Comprehensive Eye Examination

Your appointment could be as brief as half an hour, but it will more likely require an hour or longer. The length of your exam depends on several factors: What is the doctor looking for? Which, among the dozens of tests available, will be used? In a routine comprehensive exam, the doctor will probably check your eyes for:

. myopia (nearsightedness; seeing near objects better than far objects)

. hyperopia (farsightedness; seeing distant objects more clearly than near ones)

. presbyopia (a form of farsightedness that begins at age forty to forty-five)

. astigmatism (irregular lens shape that distorts your vision slightly)

. strabismus ("cross-eye") and amblyopia ("lazy eye")

. glaucoma (high fluid pressure within the eye)

. cataracts, color blindness, blocked tear ducts, eye injury, defects on the cornea, and damage to the retina or optic nerve

Visual Acuity Test
A test for visual acuity refers to the clarity of your vision. (Acuity is from the Latin acuitas, which means "sharpness.") In other words, how well do you see?

Your eye doctor will probably use the familiar eye chart to test your vision at various distances. The eye chart most of us know is called Snellen's chart, after the nineteenth-century Dutch ophthalmologist Hermann Snellen, who invented it. It consists of rows of black letters-very large at the top, very small at the bottom-against a white background. Each eye will be tested separately while the other eye is covered. If you've had a problem with glare, the doctor will probably test your visual acuity using a variety of lighting sources.

The results of your visual acuity test are expressed by phrases such as "20/20 vision" and "20/40 vision," which some people find confusing. The first number in the phrase, in the United States at least, is always 20-which is the distance, in feet, you're standing from the eye chart. (Where the metric system is used, the first number is 6, indicating that the patient is standing 6 meters from the chart.)

The second number conveys how much your visual acuity differs from "normal" eyesight. If you have 20/20 vision, you can see at 20 feet what other people with good vision can see at 20 feet. If your vision is 20/40, you can see at 20 feet what people with good vision can see at 40 feet. The higher the second number, the worse your visual acuity. If your vision is 20/70 or worse, you have low vision. At 20/200-meaning that someone with "normal" vision standing 200 feet away can see the chart as well as you can at 20 feet away-you are considered legally blind. (Only about 10 percent of legally blind Americans have zero visual acuity; the rest have some degree of sight.)

Eye-Movement Examination and Cover Tests
There are other low-tech procedures that are probably familiar to you. These tests don't require fancy equipment, but they give the doctor a lot of important information, including whether you have cross-eye or lazy eye and how good your depth perception is.

He or she will ask you to look upward and downward, and to the right and the left. Then you'll be asked to stare at an object-first at a distance and later up close. The doctor will cover one of your eyes and quickly note how much the uncovered eye moves to adjust, then repeat the process with the other eye. He or she will probably hold an object, perhaps a pencil, near your eyes and ask you to "follow" it as it moves from side to side.

Iris and Pupil Examination
The doctor will check the appearance of the iris. Is it symmetrical? Does the pupil respond correctly to light, dilating and constricting as needed? What is the size of the pupils?

Refractive Error
If your vision is worse than 20/20, the doctor will perform a variety of tests to determine the correction needed-that is, to come up with an accurate prescription for eyeglasses or contact lenses.

The degree of farsightedness, nearsightedness, astigmatism, or presbyopia is called refractive error. To measure refractive error precisely, the doctor will probably use another rather old-fashioned device called a phoropter.

If you've had an eye exam, you're probably familiar with a phoropter. It is a complete range of corrective lenses that can be adjusted to offer you hundreds of combinations. The doctor adjusts the lenses and asks you to indicate which of two combinations is better. By continually changing the lenses, the doctor can arrive at a combination of lens strengths that will be the basis of your prescription.

Though there are automated devices for testing refractive error, many eye doctors report getting the best results by using them in conjunction with the more-subjective phoropter. An autorefractor emits a pinpoint beam of light that reflects off the retina and measures the eye's response. Autorefractors are especially useful when the patient is a small child or, perhaps, an adult who is unable to respond accurately to phoropter combinations. Some ophthalmologists use advanced computerized equipment, such as a high-tech scanner called a wavefront aberrometer, for more-detailed results.

