Inversion of the F-waves in Median Neuropathy at the Wrist (Carpal Tunnel Syndrome):
An Adjunctive Electrodiagnostic Method

by Daniel L. Menkes, M.D., Daniel C. Hood, M.D. and Anneke C. Bush Sc.D. M.H.S.

Published: 18 March 1997

The role of F-waves in the diagnosis of carpal tunnel syndrome (CTS) remains controversial. A comparison of ipsilateral-median and ulnar-nerve F-wave minimal latencies (FWML) usually reveals that the median F-wave is shorter than its ulnar counterpart, a pattern that should reverse in CTS associated with median neuropathy at the wrist. Prospective studies were undertaken on 30 controls and 57 patients with symptoms and electrophysiologic evidence of CTS. The mean median FWML minus the ulnar FWML value in the control group was ­0.74 msec, with a standard deviation of 0.677 yielding an upper limit of normal of 0.98 msec (mean, + 2.5 SD). In the cases examined, when the median F-wave was absent or the FWML exceeded a normal ipsilateral ulnar FWML by 1 msec, "inversion of the F-waves (FWIN) was documented. CTS was diagnosed when median palm-to-wrist test results were abnormal (compared with ulnar, or abnormal in absolute terms) in patients with symptoms of CTS. Inversion of the F-waves was present in 72 of 95 limbs in the patients, and none of the controls yielding a specificity of 100% or a sensitivity of 76%. The sensitivity of this test was superior to all standard tests of median motor-nerve function and greater than or equal to all other tests for CTS save the mixed-nerve palm-to-wrist comparison. We conclude that inversion of the F-waves is a useful adjunctive test in the diagnosis of CTS.

Keywords: Carpal Tunnel Syndrome, F-Waves, Median Nerve, Entrapment Neuropathy, Electromyography