- How it Works
September 11, 2017
You’ve likely seen the promos from optical centers advertising free or low-cost comprehensive eye exams. Sometimes the exam is part of a package deal when you purchase eyeglasses or contact lenses. Understanding the different components comprising a comprehensive eye exam can help alleviate some of the confusion and ensure you receive a thorough exam.
Visual acuity is a core test, typically performed at the beginning of the exam. A standardized Snellen chart is placed or projected on the wall 20 feet away. For near vision, a small chart is held 14 inches from the face. One eye is tested at a time, so the eye doctor provides a small paddle or opaque piece of paper to block the other eye. You will be asked to read progressively small letters until you can no longer clearly distinguish letters. The 20/20 line is usually the fourth line down on a Snellen chart. “Normal” or 20/20 vision means you can clearly read these specific letters from 20 feet away.
A device called a phoropter or refractor is a quick means of determining the refractive error and obtaining an accurate eyeglass prescription. The device contains lenses of different strengths which are moved into the patient’s view. The eye doctor asks which of the two combinations of lenses looks clearer until the best correction is found for each eye. This test is also useful for verifying the strength of reading glasses in people who have difficulty only with near vision.
This is a common test performed during a comprehensive eye exam. The head is placed in the chin rest of the slit lamp and the patient looks inside while the device meticulously scans the eyes. A combination of bright light and different magnifying lenses provide a 3-D view of the entire physical structure of the eye from the inside. The test is not a definitive diagnostic tool for eye disease, however, it is effective at spotting possible signs of cataracts, glaucoma, diabetic eye disease, age-related macular degeneration, blood disorders, and certain cancers.
Tonometry is the measurement in mmHG of the intraocular pressure (IOP) inside the eye, which is elevated in people with glaucoma. The most common method used during a comprehensive eye exam is known as the puff or air puff test. A numbing drop may be placed in each eye before this test, which wears off quickly. The patient sits with their chin resting on a brace and the eye doctor points a small jet directly at the eye. The jet delivers a quick puff of air onto the surface of the eye. This provides a rough measurement of IOP, however, it’s not considered accurate. If the test reveals an elevated IOP, additional tests will be scheduled.
An eye doctor often uses a visual field test to check central and peripheral (side) vision. A common method is to hold up one or two fingers or a small disc on a stick. While looking straight ahead, the patient tells the eye doctor when they see the target in their peripheral vision.
Patient completing a visual field test.
During an ocular motility test, the head is kept still and the slow movement of a handheld light or other target is followed with only the eyes. This determines how well eyes follow a moving object and/or quickly move between objects, and also checks the accuracy of fixating on two separate targets. A cover/uncover test involves looking at a distant object with one eye covered for a few seconds, then this is repeated for the other eye. How the eye moves after it is uncovered can reveal problems like strabismus.
You may not receive a dilated eye exam at an optical center unless you specifically ask for one. Annual comprehensive dilated eye exams are recommended starting at age 60. African Americans are advised to have their first comprehensive dilated eye exam at age 40 due to their higher risk of glaucoma. People with diabetes should also have a comprehensive dilated exam at least once a year. During dilation, drops are placed in each eye to widen the pupil. The pupil is the opening in the center of the iris (colored part of the eye). Dilation enables more light to enter the eye, thereby providing the eye doctor with a clearer view of tissues at the back of the eye, including the retina, macula, and optic nerve. If you’re having a dilated eye exam, have someone else drive and bring a pair of sunglasses. Vision is blurry and the eyes are more sensitive to light after dilation. The full effects of dilation take 4-6 hours to wear off.
Dilated eye exam.
This test may or may not be performed during a comprehensive eye exam. The most widely used color blindness test is the Ishihara Color Vision Test. This consists of 38 different circular patterns (or “plates”) comprised of many dots of various colors, brightness, and sizes. A person with normal color vision will see a single-digit or two-digit number among the seemingly random array of dots. A colorblind person will either be unable to see the number or will see a different number than intended. If this test is done during an eye exam, the eye doctor only uses a small sample.
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