Amplitude ratio of ulnar sensory nerve action potentials in segmental conduction study: reference values in healthy subjects and diagnostic usefulness in patients with ulnar neuropathy at the elbow.

Objective: To determine normal values for the amplitude ratio of sensory nerve action potential (SNAP) from an ulnar sensory segmental nerve conduction study, and to the evaluate usefulness in the diagnosis of mild-degree ulnar neuropathy at the elbow (UNE).

Design: Segmental sensory conduction study of the ulnar nerve was performed in 71 healthy subjects. Peak latency and baseline to peak amplitudes were measured. The amplitude ratio of below-elbow to wrist (BE/W) stimulations and above-elbow (AE) to below-elbow stimulations (BE) were calculated. Normal cutoff values were obtained by subtracting 2 SD from the mean value, and these values were applied to 22 symptomatic UNE cases. The amplitude ratio was also obtained in six subjects with C8 radiculopathy.

Results: The amplitude ratios of BE/W and AE/BE were 0.61 +/- 0.08 and 0.82 +/- 0.08, respectively. The cutoff value of BE/W was 0.45, and that of AE/BE was 0.65. The amplitude ratio of BE/W showed a weak correlation to the length of the forearm segment (r = -0.25, P < 0.05). Five of the 22 UNE patients revealed only reduced amplitude ratios of SNAP across the lesion, whereas all the patients with C8 radiculopathy showed normal amplitude ratios of ulnar SNAP.

Conclusions: The amplitude ratio of ulnar SNAPs may be useful in the diagnosis of mild ulnar neuropathy with only sensory symptoms and normal segmental motor conduction.