THE H-REFLEX TO MAGNETIC STIMULATION

THE H REFLEX TO MAGNETIC STIMULATION Y. Zhu, M.D., K.G. Woodward, M.D., S.H. Su, M.D., and A. Starr, M.D. (University of California, Irvine, California) We elicited the gastrocnemius-soleus H reflex with mag- netic stimulation to tl1e posterior tibial nerve in 5 11or- mal subjects. H reflexes were studied with the subjects prone. For conventional electric and magnetic stimulation, stan- dard electrode placement was used, and the posterior tibial nerve was excited at the popliteal fossa. Electric stimulation was done with the cathode proximal and a stimulus duration of 1000 µ,s. Magnetic stimulation was affected by 70 µs pulses, 2 T at 100% power, into a 9- cm round coil or a 5-cm round coil with a focal point. For both coils, each face was arbitrarily labelled A or B. With either coil, the H response rarely appeared alone; the H threshold was usually above the M thresh- old. The 9-cm coil was more effective, eliciting higher amplitude M and H responses at the same power; even so, maximal M and H amplitudes were lower with mag- netic than electric stimulation. H latencies to electric and magnetic stimulation were not significantly cliff er- ent. The H threshold varied witl1 coil orientation (face A and B) and was lower with active ipsilateral or con- tralateral plantar ftexion. H reflexes are reliably obtained in normal subjects with a 9-cm coil. The 5-cm coil is inadequate for H re- flex testing.