Eye Exams - Low Vision

Low vision is a condition that exists when ordinary eyeglasses, contact lenses or intraocular lens implants do not provide clear vision. Not to be confused with blindness, people with low vision still have useful vision that can often be improved with visual devices. The terms “legally blind” or “partially sighted” are often used in association with low vision. A person is considered legally blind when the best corrected vision in the better eye is no more than 20/200 and/or the field of view is less than 20 degrees. A person with best-corrected vision of no more than 20/70 in the better eye is considered partially sighted or visually impaired.

Though most often experienced by mature adults (60+), people of all ages may be affected with low vision because of birth defects, inherited diseases, injuries, diabetes, glaucoma, or cataracts. The most common cause of low vision is age-related macular degeneration, a disease of the retina that affects central, or straight-ahead, vision. Low vision may also result from decreased side (peripheral) vision or a loss of color vision. Or, the eye might lose the ability to adjust to light, contrast or glare. Different types of low vision may require different kinds of assistance.

For those who experience the symptoms of low vision, a complete eye examination by an ophthalmologist or optometrist is essential. Once the causes are determined, the doctor can suggest various treatments including the use of low-vision devices, or the eye care professional may refer the patient to a low-vision specialist or agency for help.

Patient History

The low-vision examination begins with a complete medical history, including address and telephone numbers, insurance information, names of other doctors, list of medications being taken and past medical records if available. Because the exam includes dilating the eyes, it’s a good idea to bring sunglasses to wear during the trip home.

In addition, the practitioner may ask for input from other health care providers or therapists, family members and teachers, as is appropriate and permitted by the patient. Included in this information will be the nature and duration of the problems, visual difficulties and chief complaint, ocular and visual history, general health history, medications, allergies, and any relevant data regarding occupation and avocation.

Ocular Examination

Because every low-vision case is different, the elements of a low-vision examination depend upon several factors, including the disease or disorder responsible for the visual impairment, the chronological and developmental age of the patient, the specific visual abilities of the patient, and the doctor’s clinical judgment. Every low-vision exam, however, normally includes tests of visual acuity, depth perception, color vision, contrast sensitivity and curvature of the front of the eye.

Visual Acuity

The results of visual acuity testing are the basis for determining initial magnification requirements and the potential for specific rehabilitation strategies. The measurement of visual acuity in a low-vision patient involves the use of charts with a larger range of letters. In addition to the standard measurements taken during a visual acuity test, the doctor will watch the patient carefully to see if changes in posture, tilt of the head, or viewing position make a difference in visual ability. Visual acuity will also be checked under various lighting conditions to determine the effects of glare and to measure the potential need for filters to reduce photophobia, which is abnormal sensitivity to, and discomfort from, light.

There are times in low-vision examinations when usable vision cannot be measured with charts, even with those designed for low-vision testing. In those cases, an attempt will be made to measure visual acuity based on the ability to detect objects based on distance, size and light perception. If a near-vision test is warranted, the doctor will use acuity charts designed for visually impaired patients.

Refraction

Although traditional procedures for the assessment of refractive error are sometimes less effective in low-vision patients, a refractive examination is still administered to ensure optimal correction for best visual acuity and to determine the amount of magnification needed for certain tasks. The refractive exam will also detect presbyopia or any significant refractive error, which may affect success with low vision devices.

An autorefractor, which provides an “objective” measurement of vision, is often used to provide an accurate starting point, especially when high refractive error or opacities (cloudiness) of the eye are present. A retinoscope, which is a hand-held instrument for measuring an eye’s refractive error, may also be used when opacities are present, pupils are small, or reflexes are less than normal.

The doctor may also magnify distance vision with various telescopic systems, assess the response to filters to control glare, and test various reading aids including strong reading eyewear, magnifiers, electronic magnifiers and electronic reading machines.

Contrast Sensitivity

Contrast sensitivity is an element of visual ability that is not measured by standard visual acuity tests, which presents an array of black letters of different size on a white background. However, most of what people see every day see comprises objects that may not be outlined clearly or that do not stand out from their background. The ability to detect such objects is contrast sensitivity, which often is limited in cases of low vision. Typically, a contrast sensitivity test is performed to measure a patient’s ability to detect a number of different grating patterns at different spatial frequencies. The test determines the patients contrast detection threshold, the lowest contrast at which a pattern can be seen.

Visual Field Assessment

A low-ision exam includes a visual field test that measures the ability of the eye to see straight ahead and to the side (peripheral vision). There are two versions of this test, and in each version, one eye is temporarily covered and the patient is asked to look straight ahead at a fixed space.

In a moving target test, the targets are moved from the side, where they are not visible, towards the center of vision until the patient sees them. This test can be done with either a black screen on the wall or with a large bowl-shaped instrument.

In a fixed target test called computerized static perimetry, small points of light appear bright or dim but do not move. During this test, a patient sits in a chair facing either a bowl-shaped instrument or a computer screen.

The visual field examination is important in detecting many disorders of the eye, optic nerve and brain.

Ocular Motility and Binocular Vision Assessment

Depending on the particular low-vision symptoms demonstrated by a patient, several tests will be performed on the oculomotor system, the muscles and nerves that control the movement of the eyes. Common disorders include nystagmus, ocular motility dysfunction, strabismus, substandard binocularity, and diplopia. A variety of additional tests may be performed to detect defects in the oculomotor system.

Ocular Health Assessment

Because knowing the physical cause of low vision is crucial to suitable treatment, a thorough assessment of the health of the eyes is an important component of a comprehensive low vision examination. Making or confirming a diagnosis will assist the doctor in understanding the impairment and establishing a treatment. The doctor will conduct a thorough examination of the exterior and interior of the eye using a variety of instruments.

Supplemental Testing

In some patients, additional testing may be indicated because of a specific disease or condition, a patient complaint, educational or work-related needs, inadequate response to magnification, or other findings. Such tests could include color vision testing, visually evoked potential, electroretinogram, electro-oculogram, and fluorescein angiography.

Management of Visual Impairment

At the conclusion of a low-vision examination, the doctor will discuss the results with the patient, make recommendations for treatment, and may refer the patient to a specialist or to an organization designed to support people with low vision. The patient should take time to learn about the many types of devices now available to assist those with low vision problems.

There are two types of low-vision devices, optical and nonoptical. Optical devices use lenses or combinations of lenses to provide magnification. Nonoptical devices comprise large-print books, large playing cards, enlarged telephone dials, high-contrast watch faces, and machines that can talk, scan print and read aloud.

Several governmental and private agencies provide social services for people with low vision. These include talking books, independent home-living instruction and, in some cases, orientation and mobility training.

See Age-related Macular Degeneration, Cataracts, Contrast Sensitivity, Diabetes, Glaucoma, Visual Acuity, and Visual Field in Eye Care Encyclopedia.