Pupillary fatigue in myasthenia gravis.

By shining a narrow beam of light upon the pupillary margin, a sustained oscillation of pupil size or hippus, may be induced; the frequency of which is dependent upon the rate of conduction along the neuronal pathways subserving pupillary function. This manoeuvre has been carried out upon a series of twelve patients with myasthenia gravis, eleven of whom demonstrated a gradual diminution in oscillation frequency. This is compared with twelve control subjects, only one of whom demonstrated a significant decline in oscillation rate. The mean gradients were 0.0 +/- 0.4 cycles/min for the control subjects and -2.4 +/- 0.4 cycles/min for the patients with myasthenia. These gradients differ significantly p less than 0.0001. It is suggested that the pupillary reaction to light may be fatigued by exercising it at its maximum intrinsic rate in patients with myasthenia gravis.