The H [Hoffmann] reflex latency obtained from the triceps surae following tibial-nerve stimulation was evaluated for detecting and differentiating S1 from L5 radiculopathy. Of 32 patients studied, 15 had a definite unilateral L5 radiculopathy and 17 had a definite unilateral S1 radiculopathy. There was a mean H reflex latency difference of 0.03 ms (range, -0.1 to 0.2 ms) between the affected and unaffected extremities for the 15 patients with L5 radiculopathy and of 2.9 ms (range, 1.3-4.1 ms) for the 13 patients with S1 radiculopathy where an H reflex could be recorded. The H reflex was unobtainable in the affected extremity in 4 patients with S1 radiculopathy. Apparently H reflex latency testing is a valuable tool in helping to differentiate S1 from L5 radiculopathy.
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