A CHILD'S EYES


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Foreward

One of the defining characteristics of medical care delivery in the coming era is restriction of access to specialized care. What this means to primary care physicians -- to pediatricians and family practitioners – is new responsibility to diagnose and to treat medical problems formerly the province of more specialized physicians. One of the new challenges is that they must decide who should be referred to specialists in pediatric ophthalmology. And increasingly, they must assume the role of educating the parents of their young patients with eye problems.

A Child's Eyes provides a quick desk reference for non-ophthalmological practitioners. It covers the multitude of disorders affecting the eyes of children and gives straightforward descriptions of all the common disorders encountered in pediatric ophthalmology. It is an ideal resource that belongs on the bookshelf of every pediatrician and family practitioner. The coverage is concise, yet complete, and the book is well illustrated to support the written descriptions. The excellent index and margin references will bring the subject needing review and the practitioner together in an instant.

Eye problems can be difficult for primary care providers, but they can be completely baffling to parents. When a physician is faced with the need to educate parents, A Child's Eyes will provide the needed information in layman's English.

Both authors are seasoned educators and renowned pediatric ophthalmologists and strabismologists. They have many years of practice in this field, providing care to untold thousands of children and, in the process, investing enormous time interacting with and educating their parents. Although they have addressed this book principally to primary care physicians, it is designed to be shared with interested parents, as well as with educators, occupational therapists, and the many other professionals concerned with children.

A Child's Eyes is a valuable tool for everyone who is interested in acquiring an understanding of this specialty.

Marshall M. Parks, M.D.

Preface

During the course of our careers as pediatric ophthalmologists, we have encountered countless parents who are genuinely perplexed about their children's eye problems. We wrote this book with the realization that, more and more, it will be primary care practitioners who will have to provide answers to their questions. More importantly, these doctors will also be faced with treating certain patients that they previously referred, and will need the confidence to decide when a patient absolutely requires referral. Insurance administrators and others interested in medical care utilization will want to know this information as well.

We've tried to address this book on two levels: first, for primary care physicians, and second, for parents and other nonmedical professionals who have regular contact with our patients. It's not always been easy. There are doubtless some instances when the text will be too elementary for our medical readers and others when it will be too sophisticated for our lay readers. We trust both will be understanding.

Finally, a note about eye doctors. We're both ophthalmologists, which means we completed medical school, one year of post-graduate training in primary medical care, and three years of residency training in ophthalmology – before subspecializing in pediatric ophthalmology. There are also eye doctors of optometry. They have earned a professional degree (O.D.), in a college of optometry. In general, ophthalmologists do optical treatments (glasses and contact lenses), medical management, and surgery for eye problems. Optometrists spend more of their time on optical treatments, and may specialize in such fields as contact lenses and low vision. The differences in scope of practice between ophthalmology and optometry have narrowed in recent years, causing a certain amount of controversy. We want to side-step this controversy, and so have referred to "eye doctor" in many places, specifying "ophthalmologist" only when the issue is clearly surgical.

John W. Simon, M.D.
Joseph H. Calhoun, M.D.

Introduction

Little Heather was an adorable two-year-old who had her dad – and almost everybody else – wrapped around her little finger. She also had an eye problem.

One day, for no apparent reason, Heather's right eye began to turn inward, or cross. At first Heather's parents weren't sure there really was a problem because her eye crossed only when she was very tired. But gradually the crossing became more pronounced and more frequent. Then it began to happen not only when Heather was tired, but almost all day long. Looking at her picture books made it worse, and she tended to close or cover her right eye.

Her parents began to panic. Her pediatrician referred them to an ophthalmologist, who said that Heather had strabismus. The doctor prescribed a pair of glasses and an eye patch for her left eye. Heather, of course, had no use for either one. Whenever her parents left the room she pulled off the glasses, and the patch came off soon after. Every time, her right eye immediately crossed in again.

Heather's parents were torn. They couldn't stand to see their little girl so uncomfortable. But they were afraid that if they didn't force her to wear the glasses and patch, her vision would be ruined for life. Maybe the doctor had made a mistake. A neighbor said that his son Samuel had once had the same problem as Heather, but a simple operation had corrected it. Surgery sounded awful. But maybe it was better than making Heather do something that made her unhappy and didn't seem to be doing any good.

Heather's story, with minor variations, is repeated every day. Parents are understandably confused and anxious. Eye problems can be complicated, and the technical jargon used to describe them makes them seem even more so. Many parents are afraid that any mistake on their part might damage their children's eyes for the rest of their lives.

