Short communication Posterior antebrachial cutaneous nerve conduction study in radial neuropathy Y.L. Lo a, * , K.M. Prakash a , T.H. Leoh b , Y.E. Tan b , Y.F. Dan b , L.Q. Xu b , P. Ratnagopal a a Department of Neurology, National Neuroscience Institute, Singapore General Hospital, Outram Road, Singapore 169608, Singapore b Department of Neurology, Neurodiagnostic Unit, Singapore General Hospital, Singapore Received 31 December 2003; received in revised form 29 April 2004; accepted 3 May 2004 Available online 20 June 2004 Abstract Radial neuropathy most commonly occurs as a result of external compression at the spiral groove region. The posterior antebrachial cutaneous nerve (PACN) conduction study was performed in 15 consecutive patients with radial palsy. Unilateral PACN abnormalities were present in 11 patients. A normal PACN study was correlated with clinical improvement at 3 months. Conversely, PACN abnormality was correlated with radial motor axon loss and a poorer prognosis. The PACN study is a simple adjunct which provides additional information relating to the diagnosis and prognosis of radial lesions. D 2004 Elsevier B.V. All rights reserved. Keywords: Posterior antebrachial cutaneous nerve; Radial neuropathy; Nerve conduction study; Diagnosis; Prognosis 1. Introduction Radial neuropathy, together with the carpal tunnel syn- drome and ulnar neuropathy, are commonly encountered upper limb entrapment neuropathies [1]. The most common site of injury is at the spiral groove of the humerus [2], usually as a result of fracture or external compression. However, electrophysiological localization of this entrap- ment neuropathy may not be straightforward. The posterior antebrachial cutaneous nerve (PACN) is a sensory branch of the radial nerve from the level of the spiral groove. It is formed from cell bodies mainly in the C7 dorsal root ganglion. Nerve fibers traverse the middle trunk and posterior cord of the brachial plexus [3]. As its origins lie in close proximity to the spiral groove, we evaluate the use of its nerve conduction study (NCS) in radial nerve lesions at this location. 2. Methods Over a 2-year period, 15 patients (4 females; age range: 18 to 63) presenting consecutively with unilateral radial nerve palsy were prospectively entered into the study. All patients underwent routine (NCS) and electromyography (EMG) in the evaluation of unilateral radial neuropathy. Routine NCS included median, ulnar, superficial radial sensory and radial motor studies. For superficial radial sensory studies, the active recording electrode was placed over the ‘anatomical snuffbox’ region at the extensor pollicis longus tendon. The reference electrode was placed over the metacarpal head of the index finger. Antidromic stimulation was applied at the lateral border of the radius 12 cm proximal to the active recording elec- trode [4]. For radial motor studies, extensor indicis surface record- ings were obtained from stimulating at mid-forearm, 4 cm above the lateral humeral epicondyle and mid-upper arm above spiral groove, using a 0.5 ms square-wave pulse. The muscle was palpated to ascertain the active recording elec- trode position, with the reference placed 3 cm distally. Conduction block was defined as 50% drop in compound muscle action potential (CMAP) amplitude across proximal and distal sites. Comparison was made with the opposite side for absolute CMAP amplitudes. A 50% reduction in ampli- tude was defined as an axon loss lesion in the affected side [5]. For PACN conduction studies, antidromic stimulation was performed at the elbow 2 cm medial to the lateral epicondyle, between the triceps and biceps muscles. The 0022-510X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jns.2004.05.004 * Corresponding author. Tel.: +65-63265003; fax: +65-62203321. E-mail address: gnrlyl@sgh.com.sg (Y.L. Lo). www.elsevier.com/locate/jns Journal of the Neurological Sciences 223 (2004) 199 – 202