F wave duration in mild S1 radiculopathy: comparison between the affected and unaffected sides.

OBJECTIVES: The aim of this study was to investigate the F wave duration (Fdur) of the tibial nerve in mild S1 radiculopathy. We evaluated the difference in this parameter between the affected and unaffected sides, and discussed the clinical significance of this difference. METHODS: Bilateral tibial F waves were obtained from 46 normal subjects (control group) and 27 patients with L5/S1 intervertebral disc herniation (patient group). Minimum latency of F wave (Fmin) and Fdur were analyzed. RESULTS: Fmin and Fmin corrected by the subject's height (Fmin/H) were both significantly longer on the affected side than on the unaffected side and in the normal group, but the incidence of abnormality was very low for both parameters. Even patient showing normal Fmin or Fmin/H sometimes exhibited Fdur values prolonged beyond the reference range. Judgment of abnormal Fdur based on the difference between the two sides was the most sensitive method for detecting the root injury. No patients showed decreased Fdur. There was no significant difference between the two groups in the amplitude of compound muscle action potential of the abductor hallucis. CONCLUSIONS: Our study suggested that evaluation of the difference in Fdur between the two sides was a valuable means of decreasing false-negative results in F wave study in mild S1 radiculopathy cases.