RESUMEN
OBJECTIVE: To investigate the source of an unusual and previously unreported volume conducted potential on motor nerve conduction studies. In a case of subacute ulnar neuropathy at wrist (UNW) selectively involving the deep motor branch, we recorded from the hypothenar eminence a large positive wave (2.5 ms-2 mV) preceding the negative takeoff of the delayed distal ulnar motor response. METHODS: We performed multiple channels motor and sensory ulnar nerve (UN) conduction studies; these included selective electrical stimulation and anaesthetic block of UN branches and also selective recording of motor responses by single fibre needles; data were confirmed by an intraoperative neurophysiological study and correlated with MRI and surgical findings. RESULTS: Detailed neurophysiological investigation demonstrated the generation of this waveform from the palmaris brevis (PB) muscle. MRI and surgical exploration documented a hypertrophy of this muscle. CONCLUSIONS: In type II degrees UNW, depolarization of a spared palmaris brevis muscle may be recorded as a positive wave preceding the delayed abductor digiti minimi motor response. SIGNIFICANCE: We underline the peculiar localizing value of this volume conducted 'meaningful artefact' in that particular setting. It actually represented an early neurographic analogue of what is known as the clinical 'Palmaris Brevis Sign' in long standing type II degrees UNW.