Topical anesthesia: H-reflex recovery changes by desensitization of the skin

The H-reflex recovery curve of the soleus muscle was studied in 53 normal subjects before and after the application of topical anesthesia (20% benzocaine) to various skin areas and dermatomes of the lower limb. Skin areas were those overlying the calf, tibialis anterior, quadriceps and hamstrings muscles, and the dermatomes were L2, L3, L4, L5, S1, S2. In 42 subjects the H-reflex initial recovery decreased by 10-80 msec with a mean of 20 +/- 17 msec 30 min after application of topical anesthesia. Also, the test reflex (the second sequential H-reflex) had a faster recovery with less inhibition than that before anesthesia. It was also noticed that the longer the inhibition period before anesthesia, the greater the decrease in recovery time. These observations had no correlation with the skin area or dermatome desensitized by the anesthesia. furthermore, these observations were not noted when a placebo was sprayed on the skin. The results from this study indicate that input from cutaneous afferents may have an important inhibitory effect on the alpha-motoneurons of the soleus muscle. This effect is possibly more profound on the large phasic motoneurons. These results are discussed in relation to clinical interpretations of the H-reflex recovery curves.