The effect of levodopa on the habituation of the acoustic-palpebral reflex in Parkinson's disease.

The aim of the present study was to assess the habituation of the blink responses evoked by repetitive auditory stimuli in patients with Parkinson's disease in different clinical states. We studied 28 parkinsonian patients. Eighteen patients without motor fluctuations were studied off and on levodopa, on two different sessions, one week apart. The remaining 10 parkinsonians had the wearing off phenomenon and were studied while being off their medication and while ON and OFF, as determined clinically, after a single dose of levodopa. Twelve age-matched controls were also studied in a single session. Unilateral auditory 1 kHz, 105 dB stimuli were delivered at a frequency of 0.2 Hz. The number of responses (R) from the orbicularis oculi was registered. R values were significantly smaller in controls and in treated parkinsonians than in untreated patients. R was also significantly smaller when ON than at baseline and when OFF in parkinsonians with the wearing off phenomenon. These results suggest a failure of the inhibition of the acoustic-palpebral reflex in Parkinson's disease. The improvement with levodopa and the similarities between the clinical and the electrophysiological changes suggest a dopaminergic role in the inhibition of this brainstem reflex.