Regional differences in ulnar nerve excitability may predispose to the development of entrapment neuropathy Arun V. Krishnan a, * , Susanna B. Park b , Mike Payne c , Cindy S.-Y. Lin d , Steve Vucic e , Matthew C. Kiernan b a Translational Neuroscience Facility, School of Medical Sciences, University of New South Wales, Australia b Prince of Wales Medical Research Institute, Prince of Wales Clinical School, University of New South Wales, Australia c University of Western Ontario, London, Ontario, Canada d Department of Health and Exercise Science, School of Medical Sciences, University of New South Wales, Australia e Western Clinical School, University of Sydney, Sydney, Australia See Editorial, pages 1–2 article info Article history: Available online 29 June 2010 Keywords: Membrane potential Na + /K + pump Nerve excitability Sodium channel Ulnar neuropathy abstract Objective: To assess whether there are differences in nerve excitability properties between proximal and distal stimulation sites in the ulnar nerve in healthy controls, which may provide information on whether alteration in ion channel function predisposes to the development of ulnar neuropathy at the elbow. Methods: Nerve excitability studies were undertaken in 11 healthy controls. Studies were undertaken with stimulation of the ulnar nerve at the elbow and wrist. Recordings were obtained from abductor digiti minimi in both sets of studies. Results: Recordings obtained following stimulation of the nerve at the elbow demonstrated significant differences to those obtained following stimulation of the nerve at the wrist. Specifically, there was a left shift in stimulus–response curves at the elbow compared to the wrist, with prolonged strength-duration time constant, and reduced rheobase (P < 0.05). These changes were accompanied by increased refracto- riness and reductions in superexcitability and late subexcitability (P < 0.05). Conclusions: The present findings may suggest relative depolarization of ulnar nerve axons at the elbow. Significance: These changes may reflect regional differences in axonal Na + /K + pump function and thereby predispose the ulnar nerve to conduction failure and axonal degeneration when exposed to trauma. Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. 1. Introduction Entrapment of the ulnar nerve entrapment at the elbow is the second most common entrapment neuropathy. The most frequent site of entrapment is in the humeroulnar arcade, between the heads of the flexor carpi ulnaris muscle. Clinical features include paraesthesiae affecting the medial digits IV and V, with weakness of intrinsic hand muscles developing in more severe cases. While the prognosis of ulnar neuropathy at the elbow (UNE) remains var- iable, persistence of symptoms and ongoing disability is evident in 40–50% of patients at 2-year follow-up (Dunselman et al., 2008; Evoli et al., 2003). The key predisposing factor to the development of UNE relates to the superficial location of the nerve at this site (Dellon and Mackinnon, 1988). In addition to anatomical predisposition, it re- mains conceivable that regional differences in biophysical proper- ties may also contribute to the development of neuropathic processes (Bae et al., 2009; Krishnan et al., 2004). For example, length-dependent gradients in biophysical properties have been postulated to contribute to the development of distal neuropathy (Krishnan et al., 2004). These differences were demonstrated through the use of nerve excitability techniques, clinical tech- niques that provide information about axonal ion channel proper- ties and membrane potential (Bostock et al., 1998; Krishnan et al., 2008; Kuwabara et al., 2002; Park et al., 2009). Consequently, the present study utilized nerve excitability to investigate whether there were differences in ulnar nerve biophysical properties 1388-2457/$36.00 Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2010.04.038 Abbreviations: ADM, abductor digiti minimi; APB, abductor pollicis brevis; CMAP, compound motor action potential; FCR, flexor carpi radialis; SEM, standard error of the mean; UNE, ulnar neuropathy at the elbow. * Corresponding author. Address: Translational Neuroscience Facility, School of Medical Sciences, University of New South Wales, Sydney NSW 2052, Australia. Tel.: +61 2 93852756; fax: +61 2 93851485. E-mail address: arun.krishnan@unsw.edu.au (A.V. Krishnan). Clinical Neurophysiology 122 (2011) 194–198 Contents lists available at ScienceDirect Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph