Ординатура / Офтальмология / Английские материалы / The Eye Care Sourcebook_Lavine_2001

T H E E Y E C A R E S O U R C E B O O K

During one hot summer, a man made an urgent appointment to see me because his eyelid began twitching. I remarked that he had probably been drinking a lot of soda. Sure enough, he entered my office carrying a can of cola!

Treatment of this harmless but annoying symptom includes elimination of caffeine or other stimulants from the diet and treatment of any underlying dry eye syndrome or eyelid inflammation (blepharitis) problem. Occasionally treatment can include a little antihistamine medication or, as a last resort, an old remedy called quinine.

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Anatomy of the Eye

THE EYE RESTS IN THE BONY SOCKET THAT WE CALL THE ORBIT.

The rims of the orbit help protect the eye from injury. Fat in the orbit behind the eye serves as a cushion to keep the eye from being traumatized by the bones of the orbit when the eye or the head is jostled. The optic nerve, an extension of the brain, enters the back of the orbit through a bony canal and then enters the eye, where its nerve fibers become distributed over the surface of the retina. Also entering the orbit are the various blood vessels that supply all the parts of the eye, including its muscles, and the nerves that transmit messages back and forth between the eye and the brain.

It is sometimes helpful to think of the eye in terms of its coats or layers. The outer coat consists of the cornea, the clear window of the front of the eye, and the sclera, the tough, white tissue that begins where the cornea ends. The middle layer is the uvea (Greek for “grape,” because of its coloration). The uvea includes the ciliary body; the iris, which surrounds the pupil; and the choroid, the blood vessel–rich layer in the back half of the eye. Finally, the retina forms the inner coat and is located beside the choroid. The retina is in contact with the vitreous humor, the gel-like substance that fills the interior of the back of the eye, while the cornea is in contact with the aqueous humor, the fluid inside the front part of the eye. We will now look at each part of the eye in more detail.

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Copyright 2001 by Jay B. Levine. Click Here for Terms of Use.

T H E E Y E C A R E S O U R C E B O O K

FIGURE 3.1 The Human Eye

Source: “The Human Eye,” by C. Bohlman Patterson, in Opthamology Principles and Concepts, 8th edition, 1946, p. ii; reprinted with permission from The C. V. Mosby Company (Harcourt Health Sciences).

The Cornea

The cornea is transparent and fairly thin, only about half a millimeter in thickness near its center. It serves as a lens, changing the pathway of incoming rays of light, making them converge so that they ultimately focus on the retina. The cornea also has a protective function in terms of infection and foreign bodies that might enter the eye.

The outermost layer of the cornea is called the epithelium and is only about six cell layers thick. These cells may be thought of as the “skin” of the

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eye. They prevent most bacteria from entering the cornea or the interior of the eye. When some of the cells of the epithelium are lost, as occurs with a corneal abrasion, the cornea becomes susceptible to infection, and a corneal ulcer may form. The tear film, the layer of tears that coats the surface of the cornea, protects the epithelium, keeping it from drying out, and also contains antibodies and other substances that resist infection.

The corneal surface is extremely sensitive because of the numerous nerve endings there. If the eye is abraded and epithelial cells are lost, the exposed nerve endings produce sharp, fairly severe pain. Other insults to the surface of the cornea, such as overwearing a contact

lens, can produce similar pain. Sensitivity of the cornea may be reduced in some circumstances, for example, with herpes infections of the cornea or in diabetics. Fluid buildup

(edema) in the epithelium can occur when the pressure in the eye becomes very high; when the endothelial (inner lining) cells of the cornea are not functioning properly; or as a result of injury, infection, or inflammation. This edema gives the corneal epithelium a cloudy appearance. The vision becomes blurred, and the person may see halos around lights.

The eye tends to replace lost epithelial cells very quickly, as they can divide rapidly and slide over to fill in any gaps. In diabetics, however, the epithelial cells may not “stick” down as well to the layer of the cornea behind them.