Dilating the Pupils
For certain additional tests-to examine your general eye health and the retina, optic nerve, and blood vessels-the doctor will need to dilate your pupils using eye drops. These drops take about twenty minutes to fully open the pupils, giving the doctor a much wider view of the inside of your eyes than would be possible with constricted pupils.

After dilation, your vision might be blurred and highly sensitive to light for several hours. You won't want to walk out into bright sunlight with your eyes uncovered. If you don't have sunglasses, most eye doctors will give you disposable sunglasses to wear on the way home. Since there's no way of knowing how long it will take your eyes to return to normal, the best course is to arrange for someone to drive you home.

Ophthalmoscopic Examination
An ophthalmoscope is a specialized device through which your doctor can inspect the blood vessels and the optic nerve at the back of the eye. He or she will also examine the retina for detachment and tears, and the small areas on the retina responsible for sharp vision (the fovea) and central vision (the macula).

Slit-Lamp Examination
A slit lamp, or biomicroscope, allows the doctor to see signs of infection or disease at the front of the eye, including problems in the eyelids, cornea, conjunctiva (the thin, transparent membrane that protects the front of the eye), and iris. Using a higher-powered lens, he or she can also see to the back of the eye, detecting macular degeneration and other problems. During a slit-lamp exam, your head will be comfortably stabilized on the lamp's chin rest.

As its name suggests, the slit lamp shines slits of light into the eye. The size of the slit is adjustable, so the doctor can see very small sections of the eye at very high magnification. The lens nucleus is clearly visible, as are the lens position, the other layers of the lens, and the degree of brunescence (browning of the lens), which is responsible for some cataract patients' inability to distinguish blues and purples.

Visual Field Measurement
The simplest way to test your field of vision-how far you can see to the left and right out of the corners of your eyes-is for you to focus on the doctor's face while he or she moves a finger slowly to the side and asks you to signal when you can no longer see it. An instrument called a perimeter, which emits flashes of light, can be used to reveal blind spots. You simply stare at an image and tell the doctor when you see a flash.

Tonometry-Intraocular Pressure (IOP) Measurement As part of a routine eye exam, your eye doctor will probably use a tonometer to screen your eyes for glaucoma. A noncontact tonometer is generally used for screening. It expels a puff of air toward the eye and measures the resulting small, instantaneous indentation. The size of the indentation indicates the intraocular pressure (IOP) inside the eye. Other types of tonometers are placed directly on the cornea after the eye is numbed with eye drops.

Potential Acuity Testing
If you have cataracts, the doctor may perform potential acuity testing, a measure of what your vision would be like if the cataracts were removed. Potential acuity testing is especially useful in determining how much of your vision loss is due to cataracts. One way of testing potential acuity is with a pinhole acuity meter, which projects an eye chart directly onto the retina, bypassing the cataract.

Contrast Sensitivity Testing
If cataracts make it hard for you to differentiate shades of gray, you have low contrast sensitivity. Your eye doctor might measure this by using a low-contrast visual-acuity chart, or possibly a chart with different contrast levels on symbols of the same size.

Calculating the Power of Your Intraocular Lens
Cataract patients planning to undergo lensreplacement surgery will have tests that examine (a) the power of the cornea and (b) the length of the eye-the primary measurements needed to formulate the synthetic lens to be implanted. The meticulous corneal examinations are also useful for patients with astigmatism and glaucoma.

. Corneal topography uses highly sophisticated technology to create a precise three-dimensional map of the cornea.

. Corneal pachymetry uses ultrasound to measure the thickness of the cornea.

. Ultrasound biometry ("A-scan") measures the length of the eye with ultrasound.

. Optical coherence biometry also measures the length of the eye but uses light instead of ultrasound.

The ophthalmologist will use all this data to calculate the best intraocular lens (IOL) type and power to be implanted. For the most accurate measurements, keep soft contact lenses out of your eyes for three days before your evaluation. If you wear hard contacts or rigid gas permeable (RGP) contacts, stop wearing them three weeks before the evaluation.

Once your cataracts are diagnosed, it will be up to you to decide when they are unacceptably interfering with your work and your lifestyle. The next chapter contains guidelines for the timing of surgery and explains the procedure in detail.