Just as we are advised to watch our blood pressure as we get older, the pressure within our eyes is equally at risk with advancing years. Changes in the ageing eye can lead to less-effective drainage of the clear fluid (aqueous humor) in the eye, resulting in a buildup of intraocular pressure (IOP). A dangerously elevated IOP, or ocular hypertension (OHT), is a major risk factor for glaucoma.
The condition is the third-leading cause of blindness in the world. It can strike at any age, but the elderly are particularly susceptible. Current estimates of glaucoma are a staggering 100-150 million cases worldwide. With our ageing population - increasing by 50 per cent in the US alone over the next 15 years - the numbers are expected to soar dramatically (Arch Ophthalmol, 2004; 122: 532-8).
Notorious for its lack of symptoms, glaucoma can cause progressive damage to the optic nerve without your realising it. The damage often involves loss of peripheral vision, which is not easily apparent. It’s only when your field of vision has been seriously reduced (when patients complain of bumping into things a lot) that the sufferer is likely to finally head off to see a doctor (see box, page 2).
As much of the damage to the optic nerve is permanent, the conventional management strategy is to prevent any further visual loss by controlling the ocular hypertension - and there are no prizes for guessing what doctors turn to as the first line of treatment.
Drugs are big business in the glaucoma field. The way doctors prescribe eyedrops (sometimes combining two or more types) to glaucoma patients has helped to rake in big profits for the ophthalmic drugs industry. Worldwide sales of medicated eyedrops for glaucoma average $3 billion (USD) each year, and account for almost half the total ophthalmic pharmaceutical market. Given the predicted rise in glaucoma cases in tandem with the ageing population, that sound you can just about hear is the drug companies rubbing their hands in anticipation.
But these seemingly innocuous drops - which, in most cases, have to be taken for life - cause a laundry list of side-effects, and are often as dangerous to the body as drugs taken by mouth.
Keeping the risks in sight
A cornucopia of medicated eyedrops is available for glaucoma, and the mechanism by which they lower eye pressure depends on the class of agent they contain. Needless to say, they all come with their own rather extensive menu of side-effects.
* Miotics such as pilocarpine work by constricting the pupil, and stimulating the ciliary muscles to increase the drainage of fluid from the eye.
Downside: Because miotics reduce the size of the pupil, a common complaint is blurred or dim vision. This could also artificially induce nightblindness. Also, as these eyedrops need to be administered four times a day, patients may find it difficult to keep track regularly.
* Carbonic anhydrase inhibitors are available as eyedrops (Trusopt, Azopt) or in an oral form (Diamox). These agents inhibit the enzyme involved in producing the aqueous humor, thereby decreasing IOP.
Downside: Chronic use of these drops can cause an allergic response, with redness and itching of the eye (conjunctiva) as well as scaling on the lower eyelids. When taken orally, the side-effects stretch considerably to include frequent urination, tingling in the fingers/toes, skin rash, gastrointestinal disorders, depression, fatigue, impotence, weight loss and lethargy.