The central loop of H-reflex in the S1 spinal nerve: normal values and constitutional influencing factors

To investigate the central loop of H reflex in the S1 nerve and the constitutional influencing factors in normal healthy individuals.

The study was performed on 39 apparently healthy volunteers. To obtain central H-reflex, the cathode electrode was inserted at a point 1 cm medial to the posterior superior iliac spine, perpendicular to the frontal plane. After the needle touched the sacrum, it was slightly retracted to avoid direct contract to the sacrum. The anode electrode was placed over the anterior iliac spine. The active pick-up electrode was placed at the middle of the line connecting the popliteal crease to the medial malleolus. The reference electrode was placed 2 cm distal to it. The ground eletrode was placed near the active pick-up electrode over the calf muscles.

Mean +/- SD for minimum necessary stimulation to obtain central H-reflex was 23 +/- 13 mA. This was significantly higher than the minimum necessary stimulation to obtain peripheral H reflex (p < 0.0001 using paired t test). Mean central loop of H reflex was 6.9 msec with SD of 0.4 msec. There were no significant differences between the two sides. Mean amplitude of M-wave was 2.2 mV with SD of 1.5 in central loop study. Mean amplitude of H-wave was 2.4 mV with SD of 1.5 in central loop study. The Mean time interval between the onset of M-wave and the end of H-wave (M-H duration) was 18.5 msec with SD of 2.3. After stepwise reduction of variables, considering the correlation between leg length and other variables, the leg length was the only variable strongly correlating with central loop of H-reflex.

Lack of major influencing factors after correction for leg length, elimination of a major fraction of the afferent segment of the loop, and its ability to differentiate peripheral from central lesions make central H-reflex studies an invaluable diagnostic tool.