The stimulus intensity modifies the blink reflex recovery cycle in healthy subjects and in blepharospasm

Objective: The excitability of human brainstem interneurons is measured by the blink reflex recovery cycle and is abnormal in blepharospasm. We wondered whether the results of this paradigm depend on the stimulus intensity. Methods: We investigated the blink reflex recovery cycle in 13 healthy control subjects and in 13 patients with blepharospasm (7 of them treated with botulinum toxin) using 4 levels of stimulus intensity (5, 12, 19 and 26 mA) and two interstimulus intervals (ISIs, 150 or 250 ms). Results: In all groups the inhibition of the second R2 response was reduced with strong stimulus intensities: In controls, the inhibition of the second R2 decreased significantly (e.g. young controls, ISI 150 ms, from 89.6±15.6% at 5 mA to 21.9±49.7% at 26 mA, mean±standard deviation). In patients the R2 inhibition found at 5 mA was converted in an R2 facilitation at 26 mA, irrespective of the status of treatment. In addition, the patients’ results at 5 mA did not differ significantly from the controls’ results at 26 mA. Conclusions: The R2 inhibition and its sensitivity to detect abnormal interneuronal excitability depend on the stimulus intensity, which may act by modifying the excitability of the R2 interneurons.