visual acuity
Visual vs. Acuity Contrast Sensitivity
Which test provides the most valuable information on a patient's visual state?

In 1862, when the Dutch ophthalmologist Herman Snellen introduced his revolutionary new visual acuity chart and the concept of the Snellen fraction, spectacle correction was already ingrained in our culture. The eye chart and the Snellen fraction have become classics.

Snellen's concept was to develop a simple and quick method by which to quantify the resolution of the eye. His visual acuity charts not only measured how well a patient could distinguish details of objects, but also allowed us to measure the patient's clarity or clearness of vision at a specific distance. He developed, as well, a specialized notation that in time became known as "Snellen acuity" or the Snellen fraction. The Snellen acuity number corresponds to the smallest letter or line a patient can see on the eye chart and enables the clinician to follow any changes in the patient's vision over time. The fraction consisted of a numerator, which represents the distance of the observer from the chart during the test, over a denominator, which represents the size of the smallest letter the patient can see clearly. The Snellen fraction is now as the standard for measuring vision.

But the visual acuity chart measures vision under near ideal contrast conditions (100% contrast with black letters on a white background), a situation rarely encountered outside the examination room. Nearly 100 years later, we discovered that visual acuity is only one aspect or component of visual function. Other than visual acuity, visual function is actually comprised of many other components, including but not limited to: visual field, color perception, stereoacuity, glare recovery, dark adaptation, fixation and contrast sensitivity function (CSF).

Shortly after the introduction of the first clinical tests of contrast sensitivity function, in 1981 (prior to that time, CSF testing was time consuming and was mostly confined to research laboratories), it became apparent that CSF may actually be a better indicator of how patients sees under non-ideal conditions, as well as how their eyes function in real-life situations. In fact, CSF may be one of the most important components of visual function since it helps to predict patients' ocular functions in the real world. Geoffrey Arden, M.D., clearly indicated this when he defined CSF as the ability to see low contrast patterns, or to put it another way, how much contrast does an object have to have to be visible?

Yet it has taken more than 25 years for the measurement of the CSF to finally be recognized as an important factor for an individual's quality of life. Eleanor Faye, M.D, and I introduced the use of a clinically relevant contrast sensitivity test, the Vision Contrast Test System (VCTS) for low-vision patients in 1981 at The Lighthouse Low Vision Service in New York. We soon realized that CSF testing could reveal new information not provided by the visual acuity chart. Inclusion of CSF testing also aided in decision regarding patient management and case disposition. Decreased CSF may affect your patient's everyday activities. The world may world appear hazy to them, they may have difficulty driving (especially at night), recognizing faces or seeing labels on the medication bottles.

Contrast sensitivity can also effectively measure vision changes over weeks, months, or years. This could be especially beneficial in monitoring progressive pathological conditions, such as cataracts, age-related macular degeneration, diabetic retinopathy and glaucoma. CSF could also provide feedback as to whether therapy is effective or if surgery may be indicated

The value of visual acuity

Yet visual acuity has maintained its status as the benchmark and standard by which to measure eyesight, from the effectiveness of a refractive correction, how we determine legal blindness in the United States and the rest of the world. In addition, visual acuity has been recognized as especially important in the management of ocular disease.

But the profession of Optometry has changed dramatically over the past 25 years. There has been a dramatic upswing in optometric involvement in the medical management and co-management of patients with ocular disease. Optometrists increasingly manage patients with glaucoma and provide pre- and post-surgical care of patients who have undergone cataract, laser, or even corneal surgery. The low-vision clinician is also involved in the evaluation and treatment of patients with unstable pathological conditions, ranging from age-related macular degeneration (AMD) to venous occlusion to inoperable cataracts.

In addition to visual acuity, medical management has become more dependent on the objective information provided by other diagnostic tests. In fact, in the past few years we have witnessed a dramatic increase in the use of diagnostic instrumentation such as the Heidelberg Retinal Tomograph (HRT), GDx Nerve Fiber Analyzer, pachymeter and Optical Coherence Tomography (OCT). Despite the value of these new instruments and tests to quantify vision loss, they do not address one very important patient concern: the all-important quality-of-life concern, of special importance to the older population.

The importance of contrast sensitivity as an important measurement of vision function was finally recognized when it was included in an initial clinical trial for the treatment of choroidal neovascular AMD. There was enormous excitement when the results of the early clinical trials indicated verteporfin (Visudyne, Novartis) in photodynamic therapy could preserve an AMD patient's vision.

The results from the Treatment of Age-Related Macular Degeneration with Photodynamic Therapy (TAP) study group revealed that patients lost 12-15 letters on the ETDRS visual acuity chart over a two-year period compared with 25 letters for the control group. This same retention of vision has held true in the five-year follow-up study, with patients losing 10 letters while the placebo patients lost 25 letters.

More important and just as dramatic (yet not fully grasped by the professional community) was the evidence that verteporfin therapy stabilized CSF as well. In fact, patients in the study had no loss of CSF on the Pelli-Robson test, while the placebo group lost about seven letters. The loss of vision in the placebo group is comparable to a loss of visual acuity on the eye chart from a baseline of 20/40 to 20/80, or from a baseline of 20/100 to 20/200. Investigators found the preserved vision in the treatment group might correlate with reduced emotional distress and depression, as well as the ability to be able to live independently.

It would therefore be reasonable to assume that CSF is a better predictor of patient performance in the real world than visual acuity. For example: A patient presents in your office with early cataracts and glaucoma. Some of the patient's visual complaints include difficulty reading the menus in restaurants with "romantic" lighting, reading the newspaper and medication bottles, as well as in walking down stairs in a poorly lit stairway. The results of a CSF test in this instance would most likely be a better predictor of patient performance as well as difficulty coping with everyday activities.

CSF has already gained some acceptance and is being used extensively in low vision rehabilitation to monitor cataract progression, fit contact lenses and monitor the effectiveness of glaucoma and vision therapy.

Incorporating CSF into your exam

There are a number of contrast sensitivity tests commercially available (see chart on page 78). The latest CSF test, the Mars Letter Contrast Sensitivity test, was critically analyzed and acclaimed at the international meeting, Vision 2005 in London. It is a small-format letter near contrast sensitivity test, with improved accuracy over the Pelli-Robson test. It utilizes Sloan letters with each letter decreasing in contrast decrements by 0.04 log units. The Mars test is easy to administer, takes less than a minute and has an unambiguous scoring procedure. More importantly, patient responses on the MARS test indicate an early, moderate, severe or profound vision loss, which allows for more effective successful treatment strategies.

There is new software available as well. EyeView Functional Analysis Software from

Vision Sciences Research can be used with the company's Functional Acuity Contrast Test (FACT) contrast sensitivity charts as well as ETDRS acuity charts. It produces pictures that demonstrate how the world looks to your patient based on their contrast sensitivity scores.

Contrast sensitivity testing can add a whole new dimension to your practice by helping you predict visual performance issues as well as design a more effective treatment plan. It's a great practice management tool to assist in your explanation to the patient, as well as family members, the reasons for some visual complaints. It's another effective way to monitor vision over time and can be especially helpful for low-vision patients.

And that's my contrasting opinion too.

Dr. Rosenthal serves as the Chief of the Low Vision Clinical Practice at Lighthouse International, is an adjunct professor at Mt. Sinai Hospital, a distinguished professor at the State University of New York and Chair of the Scientific Advisory Board of the AMD Alliance International. He lectures throughout the world and has authored eights books on visual impairment and macular degeneration as well as over 100 scientific articles.