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continuing education course

Approved by the American Board of Opticianry and The National Contact Lens Examiners

Anatomy and Physiology of the Eye

National Academy of Opticianry 8401 Corporate Drive #605

Landover, MD 20785 800-229-4828 ph 301-577-3880 fax

www.nao.org

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National Academy of Opticianry 8401 Corporate Drive #605

Landover, MD 20785 800-229-4828 ph 301-577-3880 fax

www.nao.org

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Anatomy & Physiology of the Eye

PREFACE

This continuing education course for opticians was prepared under the auspices of the National

Academy of Opticianry and is designed to be convenient, cost effective and practical for the

optician.

The skills and knowledge required to practice the profession of opticianry will continue to

change significantly in the future, as advances in technology are applied to this eyecare specialty.

Higher rates of obsolescence will result in an increased tempo of change and opticians at all

levels will have to devote more time to acquire the necessary skills and knowledge to meet these

changes. The National Academy of Opticianry recognizes the need to provide a Continuing

Education Program for all opticians to be taken by home study. This course has been developed

as a part of the overall program to enable opticians to develop and improve their technical

knowledge and skills in their chosen profession,

The National Academy of Opticianry

INSTRUCTIONS Read and study the material presented in the following pages. After you feel that you understand

the material thoroughly, take the test following the instructions given at the beginning of the test.

Upon completion of the test, mail the answer sheet to the National Academy of Opticianry, 8401

Corporate Drive #605, Landover, MD 20785

CREDIT The American Board of Opticianry (ABO) and the National Contact Lens Examiners (NCLE)

have approved this course for Continuing Education Credit toward certification renewal. The

number of credit hours for this course are 3 (three) hours. These courses may be eligible for

credit to meet the Continuing Education Requirements of various states. To earn this ABO

and/or NCLE credit you must achieve a grade of 80 percent or higher on the test. The Academy

will notify all test takers of their score and issue certificates of credit to those who pass. The test

passer must then forward the certificate of credit to the ABO or the NCLE.

AUTHOR David D. Michaels, M.D., is a Clinical Professor of Ophthalmology at the University of

California School of Medicine, Los Angeles, California. He is also Chairman of the Department

of Ophthalmology of San Pedro and Peninsula Hospital, San Pedro, California. Dr. Michaels is a

noted lecturer and the author of Visual Optics and Refraction, A Clinical Approach and has been

honored by being elected Professor Emeritus at two Universities.

INTENDED AUDIENCE Anatomy & Physiology of the Eye is designed for the basic individual

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COURSE DESCRIPTION Lectures, models, demonstrations and slides covering basic gross and microscopic anatomy of

the eye and orbit. Functional correlations emphasize mechanisms of acuity, refractive error,

corneal oxygenation, adaptation, binocular vision and intraocular pressure.

INSTRUCTIONAL OBJECTIVES Upon successfully completing this course, the participant should be able to:

o Describe the anatomic organization of the eye and orbit

o Explain the structure-function relationship in refractive error, strabismus, glaucoma,

presbyopia, and contact lens wear.

o Define anatomic terms frequently used in an ophthalmic practice

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Anatomy & Physiology of the Eye

By David D. Michaels, M.D

INTRODUCTION

The fibrous tunic consists of sclera and cornea. The sclera is the white of the eye. It is composed

of tough inelastic fibers, tightly laced together. It thus resists the intraocular pressure without

losing shape, and provides an anchor for the attachment of extraocular muscles. The cornea fits

into the scleral shell like a watchglass into its case. Unlike the sclera, the fiber bundles making

up the cornea are all parallel. This makes it transparent without losing stability. So as not to

interfere with transparency, the cornea contains no blood vessels, but receives its nutrition by

diffusion from the surrounding area, aqueous humor; and the choroid which is composed almost

entirely of blood vessels interspersed with dark pigment.

The neural tunic consists of the retina. The retina is a complex tissue containing photoreceptor

cells (rods and cones), intermediate nerve cells (bipolars), and terminal nerve cells (ganglions)

whose extensions make up the optic nerve. There are about 120 million rods and 6 million cones

in each retina. All the nerve fibers traverse the retinal surface to exit through the same opening--

the optic disc. Unlike the film in a camera which is equally sensitive throughout, the human

retina has one area about the size of a pinhead which is exquisitely designed to distinguish

details--the fovea. When we turn our eyes to look at an object, the region we look with is the

fovea.

UVEAL TRACT FIG. 5-2

The vascular tunic (also called uveal tract) (fig.

