Harrison TB and Benatar M (2007) Accuracy of repetitive nerve stimulation for diagnosis of the cramp–fasciculation syndrome. Muscle Nerve 35: 776–780

Cramp–fasciculation syndrome (CFS) has been suggested to be caused by hyperexcitability of peripheral nerves. Reasons for this hypothesis include an overlap in symptomatology of CFS and acquired neuromyotonia, and the fact that afterdischarges and cramp potentials following repetitive nerve stimulation (RNS) occur more readily in patients with CFS than in individuals without this condition. Conclusive evidence for this hypothesis, however, is lacking.

In a retrospective cohort study, researchers from Emory University, Atlanta, GA, examined peripheral nerve excitability in 108 consecutive patients who had been evaluated with posterior tibial RNS. Among this patient population, 36 individuals met clinical criteria for the diagnosis of CFS. RNS results were classified as normal or abnormal by two investigators on the basis of the presence or absence of afterdischarges, cramp potentials or continuous motor unit activity. At all stimulation frequencies (1, 2 and 5 Hz), abnormal RNS was significantly more frequent among those with CFS than among those without this syndrome (P <0.002 for all). The specificity of abnormal RNS for the diagnosis of CFS was highest (76%) when abnormal RNS was defined on the basis of afterdischarges, cramp potentials or continuous motor unit activity at 1 Hz; however, sensitivity was highest (83%) when abnormal RNS was defined on the basis of abnormalities at any frequency. Receiver operating characteristic curve analysis revealed that tibial RNS correctly classified CFS in 75% of subjects. These results indicate that CFS is a form of abnormal peripheral nerve excitability, and that RNS might be a clinically useful test for CFS.