 measure of a visual field in about three minutes. Looking into the automated perimeter, you signal when you see flashes of light. The computer maps your field of vision based on which flashes you see and which you miss.
The next part of a routine eye exam is an external exam, which is a visual inspection of the parts of the eyes that can be seen with just a flashlight. An external exam can be performed quickly. The eye doctor observes the condition of the eyelashes; the position, motions, and skin condition of the eyelids; the actions of the eye muscles (assessed by watching the movements of the eyes); the appearance of the whites of the eyes and the conjunctiva; and the size of the pupils and their reactions, particularly to light.
To see the internal structures of your eyes, the doctor will next ask you to rest your chin on a chinrest and press your forehead against a strap, while he aims an instrument at you called a slit lamp. The slit lamp is both a high-powered microscope and a light source that is focused to form a flat sheet. Because the front parts of the eye are fairly transparent, the sheet of light can show a cross section of the front structures of the eye, the way a sunbeam shining across a room can show the dust in the air. Depending on the width of the light beam and the lens, the slit lamp can give a magnified, three-dimensional view of the cornea, the iris, or the lens, or it can show a cross section from front to back of the eye, through the cornea, aqueous humor, lens, and vitreous. With an additional lens (either a handheld lens or one that fits directly against the cornea), the doctor can see all the way to the retina, blood vessels, and optic nerve at the back of the eye.
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