5-2) consists of three parts; the iris which acts

as a diaphragm to regulate the amount of light

entering the eye; the ciliary body which

provides the musculature to focus on near

objects (accommodation) and secrete Situated

behind the pupil is the crystalline lens. About

the size of a large pea, it is a transparent

structure capable of changing its shape when

acted on by the ciliary muscle.

FIG. 5-1

The eye is like a living camera. It has an

outer protective coat (fibrous tunic); a

middle coat to insulate it from stray light

and nourish it (vascular tunic); and an

inner photosensitive coat which acts like

a photographic film (neural tunic). (fig.

5-1 )

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This change in curvature is called accommodation. The lens is held in place by fine inelastic

fibers; the suspensory ligaments. The ciliary muscle pulls or releases tension on these ligaments,

which in turn flatten or increase the curvature of the crystalline lens.

FIG. 5-3

In order to rotate the eyeball, six extra-ocular muscles (fig. 5-4) are firmly attached to the sclera.

Four are recti (meaning straight) and two are obliques. Some branches of their blood vessels

supply the front of the eyeball (anterior ciliaries). The back half of the globe receives branches

(posterior ciliaries) from the ophthalmic artery which is in turn a division of the carotid---the

major artery to the brain. The blood drainage of the eyeball is by four vortex veins which empty

into the ophthalmic veins.

The nerve supply to the eyeball consists of a sensory optic nerve which relays the light-induced

messages from the retina to the back part of the brain; autonomic nerves (both sympathetic and

parasympathetic) which control pupil size, ciliary muscle action, lacrimal secretion, and blood

vessel diameter; and branches of the trigeminal (V cranial) nerve which provide for pain, touch,

and temperature sensation. In addition, there are motor nerves to the extra-ocular and lid

muscles.

The eyeball, together with its blood and nerve supply, is surrounded by a tough covering called

Tenon's capsule. The capsule acts somewhat like a ball and socket joint to allow free movement

of the globe. Surrounding the muscles and Tenon's capsule is the orbital fat which, in life, is

more like thick oil to minimize friction.

It will be evident that the interior of the eyeball

(fig. 5-3) can be divided into three spaces; the

space between the back of the cornea and the

iris (anterior chamber); the space between the

iris and the lens and suspensory ligaments

(posterior chamber); and the largest or vitreous

space between lens and retina. The anterior

and posterior chambers are filled with a watery

fluid which is constantly produced and

drained; the aqueous humor. The vitreous

chamber is filled with gel-like fluid that

remains unchanged throughout life; the

vitreous humor.

EXTRA-OCULAR MUSCLES FIG. 5-4

The eyeball and its adjacent tissues lies

in a bony cavity of the skull called the

orbit. (fig. 5-5) The bones making up the

orbit are partly those of the face, partly

those of the cranial cavity, and partly

those of the sinuses.

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Above and behind the orbit is the brain; below is the maxillary sinus; on the inner side is the

ethmoid sinus and nasal cavity; on the outer side is the zygomatic arch and temple.

[C:\AnaPhyDoc\AnaP0011.TIF]

The conjunctiva thus consists of three parts; the portion lining the lids (palpebral), the backward

curving portion (fornix), and the portion adherent to the eyeball (bulbar).

The conjunctiva is a mucous membrane because it contains cells that produce mucus (goblet

cells). Their function is to lubricate the movements of the lids as they blink 10 to 16 times a

minute. To further minimize friction, the lacrimal gland secretes tears which are distributed over

the cornea by the blink reflex. The tears accumulate in the inner cornea of the eye, from where

they are drained by a system of canals into the nose.

The following important average dimensions should be committed to memory: the globe is 25

mm in diameter; the cornea is 12 mm in diameter and 0.5 mm thick in the center; the separation

between eyes can range from 55 to 75 mm; the pupil diameter under average light is 3.5 mm; the

anterior chamber depth is about 3.5 mm; the diameter of the optic disc is 1.5 mm.

Having outlined the general anatomic plan, let us now return and look at each part in more detail.

Cornea

If the eye is the window of the soul, the cornea is the window to the eye. Although the tissue

looks crystal clear, it is actually made up of five distinct layers: epithelium, Bowman's

membrane, stroma, Descemet's membrane, and endothelium. (fig. 5-6) Each can be seen easily

with the slit lamp biomicroscope (available in most offices). The epithelium is a stratified layer

of cells which is continuously renewed; Bowman's and Descemet's membranes are structureless;

the stroma makes up 90% of the corneal thickness and is composed of band-like fibers of

uniform size and arrangement. Between the fibers are the actual corneal corpuscles (keratocytes)

squeezed almost fiat. The innermost endothelium is a single layer of cells which also serves as a

lining for the anterior chamber.

