What is ophthalmology? |

Science and Profession

Among the sense organs and functions in the body, probably the most complex are the eye and the process of vision that it supports. Ophthalmologists study both the anatomy and the physiology of the eye in order to understand and treat common and rare eye infections and disorders.

The principal anatomical element of vision is the eyeball, or eye globe, located in the right and left orbital openings of the skull. It is embedded in a complex system of tissues surrounded by ocular muscles that control its movement. Adjacent to the eye and also within the bony orbit is the lacrimal gland, which is responsible for keeping the eye moist. Only the front third of the globe is exposed. This exposed area is made up of the central transparent portion, the cornea, and a surrounding white portion, which is only part of the sclera, the main component mass of the globe itself. The sclera is a very dense collagenous (protein-rich) structure which has two large openings (the anterior and posterior scleral foramina) and a number of smaller apertures that allow for the passage of nerves and blood vessels into the eye. It is through the posterior scleral foramen that three main components sustaining the eye’s functions pass: the optic nerve, the central retinal vein, and the central retinal artery.

The eye has three main layers, within which are further specialized divisions. The outer layer consists essentially of the transparent cornea and opaque sclera. The middle layer, called the uvea, is made up of the choroid, which is the outer coating of the layer; the ciliary body, which contains key eye muscles that affect the degree of curvature in the lens; and the iris, which, with the lens located immediately behind it, separates the anterior from the posterior chambers of the eye. This iris itself has two layers, the stroma and the epithelium. The latter is immediately recognizable to the layperson, since its cells are markedly pigmented, giving to each individual a characteristic eye color.

It is the opening in the iris, called the pupil, that allows the passage of light into the inner layer of the eye, which contains the key sensory portion of the organ, the retina. Before light reaches the inner layer and the retina, it passes through the lens of the eye, located immediately behind the iris (which it supports), and through the largest area of open space within the eye, the vitreous cavity. This posterior cavity, like the smaller forward cavity of the eye, is filled with a transparent hydrogel called aqueous humor, made up mainly of water (about 95 percent of its total mass) in a collagenous framework within which the main component is hyaluronic acid. The aqueous humor is very similar to plasma but lacks its protein concentration. The pupil of the eye serves a purpose similar to the diaphragm (or f-stop) on a camera; it opens wider (dilates) or closes (contracts) according to the intensity of light striking the eye. (This reaction explains why, after a few minutes in an apparently totally dark room, the eye adjusts at least in part to the lower intensity of light.) For purposes of examining the internal structures of the eye, ophthalmologists sometimes place special drops in the eye to cause the pupil to dilate.

The lens of the eye, which is held in place behind the pupil by zonular fibers, consists of onionlike lens fibers. These are the product of epithelial cells that “migrate” from their place of origin in a germinative zone next to the edges of the lens to the anterior portion of the concentric structure of the lens. The central or internal layers of lens fibers, called the embryonic nucleus, represent the earliest cell specialization processes before birth. By contrast, the anterior and posterior lens fibers are constantly renewed at the surface.

As light passes through the transparent lens fibers, the phenomenon of refraction results, in the simplest possible explanation, both from the concentric shape of the lens itself and from a differential in the index of refraction occurring in the “younger” outside layers of lens fibers and that of the “older” central layers; the latter have a greater index of refraction than the former. Another phenomenon that increases the refractive power of the lens occurs when the zonular fibers that hold it in place relax under the influence of the ciliary muscle, making the lens more spherical in shape. The resultant increase in refractive power is called accommodation.

It is the retina, located in the last layer of the eye, that receives the light images passing through the lens and transmits them to the brain via the optic nerve. Physiologists consider the nerve-related function of the retina to be comparable in many details with all other sensory phenomena in the body, including touch and smell. The retina itself consists of a very thin outer layer, called the retinal pigment epithelium, and an inner layer, the sensory retina. On the surface of the retina, one finds a layer of photoreceptor cells. Once affected by the absorption of light rays reaching them from the lens, these cells form synapses with an intermediate layer of modulator cells. A synaptic relationship may be defined as an excitatory functional contact between two nerve cells, causing either a chemical or an electrical response. The modulator cells—referred to as neurons when their function is to receive synaptic transmissions from receptor cells—in turn pass the “message” of light to ganglion cells forming the innermost cellular layer of the retina. These cells transmit electrical discharges through the optic nerve to the brain, where they are registered as images.

