In order to protect your eye health, it is important to have a complete eye examination regularly. An eye examination can be performed by either an optometrist or an ophthalmologist and includes a number of tests that measure vision, eye pressure, visual fields and the health of different parts of the eye.
Visual Acuity Test
The visual acuity test measures distance vision. If a patient already wears glasses, the test would be performed while wearing the glasses or contact lenses. The patient covers one eye and reads rows of letters aloud from a chart, known as the Snellen chart, 20 feet away and then switches eyes. The smallest row that can be read is recorded as the vision in that eye.
Visual Field Testing
The visual field exams measure peripheral, or side, vision. During a general eye examination, the doctor will perform a confrontational visual field exam. The patient covers one eye and looks straight ahead while the doctor holds an object to the side; the patient reports when the object cannot be seen anymore. There are also machines such as automated perimeters that can also be used to measure the visual field as well.
Refraction
During the refraction examination, the eye doctor determines the eyeglass prescription. The doctor often uses a phoropter, which is a machine that contains a series of lenses of different prescription powers. While showing the patient a Snellen chart, the doctor shows the patient different lenses in one eye at a time and asks which lens makes the letters on the chart look clearer. The doctor repeats this step until the combination of lenses that give the best possible visual acuity is found.
Slit Lamp Examination
A slit lamp is a type of microscope that allows the doctor to look closely at the patient's eyes. The patient sits at the slit lamp and the eye doctor sits at the other side. Using the slit lamp, the doctor can examine the eyelids, conjunctiva and cornea as well as structures inside the eye.
Eye Pressure Measurement
The eye doctor typically measures the pressure inside the eye using one of two methods: non-contact tonometry or applanation tonometry. With non-contact tonometry, a puff of air is blown into the eye to measure the eye pressure. With applanation tonometry, the doctor places a topical anesthetic into the eye and while the patient looks straight ahead, a device called a tonometer is placed against the eye to measure the pressure.
Retinal Examination
The eye doctor can examine the back portion of the eye—the retina and optic nerve—with direct or indirect ophthalmoscopy or using the slit lamp. Typically the doctor dilates the pupils using eyedrops and looks at the rear of the eye using a hand-held ophthalmoscope, which is direct; a light mounted on the doctor's head with a hand-held lens, which is indirect; or the slit lamp with a magnifying lens.
Other Tests
There are additional tests that may be performed during an eye examination. One is the color vision test, which measures color blindness. Doctors can also measure the balance of the eye muscles using the eye muscle test in which the doctor asks the patient to look in different directions. Sometimes, the eye doctor uses a machine to automatically determine the glasses prescription before using the phoropter. There are more specialized tests that an eye doctor may perform depending on the patient.