The middle layer of the cornea, called the stroma, makes up most of the thickness of the cornea. It is composed of collagen, the same protein that is in skin and nails. In the case of the cornea, however, the collagen is laid down in such a way that it remains transparent. The part of the stroma immediately next to the epithelium is fairly dense and is called Bowman’s membrane, although it is not really a membrane. The corneal epithelial cells form over Bowman’s membrane. If this “membrane” becomes damaged, the epithelial cells may have difficulty adhering to it.

If the stroma becomes damaged by injury (such as laceration), ulceration, or inflammation, it usually forms a scar in the affected area rather than returning to its pristine, transparent state. Such scarring interferes with vision if it occurs near the center of the cornea. It can also make the curvature of the cornea irregular, causing irregular astigmatism, a kind of blurring that ordinary eyeglasses do not correct very well. Corneal edema, or

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fluid buildup in the cornea, can thicken the stroma and sometimes even make it cloudy.

Along the back surface of the stroma is a thin membrane called Descemet’s membrane. A single layer of cells, the endothelium, is present on Descemet’s membrane. The endothelium represents the innermost layer of the cornea. The endothelial cells have a very important function. Because of the fluid pressure inside the eye, water is always trying to enter the cornea. If it did so, the cornea would lose its transparency. The endothelial cells constantly pump any water that gets into the cornea back into the anterior chamber of the eye. Although endothelial cells are lost with age and as a result of eye surgery or injury, they do not regenerate as the epithelium does. If too many are lost, the cornea develops edema.

The Sclera and Conjunctiva

The sclera is the tough white coat of the eye. It begins where the cornea ends and extends all the way around to the back of the eye. A small gap in the sclera allows the optic nerve to enter the eye at the very back. The sclera is also perforated by tiny blood vessels and nerves. Like the cornea, the sclera is composed of collagen, but since the collagen fibers are laid down differently, the sclera is opaque rather than transparent. Because of its density, the sclera imparts some rigidity to the eye. It can be lacerated by sharp objects or even ruptured by sharp blows to the eye, but it usually halts minor foreign bodies in their tracks.

Covering the sclera in the front part of the eye is a transparent, blood vessel–containing mucous membrane called the conjunctiva. The conjunctiva is reflected from the sclera onto the inside surface of both the upper and lower eyelids. In the area where the conjunctiva

leaves the sclera and becomes the lining of the eyelids, a narrow pouch, or cul-de-sac, is formed. People who fear that their misplaced contact lenses may travel behind their eye will be reassured to know that this conjunctival

cul-de-sac is what prevents that possibility. The blood vessels in the conjunctiva are the thin, thready red lines you see over the sclera. When these blood vessels dilate, the eye develops its familiar bloodshot appearance. An interesting fact is that the conjunctiva is the one place in the body where the blood

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vessels are visible externally. The appearance of these vessels (for example, narrowing) can help indicate whether blood vessel damage is occurring in people with high blood pressure or diabetes. Inflammation of the conjunctiva is called conjunctivitis, and this is the term used to denote ordinary eye infections.

The Anterior Chamber

The anterior chamber is the fluid-filled space behind the cornea and in front of the iris. The watery fluid that fills the anterior chamber is called the aqueous humor. Aqueous humor is produced by the ciliary body and is continuously secreted into the posterior chamber, the small space behind the iris and in front of the lens. The aqueous humor flows from there through the pupil to enter the anterior chamber. The anterior chamber always maintains about the same depth, although it may become shallow or even “flat” as a result of abnormal events— for example, an injury that perforates the cornea, allowing the aqueous humor to leak out. Surgical complications, especially accompanying glaucoma surgery, can also cause a flattening of the anterior chamber.Whatever the cause, urgent action is often necessary to restore the anterior chamber to its normal depth.

When iritis, an inflammation of the iris, is present, white blood cells and protein can be detected in the aqueous humor. Bleeding (hyphema) may occur in the anterior chamber as well, most commonly as a result of injury.

