The optometric assistant plays an important role in the care and diagnosis of low vision patients. Here’s how you can assist with the low vision evaluation.
LOW VISION BASICS
A patient is considered to have low vision if the level of vision is so reduced that it cannot be fully corrected with glasses or contact lenses. Patients with low vision can have some useful sight, but their vision is compromised enough to affect daily activities such as reading or driving. Someone is said to have low vision if their best corrected acuity is 20/200 or worse. Also, those patients who find it increasingly more difficult to perform daily tasks due to their inability to see, or those who have severely reduced contrast sensitivity or visual field defects are also considered to have low vision.
Many patients with the condition are elderly, but it can affect individuals of any age. The most common causes of low vision are:
•Glaucoma. High intraocular pressure can cause damage to the optic nerve and blind spots in the visual field.
•Macular degeneration. The condition can cause loss of central vision.
•Diabetic retinopathy. Out-of-control blood sugar can cause damage, swelling, and leaking of the retinal vessels which ultimately can lead to loss of vision.
•Cataracts. Clouding of the anatomical lens can cause a decrease in visual acuity.
•Trauma. An eye that has experienced trauma may have permanently damaged vision.
The earlier the signs of low vision are detected, the quicker treatment and therapy can be prescribed to:
•aid the patient to maintain the current level of vision
•treat the condition causing the impaired vision
•assist and provide support in teaching the individual how to maintain as much independence as possible.
The most common signs of low vision are having difficulty reading; seeing things in the distance; recognizing people’s faces; and perceiving objects in their path such as furniture, steps, or street curbs.
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LOW VISION EVALUATION
An in-depth assessment of the functional vision of the patient, the low vision evaluation includes a comprehensive history, visual acuity assessment, and evaluation of ocular motility. The goal is to determine if surgery, lenses, or devices can help to improve visual acuity to best suit the patient’s needs.
History: The optometric assistant can aid the evaluation by performing an extensive history. This history includes asking questions about when/what caused the vision loss, what treatment has already been done for the vision loss, and if there is a family history of vision loss. The assistant should also ask about and document the patient’s full medical, ocular, and social history, current medications, and allergies. In addition, ask questions about what functional problems the patient has secondary to the decreased visual acuity such as:
•What difficulties does the patient experience in performing daily activities?
•Are there problems functioning in very bright light or glare? If yes, does the patient wear sunglasses?
•Does the vision impairment affect the patient’s mobility or ability to travel?
•Does the decreased acuity affect the patient’s ability to do their job? Did the patient leave their job because of
•How is the patient dealing with the emotional impact of losing their vision? Does the patient have emotional support from the family or community? What difficulties has the patient had in adapting to the vision loss?
After completion of the comprehensive history, the next step is to move onto the clinical portion of the evaluation.
The measurement of visual acuity includes not only checking the patient’s vision on the Snellen chart, but also in performing tests such as color vision, depth perception, contrast sensitivity, visual field assessment, and corneal curvature measurement.
Visual acuity can be measured by using the Snellen chart, ETDRS chart, or for non-verbal patients or infants, the Teller acuity cards. In addition, measure visual acuity for distance, near, and binocularly. Since the visual acuity in the examination room does not duplicate the lighting, contrast, and glare the patient experiences in real life, the assistant should also perform contrast sensitivity testing. Testing of the binocular vision refers to how the eyes work together. The coordination and merging of the two images sent to the brain by the eyes determines if a patient has depth perception. The stereo fly test uses polarized glasses along with a polarized vectograph plate to evaluate binocular vision and depth perception.
The contrast sensitivity test simulates the subtle changes in contrast that occur in the real world. Often these tests are better for determining the patient’s functional vision than the standard visual acuity tests. The test measures the eye’s ability to distinguish differences in brightness or contrast, rather than the black/white contrast of the eyechart. The test may also show how much light the patient needs to distinguish objects that have similar brightness or color by using gratings that gradually decrease in contrast. Detecting the decrease in contrast is important to the patient’s daily activities such as:
•Walking outdoors in dim or very bright light and detecting stairs, curbs, cars, etc.
•Reading and writing
The most commonly used contrast sensitivity tests are the Pelli-Robson chart, the Regan chart, and the functional acuity contrast test.
Testing of the low vision patient’s color vision can provide useful information about abnormalities or deficiencies of color vision. The Farnsworth-Munsell D15 is a quick, easy to administer, and informative screening test.
An eye turn or lazy eye can affect the quality of the image sent to the brain from one eye and the patient’s visual performance. The patient’s ocular motility should be carefully evaluated. The eyes should be evaluated for the presence of nystagmus, strabismus, and diplopia (double vision).
VISUAL FIELD TESTING
Visual field testing includes confrontation, manual plus computerized perimetry, and Amsler grid testing. These tests can be used to diagnose and predict how the patient functions in day-to-day activities, and can detect areas of visual field loss and decreased central acuity, blind spots, and distortions. Patients that have compromised visual fields may: bump into objects at home, such as furniture; become startled by objects or people not seen in the periphery; be very nervous in crowded or unknown environments.
Low vision patients can benefit from rehabilitation with the ultimate goal of promoting independent functionality. For some patients, modification of the environment with improved illumination, wearing sunglasses to reduce glare, or increasing color contrast of frequently used objects can help the low vision patient. Patients who suffer from low vision can also benefit from the use of low vision optical devices. These devices are relatively inexpensive, portable, and can be used to magnify words on the page when reading or to improve distance vision. The most commonly used devices are handheld or stand magnifiers, spectacle clip-on high magnifiers for reading, or handheld or spectacle-mounted telescopes for distance.
Janet Hunter, president of Eye Source, LLC, specializes in ophthalmic technician training.