Skin receptors and intradermal nerves do not generate the sensory double peak.

INTRODUCTION: The cause of the double peak observed at submaximal stimulation of sensory nerves is unknown. The first peak is generated under the cathode and the second under the anode. The double peak is thought to arise from intradermal nerves or skin receptors, and in this study we tested this assumption. METHODS: We studied the effect of different stimulus durations on anodal peak latency in volunteers. Biphasic anodal stimulation was used to investigate the latent additive effect of the trailing negative phase on the partial depolarization induced by the initial positive phase. We further tested the maximal amplitude of anode-generated potentials to estimate the number of neural structures involved in their generation. RESULTS: Increased stimulus duration caused anode-generated potential delay. Biphasic stimulation increased anode-generated amplitude 4-fold compared with monophasic stimulation. The anode-generated potential produced up to 85% of the supramaximal cathode-generated amplitude. CONCLUSIONS: The results suggest that the double peak arises from anodal break excitation and not from intradermal nerves or receptors.

Supraorbital nerve conduction study in normal subjects.

There is currently no examination technique that allows direct measurement of supraorbital nerve conduction velocity and amplitude. Therefore, in this study we describe a novel nerve conduction technique that allows measurement of the supraorbital sensory nerve action potential (SNAP) distal to the supraorbital foramen. Supraorbital SNAPs were recorded bilaterally from 17 healthy volunteers using an antidromic technique. The SNAPs were consistently recordable over the site 6 cm lateral to the midline point that was marked 10 cm above the nasion. Measured parameters included peak latency (mean 2.3 ms, SD 0.3), amplitude (mean 14.6 µV; SD 10.5), and velocity (mean 51.3 m/s, SD 6.8). The mean percentage of interside difference in amplitude was 25.6% (SD 17.3). Cut-off values (97th percentile) were 2.7 ms (peak latency), 3.3 µV (amplitude), 41.9 m/s (conduction velocity), and 54.9% (interside difference in amplitude). Supraorbital SNAPs can be recorded in all normal subjects and used as a quantitative measure of the functioning large fibers in the nerve.

Carpal tunnel syndrome caused by gouty tophus of the flexor tendons of the fingers: sonographic features.

We report the case of a 23-year-old male presenting with carpal tunnel syndrome and a swelling over the flexor surface of the wrist. MRI findings were initially suggestive of a median nerve schwannoma but sonography (US) showed a heterogenous mass infiltrating the flexor tendons of the fingers and displacing the median nerve in the carpal tunnel. US findings were confirmed by surgical exploration, which revealed a gouty tophus of the flexor tendons of the fingers at the wrist with secondary median nerve displacement and compression.