Cataracts cause varying degrees of visual loss. The main indication for
cataract surgery is when a person’s
visual function no longer meets his or her needs and cataract extraction will provide improved vision. This level of visual impairment is subjective and differs considerably among individuals. Nevertheless, insurance carriers typically choose 20/40
Snellen visual acuity as the threshold for cataract surgery, since this level of sight is required for driving. However,
visual acuity can be quite variable depending upon the testing conditions, test distance, and type of cataract. Therefore it may be difficult to appreciate the impact of a cataract on the patient’s vision especially when he or she can see better than 20/40.
We are often limited in our ability to adequately comprehend what a patient describes as decreased, blurry, or poor vision, because we can only test isolated components of visual function like Snellen acuity, contrast sensitivity, glare, color vision, and visual field. These tests give us incomplete information about a patient’s visual performance. For example, a patient may see 20/20 in each eye, but complain that while the vision in one eye is sharp and crisp, the vision in the fellow eye is blurred. It can be difficult to determine the visual significance of such a cataract since our tests do not enable us to accurately understand the quality of the patient’s sight.
Visual acuity is tested monocularly with high contrast (i.e., black letters on a white background). While this is the standard method of assessing vision, it is quite artificial and is not representative of real world conditions. Contrast sensitivity testing is more realistic but less commonly measured. Contrast sensitivity evaluates the ability to differentiate between an object and its background using low contrast letters or sine-wave gratings with different spatial frequencies. Whenever visual acuity is decreased, so is contrast sensitivity, but sometimes contrast sensitivity can be affected significantly more than visual acuity. Therefore, to better assess a patient’s visual difficulty, contrast sensitivity should be tested when the visual acuity is better than expected based on the patient’s complaints.
Another helpful measurement is glare testing. This is often performed with a brightness acuity test (BAT) to simulate glare from a light source. Patients with cataracts may have good distance visual acuity in a dim room, but experience a significant reduction in acuity from a bright light. This is the characteristic situation with a central posterior subcapsular cataract that scatters light and blocks the entrance pupil when the pupil constricts.
Other tests can be performed to assess the visual potential in a cataract patient with coexisting ocular pathology or to rule out macular or optic nerve pathology when the cataract interferes with a clear view of the posterior pole. The potential acuity meter (PAM) test projects an eye chart directly onto the retina through lens opacities. This is often useful in estimating how much the cataract is contributing to the patient’s visual loss. Alternatively, color vision can be used to assess the optic nerve and macula. Color vision testing is a sensitive indicator of optic nerve function and can be tested quickly with Ishihara pseudoisochromatic or Hardy-Rand-Rittler plates. In addition, gross macular function can be evaluated with red perception by asking the patient to identify the color of a red eye drop cap. Optical coherence tomography (OCT) is probably the most helpful test for diagnosing macular pathology, especially subtle findings and when a premium IOL implant is being considered. Visual field testing is also helpful since it will identify scotomas from suspected retinal or optic nerve pathology. The presence of generalized depression in an otherwise normal field test is characteristic of a significant cataract. Finally, we should not forget the basics: pupil testing is performed in all patients with decreased vision because the presence of a relative afferent pupillary defect indicates optic nerve or widespread retinal dysfunction.
Therefore, although we usually diagnose a visually significant cataract from the patient’s symptoms, the Snellen acuity, and the presence of a cataract on slit-lamp examination, there are a variety of other tests at our disposable. These are particularly helpful when the visual acuity is better than expected, the acuity does not correlate with the severity of the patient’s visual complaints, or there is other ocular pathology that may be contributing to the reduced vision.