|Anatomy, Physiology and
||Ted M. Montgomery,
<!>One-sixth of the outer layer of the eye (called the tunic fibrosa or fibrous tunic) bulges forward as the cornea, the transparent dome which serves as the outer window of the eye. The cornea is the primary (most powerful) structure focusing light entering the eye (along with the secondary focusing structure, the crystalline lens).
The cornea is composed, for the most part, of connective tissue with a thin layer of epithelium on the surface. Epithelium is the type of tissue that covers all free body surfaces.
The cornea is composed of 5 layers, from the front to the back:
The transparency of the cornea is due to the fact that it contains hardly any cells and no blood vessels. However, blood vessels can creep in from around it, if it is constantly irritated or infected, which can interfere with vision.
On the other hand, the cornea contains the highest concentration of nerve fibers of any body structure, making it extremely sensitive to pain. The nerve fibers enter on the margins of the cornea and radiate toward the center. These fibers are associated with numerous pain receptors that have a very low threshold. Cold receptors also are abundant in the cornea, although heat and touch receptors seem to be lacking.
Along its circumference, the cornea is continuous with the sclera: the white, opaque portion of the eye. The sclera makes up the back five-sixths of the eye’s outer layer. It provides protection and serves as an attachment for the extraocular muscles, which move the eye.
Coating the outer surface of the cornea is a “pre-corneal tear film.” People normally blink the eyelids of their eyes about every six seconds to replenish the tear film. Tears have four main functions on the eye:
The tear film resting on the corneal surface has three layers, from front to back:
The most external layer of the tear film is the lipid or oil layer. This layer prevents the lacrimal layer beneath it from evaporating. It also prevents the tears from flowing over the edge of the lower eyelid (“epiphora”).
The lipid component of the tear film is produced by sebaceous glands known as “Meibomian” glands (located in the tarsal plates along the eyelid margins) and the glands of “Zeis” (which open into the hair follicles of the eyelashes). An enlargement of a Meibomian gland is known as a “chalazion,” while an infection of a Zeis gland is known as a “hordeolum” or “sty(e).”<!><!>
Beneath the lipid layer is located the lacrimal or aqueous layer of the tear film. This middle layer is the thickest of the three tear layers, and it is formed primarily by the glands of “Krause” and “Wolfring” and secondarily by the “lacrimal” gland, all of which are located in the eyelids. The lacrimal gland is the major producer of tears when one is crying or due to foreign body irritation.
Lacrimal fluid, containing salts, proteins, and lysozyme, has several functions:
If the eye’s tears are “isotonic,” there will be no change in water volume in the cornea and vision will remain normal. (Tears normally have a tonicity equal to .9% saline.) If the tears are “hypotonic,” water will flow into the cornea (such as when crying or swimming in a pool) and it will swell, causing it to become more myopic. If the tears are “hypertonic,” water will flow out of the cornea (such as when swimming in the ocean) and it will shrink, causing it to become more hyperopic.
The epithelial surface of the cornea is naturally “hydrophobic” (water-repelling). Therefore, for a tear layer to be able to remain on the corneal surface without rolling off, the “hydrophilic” (water-attracting) mucoid or mucin layer of the tear film is laid down onto the surface of the cornea by “goblet cells,” which are present in the bulbar conjunctiva. In turn, the lacrimal layer of the tear film, located above the mucoid layer, can defy gravity and remain on the front of the eye.
A deficiency of any of the three layers of the tear film can lead to a “dry eye” or “bleary eye” condition, causing anything from mild eye irritation to severe pain. Interestingly, in some cases, excessive tearing or watering of the eyes can be a symptom of a dry eye condition. This is because when there is an inadequate normal tear layer on the eye, irritation results; this can cause an overproduction of the lacrimal gland and a flooding of lacrimal fluid into the eye (“reflex tearing”). When the eyes are too dry or too wet, vision can be adversely affected.
