Screening for visual acuity


Visual impairment of some form affects 13% of elderly people; almost 8% of them have severe impairment (blindness in both eyes or inability to read newsprint even with glasses).

About 1% of people over 40 years have bilateral blindness. In 1989, there were 63,576 people registered as legally blind in Canada. The leading causes of visual impairment in elderly people are presbyopia, cataracts, age-related macular degeneration (ARMD), glaucoma and diabetic retinopathy.

Although people may notice reduced visual acuity while reading, watching television, recognizing people or in other activities, surprisingly such deficits often go unrecognized by the people and their physicians.

In one study, up to one third of elderly day patients were found to have unrecognized severe visual loss, and in another, one quarter were wearing inappropriate corrective lenses.

The question "When wearing glasses, can you see well enough to recognize a friend across the street?" had a sensitivity of 48% in detecting visual acuity of less than 20/40 (6/12).

Other questions have been found to have a sensitivity of only 20% to 30%. Impaired visual acuity is readily detected with the use of a Snellen sight chart, which can be used in a family physician's office or, in a reduced size, as a portable tool.

A portable visual acuity box was used in the Visual Acuity Impairment Survey Pilot Study. Nearly 1200 subjects were screened, and 123 (10.5%), mostly elderly people, were found to have poor acuity of some degree. Less than half of the subjects were then examined at an eye clinic.

Compared with the clinic examination, the portable unit had a sensitivity of 94% and a specificity of 89%. Lowenstein and associates found that when the sight chart was viewed through a pinhole (which minimizes refractive error) the sensitivity was 79% and the specificity 98%.

In a study in Wales 202 elderly patients attending an outpatient clinic were questioned about their use of eyewear and their vision and then were tested with the use of a Snellen sight chart viewed through a pinhole.

Over one third of the patients were found to have impaired vision; 30 had refractive errors. Of the 42 patients with nonrefractive errors 27 had a treatable condition (most often cataracts or glaucoma) discovered by an ophthalmologist, and 15 had an untreatable, serious condition (usually ARMD). Only 9 of the 42 believed that their vision was inadequate.

Over 400 consecutive patients attending a primary care general medical clinic in Baltimore were asked to complete a brief questionnaire and undergo a standard vision test with a Snellen sight chart.

Nearly two thirds did not meet predefined criteria and were referred to an ophthalmology clinic. Of the 101 who showed up for the evaluation at the clinic, 96 were found to have serious eye disease, the most prevalent being cataracts, diabetic retinopathy, glaucoma and ARMD.

Immediate medical therapy was required for 14% and surgical intervention for 18%. The vision test alone failed to identify most cases of diabetic retinopathy and glaucoma.

Major causes of visual impairment

Presbyopia : As people get older their lenses become thicker and less flexible, which results in diminishing accommodation and often refractive errors. The process is universal with aging but does not usually result in blindness.

Refractive errors due to presbyopia are readily corrected with eyeglasses or contact lenses. In the Baltimore Eye Survey visual acuity was measured with the person wearing his or her corrective lenses, if any. Following appropriate correction 54.0% of the subjects had improved vision by at least one line on the Snellen sight chart, and 7.5% had improved vision by three or more lines.

Cataract : The presence of any opacity in the lens is defined as cataract. Cataracts appear in different forms and sizes and may result from trauma, disease (e.g., diabetes or hypoparathyroidism), ionizing radiation or the use of medications (e.g., corticosteroids or antineoplastic agents). In most cases they are idiopathic or "senile."

Symptoms include deterioration in visual acuity, increased glare in bright light and a "halo" seen around objects. In hyperopic patients "second sight," a temporary improvement, results from a myopic shift with the onset of cataract formation. Cataracts are readily detected by means of ophthalmoscopy. The test characteristics of examination in primary care are unknown.