It often comes as a surprise to parents that so many different problems can affect how their children see and that so many kinds of treatment may be recommended for problems that, at least at first glance, seem to be identical. When Heather's parents were told that Samuel had had "exactly the same problem" as Heather, the chances are good that the problems weren't really the same at all.

Myths and misconceptions about children's eye problems are widespread. All parents have heard stories about children with "lazy" eyes or "pink" eyes and even such diseases as glaucoma or cataracts. They hear about children who need glasses by age 2 and newborns who need eye surgery. They're convinced that sitting too close to the television or reading with a flashlight under the covers will do permanent damage to their children's eyes.

Parents are surprised to learn:
  • when an eye turns in childhood, its vision may suffer;
  • glasses sometimes make crossed eyes straighten;
  • "vision" is what an eye can see, not what its glasses prescription is;
  • a right eye that is crossed can be straightened by surgery on either eye (or both eyes).
This book will help you answer the questions, explain the unexplained, and dispel the myths and misconceptions about children's eye problems. After reading it you should understand enough about the subject not to panic when something unusual is noticed about a child's eyes or visual behavior. You should know when and where to turn for help and what kinds of tests are likely to be performed. You will be able to answer the questions parents ask in language they can understand.

THE EYE:
ANATOMY AND FUNCTION

Before delving into the details of the various problems that can affect children's eyes, we will begin by explaining the parts of the eye itself.

A marvelously adapted organ, the eye is able to give us many times more information about our surroundings than all our other senses combined. Its structure is remarkable, not only for what it can do, but for its inherent beauty as well. Because the eye is the only part of the body where nerves and tiny blood vessels can be seen directly, examining the eye can provide important clues about the health of the entire body.

The eye is often compared to a camera, and the similarities are striking. Just as a camera focuses light on the film, an eye focuses light on the retina. The eye has a variable-focus lens suspended in place just behind the pupil, with more focusing power provided by the curvature of the cornea. Like the aperture of an automatic camera, the pupil opens in dim light and closes in bright light to control the amount of light entering the eye.

Much like the images on a roll of film, images formed on the retina must then be "developed." For this purpose they are transmitted by the optic nerve, which leads from each eye toward the visual area in the back of the brain.

THE NORMAL EYE. The transparent cornea reflects a small portion of the incoming light, for example from a camera's flash. That reflection is seen as a white spot at the 10:30 o'clock position in front of the iris and pupil. THE NORMAL EYE IN CROSS SECTION. The outermost of the three layers includes the transparent cornea and the opaque white sclera. The middle layer is the vascular uvea, made up of the iris, the ciliary body, and the choroid. The innermost layer is the retina.
How the eye functions

To understand the eye, let's begin by looking at some illustrations of it (opposite page). Note the iris, pupil, conjunctiva, sclera, cornea, choroid, ciliary body, zonules, lens, retina, macula, optic nerve, two kinds of fluid and six muscles. Each of these must be in good working order for the eye to function properly.


PUPILLARY LIGHT REACTION. Both pupils dilate in dim light and constrict in bright light. 		The iris and the pupil

Even though it's actually inside the eye, the first part people tend to look at is the iris (plural: irides). The iris is a muscular structure shaped like a doughnut. Its color is determined by pigment cells, which tend to darken during infancy. Generally by 6 to 12 months of age, the color of the iris is determined for life.

The black hole at the center of the iris is the pupil. In darkness it dilates and in light it constricts, due to the involuntary relaxation or contraction of the iris muscles. The pupils also change size in response to looking at things at close range, to different emotional states, and to various kinds of medications and eyedrops.

The sclera and the cornea

The "white" of the eye – the sclera – is the tough outer coating that gives the eye its spherical shape (the eyelids conceal much of its shape). During the first few years of life, the sclera is still soft and growing.

The sclera is covered by the conjunctiva (plural: conjunctivae), a thin membrane, translucent like waxed paper, that also lines the undersides of the eyelids. The conjunctiva, along with the lacrimal glands, makes tears, which keep the eye moist. But it is probably best known for its tendency to get inflamed when it is infected or irritated. This condition, called conjunctivitis, is very common and is generally only an annoyance.

The cornea is in front of the iris. Though it is part of the same layer as the sclera, it is different in that the cornea is transparent. Light must pass through it to get to the pupil and the inside of the eye. As you can see from the cross section, the cornea is curved like a watchglass. This curvature accounts for about two-thirds of the focusing power of the eye. The cornea is a very sensitive structure. If you have ever had something in your eye, had a scratch on the cornea, or worn a contact lens too long, you are well aware that it is richly supplied with pain nerves. Unlike inflammation or infection of the conjunctiva, damage to the cornea can often threaten eyesight.