Diagnostic and Treatment Techniques

Ophthalmologists must deal with a wide variety of problems affecting the eyes, ranging from injuries to the diagnosis of vision problems that can be corrected with eyeglasses or contact lenses. Perhaps the most important area of applied ophthalmology, however, involves treating the diseases that may occur in several areas of the eye.

An entire category of diseases can appear in the conjunctiva, the thin mucous membrane that lines the inner portion of the eyelid and covers the exterior of the sclera. Conjunctivitis refers to inflammatory conditions that may attack this membrane. Some conditions cause mere irritation, while others may lead to serious infections. In acute catarrhal, or mucopurulent, conjunctivitis, the conjunctival blood vessels become congested with mucus and then with pus, which accumulates on the margins of the eyelids. If untreated, this form of contagious, easily transmitted infection begins to affect the cornea, by causing prismatic distortions and eventually abrasions that may infect the cornea itself. A more serious form of conjunctivitis is referred to as purulent conjunctivitis; it is sometimes associated with complications of the sexually transmitted disease gonorrhea.

Inflammation of the cornea, or keratitis, usually comes from the passage of virulent organisms from the conjunctival sac, which, although exposed to the external environment, may not itself react to the presence of bacteria. There are many different types of keratitis. Individuals may be vulnerable to infections in the cornea as a result of abrasions (one of the main reasons that all ophthalmologists recommend against rubbing the eye to remove irritating particles) or because of abnormal conditions affecting the surface of the cornea. Among the latter, ophthalmologists list excessive dryness in the eye and the side effects of malnutrition leading to a condition called keratomalacia, which is common in underdeveloped countries.

Bacteria such as pneumococci (the primary contributor to pneumonia in the lungs) may cause infections that result in corneal ulceration, the most common form of keratitis. In such cases, the area affected by the ulceration may increase considerably as epithelial tissue in the cornea attaches itself to the ulcer. Corneal ulcers may be removed by surgery, although the effect of remaining scar tissue may reduce the level of vision. The prospect of success in corneal transplantion has not eliminated the need for ulcer removal surgery, since transplants depend on the availability of “fresh” cornea donors.

Another form of corneal infection, herpes zoster (a form of skin rash also called shingles), is caused by the virus that causes chickenpox; it is common among aged patients whose cellular immunity systems suffer from decreased efficiency. In herpes zoster ophthalmicus, an infection that begins in the eye spreads via the nasociliary branch of the ophthalmic nerves and appears as red blotches on the surface of the skin (usually near the eye orbits on the side of the infection only). Zoster attacks are accompanied by rather severe pain. Ophthalmologists use several key drugs to treat this condition, including Distalgesic, Fortral, or Pethidine. Resultant depression in the patient may be relieved by prescribing amitriptyline.

Inflammation and possible infection of other regions of the eye also occur. Some zones, such as the sclera, tend to be more resistant to invasion because of the density of their fibrous tissues. Superficial inflammation of the sclera, called episcleritis, may be transitory but recurrent. Ophthalmologists will prescribe the anti-inflammatory drug Tandearil in the form of drops. More serious but much less common is the condition called scleritis, which extends much deeper into the tissue of the sclera and may affect the cornea and the uveal tract in the middle layer of the eye. Treatment of scleritis involves the use of steroid therapy, such as the corticosteroid drug prednisolone, often supplemented with Tandearil.

Uveitis is a term that applies to inflammations that occur in the uveal tract. The name suggests that such complications are not limited to one or another of the parts of the uveal zone (the iris or the ciliary body): All are affected and must be treated simultaneously; some natural treatments for uveitis have been suggested.

The most common vision problem is myopia (nearsightedness). While most people still choose to correct nearsightedness with contact lenses or glasses, laser techniques such as photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) have shown some promise in treating myopia. Early enthusiasm for radial keratotomy has waned because of erratic results.

The most widely known eye disorders are probably glaucoma and cataracts. Glaucoma occurs when pressure caused by an excessive amount of aqueous humor increases inside the eyeball, specifically in the area of the retina. Impairment of vision may be slight, occurring at first in the peripheral area of sight. Further deterioration, however, may lead to blindness in the eye. Regular treatment with drugs that reduce the production of aqueous humor is necessary in patients suffering from chronic glaucoma. Acute glaucoma, which is very sudden, represents only about one-tenth of recorded cases. It must be treated within less than a week to avert permanent blindness.