THE BONY ORBIT FIG. 5-5

We see that the eye has bony protection on all

sides except the front. The forward protection

is provided by the lids. The upper and lower

lids consist of a fibrous skeleton (tarsal

plates), muscles which open them (levator)

and muscles which close them, (orbicularis

oculi). On the surface, the skin of the lids is

continuous with the skin of the face. On the

inner side, the lids are covered by a mucous

membrane---the conjunctiva. The conjunctiva

is reflected backwards to cover the exposed

parts of the sclera.

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The cornea is the most powerful refractive component of the eye. Of the total 60 D. power of the

eye, the cornea contributes 43 D. and the crystalline lens 17 D. This is not because the corneal

radius of a curvature is so small, but because the refractive index difference between air and

cornea is so large (1.00 and 1.37). You should be able to calculate from these figures that the

average corneal radius is about 8.6 mm.

The cornea is bathed on each side by watery fluids. On the front is the tear film which smoothes

out any epithelial irregularities; on the back is the aqueous humor. To keep it from absorbing

water and swelling, endothelial cells act like tiny bilge pumps (deturgescence). This metabolic

work requires a source of energy and oxygen. The cornea gets its energy by way of glucose from

the aqueous and its oxygen directly from the air. When the oxygen supply is cut off by an

improperly fitted contact lens, or if the endothelial cells are injured by intraocular surgery or lost

from old age, the cornea swells (edema). As the water percolates into and between the surface

epithelial cells, they break up to produce painful erosions (bullous keratopathy). Since surface

irregularity is compromised, vision is also impaired.

The cornea is not only a barrier to injury and infection, but also to drugs. When ointments or

drops are instilled into the conjunctival sac, they must negotiate the corneal layers to gain access

into the interior (e.g., to dilate or constrict the pupil or produce cycloplegia). Because it is

exquisitely sensitive, a topical anesthetic is usually necessary to measure intraocular pressure.

CORNEA IN CROSS

SECTION FIG. 5-6

The cornea is about 1/2mm

thick at its center and is

slightly thinner at the center

when compared to the

periphery.

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Sclera

Corneal incisions in cataract or other intraocular surgeries are generally made at the limbus. It is

also a point of reference in measuring the insertions of the extraocular muscles. Umbal blood

vessels are the source of diffusional nutrients to the cornea. In chronic inflammation or anoxia,

limbal vessels may actually invade the cornea. While this attempt to restore metabolic balance is

admirable it also interferes with vision. Moreover, it destroys the immunologic isolation which

makes corneal transplants successful.

Tear film

An understanding of the tear film is important because of its role in debilitating dry eye

syndromes on the one hand, and happy contact lens wearers on the other.

Two types of tearing are recognized; tearing as a reflex response to pain, emotion, freshly cut

onions, etc., and basic secretion which goes on at a steady rate the rest of the time. The lacrimal

gland supplies tears via small canals into the upper fornix for reflex tearing. Basic tear secretion

is carried out by scattered small tear glands in the conjunctiva (accessory lacrimal glands).

The tear film covering the cornea is composed of three distinct layers. (fig. 5-9)

CORNEAL GEOMETRY FIG. 5-7

FIG. 5-8

The sclera is a dense, collagenous connective tissue

about 1 mm thick. It is pierced by various vessels and

nerves which enter or leave the globe. The optic nerve

fibers cross a sieve-like membrane at the optic disc

(lamina cribrosa). Because of its close resemblance to

connective tissue, scleritis may be a complication of

diseases like rheumatoid arthritis.

The 1 mm transition zone between cornea and sclera is

the limbus--an important surgical landmark. (fig. 5-8)

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The corneas of patients wearing hard contact lenses get their oxygen from the tear pool

underneath the lens. This pool must be constantly renewed by proper blinking and adequate tear

production. Soft contact lenses transmit oxygen directly and thus require little movement. But

enough tears must be available to keep them hydrated.

Anterior chamber

FIG. 5-9

An outer oily layer supplied by glands of

the lids; a middle watery layer which makes

up the bulk thickness and is supplied by the

tear glands; and an inner mucin layer,

supplied by conjunctival goblet cells. The

mucin has somewhat the same function as

"wetting" solution used on hard contact

lenses. Dry eyes can therefore be caused by

insufficient oil gland secretion (e.g., lid

disease), insufficient aqueous tears (not

infrequent in old age), or inadequate mucin

production (conjunctival disease). Dry eye

manifestations are chiefly corneal; dry

spots, erosions, filaments, and even

ulceration.