 The choroid, the ciliary body, and the lens

Hidden beneath the sclera is a second layer, the choroid. Its function is to supply blood to the other parts of the eye, especially the retina. In fact, blood vessels in the choroid are more densely packed than anywhere else in the body.

The retina needs a rich supply of blood for two reasons. First, it is metabolically active and therefore needs a great deal of energy. Second, the focusing of light onto the retina creates heat, much like the focusing of the sun's light by a magnifying lens. The rich blood supply of the choroid carries this heat away and protects the eye from injury.

The choroid is in the same layer as the iris. Also in this layer is the ciliary body, which lies just behind the junction of the cornea and the sclera. Like the iris, the ciliary body is a muscular structure, but its central opening is much larger. The ciliary body produces aqueous humor.

The lens, made of nearly pure protein, is a transparent structure with two convex surfaces. Special "guy-wires" called zonules connect the lens to the ciliary body and suspend it so it is centered behind the pupil. When the ciliary body constricts, it relaxes the pull on the zonules so that the shape of the lens changes. This process, called accommodation, focuses light on the retina so we can see near objects clearly. When the muscles of the ciliary body are relaxed, tension is placed on the zonules and the focus of the lens is readjusted so we can see things at a distance.

Often in older people, and occasionally even in children, the normally transparent lens becomes cloudy. If it interferes with vision, this clouding of the lens is called a cataract.

 The retina

The innermost layer of the eye is the retina, which contains specialized cells called photoreceptors (rods and cones). These cells turn light energy into nerve impulses. In the center of the retina is the macula (the very center is called the fovea). The central part of the retina provides the sharpest vision because a very large number of photoreceptors are crowded closely together. For that reason, people subconsciously direct their foveas at whatever they're looking at.

The fovea has no blood vessels, but it looks red because of blood that shows through from the choroid. The parts of the retina farther away from the macula are important because they provide us with peripheral vision - a wide visual area around what is directly looked at.
The optic nerve
Nerve impulses from the photoreceptors in all parts of the retina are passed to the optic nerve, which can be seen inside the eye as a pinkish-yellow disc.

You can "find" your own optic disc by doing a little experiment. Hold your two forefingers in front of you at arm's length. Now close your left eye, concentrate on the tip of the left finger, and move the right finger slowly to the right. When your fingers are about 6 inches apart, the right fingertip will disappear, only to reappear when the fingers are 7 or so inches apart.

You've just demonstrated the "blind spot" caused by the optic disc, which contains no photoreceptor cells. It does contain some one million nerve cells, however, which transmit all of the visual information from the eye to the brain. In fact, anatomists consider the optic nerve a part of the brain.		 THE NORMAL OPTIC NERVE AND RETINA. Viewed through the indirect opthalmoscope, the healthy optic nerve appears as a pinkish-yellow disc. The youthful retina reflects light as a sheen, which in this picture nearly encircles the macula, partially seen about two and one-half dissc-diameters to the right of the optic nerve.

 The fluids inside the eye

The front third of the eye – the part behind the cornea and surrounding the lens and iris – is filled with aqueous humor (also called aqueous). Aqueous is a watery fluid produced by the ciliary body. It circulates through the pupil and filters out of the eye at the angle between the iris and the cornea. The balance between the production and drainage of aqueous maintains the normal pressure inside the eye. If the pressure is too high, usually because of poor drainage, glaucoma can develop and the optic nerve can be damaged.

Vitreous humor (also called vitreous) fills the back two-thirds of the eye. Vitreous has an unusual consistency, much like uncooked egg white.

Both the aqueous and the vitreous must remain clear for vision to be sharp. Inflammation or blood inside the eye, for example, would likely blur the vision.


THE NORMAL EYE AND ORBIT IN LONGITUDINAL SECTION. The eye sits in a bony cavity called the orbit (the eye socket). The optic nerve carries visual information to the brain. This view shows the superior rectus and inferior rectus muscles. The medial rectus and lateral rectus muscles are out of the plane of the drawing.

 The eyelids and the orbit

In front of the eye, of course, are the eyelids. The lids keep the cornea moist by moving the tears across the surface of the eye with each blink and by preventing the evaporation of tears during sleep. Without normal eyelids, corneal damage often occurs and can be severe.

