 to the eye, until it actually does touch the front of the eye. It cannot be felt, however, because the doctor applies a drop of local anesthetic before the test. The Tonopen is a smaller, handheld version of the applanation tonometer. There is also an air puff tonometer, which shoots a jet of air at the eye and often makes people jump.
It used to be said that normal intraocular pressure is exactly 21 millimeters of mercury, and that any pressure above that indicated glaucoma. In retrospect, that was as rigid as saying that normal body temperature is precisely 98.6°F. We now know that normal eye pressure varies from person to person. Many people with eye pressure in the low twenties do not have glaucoma, while some people with glaucoma have eye pressure in the high or even mid-teens.
The diagnosis of glaucoma is not made by eye pressure alone, but by characteristic changes in the appearance of the optic nerve and in the visual field. If eye pressure is normal but the visual field is reduced, the doctor may suspect glaucoma. Some people are marked “glaucoma suspects” for a while, meaning that it’s not absolutely clear that they have the disease. If a diagnosis cannot be made definitively, it’s reasonable to wait to begin treatment; if it turns out that the person does have glaucoma, it is still early enough that it has done no detectable damage. As soon as damage is noticed, though, treatment must be started immediately.
Treatment usually begins with medicated eyedrops, although some drugs can be taken orally. There are some kinds of glaucoma that require surgical or laser procedures. A laser procedure also may become necessary if eyedrops alone are not working well enough.
When we are young, the lens of the eye is almost perfectly transparent, but during the normal course of aging, it gradually changes color and becomes cloudy. A clouded or opaque lens is called a cataract. There is no definite dividing line between normal aging changes and early cataract. Most people develop some cataract as they get older; people with diabetes are more likely than others to have cataract.
If a cataract gets bad enough to interfere with a person’s lifestyle and abilities, it can be removed with surgery and replaced with an intraocular lens, or implant. It used to be said that a cataract should be removed when it was “ripe,” but the decision to remove a cataract is really based on how much disability it causes a person. If it does not significantly impair your vision, it’s OK to wait. (It is possible to wait too long to have a cataract removed, but that is after the person can no longer see through it.) Cataract surgery is so quick and successful these days that there is no reason at all to dread or avoid it.
An eye problem that cannot wait to be treated is a retinal detachment. Most of eye’s interior is filled with vitreous, a gelatinous substance with fibers and cells that is firmly attached to the retina. Over time, the vitreous gradually becomes more liquid and less gelatinous. As the gelatin liquefies, the thin membrane at the back of the vitreous (the posterior vitreous membrane) can peel away from the retina and flap into the vitreous. This is called a posterior vitreous detachment, or PVD. It can occur without the person noticing it; alternatively, it can create dramatic flashes of light inside the eye, or brown or black floaters, which are actually the shadows cast on the retina either by the posterior vitreous membrane itself or by some leakage of blood into the vitreous.
While a PVD itself is not harmful, if part of the membrane is caught or has scarred to a spot on the retina, it can tear a hole in the retina as it peels away. If liquid vitreous then leaks through the hole and under the retina, the retina itself starts to separate from the outside of the eye. This is called a retinal detachment. Gradually, a person will lose sight in that eye, as if a curtain were slowly being drawn over it.
Most posterior vitreous detachments do not lead to retinal detachment, and many small retinal holes can be treated easily with a laser o