It increases the depth of focus when constricting.

(B). ACCOMODATION:

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 image.

it is part of the Near Reaction that involves accomodation along with convergence and pupillary constriction.

(C). REFRACTIVE ERRORS:

Emmetropia: Normal eye. No refractive error.

Anisometropia: A refractive error is present.

1. Hyperopia

2. Myopia

3. Presbyopia

4. Astigmatism

I. HYPEROPJA (Hypermetropia, farsightedness)

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 hyperopia:
- 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 shorter
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 blurring
of the disc margins, hyperemia of the disc, hemorrhages...

b. Curvature hyperopia:

When either the lens or cornea has a weaker than normal curvature, lower refractive power or convergence occurs.

c. Index of refraction hyperopia:

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 refraction

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.

—Symptoms of hyperopia:

1. Blurred vision for distance

2. Frontal headache aggravated by prolonged use of near vision.

3. Asthenopia: 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:

Convergent or (+)lenses.

2. MYOPIA nearsightedness)

The focused image if formed in front of the retina.

a. Structural or axial myopia:

The AP diameter is longer than normal. Patients may have pseudoproptosis due to the larger globe.

b. Curvature myopia:

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 of refraction:

Occurs with nuclear sclerosis making the eye myopic

d. Anterior displacement of the lens:

Occurs after trauma or after glaucoma surgery.

Symptoms of myopia:

I. Blurred vision for distance

2. Squint (due to blepharospasm-like action to act as a pinhole)

3. Headache (rare)

Myopia is usually detected at the age of 9-10 years and keeps increasing till mid-teens when it stabilizes at -5d. or less

Progressive myopia:

- rare form of myopia

- 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 liquefaction

- usually 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.

 

Congenital myopia:

- more than —10d. in infants

- generally not progressive

- should be corrected as soon as detected.

Treatment:

Always give full correction with (-) lenses.

3. ASTIGMATISM:

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 steeper
Regular astigmatism: Principle meridians are 90 degrees apart

Irregularastigmatism: 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.

Symptoms of astigmatism:

- blurred vision for far and near

2- squint (for pinhole effect)

3- asthenopic symptoms

4- frontal headaches

5- tilting of the head (for oblique astigmatism)

Treatment is with cylindrical lenses.

4. PRESBYOPIA:

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 lens.

Symptoms include:

- larger reading distance required

- inability to focus on close work.

- Excessive illumination required for close work.

Treatment: Add positive lenses to far correction according to age.

 

 

 

5. AMBLYOPIA:

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 involved eye.

Causes include:

- strabismus (most common cause)

- anisometropia

- high hyperopia

- opacities: corneal scars, cataract

- optic nerve disease

- retinal disease

 

 

 

(D). LENS PRESCRIPTION:

 

- Retinoscopy

- Sphere, cylinder X axis

 

- Transposition:
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 CALCULATION:

 

The power of the intraocular lens to be replaced after cataract surgery is usually calculated according to the SRK ( Sanders-Retzlaff-Kraff) equation:

 

IOL power = A-2.5L-0.9K

 

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)

 

 

 

 

USE OF LASERS IN OPHTHALMOLOGY:

 

 

THE ARGON LASER:

It has a thermal effect and is used for.

a. photocoagulation of the retina in diabetic retinopathy

b. macular degeneration (if neovascularization is present)

c. sealing of retinal holes and tears

d. trabeculoplasty in open angle glaucoma

e. suture lysis

 

 

 

THE NEODYNIUM-YAG LASER:

It causes photodisruption of tissues and is used for:

a. iridotomy in angle closure glaucoma

b. capsulotomy: cutting a hole in the posterior capsule of the lens which remains after ECCE and which thickens

c. cutting vitreous bands and opacities

 

 

THE EXC1MER LASER:

it causes photoablation of tissue and is used for:

a. photorefractive surgery to change the surface of the cornea in order to correct errors of refraction.

CLINICAL &CASES

 

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 refraction?

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:

-3.50 ± 1.50 X 120

-1.00 + 2.25 X 85

 

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 cause.

(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:

±1.50±0.50X90 20/100

+5.00 + 3.50 X 100 20/i 00

(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.