|Anatomy, Physiology and
||Ted M. Montgomery,
The conjunctiva is a clear mucous membrane that lines the inner surfaces of the eyelids and and continues on to cover the front surface of the eyeball, except for the central clear portion of the outer eye (the cornea). The entire conjunctiva is transparent.
The conjunctiva is composed of 3 sections:
Although the palpebral conjunctiva is moderately thick, the bulbar conjunctiva is very thin. The latter also is very movable, easily sliding back and forth over the front of the eyeball it covers. Since it is clear, blood vessels are easily visible underneath it.
Within the bulbar conjunctiva are “goblet cells,” which secrete “mucin.” This is an important component of the pre-corneal tear layer that protects and nourishes the cornea.
An “infectious conjunctivitis” (or “pink eye”) is an inflammation of infected conjunctiva. An infection typically is caused by a bacterium or a virus. Less commonly, a fungus or an amoeba can cause the infection.
The same bacteria and viruses that cause colds, ear infections, and sinus infections also can cause an infectious conjunctivitis. Even bacteria that cause sexually transmitted diseases (STDs) such as chlamydia and gonorrhea can cause an infectious conjunctivitis.
Someone with an infectious conjunctivitis must be careful not to touch the infected eye. If that occurs, it is imperative to wash the hands well, because the infection easily can be transferred to the other eye and/or to the eyes of other people.
A “noninfectious conjunctivitis” is an inflammation of noninfected conjunctiva. Unlike an infectious conjunctivitis, a noninfectious conjunctivitis cannot be spread to someone else by hand-to-eye contact.
One type of noninfectious conjunctivitis is “irritant conjunctivitis.” It is caused by irritants to the eye, such as pollution particles in the air, chlorine in swimming pools, or an acid or base accidentally dropped or rubbed onto the eye.
More frequently, a noninfectious conjunctivitis is caused by an allergic reaction to something. It can produce conjunctival redness, extreme itching, and excessive ocular mucous production. This reaction, called “allergic conjunctivitis,” commonly is caused by a seasonal allergy to pollen or plant bi-products, most often in the spring and summer (“vernal conjunctivitis”). An allergic conjunctivitis also can be caused by an allergic reaction to preservatives in eye drops or solutions.
An allergic conjunctivitis also can result from a reaction to proteins or other contaminants deposited on the surface of contact lenses, most commonly extended wear soft lenses. The latter can result in “giant papillary conjunctivitis” (GPC), mostly evidenced by the appearance of large “papillae” on the superior conjunctival tarsal plate (underneath the upper eyelid). Each papilla is a collection of lymphocytes and plasma cells.
Elimination of conjunctival papillae often is not easy. Obtaining new contact lenses, with reduced wearing time and with regular enzymatic cleaning of the lenses, is recommended. Sometimes it is best to be refit with disposable soft lenses or with rigid gas permeable (RGP) lenses. With these lenses, protein build-up is not as much of a problem as it is with extended wear lenses, though it still can occur.
Many people develop callous-like thickenings of the conjunctiva on the front of the eye, usually located on the nasal portion of the conjunctiva. Such eyes are susceptible to irritation caused by dry climates (especially with windy conditions), as well as toxic vapors, salt water spray, excessive exposure to the sun (ultraviolet radiation), and even inadequate natural lubrication of the eye (tears).
There are two types of these raised, yellowish or yellowish-white patches. One type is a “pinguecula” and the other a “pterygium.”
A pinguecula often is referred to as a fatty degeneration of the conjunctival tissue. The fine, nearly transparent collagen fibers of the conjunctiva degenerate and are replaced by thicker, yellowish, more durable fibers, sometimes containing calcium crystals. This causes the elevated, yellow and sometimes glistening whitish area located near the cornea.
There is no effect on vision from a pinguecula, which can appear after only a brief exposure to damaging irritation, such as excessive dryness or sun (ultraviolet radiation). The tissue damage increases with continued exposure. It might take only a day or two to notice a new pinguecula but weeks or months for it to resolve.
A pterygium, although produced by the same things which cause a pinguecula, often has inflammed blood vessels infusing into it. A pterygium does not emerge from a pinguecula.
Unlike a simple pinguecula, a pterygium often is progressive and will involve the cornea, if left unchecked. It is triangular in shape, with the base of the triangle located in the conjunctiva and the apex of the triangle encroaching onto the cornea. A pterygium virtually always is located on the nasal conjunctiva.
With corneal involvement, even if arrested surgically, a pterygium can affect vision by warping the surface of the cornea and inducing astigmatism. In some cases, the pterygium actually may grow all the over to the central cornea, in front of the pupil, and obstruct the entering light.
A somewhat common condition, caused by direct or indirect trauma to the eye, is a “subconjunctival hemorrhage.” This manifestss as a spot or pool of blood underneath the clear conjunctiva. It can be seen in distinct contrast to the sclera or white part of the eye. The hemorrhage may be present on only one side or on both sides of the cornea, and it almost always is on only one eye, unless the trauma has affected both eyes.
The trauma causing the hemorrhage may be due to a blunt hit, hard coughing, pushing, straining, heavy lifting, or even hypertension. Any of these things can cause a small blood vessel to break and to leak blood underneath the conjunctiva. A subconjunctival hemorrhage is one of the worst looking things that is harmless and will not affect vision. No treatment is necessary. The blood should reabsorb and disappear in 1-2 weeks, depending on the extent of the bleeding.
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