Effect of photic conditioning on blink reflex recovery function in blepharospasm.

Patients with blepharospasm may have selective sensitivity to photic stimuli, for their symptoms are often aggravated after exposure to brightness. To investigate the pathophysiology underlying this phenomenon, we compared the effects of electric and photic conditioning on electrically-evoked blink reflex in 21 patients with blepharospasm and 11 age-matched control subjects. With electric conditioning, R1 was facilitated at inter-stimulus intervals (ISIs) of up to 100 ms in both groups. R2 was inhibited at all ISIs longer than 20 ms in the normals, but to a much lesser extent in the patients, suggesting increased excitability of the multi-synaptic reflex pathway. Compared to electric conditioning, photic conditioning was less potent in inhibiting R2 in the normals. In contrast, photic conditioning in the patients produced R2 inhibition as powerful as electric conditioning. These findings indicate that, in patients with blepharospasm, photic input has a relatively more profound inhibitory effect on the test blink reflex than electric conditioning as compared with normal subjects. The greater susceptibility of the brainstem interneurons to photic conditioning in blepharospasm may be related to the clinically-observed light sensitivity.