Cataracts occur when there is a loss of full transparency in the lens of the eye. Cataracts occurring among children are congenital or hereditary in origin. Cataract-like damage to the lens of the eye may also result from exposure to the sun’s rays (which is especially dangerous when one views the sun without protection during eclipses), extreme heat, x-rays, or nuclear radiation. Most characteristically, however, cataracts (from slight to advanced stages) are associated with the aging process. Formerly, cataract surgery was difficult and the recovery period slow, so patients were advised to wait as long as possible to have cataracts removed. Improvements in surgical techniques and materials mean that patients no longer need to wait until their vision is severely impaired to have this surgery. Most cataract extractions are combined with implantation of an intraocular lens, so that patients do not need to wear specially prescribed contact lenses or thick glasses following surgery.

Ophthalmologists make use of laser surgery for an increasing number of eye disorders. Lasers are used to treat eye problems caused by diabetes and hypertension, to treat or prevent some types of glaucoma, and to treat other, rarer eye conditions. Macular degeneration, an important cause of decreased central vision, may be arrested by laser therapy, but the technique does not repair existing damage.

Microsurgical techniques have further revolutionized eye care and have led to more effective management of conditions (such as retinal detachment) that formerly caused blindness.

Perspective and Prospects

Knowledge of the anatomy and physiology of the eye evolved gradually through history and then spectacularly in the latter half of the twentieth century. The most extraordinary advances in the later period were made in the field of eye surgery. For an understanding of how vision itself worked, it took centuries for surprisingly unscientific views to cede to the first modern theories and then, with the advance of anatomical dissection, the practical possibility of examining both normal and abnormal conditions of the organ in the laboratory.

An early but not widespread theory of how the eye sees, held into the Middle Ages, depended on what now seems to be the fantastic conception of eidola, or “skins.” Those who believed this theory held (in part correctly) that something must be leaving the objects that one perceives through the eyes. This “something” was thought to be a skinlike picture that, once detached from the object in question, actually entered the eye (after an unexplainable physical contraction) through the pupil, the aperture in the eye that is visible in many different animals. Another widespread theory was a prescientific version not of light rays but of “visual rays” that were thought to leave the interior of the eye, returning to record the colors and shapes of objects encountered.

Historians generally agree that the tenth-century Arab scientist ibn al-Haytham, known in the West as Alhazen, was the first to suggest that rays of light entered the eye to stimulate what he called the “sensorium.” Although Alhazen’s theory predated a scientific explanation of the nature of light itself, he based his views on the phenomenon of the lingering image on the eye’s “sensorium” of strong light, particularly that of the sun, even after the eyelids closed out the object emitting light. He even proposed a basic theory of refraction of light inside the eye. According to his theory, the sensorium recorded images according to an exact formula that reconstituted both the “shape” and the “order” in which rays are received by the eye, depending on the angle at which they strike the spherical surface of the cornea. Alhazen even warned that although the eye’s sensorium always duplicated this formula exactly, the observer (actually, the observer’s brain) could be “tricked” by the reproduction of certain ray patterns that might resemble something that was not “real”—the optical illusion.

Alhazen’s views would be examined and extended during the late sixteenth and mid-seventeenth centuries in the West by the scientific pioneers of optics, specifically the Italian Francesco Maurolico (died 1575) and the famous German astronomer Johannes Kepler (1571–1630). Kepler’s best-known work complemented that of his Italian contemporary Galileo Galilei (1564–1642), marking a breakthrough in the science of optics and the use of lenses to make telescopes in order to explore the skies. Only in later generations, however, did the ophthalmological relevance of some of his findings concerning the measurement of light reflected off the objects “seen” by a lens become clear.

As specialized interest in the eye progressed along with the constant advance of science in the eighteenth and nineteenth centuries, exact observation of the internal features of the organ of vision hinged on both the historical progress of anatomical dissection and the development of instruments to look into the living eye. One of the principal figures who contributed to the latter field was the Swedish ophthalmologist Allvar Gullstrand (1862–1930). Gullstrand received the Nobel Prize in Physiology or Medicine in 1911 for his application of physical mathematics to the study of refraction of light in the eye. He gained additional worldwide attention for his research on astigmatism (the failure of rays to be focused by the lens accurately on a single central point) and for devising the so-called slit lamp for viewing the interior of the eye through the use of an intense beam of light.

In the area of eye surgery, a major landmark was achieved in the 1960s when the Spanish ophthalmologist Ramón Castroviejo began to develop a method for surgical transplant of fully transparent corneas from deceased donors to replace damaged corneas in eye patients.

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