FIG. 5-10

The depth of the anterior chamber varies with age, the

size of the eye, whether a cataract has been removed,

or an intraocular implant inserted. (fig. 5-10) Gradual

shallowing of the chamber with advancing age is one

factor which increases the susceptibility of the elderly

to glaucoma. The most important part of the anterior

chamber is the angle where iris and cornea meet

circumferentially. The angle is filled with a meshwork

of connective tissue (trabeculae) (fig. 5-11) through

which aqueous is filtered on its way to Schlemm's

canal.

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Iris The iris (fig. 5-12) is a thin vascular membrane whose color varies according to the amount of

pigment it contains (blue irises simply have less pigment--not blue pigment). Its central aperture

the pupil, is controlled by fine muscles. The muscles regulate the size of the pupil according to

the prevailing light. Drugs which dilate the pupil are called mydriatics; drugs which constrict the

pupil are called miotics. These drugs act by their effects on the autonomic nerve fibers. When the

iris is inflamed (iritis), there is an outpouring of protein and cells into the anterior chamber. This

can be seen as a "flare" with the fine beam of the biomicroscope, analogous to the visibility of a

beam of light in a dusty room.

The response of the pupils to light provides an important clinical sign of the integrity of the

retina and the function of the nerve fibers to the iris. Three reflexes are generally elicited; direct

pupil constriction to light; the simultaneous constriction of the pupil of the opposite eye even

though it is not illuminated (consensual reflex); and the bilateral pupil constriction when looking

at a near target (not illuminated). If the eye is blind, for example, the direct and consensual

reflexes will be absent, but the near reflex will be present.

FIG. 5-11

From Schlemm's canal, collector channels

carry the aqueous into a venous plexus in the

sclera and thus into the general circulation.

On the basis of the anatomic configuration,

glaucoma is classified into open angle and

closed angle. Open angle glaucoma is by far

the most common (about 98%) and is a

slowly progressive disease which seldom

produces pain or other symptoms. It is

caused by an as yet undetermined block in

the trabeculae. Closed angle glaucoma, on

the other hand, is due to adhesion between

iris and cornea, usually from pupil dilation,

which prevents aqueous from gaining access

to the trabeculae. In closed angle glaucoma,

there is a sudden large rise in intraocular

pressure, causing severe pain and rapid

visual loss.

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Ciliary body

FIG. 5-12

In cross-section, the ciliary body (fig. 5-12) has a triangular shape, with finger-like extensions

toward the crystalline lens (ciliary processes). The ciliary processes are extremely vascular and it

is from these vessels the aqueous humor is elaborated by the ciliary epithelium. The exact

mechanism of production is not known, but is believed to be a process of secretion by the cells in

which they transfer fluid from the blood into the posterior chamber. The aqueous then circulates

through the pupil into the anterior chamber, and is drained through the trabeculae. In

inflammations where the pupillary margin becomes adherent to the crystalline lens, the aqueous

is blocked and pressure builds up in the posterior chamber (pupillary block glaucoma). To

prevent such adhesions, the pupil is dilated in patients with active iritis.

Fine non-elastic fibrils (suspensory ligament or zonule of Zinn) extend from the valleys between

ciliary processes to the crystalline lens capsule. The tension of these fibrils is regulated by the

ciliary muscle. When the muscle contracts it reduces in size, causing it to move forward and

inward. This decreases the tension on the fibrils and allows the lens to become more convex,

providing more plus power to bring near objects into focus (accommodation). As we get older,

this focusing ability begins to fail (presbyopia). But not because the ciliary muscle gets weaker--

rather because the lens becomes less elastic.

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Choroid

CHOROID FIG. 5-13 FIG. 5-14

The choroid is the largest portion of the uveal tract. (fig. 5-13) It is a thin membrane made up

almost entirely of blood vessels. The vessels get progressively smaller from the scleral to the

retinal side. Next to the retina, they have the diameter of capillaries (choriocapillaries). A thin

but important membrane separates the choriocapillaries from the rods and cones of the retina

(Bruch's membrane). In some older people, this membrane cracks and degenerates, perhaps

because of partial obstruction of the capillaries. The vessels can now leak blood into

the retina. This is the main mechanism of senile macular degeneration.

