Pupil cycle time: a simple way of measuring an autonomic reflex.

Persistent regular oscillations of the pupil of the eye can be induced using a slit-lamp. The period of these oscillations, pupil cycle time (PCT), is simple to measure with a hand-held stop-watch. Evidence from pharmacological testing suggests that PCT is a sensitive measure of dysfunction of the parasympathetic efferent limb of the pupillary light reflex arc. This is confirmed by the finding that PCT is prolonged in a high proportion of patients with evidence of autonomic neuropathy judged by abnormalities of their cardiovascular reflexes. The method provides a simple, non-invasive way of quantifying an autonomic reflex and it may be of value in investigating patients with diseases affecting the autonomic nervous system.