Characterization of a novel C8 phasic muscle stretch reflex

Abstract

Muscle stretch reflexes (MSRs) are fundamental to the neurologic examination. Although the fifth, sixth, and seventh cervical roots have appropriate and easily elicited MSRs, the C8 level does not. The C8 reflex is obtained by tapping the thenar eminence, which produces flexion at the interphalyngeal joints of the fingers. To confirm that finger flexion represented an MSR, an externally triggered hammer-skin contact set-up was employed. Surface recording electrodes were placed at one third of the distance between the medial epicondyle and ulnar styloid. This montage produced distinct T-waves with latencies compatible with a reflex arc. Normative data were collected for 75 healthy subjects (150 arms), including 15 from each of five age groups (20-29, 30-39, 40-49, 50-59, > or =60 years). Distance 1 (D1) was measured from the FPL to the C7 spinous process and distance 2 (D2) from C7 to the G1 electrode. A high median neuropathy abolished the T-wave. A distal median nerve block at the carpal tunnel had no effect. Mean latencies (+/- SD) from the respective age groups were as follows: 21.73 +/- 1.16 ms; 22.50 +/- 1.70 ms; 22.19 +/- 1.52 ms; 22.66 +/- 2.13 ms; and 23.97 +/- 1.91 ms. Reflex latencies significantly correlated with age (r = 0.33, P = 0.004), height (r = 0.51, P < 0.0001), and D1 + D2 or arc length (r = 0.74, P < 0.0001). This novel MSR is generated from the FPL tendon and stimulates contraction of the long finger flexors. It therefore represents a clinically useful C8 MSR.