The junction of the choroid with the ciliary body forms a dentate border (ora serrata). Note that

the ora serrata (fig. 5-14) is considerably forward of the equator. It can only be visualized

clinically by the indirect ophthalmoscope and scleral depression. Such visualization is important

because many holes which lead to retinal detachment begin here.

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Fig. 5-15

CATARACT FIG. 5-16

Crystalline lens

The lens is a biconvex, semisolid, transparent

body consisting of an elastic capsule

surrounding the lens substance. (fig. 5-15) On

the front side (only) is a single layer of

epithelial cells which continuously gives rise

to new lens fibers. The fibers are laid down

like the layers of an onion, a process that goes

on throughout life. This means the lens gets

larger as we get older, and this is the reason the

anterior chamber becomes shallower.

At birth, the lens consists only of a few fibers

(nucleus). With growth, the nucleus is covered

by new fibers, like the rings on a tree. One can

therefore time a small injury by its depth

within the lens.

The lens fibers actually are not long enough to

grow entirely around the nucleus. Fibers from

each direction meet about the middle, and

where they do give rise to "suture lines." Since

there are more fibers in later life, the suture

lines get progressively more complex. All

layers of the lens, epithelium, capsule, and

suture lines are readily seen with the

biomicroscope.

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Opacities of the crystalline lens obstruct and scatter light, and hence interfere with vision. Any

opacity is technically a cataract, but it takes many opacities before vision is seriously

compromised. (fig. 5-16) The time to operate for cataract depends, not on the lens, but on a

particular patient's visual disability. That cataracts can only be removed when they are "ripe" is a

common, but untrue myth.

Cataracts are usually senescent changes caused by complex biochemical alterations in the lens

substance. These are probably universal if we live long enough. Some people, such as diabetics,

may develop opacities at an earlier age. But the onset and rate of progression is not predictable.

In early stages of cataract, the lens may swell rather than become opaque. This induces an

artificial myopia, and the patient may find himself able to read without glasses ("second sight").

Not all cataracts are senescent, however. Lens changes may also be congenital, or caused by

injuries, drugs, radiations, or intraocular disease (complicated cataract).

Vitreous

The vitreous body is a gel-like fluid which vegetates in splendid isolation from birth to death. It

neither circulates nor carries out any work except to hold the retina against the choroid. With

advancing age, however, it tends to shrink and collapse. Small aggregates form and cast shadows

on the retina, especially in bright light. These shadows are the annoying floaters most of us

acquire with age. The collapsed vitreous may also bounce against the retina, causing flashing and

scintillations. Or residual adhesions actually tug on the retina and may cause it to detach. Only

careful (and prompt) examination can distinguish between inocuous flashes and those which

precede more ominous detachments. When the retina does detach, the patient may note a veil or

curtain in his field of vision. Every vitreous hemorrhage should be suspected of hiding a retinal

detachment in older people. More rarely, detachment and hemorrhage are caused by an

intraocular tumor. Vitreous bleeding without detachment is usually due to diabetic retinopathy.

Retina

The retina is embryologically an outgrowth of the brain. Like brain and spinal cord nerve fibers,

once damaged it cannot regenerate. The optic nerve, in fact, is not a true nerve, but a "tract"

connecting one part of the brain to another. It too will not regenerate, and any damage it sustains

is permanent.

The entire eyeball is designed around the retina. The optics of the cornea and lens bring the

image of an object into focus on the retina; the sclera protects it; the choroid nourishes it and

keeps out extraneous light; and the intraocular pressure maintains its stability. It would not do to

have a camera which collapses at unpredictable moments. The retina, therefore, is the film for

our living camera.

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Optic nerve The optic nerve is made up of the axons of retinal ganglion cells. Since there are only one

million optic nerve fibers and 126 million photoreceptors, it must mean that many rods and cones

hook up (by way of bipolars) to a single ganglion. This accounts for our ability to detect very

small amounts of light. Only in the fovea is each cone connected to a single optic nerve fiber;

this is the anatomic basis for its resolving power.

Lids Fig. 5-17

The lids are folds of tissue (fig. 5-17)

whose function is protection, tear

distribution, and supplying oxygen to the

cornea (via its conjunctival capillaries)

during sleep. When the lids are open, the

space is termed the palpebral aperture.

Note that it is not always symmetric or

equidistant from the nose. Drooping of the

lid (ptosis) will make it smaller and might

occur in III nerve palsy, myasthenia, or lid

tumor. A large palpebral aperture is

usually caused by lid retraction, and is

often seen in Grave's disease. It is

characterized by the fact that sclera shows

above the eye, whereas normally the upper

lids cover about 1/4 of the cornea.

