The Value of F-wave in the Diagnosis of Cervical and Lumbosacral Root Compression Syndromes

The F-wave is a small muscle potential which follows the direct M-wave on supramaximal stimulation of a mixed nerve. Many authors have convincingly demonstrated that the F-wave travels at first from the stimulating point via motor fibers to the spinal cord and activates the anterior horn cell antidromically, which results in the delayed muscle potential. The F-wave changes can thus reveal the lesions of proximal nerve segments which are not accessible to conventional nerve conduction studies. The present study was undertaken in search for possible changes of the F-wave parameters in spinal root compression syndromes. In patients with unilateral cervical or lumbosacral root compression syndromes, F-waves were elicited in the corresponding myotomes. As compared to the intact sides, the minimum-latency of the F-waves was significantly delayed (P<0.05) in the presence of S1, L5 and C8/Th1 root compression syndromes. On the other hand, the delay of the minimum-latency of the F-wave recorded from the thenar muscles by the stimulation of median nerves did not statistically correlate with C7/C8 syndromes, probably because multisegmental innervations to the thenar muscles may be more distinct than to other muscles. Furthermore, the limits of the minimum-latency differences in normal subjects were calculated at the 5% level. Using these limits, the false positive rate may be 5%, and the false negative rate may range from 12.5% (Ulnar N. F-wave) to 36.9% (Tibial N. F-wave).