Electrophysiologic correlates of anomalous course of the median-nerve thenar motor branch.

We report the case of a right-handed 67-year-old uremic woman with severe left carpal tunnel syndrome (CTS) in whom preoperative neurophysiologic data suggested that the thenar motor branch (TMB) of the median nerve ran an anomalous course that was confirmed during surgical release. The patient had undergone periodic hemodialysis for 10 years through a vascular graft at the left forearm. The patient presented with a 2-month history of severe paresthesias, pain, and mild edema of the left hand. Electrophysiologic investigation disclosed absent sural sensory nerve action potentials (SNAP) and reduced radial and ulnar orthodromic sensory conduction velocities and SNAP amplitudes bilaterally (32 m/s, 2 μV and 30 m/s, 1 μV). Peroneal and tibial motor conduction studies and needle electromyography of the tibialis anterior muscles were normal, but electromyography of the foot extensor digitorum longus muscle showed slight chronic neuropathic findings bilaterally. Median nerve assessment showed absent I–III digit SNAP and normal distal motor latencies and amplitudes of thenar compound muscular action potentials (CMAP) bilaterally (Fig. 1). Proximal median and ulnar motor conduction studies were normal bilaterally. Median and ulnar nerves were compared by positioning the active electrode over the palmar second intermetacarpal space to record CMAP from the second lumbrical and first interosseous palmaris muscles. Median and ulnar nerve stimulation of the right (asymptomatic) hand elicited normal CMAP. The second lumbrical CMAP in the left (symptomatic) hand was absent, whereas the first interosseous palmaris CMAP was normal (Fig. 1). Needle electromyography of the left second lumbrical muscle disclosed fibrillation potentials but no abnormality in the right hand or thenar muscles bilaterally. Abnormal median-ulnar forearm communications were excluded. These findings suggested that the left TMB took an anomalous route outside the carpal tunnel and led to a diagnosis of left CTS with “atypical” sparing of the thenar median motor fibers. An open surgical release of the left median nerve at the wrist markedly reduced the paresthesias and pain. Operation disclosed a single TMB leaving the median nerve radially, 3–4 cm proximal to the wrist © 2001 John Wiley & Sons, Inc. Figure 1. Preoperative and postoperative neurophysiologic data. The normal thenar CMAP after median nerve stimulation despite an absent left second lumbrical CMAP suggested an anomalous thenar TMB, subsequently confirmed during surgery. Postoperatively, the left second lumbrical CMAP appeared. Filters used for acquisition: 10 Hz–10 kHz.