The Trabecular Meshwork

In the angle of the eye, the region at the edge of the anterior chamber where the ends of the iris and cornea can be found, two important structures can be seen. One is a portion of the ciliary body,

discussed next. The other is the trabecular meshwork, the series of drainage channels through which the aqueous humor percolates out of the anterior chamber. From the

trabecular meshwork, the aqueous humor enters a channel called the canal of Schlemm.

From there, the fluid is transported through microscopic vessels and eventually reaches tiny veins near the surface of the eye. Clearly, if the angle closes down so that the aqueous humor cannot reach the trabecular meshwork and

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therefore has no way to exit the eye, the pressure in the eye becomes very high, in a form of glaucoma called angle-closure glaucoma. In the most common form of glaucoma, chronic open angle glaucoma, resistance in the drainage pathways is present at the microscopic level.

The Ciliary Body

The ciliary body, along with the iris and the choroid, are pigmented tissues that form the uveal tract of the eye. (Uveitis refers to inflammation of these structures.) As already noted, one of the functions of the ciliary body is to produce aqueous humor, the fluid that fills the anterior and posterior chambers. The ciliary body also contains the ciliary muscle, which is involved in a focusing mechanism called accommodation. When you

change your gaze from distance to near, this muscle contracts, releasing tension on the zonules, the fibers that suspend the lens in place. As this occurs, the lens becomes more

curved so that light coming from objects close to the eye is focused on the retina. Aging changes in the muscle result eventually in presbyopia, the difficulty in focusing on materials at the reading distance that most people experience when they reach their forties. Contraction of some of the ciliary body muscle fibers can also affect the drainage channels of the eye, allowing the aqueous humor to drain out of the eye more easily.

Some eyedrops used to treat glaucoma, for example, pilocarpine, lower the eye pressure by making these muscle fibers contract. Other glaucoma eyedrops lower eye pressure by suppressing the formation of aqueous humor by the ciliary body.

The Iris and Pupil

The iris is the pigmented ring of tissue that surrounds the pupil, the hole through which light travels toward the back of the eye. The color of the eye depends on the amount of pigment (melanin) in the iris. An iris with a large amount of pigment appears brown, whereas one with only a little pigment looks blue. Of course, there are varying degrees of pigment in the eyes of different people.

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The iris contains muscles that dilate and constrict the pupil. It dilates in the dark and constricts in bright light. The pupil also becomes smaller when you look at something up close. Pupils vary in

size from one person to another, although they tend to become smaller with age. Usually, a person’s two pupils are about the same size, although small differences are common. If a pupil changes in size, the problem must be

investigated. The change could be the result of a very benign problem or a more serious problem in the brain.

The iris itself can be involved with various diseases. Growth of abnormal blood vessels (rubeosis iridis) can occur in diabetics or after major vein occlusions of the retina, and these blood vessels can scar shut the trabecular meshwork area, resulting in glaucoma. When there is inflammation in the eye, the iris may stick down to the lens behind it or to the cornea in front of it, causing problems. Pigment can sometimes be lost from the iris, for example, in a form of glaucoma called pigmentary glaucoma, giving the iris a bit of a moth-eaten appearance. A pseudoscience called iridology is based on the belief that the appearance of the iris can be used to diagnose all sorts of bodily diseases. There is no scientific basis for iridology, however, and its ability to diagnose disease has been refuted in at least one study.

The Lens

The lens focuses incoming light rays onto the retina so that the image can be clearly seen. Its shape can change to allow the eye to focus both on objects in the distance and on those up close. In this respect, it is like a zoom lens on a camera. However, the ability of the muscle of the ciliary body to change the shape of the lens declines with age, finally resulting in the need for reading glasses, a condition called presbyopia, or “oldsightedness.”