The lids contain oily glands, of which the

most prominent are the Meibomian glands.

(fig. 5-18) These secretions lubricate the

cornea and make up the outer layer of the

tear film. The blink reflex distributes the

tears over the cornea and activates the

lacrimal drainage pump.

FIG. 5-18

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Lacrimal apparatus

FIG. 5-19 THE LACRIMAL APPARATUS FIG. 5-20

The lacrimal glands consist of the lacrimal gland proper and accessory glands (Wolfring and

Krause glands) scattered throughout the lid. (fig. 5-19) The main gland provides reflex tear

secretion, while the accessory glands take care of basic secretion (steady state). The aqueous

tears are covered on the outside by the oily secretions of the lids, and on the inside by the mucin

secreted by conjunctival goblet cells. The precorneal tear film thus consists of three layers, each

of which may be selectively involved in disease. Abnormal lashes, lid-cornea incongruities,

pterygia, contact lenses, drugs or infections may all affect the tear film. Tear quantity can be

measured by a strip of filter paper (Schirmer test) or by observing the thickness of the tear

meniscus resting on the lower lid with the biomicroscope. Tear film quality can be evaluated by

tear film break-up time as you hold the lids apart.

Drainage of the tears proceeds through the punctum, (fig. 5-20) into canaliculi, lacrimal sac, and

from there via the lacrimal duct into the nose. Sulfa instilled into the eye may sometimes cause a

bitter taste because of this communication. In some infants, the lacrimal drainage system may

fail to canalize. The baby tears on one side and may show a chronic unilateral conjunctivitis.

This plumbing problem can usually be solved by simple probing.

Normal tears contain a variety of antibacterial (lysozymes) and immune substances that help to

prevent infection of the anterior ocular surface. These substances are depressed in tear deficiency

diseases. Thus patients with keratitis sicca also frequently suffer from blepharitis.

Conjunctiva The conjunctiva lines the inside of the lids and the front of the eye up to the limbus. (fig. 5-21)

The upper and lower reflections make up the fornices, and the entire space is called the

conjunctival sac. This sac receives ocular medications and prevents contact lenses from getting

into the orbit proper (a frequent concern expressed by patients). On the other hand, the fornices

also hide foreign bodies--even contact lenses have been "lost" into the space because the lids

were not carefully everted to look for them.

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CONJUNCTIVA FIG. 5-21

Non-infectious causes of red eyes are toxins, drugs, allergies, environmental irritants, orbital

tumors, keratitis, uveitis, and glaucoma. The pattern of hyperemia is fairly typical in

conjunctival, uveal, and acute glaucomatous disease (make yourself a table of differentiating

features). A patient with conjunctivitis complaining of photophobia, pain, and impaired vision

probably has secondary corneal involvement.

Extra-ocular muscles

EXTRA-OCULAR MUSCLES FIG. 5-22

Orbit

The orbit is the pyramidal space in the skull designed to hold the eyeball and its contents. (fig. 5-

23 It is arbitrarily divided into a roof, floor, lateral and medial wall. Various openings and

The conjunctiva is a mucous membrane by virtue

of its goblet cells. Mucin aids in the adhesion of

the tear film to the cornea. A reflection at the inner

canthus called the plica semilunaris is the vestigal

remnant of a third eyelid in lower animals. The

fleshy protuberance (caruncle) is a normal

anatomic feature but may frighten a patient into

believing he has a tumor when it becomes swollen.

Infection of the conjunctiva by bacteria or viruses

is the most common cause of a red eye. This is

usually accompanied by a discharge, tearing,

itching or burning, swelling, foreign body

sensation, and a velvety appearance to the

conjunctival surface.

The six extra-ocular muscles (fig. 5-22) rotate

each globe in all possible directions, but their

mechanism of action is somewhat complex. The

best way to learn them is to draw the insertions

on a rubber ball, and attach a string to each with

a pin. Such a model will serve to demonstrate

how the superior oblique turns the eye down

and the inferior oblique turns the eye up.

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depressions constitute landmarks. For example, the roof has a depression for the lacrimal gland,

the medial wall for the lacrimal sac, the floor for the infraorbital nerve, and the lateral wall for

the zygomatic nerve. (fig. 5-24)

THE BONY ORBIT FIG. 5-24

Visual acuity

The traditional criterion of retinal image clarity is resolving power. Thus if an eye resolves 100

lines/mm it is said to have better acuity than one which resolves only 50 lines/mm. Resolution is

highly dependent on measurement conditions; design, size, shape, color, illumination, contrast,

as well as patient cooperation.

