Anodal block in F-wave studies.

OBJECTIVE

To determine whether F-wave results differ with the anode proximal or distal to the cathode, ie, if clinical anodal block exists.

DESIGN

Prospective study of 30 healthy volunteers undergoing nerve conduction and F-wave studies in one median nerve. A needle cathode electrode was used with a surface anode placed alternately proximal and distal to the cathode. The same electromyographer performed all studies with a Dantec Counterpoint machine.

RESULTS

F-wave latencies were essentially unaffected by distal versus proximal positioning. Minimum, maximum, and mean F-wave latencies correlated extremely highly (r=.973 to .988). For both F-wave and M-response latencies and amplitudes, differences between mean values obtained using the two methods were extremely small and were neither clinically nor statistically significant. The frequency of elicitation of F-waves may (p < .05) have been slightly (3.5%) lower when the anode was in the distal position.

CONCLUSION

Anodal block is not seen in F-wave studies when using needle electrode stimulations. Reversing the stimulator does not seem to be required. Further study with surface stimulating electrodes is underway to confirm results.