Motoneuron excitability modulation after desensitization of the skin by iontophoresis of lidocaine hydrochloride.

Soleus motoneuron excitability was monitored after skin desensitization by iontophoresis of lidocaine hydrochloride in able-bodied subjects. Motoneuron excitability was measured by plotting H-reflex recovery curves before and after the application of lidocaine hydrochloride or a placebo. Significant H-reflex augmentation between 50 and 1,000 msec resulted after both iontophoretic application of the drug and the placebo (p less than .05). In addition, H-reflex initial recovery decreased by 20 to 30 msec in 75% of the subjects tested (p less than .05). H-reflex recovery curve facilitation lasted 30 minutes and returned to control values 45 minutes post-iontophoresis. H-reflex facilitation was not significantly different between the drug and the placebo treatments. It was concluded from these results that low-voltage, galvanic electric stimulation associated with iontophoresis predominantly caused the increase in motoneuron excitability, although other peripheral facilitatory factors may have been involved. Furthermore, cutaneous desensitization appeared not to have an effect on motoneuron excitability. This conclusion was based on the observation that similar H-reflex amplitude augmentation resulted after administration of a placebo as well as an active drug.