CENTRAL LOOP OF THE H-REFLEX - NORMAL VALUE AND USE IN S1 RADICULOPATHY

The H reflex in the S1 spinal nerve has been used in electrodiagnosis of S1 radiculopathy for several years. Direct stimulation of the S1 sp inal nerve has provided more complete information about the H reflex p athway by dividing it into its peripheral and central (or spinal) cond uction portions. A previous study compared spinal nerve latency with t he H reflex latency, demonstrating an abnormal S1 ratio in subjects wi th S1 radiculopathy, thereby suggesting slowing within the spinal segm ent of the nerve. No study, however, has established a normal value fo r the central (spinal) portion of the H reflex. We electrodiagnostical ly tested 20 subjects with normal clinical neurologic and musculoskele tal examinations to define a normal sample of the central loop of the H reflex in the S1 spinal nerve. The peak latencies of the M and H ref lex responses were measured after a single stimulus to the S1 spinal n erve. The data obtained established 7 +/- 0.3 ms as the normal value f or the interpotential latency difference (central loop) of the H refle x in the S1 spinal nerve in healthy subjects. Six patients with clinic al and electromyographic evidence of S1 radiculopathy all had central loop latencies of > 8 ms. The normal value of the central loop of the H reflex suggested in this pilot investigation may, therefore, be used to allow for earlier and more accurate diagnosis of an acute S1 radic ulopathy.