New technology helps in vision assessment of preverbal child

Although SSVEP is gaining acceptance, other options for vision assessment in preverbal or nonverbal children also remain valid, said Dr. Good, a principal investigator at Smith-Kettlewell Eye Research Institute, San Francisco. These options are central, steady, and maintained (CSM) fixation testing and behavioral techniques such as the Teller acuity card procedure.

In the clinic, CSM is the most frequently used of these options for the detection of amblyopia because it is reliable, fast, inexpensive, and easy to use; when no strabismus is present, a vertical prism test can be used as an adjunctive assessment, he said.

Electrophysiology measures such as SSVEP merit a place in the clinician's armamentarium, however, he said.

"SSVEPs have a great deal to offer and can yield a huge amount of data," Dr. Good said.

This testing format is not easily transportable and is rather complicated, requiring training and time before it can be correctly used, however, he cautioned.

"There are questions as to whether SSVEP measures actual acuity, but these questions, I think, are in the process of being answered in a manner that indicates validation if properly used," Dr. Good said.

Visual acuity measures often are better when measured electrophysiologically, compared with behavioral techniques, he added.

Advantages of SSVEP include the ability to yield a threshold for a variety of types of acuities. It also can "sweep" or measure a variety of functions, such as contrast, grating, vernier acuity, and more complex functions, and the data can be analyzed in terms of signal amplitudes, phase, and various harmonics.

The ability to measure vernier acuity could be particularly helpful, because data from the Cooperative Amblyopia Treatment Study showed a one-to-one correlation between Snellen acuity and vernier acuity, Dr. Good said.

"This suggests that vernier acuity may be a good way of getting at the actual Snellen acuity," he said.

Beyond visual threshold, it also is possible to measure signal amplitudes. A study of 50 children with no intraventricular hemorrhage (IVH), compared with 20 with grade I and II IVH, showed with a grating acuity test that the infants with IVH had lower signal amplitudes across the spatial frequency sweep compared with the healthy infants. The same result was found with vernier acuity, despite the fact that visual thresholds for both groups were similar.

The significance of these findings is still unclear, Dr. Good said, adding that research is ongoing.