Besides excessive tearing, symptoms associated with dry eyes can include the following:
There can be multiple causes of a dry eye condition, and these are some of the possibilities:
A dry eye problem often can be relieved with the use of over-the-counter eyedrops which behave as “artificial tears” on the eyes. These types of drops can soothe the eyes, moisturize dry spots, supplement tears, and protect eyes from further irritation. Some drops are formulated to match the pH of human tears for added comfort. Special ocular lubricant ointments, applied to the eyes for overnight use, also are available.
Artificial tears may be preserved or unpreserved. Bottle contamination is less likely with preserved drops; however, an allergic reaction to the preservatives can occur. If unpreserved eyedrops are used, care must be taken not to contaminate the bottle by touching the tip to any surface, including the eyeball.<!>
Some eyedrops contain “vasoconstrictors” (chemicals such as tetrahydrozaline or naphazoline), which constrict the conjunctival blood vessels, thereby reducing the amount of redness on the surface of the eyes. These drops may or may not contain a tear substitute component for red eyes. Overuse of such drops can cause eyes to become even more red (“rebound hyperemia”), due to a weakening of the muscles persistently constricting the blood vessels.<!>
In certain cases, artificial tear drops do not relieve the discomfort due to dry eyes. In such cases, if the discomfort is severe enough, other options are available. The most common of these involves closing the tear ducts (which act as “drains” for the tears). Using either a silicone plug or scarring the tear duct closed by cauterization (with a “hot poker”) decreases or stops the passage of the tears into the tear ducts. That way, any tears naturally produced, or artificially placed into eyes, will remain longer (until they evaporate). It can be a very successful way to make irritated eyes with a chronic dry eye syndrome feel more comfortable.
“Keratoconus”—which is a combination of two Greek words: karato, meaning cornea, and konos, meaning cone—is a non-inflammatory condition in which there is progressive central thinning of the cornea, changing it from dome-shaped to cone-shaped. An asymmetrical, cone-like bulge develops, eventually resulting in significant distortion and visual impairment.
As keratoconus progresses, the corneal tissue continues to thin and to bulge forward and downward, becoming very irregular. In advanced stages, there can be a precipitous drop in vision due to sudden clouding of the cornea, referred to as “acute hydrops,” where there is a sudden infusion of fluid into the stretched cornea. This usually resolves over weeks to months but often is followed by central corneal scarring, further impairing vision.
An uncommon disorder, keratoconus affects about 1 out of every 2,000 people. It almost always is bilateral (affecting both eyes) and typically takes years or decades to progress, usually beginning at puberty or later in the teens. Its progression can halt at any stage, from mild to severe.
Although keratoconus does not cause complete blindness, it often causes a dramatic increase in myopia (nearsightedness) and irregular astigmatism, significantly blurring and distorting vision, as well as causing significant photophobia (light sensitivity) and glare.
The cause of keratoconus can be genetic; about 13-15% of people with the condition also have other family members with it. Severe rubbing of the eyes also may exacerbate the problem. In its early stages, keratoconus can be diagnosed only by using detailed computer maps of the corneal surface, detecting subtle changes in corneal shape.
Initially, stronger eyeglasses are successful in correcting the progressive myopia and astigmatism; however, as the disease advances, rigid gas permeable (RGP) contact lenses are necessary to flatten the anterior corneal surfaces and obtain optimal visual acuity. Contact lens fitting can be difficult in patients with keratoconus, usually requiring frequent doctor visits and lens changes.
Ultimately, if good vision no longer can be attained with contact lenses, or if intolerance to the contact lenses develops, corneal transplantation is recommended. This procedure is necessary in only about 10% of people with keratoconus and is successful in greater than 90% of cases, one of the highest success rates for corneal transplantation. Although this procedure replaces the thinned central portion of the cornea (with a section of donor cornea), contact lenses and/or eyeglasses still may be required for maximum visual acuity.
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