Glaucoma

Did you know that glaucoma is a leading cause of blindness in people over 40? Almost all blindness due to glaucoma can be prevented. Glaucoma is a condition of dangerously increased intraocular (within the eye) pressure.

Physiology

The eye is a closed, fluid-filled compartment. It has many specialised solid structures, but they take up little space when compared to the amount of fluid. Some of the fluid is in the form of a gel, called the vitreous, but most is water and essential salts. This water-based fluid, called aqueous, is constantly being produced inside the eye by the ciliary body, a small circumferential structure behind the iris. It then circulates through the pupil and over the iris before being drained out of the eye at the angle between the iris and cornea through very intricate “plumbing” termed the trabecular meshwork.
This circulation of the aqueous furnishes nutrients and performs other functions for the eye. The ease of the drainage of the fluid out of the eye is the eye’s outflow facility. When too little drainage occurs relative to the amount of aqueous produced, there is an increased intraocular pressure and glaucoma begins.

Types of Glaucoma

There are three major classes of glaucoma described below. Each of these has many subtypes and can occur in association with other eye or generalised diseases.

Open Angle Glaucoma

Means the drainage angle is open, but the fluid reaching it is not able to percolate through. The “plumbing” itself is not fully functional.

Angle Closure Glaucoma

In this disease, the angle structures including the trabecular meshwork are blocked, typically by the iris. This prevents aqueous from reaching the drainage channels, although the meshwork itself is functional.

Congenital Glaucoma

Is an inborn anomaly of the drainage structures, i.e. an abnormally developed filtration angle that prevents proper outflow facility or drainage.

Signs, Symptoms and Statistics

Incidence of Glaucoma with Age

About one percent of people above 40 suffer from glaucoma. Primary Open Angle Glaucoma, the variety not associated with other diseases, comprises the large majority accounting for 60 – 70 % of the cases. It also produces about 10 – 20 % of all blindness in the US.

When both the primary and secondary forms are considered, these figures are considerably higher. Unfortunately, the symptoms are often minimal or absent. A chronic, insidious, bilateral disease, it gradually destroys the optic nerve with constriction of one’s field of vision (”tunnel vision”) as well as scotoma formation or “holes” in one’s visual field. Lastly, central or reading vision is affected, but by this time, the glaucoma is far advanced.

In contrast, Angle Closure or Narrow Angle Glaucoma is much less common. It usually presents as an acute or sudden event. The drainage angle is occluded, usually by iris, and the pressure rise is rapid and dramatic. This produces pain, blurred vision, haloes around light, redness, and pupil abnormalities. Acute angle closure attacks tend to occur in smaller, far-sighted eyes. Very rarely the attacks can be brought on by dilating drops, but they usually occur spontaneously. Some patients may have prodromal or subacute episodes. Congenital Glaucoma is exceedingly rare.

Diagnosis of Glaucoma

Along with a complete eye exam, the key to the diagnosis of glaucoma is the measurement of intraocular pressure (tonometry). Tonometry can be measured with an instrument which sends a pulse of air directed on the eye or through a microscope which contains a measurement device which touches the anesthized eye. Some people may have pressures which are suspiciously high. Your doctor may wish to retest the pressure within a few months to a year or he may need to run some additional tests in order to confirm if the pressure is causing damage.

A second important test is direct inspection of the optic nerve. A doctor looks into the back of the eye to inspect the optic nerve. Enlargement of the optic cup indicates continuing damage from elevated eye pressure. These changes are charted or photographed in order to monitor the effectiveness of treatment. Sometimes your eye may have to be dilated in order for the new eye doctor to get a good view.

A third and very important test is the test of your field of vision. In this test your head is placed in a bowl like apparatus and lights are projected to the side to see if you can detect them. A computer compares your results with what is normal. One eye is tested at a time. The computer provides a print out of your field of vision. This is  a very helpful test to see if your vision has begun to be affected – and, if so, to what extent. Gonioscopy is an additional test which involves placing a special contact lens which contains a mirror on the eye to see if the drainage area is open. Left untreated, Glaucoma progresses with increasing permanent loss of vision. All individuals over age 40 need yearly, or nearly yearly, glaucoma checks. Those specifically at risk are diabetics, highly nearsighted individuals, older people and relatives  of glaucoma patients.

Angle Closure Glaucoma cannot be detected prior an attack, but the tendency for this disease may be seen during a complete eye exam.

Treatment

Treatment involves a variety of means: medical, surgical and laser. Medical methods include topical or drop medications, occasionally pills, and rarely medicine by injection for acute angle closure. Eyedrops are by far the most common treatment. Drugs work by one of two mechanisms. They decrease aqueous production and/or increase the outflow facility. The first group includes timolol, epinephrine, and oral carbonic anhydrase inhibitors. The latter includes pilocarpine and similar drugs. All of these can be used in combination. Although side effects are infrequent, they must be monitored. Timolol, for example, should be used with care in asthmatics. The most frequent, often unavoidable side effects of pilocarpine in younger patients are headache and temporarily blurred vision. If medical management fails, then either surgery or laser therapy is the next step.  Their success rates are variable depending on the severity and subtype of glaucoma. If medical drug therapy cannot prevent continued eye damage, there is often little alternative to proceeding with surgery or laser therapy, despite the risks.

The most important factor is an early detection. Regular vision examinations are the only way to tell if glaucoma is developing. Glaucoma rarely causes symptoms until very late.