Axillary F central latency: simple electrodiagnostic technique for proximal neuropathy.

A simple technique is described for measuring the F-wave conduction time of the proximal segment of peripheral nerves. This should prove helpful in the diagnosis of a proximal neuropathy or myelopathy. The F wave (Fw) and M response at the wrist (Mw) and the M response at the axilla (Ma) can be used to calculate the conduction time, defined as Axillary F Central Latency (AFCL), between the spinal cord and the axilla, covering a horizontal distance of 25cm twice. The calculated AFCL (AFCL = (Mw + Fw) - 2Ma) in our control group of 30 normal subjects was 11.2 +/- 0.8 msec (mean and 1 standard deviation) for the ulnar or median nerve. An AFCL of more than 13.6 msec (mean plus 3SD), or a discrepancy of the AFCL greater than 2.1 msec in the same nerves between the 2 sides or between the median and ulnar nerves on the same side suggests an abnormality involving the proximal nerve segment. The prolonged AFCL has been found in patients with Guillain-Barré syndrome (GBS), brachial plexus lesions, syringomyelia and, in a few cases, with thoracic outlet syndrome (TOS).