The standard of clinical acuity is the Snellen chaff. The equation for recording the Snellen

fraction is V=d/D where d is the distance at which a given letter can just be discriminated, and D

the distance at which the same letter subtends one minute of arc. For example, 20/40 means the

minimum angle of resolution is two minutes. Care should be taken not to misinterpret 20/40 as

50% visual loss.

EYE POSITION--UPPERVIEW FIG. 5-23

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Ametropia and presbyopia

MYOPIA Fig. 5-25 HYPEROPIA Fig. 5-26

An eye is considered emmetropic, if parallel rays focus on the retina with accommodation

relaxed. The definition says nothing about biologic health--an eye may be emmetropically blind.

If the focal point of a distant target is in front of the retina, the eye is said to be myopic; (fig. 5-

25) if behind the retina, hyperopic. (fig. 5-26) Of course there is an image on the retina in all

cases, but the image is clear or blurred. The hyperope with sufficient accommodation can bring

the image into focus; the myope cannot. The myope can, however, squeeze his lids together to

create a smaller opening and the increased depth of focus helps improve vision.

Instead of defining refractive error in terms of retinal focus by parallel rays, we can turn the thing

around and ask where a target must be placed to produce a sharp retinal image in any eye,

whatever its ametropia. This position is termed the far point. The far point for an emmetropic eye

is at infinity. It is somewhere between the eye and 6 meters for a myope. It is behind the eye in

hyperopia. How can a target be behind an eye? What is meant, optically, is that the light rays

must already be convergent when they enter the hyperopic eye to form a focus. Such convergent

rays would produce a picture somewhere behind the eye. Of course, they never get there; they

enter the eye and focus on the retina instead. To distinguish between a far point in front or

behind the eye, we give it an algebraic sign. Since rays from any far point in front of the eye

must be divergent, we give it a minus sign. Similarly, convergent rays are given a plus sign.

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Thus the sign of the far point coincides with that of the correcting spectacle lens; plus in

hyperopia, minus in myopia.

To be more specific, we can actually measure the location of the far point. This measurement

constitutes clinical refraction. It may be objective, as in retinoscopy, or subjective as in the test

with trial lenses.

Where would the far point of a 2D myope be? The answer is 50 cm in front of the eye (-50 cm).

Stated differently, we are saying that if an object is placed 50 cm in front of the eye, the image

will be in sharp focus with accommodation relaxed. Or we might say that this myope must

approach to within 50 cm of an object in order to see it clearly. What about the hyperope? He

cannot see clearly at any distance unless he accommodates. And if he is over 55 years old, he

cannot even do that. How much must a 2D hyperope accommodate to see clearly at 20 feet?

Obviously 2D. How much to see clearly at 50 cm ? Another 2D, for a total of 4D. Note that the

uncorrected hyperope has to accommodate more than an emmetrope to read. You can work out

the reading requirements of a myope for yourself.

What about the far point in an astigmatic eye? Obviously such an eye has two far points/just as it

has two foci), corresponding to each of the principal meridians. If an eye is 2D myopic in the

vertical meridian and 2D hyperopic in the horizontal meridian, the vertical far point is at -50

cm. and the horizontal far point in at +50 cm. You should draw this and similar examples on

paper until this somewhat confusing subject falls into place.

ACCOMMODATION FIG. 5-27

The closest point an eye can see exerting its maximum available accommodation is called the

"near point." (fig. 5-27) An emmetropic eye with a 10D amplitude of accommodation would

have a near point of 10 cm. The near point of a 20D uncorrected myopic eye with the same

amplitude would be 5 cm. Where would the near point of a 10D uncorrected hyperope with the

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same amplitude be? (Answer is infinity). Why might a 60 year old person who is 3D myopic

object to wearing bifocals?

Presbyopia technically means the vision of old age. Actually it signifies that stage of life when

accommodation has so diminished that the patient can no longer read the size of print at the

distance he wants to. The diagnosis (and treatment) therefore depends not on accommodative

amplitude but on the person's needs. The amplitude of accommodation, as is well known,

decreases progressively from birth to old age. It is about 15 diopters in a child and zero at age

55-60.

