The most common manner in which eye care professionals measure visual acuity is the Snellen eye chart, which is quick and simple but only measures high-contrast sensitivity. This is the big “E” chart everyone has seen, and 20/20 vision which everyone understands as optimal. This has been around over 100 years and is one of those familiar and friendly parts of the routine eye exam.
The letters are in dark black and the background is white, and the screen is well-lit. The condition is standard but artificial. The real world exists in shades of grey. Why is this important? Because under real world circumstances many patients complain of difficulty in vision but test 20/20 in the doctor’s office.
If one does more careful testing with contrast sensitivity exams, the defects often become apparent. There are many such tests, but a simple one simply has letters that are various shades of grey. It obviously gets tough to identify the lighter the grey. This type of testing becomes more time consuming and thus is not usually done. In many ways contrast sensitivity is more similar to hearing tests where thresholds of each frequency are tested and a hearing diagram is the result. The hearing test would be much simpler if one was presented with a standard sound that was either on or off. As most people have taken hearing tests this analogy is useful. Hearing tests are tough.
Traumatic brain injury (TBI) is known to reduce contrast sensitivity much more than standard visual acuity. The explanation is complicated but large processing areas of the brain are involved with contrast sensitivity, and these areas are often injured in mild TBI. Thus, patients will often complain of severe difficulty in seeing under grey skies or at the end of the day as the sun goes down. Now, we all have difficulty seeing on a foggy day, but TBI victims have a measurable decrease in sensitivity compared to normals or when compared to their prior level of function. This can be assessed in an office setting but frequently is not. The complaints however, are real.
Where do these problems reveal themselves? The usual places that I have discussed in previous blogs- reading and driving. These often present situations where background lighting can vary and contrast loss becomes more critical. Contrast sensitivity is also affected by standard eye diseases such as cataracts, glaucoma, diabetic retinopathy and macular degeneration. Numerous research articles have been published on the impact of these conditions on contrast sensitivity.
The traumatic brain injury literature is also filled with such publications. Every ophthalmologist knows that a patient can have 20/20 vision but disabling glare from a cataract. The glare complaints are in part a manifestation of alteration in contrast sensitivity. I have performed cataract surgery on such patients and even though their vision is still 20/20 before and after surgery, the symptoms are gone- that patient is satisfied. But an ophthalmologist can easily see a cataract in a patient from a dilated eye exam. But the TBI often leaves no easy clues on exam. There is no white chalky-appearing lens opacity (a cataract).
An ophthalmologist interested in evaluating head injury patients must dig deeper, take more time. The treatment for symptomatic cataracts is straight forward- surgery. There is no surgery or eye drops to treat TBI. After patients undergo successful cataract surgery, they are often quite appreciative and happy. My TBI patients are usually less satisfied.
Steven H. Rauchman, M.D. is an eye physician and surgeon who has been in private practice for 30 years. He has served as a Traumatic Brain Injury (TBI) medical/legal expert for the last 6 years specializing in the area of personal injury and related traumatic brain injuries.