Tears are made by the lacrimal glands, which are located in the outer part of the upper eyelid just beneath the brow. There are also accessory lacrimal glands in the conjunctiva lining the undersides of the lids and covering the sclera.

Eyelids that droop – ptosis – are usually not dangerous, though babies with this problem should be checked for amblyopia, or poor vision.

The structures around the eye fill the eye socket, which is called the orbit. The eye is protected from injury by the orbital bones and the cushion of fat that surround it. The illustration (above) shows how the vital structures of the orbit the optic nerve, the eye muscles, and the blood vessels and nerves that supply them – fit neatly together behind the eye. Farther back in the orbit, the bones tend to be thin and can break easily.

 The eye muscles

The six muscles that attach to each eye move the eyes and align them with each other. When they don't work in a coordinated fashion, the result is loss of alignment, or strabismus. These muscles are particularly important in pediatric ophthalmology because strabismus usually begins in childhood.

Four of the muscles are oriented straight ahead. They are called the rectus muscles from the Latin word for "straight." The other two are called the oblique muscles because they pull obliquely.

THE EXTRAOCULAR MUSCLES AND THEIR ACTIONS. The principal actions of the six extraocular muscles in the right eye are shown schematically.

 The rectus muscles

The rectus muscles connect to the bones at the back of the orbit. The front end of each muscle attaches to the sclera near the cornea. When these muscles contract, they simply turn the eye (or, more accurately, the cornea) in the direction of their attachments. The superior rectus muscle attaches to the sclera above the cornea; when it contracts, the cornea turns up. Similarly, the inferior rectus attaches to the sclera below the cornea and turns the cornea down. The medial rectus attaches on the inner side and turns the cornea medially (toward the nose), while the lateral rectus attaches on the outer side and turns the cornea laterally (away from the nose).


THE RECTUS MUSCLES. This view shows the four rectus muscles of the right eye. The medial and inferior recti are partially hidden behind the eye.

 The oblique muscles

The two oblique muscles are a bit more complicated, since they attach near the back of the eye. Let's look at the inferior oblique muscle, which connects the lower half of the back of the eye to the lacrimal bone near the nose. When it contracts, the back of the eye turns down and the front of the eye turns up. In some children this muscle contracts too much and causes the cornea to turn up too far, especially when it's looking toward the nose. This condition is appropriately called "inferior oblique overaction." The inferior oblique also rotates the eye so that the 12 o'clock position turns away from the nose (excycloduction).

The superior oblique muscle attaches to the upper half of the back of the eye, extending backward through the trochlea, a bony pulley near the nose, and finally connecting to the bones at the back of the orbit. When it contracts, the back of the eye rises and the front lowers. This muscle also tends to rotate the eye so that the 12 o'clock position rotates toward the nose (incycloduction). If this muscle is weak, the eye cannot turn down well. In fact, this muscle is occasionally weak from birth, and that weakness may cause persistent head tilting.

THE OBLIQUE MUSCLES. This view shows the superior and inferior oblique muscles of the right eye. The posterior portion of the superior oblique is partially hidden behind the eye.

 The path from the optic nerve to the brain

The eye muscles surround the optic nerve as it extends from the eye to the back of the orbit, where it passes through the hole in the skull called the optic canal. But before reaching the brain, the optic nerves from both eyes partially cross each other at the optic chiasm and form the optic tracts, which end at "relay stations" in the middle of the brain called the lateral geniculate bodies. From the lateral geniculate body the optic radiations carry visual information to the occipital cortex. This is the area at the back of the brain that processes the visual information from the photoreceptors in the retina, resulting in visual perception.

Because of the way the optic nerves cross, the right side of the brain allows us to see objects on the left side of our field of vision (imaged in both eyes). Similarly, the left side of the brain allows us to see objects on the right side of our field of vision.

It is a long way from the front of the eye to the back of the brain, and much sensory and motor coordination is needed to keep the process of vision functioning normally. Fortunately, the structures just described work remarkably well together, in most cases without our even having to think about them. For example, parts of the brain subconsciously control the coordinated pull of muscles inside and outside the eyes so that we can shift our gaze in any direction and focus steadily and clearly with our eyes in any position. And the images seen by each eye are integrated by the visual cortex into a single "picture." Considering how complicated vision is and how many ways it might go wrong, it's amazing that most people have so little trouble seeing normally.


Reprinted from A Child's Eyes: A Guide to Pediatric Primary Care, copyright 1998 by John W. Simon and Joseph H. Calhoun

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