The lens is a transparent disk composed of protein and suspended in place by the zonules, fibers that are attached on their other end to the muscles of the ciliary body. The lens is similar in structure to an onion, with numerous layers formed by thin cells that continue to be laid down throughout life. The oldest portion of the lens is therefore the central portion. The inner core of the lens is called the nucleus, whereas the outer layers are

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called the cortex. The lens is entirely surrounded by a thin envelope called the capsule.

The most common abnormality involving the lens is cataract, a cloudiness in part or all of the lens caused by changes in the proteins of the lens. Thus, a cataract involving the nucleus is

called a nuclear cataract, whereas clouding of the outer layers is called a cortical cataract. A foreign object entering the eye may damage the lens, and the end result may be a cataract.

Blunt injuries to the eye may not only cause cataract but also cause some of the zonules to break. The lens may then become dislocated, floating about in the eye.

The Vitreous Humor

The vitreous humor, or simply vitreous, as it is known for short, is the gel that fills the largest chamber of the eye, bounded by the lens in front and the retina in back. It is mostly water, but it contains hyaluronic acid and a special framework of thin collagen (protein) fibers. As the eyes age, a degenerative process occurs, and the gel liquefies. Eventually, about half of the vitreous may become liquid, and half remains as a gel. As the gel portion shrinks, traction is applied to those areas of the retina to which the vitreous gel is attached. Usually the vitreous separates itself from the retina without incident, but sometimes it causes a tear in the retina, the first step in the formation of a retinal detachment.

When uveitis, or inflammation inside the eye, is present, white blood cells form in the vitreous, and they can be clearly seen when the eye is examined. Cancer cells, for example, from lymphoma, may build up in the vitreous as well. Blood in the vitreous can be seen after eye injuries or as a result of bleeding on the surface of the retina, as is sometimes seen in diabetics.

The Choroid

The choroid is part of the uvea or uveal tract, the middle coat of the eye. It is located behind the ciliary body and is sandwiched between the sclera on the outside and the retina on the inside. The choroid is rich in blood vessels and supplies blood to the outer portion of the retina, which lies next to it.A layer called Bruch’s

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membrane is shared by the choroid and the retina. The choroid can be damaged by blunt injuries to the eye, and its blood vessels may be affected by high blood pressure and other diseases. Inflammation of the choroid, choroiditis, is a form of uveitis. Scars that form in the choroid and adjacent retina (chorioretinal scars) as a result of inflammation can be quite easily seen on examination of the eye. Because of the choroid’s rich network of blood vessels, cancers that originate in other parts of the body can occasionally spread to the choroid.

The Retina

The retina is a complex, multilayered structure that forms the inner coat of the eye. Incoming light rays are focused on the retina, which is similar to the film in a camera. The light energy is then converted into nerve impulses that transmit signals to the brain. The central part of the retina, called the macula, is responsible for our central (straight ahead)

vision. The deepest layer of the retina, the pigment epithelium, lies next to Bruch’s membrane, which separates the retina from the choroid. Breaks in Bruch’s membrane from

injury or from diseases can allow blood vessels from the choroid to grow in through the breaks, where they can leak and bleed and disrupt the retina.

The portion of the retina next to the pigment epithelium is called the sensory retina. Its deepest layer, lying next to the pigment epithelium, consists of the photoreceptor cells, called rods and cones. The cones, which are most densely concentrated in the macula, help make out sharp details and colors. The cones function mainly when a moderate to high amount of illumination is present. The rods, located away from the center of the macula, help perceive images at times when there is dim illumination

The rods and cones of the retina connect with other interconnecting cells, and ultimately the nerve impulses travel along the over one million nerve fibers in each eye that make up the optic nerve. A thinning of this nerve fiber layer can often be seen when the eye is examined, especially if a green (redfree) light is used.

The retina is interrupted in the area where the optic nerve enters the eye. Major blood vessels, the central retinal artery and the central retinal vein, enter the eye through the optic nerve and then undergo a branching pattern

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