General Eye Examination

What does a general eye exam encompass?
How is visual acuity tested?
What, besides vision, does an eye exam test?

What does a general eye exam encompass?

A general eye examination assesses the overall condition of the eye or diagnoses any conditions. It should be done at least once every 1 or 2 years. Some serious eye disorders, such as glaucoma, cause no symptoms in their early stages and can be detected only by an eye examination. Vision is usually tested in each eye individually, first without glasses and then with glasses, if they are worn by the person being examined.

A thorough eye examination generally begins with an inspection of the external appearance of the eyes, the eyelids and the skin surrounding the eye. The doctor will also examine the lashes, the tear film layer of the eye, the opening of the tear ducts and ocular motility — the ability to move the eyes normally. The examiner will look for strabismus, a condition in which the movement of the two eyes is not coordinated.

How is visual acuity tested?

The examiner will test for visual acuity, the ability to see at a distance, using the familiar Snellen Eye Chart, which starts with one large "E" at the top and has a descending series of smaller letters. A reading of 20/20 means that the person being examined can see at 20 feet what is normal to be able to see at that distance. A reading of 20/60 means that the person can see at 20 feet what it is normal to see clearly at 60 feet. A reading of 20/200 — being able to see only the E at the top of the chart — means that the person can see at 20 feet what someone with excellent eyesight can see at 200 feet. Legally, blindness is defined as reading of 20/200 or worse.

The eye examination also tests the effect of glare or low-contrast conditions on an individual’s sight. It includes a test that measures the responsiveness of the outer part of the retina, which does not have as many nerve fibers as the center of the retina. This test is important when glaucoma or a neurological condition is expected. The examination was once done by moving a small white ball against a dark background in and out of the line of vision. Now it is done with electronic equipment, which produces a computer-generated printout that allows comparison of the results in subsequent examinations. It records the person’s attention span and the pattern of reactions to visual stimuli.

Refraction, which is the ability of the eye to focus on an object, can be measured in several ways. In retinoscopy, a narrow beam of light is projected into the eye. The examiner observes the light that is reflected back through the pupil from the retina to determine whether the eye is focusing normally. Or the examiner may ask the person being examined to describe changes in vision when lenses of different strength are placed in front of the eyes. Tests of refraction are used to detect and determine the extent of hyperopia, or farsightedness, when a person sees better at a distance than close-up; myopia, or nearsightedness; or astigmatism, in which vision is distorted because the front surface of the cornea is not truly spherical.

Accommodation, the ability to focus on near objects, can be measured by first correcting any refractive error detected in the examination by lenses, then determining the nearest distance at which very small print can be read. Age affects accommodation. Starting at about age 45, most people need lenses to read small print.

What, besides vision, does an eye exam test?

Another part of the test is a slit lamp examination, which magnifies the front portion of the eye. The person being tested rests the chin and forehead at the front of a magnifying device. This 10-fold magnification allows the examiner to identify a number of common problems of the conjunctiva, cornea, iris, front chamber of the eye, and the crystalline lens (See Anatomy of the Eye).

The examiner also will use an ophthalmoscope, an instrument whose deflecting prism or perforated angled mirror allows inspection of the entire retina and its blood vessels, the head of the optic nerve, and the vitreous humor, which is the fluid that fills the eye. The pupil is dilated, opened as much as possible, by administering drops such as tropicamide or phenylephrine. This examination can detect bleeding in the eye, vascular disorders such as blockage of an artery of the retina, overgrowth of blood vessel, and a number of disorders of the retina.

Tonometry is a measurement of the pressure inside the eye. The reading of intraocular pressure assesses the risk of glaucoma and overall blood pressure. After applying a quick-acting anesthetic and a drop of fluorescein to each cornea, the tomometer is pressed gently against the cornea. The pressure is gradually increased until the area of the resulting tears reaches a set limit. The force needed to produce this limit gives the reading of pressure within the eye.

Although an annual eye examination is a basic requirement for good vision, anyone experiencing changes in vision or difficulty in seeing under marginal conditions, such as low illumination, is advised to seek an examination so that an abnormal condition can be diagnosed and treated.