Binocular vision and strabismus

Binocular vision refers to the cerebral unification of two images, one from each eye, to produce a

single three-dimensional percept. The eyes are brought into alignment upon a fixation target by a

complex series of reflexes involving muscular tonus, accommodation, and vergence. This

process is called "fusion."

Intraocular pressure and glaucoma

We have already seen that aqueous is continuously produced and drained. Together with the

unchanging vitreous, this maintains a constant pressure within the globe. This is resisted by the

counter pressure of the ocular coats and the intraocular circulation. The purpose of maintaining

constant ocular pressure is to keep the refracting surfaces in precise alignment since even minor

collapse would greatly distort the optical image on the retina.

It is not possible to measure the intraocular pressure directly without sticking a needle into the

eye and balancing the pressure against a column of mercury. Instead it is measured indirectly

(hence called intraocular tension). The units, however, are still the same so many mm of Hg. The

two common ways of estimating pressure are with the Schiotz tonometer which indents the

cornea, or the applanation tonometer which flattens the cornea. The latter is generally more

precise, and can be done while the patient is sitting at the slit lamp.

Normal intraocular pressure, like normal blood pressure, varies from one individual to another,

and from time to time. The average range is from 15 mm Hg to 21 mm Hg approximately. The

person with a tension of 25 mm Hg may or may not have glaucoma, however, depending on

whether that pressure reading is excessive for that eye. How can one tell? By looking at the optic

disc and measuring the visual field. If these show abnormalities, it means that nerve fibers are

being damaged and the pressure must be reduced to a safer level.

Pressure reduction in glaucoma means life long treatment, for like diabetes, it can be controlled

but not cured. The conventional drugs used in open angle glaucoma are those which increase

drainage (outflow) such as pilocarpine, timolol, and epinephrine; those which reduce aqueous

production (diamox); or those which alter the osmotic balance with the blood (glycerol and

mannitol). Some drugs have more than one mechanism of action. Surgery is used only for those

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situations where glaucoma cannot be controlled by drugs or the patient cannot be relied on to

take medication regularly. In contrast, closed angle glaucoma is an ophthalmic emergency

because the extremely high pressure (60 + mm Hg) can lead to blindness within 24 hours. The

patient is usually hospitalized, and once the pressure is brought down, scheduled for prompt

surgery (iridectomy). If done in time, they may cure the patient so that further medication is

unnecessary. If treatment is delayed, however, the outflow is permanently compromised and the

patient now has chronic glaucoma superimposed on the closed angle mechanism. In addition to

anatomically narrow angles, aqueous outflow may also be compromised by injury (bleeding into

the chamber), inflammation (keratitis, uveitis), or tumors. These are termed secondary

glaucomas, and may be unilateral. Occasionally, a hypermature cataract may swell and block the

angle. More commonly, one sees certain people whose tension rises when they use topical or

systemic steroids for more than two weeks. These induced glaucomas can be cured by

eliminating use of the drug.

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Bibliography

Bausch and Lomb Optical Company, The Human Eye in Anatomical Transparencies, The

Bausch and Lomb Press, Rochester, N.Y., 1943

Borish, Irvin M., Clinical Refraction, Third Edition, The Professional Press, Chicago Illinois,

1970

Davidorf, Frederick H. and Hill, Donald A., Atlas of Eye Surgery and Related Anatomy,

Ophthalmology Illustrated, Keller Publishing Co., Columbus, Ohio, 1978

Hogan, Michael J., AIvarado, Jorge A. and Weddell, Joan E., Histology of the Human Eye, W.B.

Saunders Company, Philadelphia, PA. 1971

Michaels, David D., Visual Optics and Refraction, Second Edition, C.V. Mosby Company, St.

Louis, Mo., 1980

Moses, Robert A., Adler's Physiology of the Eye, Seventh Edition, C.V. Mosby Company, St.

Louis, Mo., 1981

Scheie, Harold G. and Albert, Daniel M., Textbook of Ophthalmology, Ninth Edition, W.B.

Saunders Company, Philadelphia, Pa., 1977

Stimson, Russell L., Ophthalmic Dispensing, Charles C. Thomas, Publishers, Springfield Illinois,

1979

Tisdale, Ralph, Review Outline, Anatomy and Physiology, N.Y.C. Community College,

Brooklyn N.Y., 1971

Vaughan, Daniel and Ashbury, Taylor, General Ophthalmology, Lange Medical Publications,

Los Altos Ca., 1980

Warwick, Roger, Wolff's Anatomy of the Eye and Orbit, W.B. Saunders Company, Philadelphia,

Pa., 1976

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