Stimulus characteristics affect assessment of pupil defects in amblyopia

Amblyopes do not reliably show relative afferent pupillary defects with full-field stimulation, but amblyopia has cortical involvement; hence, stimuli that engage cortex may be able to reveal pupil defects in amblyopes.

Pupillary responses were acquired with a binocular infrared pupillometer (RAPDx, Konan Medical USA, Irvine, CA) from 15 amblyopic subjects (anisometropic and small-angle strabismic) and 10 age-matched control subjects. Stimuli were a full-field white flash (330 cd/m) or a small (4 degrees) annulus at one of three contrast levels (0.3, 0.6, and 1.8) on a dim background (6.2 cd/m). Stimulus duration was 100 milliseconds, and the interstimulus duration was 2000 milliseconds.

In all four stimulus conditions, the difference in percent contraction amplitude for right versus left eye stimulation was more variable across amblyopes than across control subjects. Amblyopic eyes did not show a specific deficit for the full-field flash. However, the mid-contrast (0.6) annulus stimulus revealed a deficit in the amblyopic eye, whereas the size of the deficit did not correlate with the type or depth of the amblyopia.

Targets of appropriate pattern, brightness, and contrast that select for cortical contributions to the pupil response may be useful for eliciting pupil defects in amblyopic patients. Pupil analysis in this population could prove useful for diagnostic or prognostic value, for example, to determine which amblyopes will respond best to treatment.