Median "pseudoneurapraxia" at the wrist: reassessment of palmar stimulation of the recurrent median nerve.

OBJECTIVE: To determine the occurrence and effect of incidental deep ulnar nerve (DUN) costimulation during palmar stimulation of the recurrent median nerve (RMN). DESIGN: Observational. SETTING: Electromyography laboratory. PARTICIPANTS: Seventeen asymptomatic adult volunteers (34 hands) and 4 fresh cadaver hands. MAIN OUTCOME MEASURES: Median nerve stimulation at the wrist and careful incremental surface and subcutaneous (needle) palmar stimulation were performed while recording thenar and first dorsal interosseous manus compound muscle action potentials. Thenar palm-to-wrist amplitude difference (P -- W Delta) and palm-to-wrist amplitude ratio (P/W) were compared with published values. Correlation of DUN costimulation with falsely elevated P -- W Delta and P/W values was assessed. Multiplanar magnetic resonance imaging (MRI) and subsequent dissection of 4 fresh cadaver hands was done to measure the distance between the RMN and DUN at the palmar stimulation site. RESULTS: Two groups emerged: Group I (63%), with DUN stimulation, and II (37%), with no DUN stimulation. When DUN costimulation occurred (Group I), there was a statistically significant increase in P -- W Delta (p =.002 percutaneous, p =.02 subcutaneous) and P/W (p =.004 percutaneous, p =.007 subcutaneous) when compared with previously published data. Combining all trials on all hands, 53% and 25% had P -- W Delta values and P/W values that exceeded previously published normative data, respectively. The mean distance between the DUN and RMN at the palmar stimulation site was determined by dissection (1.2cm) and MRI (1.5cm). CONCLUSIONS: Close proximity of the DUN to the RMN causes frequent and often unavoidable DUN activation during palmar stimulation of the RMN. This inadvertent stimulation may cause a false diagnosis of median neurapraxia at the wrist.