Normal limits of side-to-side H-reflex amplitude variability.

Our hypothesis was that side-to-side difference in the tibial H-reflex amplitude can aid in the diagnosis of unilateral S1 radiculopathy with an otherwise normal side-to-side H-reflex latency difference. We studied 47 healthy subjects, (mean +/- SD, 35 +/- 10 years) using methods similar to those described by Braddom and Johnson. Only those with side-to-side latency difference less than 1.5 msec were included (n = 45). Current was increased in 0.5 to 1.0 mA increments until the H-reflex amplitude maximized and then fell; on each side the response with the maximal peak-to-peak amplitude was measured for peak-to-peak amplitude and onset latency. The amplitude on the side of the smaller response was expressed as a ratio of the contralateral amplitude; this mean value (+/- SD) was 0.74 +/- 0.17. We conclude that a side-to-side amplitude ratio smaller than 0.4 (mean - 2 SD) in the face of a normal side-to-side latency difference is probably abnormal.