Repetitive nerve stimulation for the evaluation of peripheral nerve hyperexcitability.

OBJECTIVE To examine the utility of repetitive nerve stimulation (RNS) in the evaluation of peripheral nerve hyperexcitability (PNH). BACKGROUND PNH describes a group of disorders characterized by muscle cramps, twitching and stiffness. When severe, PNH may be characterized by the presence of continuous muscle fiber activity on routine needle electromyography (EMG). In milder forms of the disease, nerve hyperexcitability may be evidenced by the presence of after-discharges or cramp potentials following RNS. METHODS Fifty-four patients were prospectively recruited and classified into one of three groups-PNH, other neuromuscular disease and controls. We recorded and quantified the after-discharges and cramp potentials following RNS at 1, 5, 10 and 30 Hz. RESULTS The proportion of nerves with after-discharges and/or cramp potentials was significantly greater in the PNH group than the control group at both 5 Hz (p=0.03) and 10 Hz (p=0.01), as well as in the neuromuscular disease group compared to controls at 5 Hz (p=0.02). There was also a significant concordance between complaints of muscle cramps and fasciculations and the finding of after-discharges and/or cramp potentials at both 5 Hz (p=0.005) and 10 Hz (p=0.004). At a stimulation frequency of 10 Hz, the sensitivity of RNS for the diagnosis of PNH (primary or secondary) was 79% and the specificity was 88%. CONCLUSION Our findings suggest that RNS at or below a stimulation frequency of 10 Hz (when positive) is a useful test for the diagnosis of PNH, whether it is primary or secondary.