Another instrument used to view the interior of the eye and the retina is the ophthalmoscope. The most familiar type of ophthalmoscope is the handheld direct ophthalmoscope, which looks like a flashlight. Doctors use it to see the central retina. They may also use an indirect ophthalmoscope, which is a head-mounted instrument like a coal-miner’s lamp that shines into the eye and condenses the out-coming light into a three-dimensional image of the retina. Looking through the lens of the instrument and a handheld lens held in front of the patient’s eye, the doctor sees a wide, panoramic view of the retina.
To obtain the best view with the indirect ophthalmoscope — and sometimes with the slit-lamp — the doctor will first dilate your pupils with eyedrops, a procedure that may be unpleasant but not painful. Because your pupils may still be dilated for some time, it’s a good idea to bring a pair of sunglasses and make arrangements for transportation after the exam.
To the person having the eye exam, the standard tests may just seem like a barrage of bright lights. But to the eye doctor, they provide invaluable information. Here are the main conditions that might be spotted during the course of the exam and some of the ways they are treated.
The vitreous and aqueous humor (the gelatinous and liquid substances that fill the eye) create intraocular pressure, a positive pressure inside the eye. Like the air inside a balloon, pressure in the eye maintains a smoothly rounded surface at the front of the cornea. Without this pressure, the eye surface would be rumpled — not a good optical lens.
If pressure in the eye becomes too high, however (due to poor drainage of the aqueous humor), it presses against the optic nerve, slowly restricting blood supply and killing the nerve cells. This is the general definition of glaucoma, although it has several distinct types. Glaucoma initially creates blind spots in the peripheral vision, but if it progresses untreated, it can cause blindness. Because peripheral vision is affected first, the field-of-vision test is an important way to screen for early glaucoma. Testing eye pressure is another.
Intraocular pressure is measured by briefly touching the eye with a tonometer. An applanation tonometer is attached to the slit lamp. You will see a mysterious-looking blue light that seems to come closer and closer 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.