Damage in peripheral nerve from continuous electrical stimulation: comparison of two stimulus waveforms

The propensity for two types of charge-balanced stimulus waveforms to induce injury during eight hours of continuous electrical stimulation of the cat sciatic nerve was investigated. One waveform was a biphasic, controlled-current pulse pair, each phase 50 microseconds in duration, with no delay between the phases ('short pulse', selected to excite primarily large axons), whereas in the second type each phase was 100 microseconds in duration, with a 400 microsecond delay between the phases (selected to excite axons of a broader spectrum of diameters). The sciatic nerve was examined for early axonal degeneration (EAD) seven days after the session of continuous stimulation. With both waveforms, the threshold stimulus current for axonal injury was greater than the current required to excite all of the nerve's large axons. The correlation between simple stimulus parameters and the amount of EAD was poor, especially with the 'short pulse' waveform, probably due to variability between animals. When the stimulus was normalised with respect to the current required to fully recruit the large axons, a good association between damage and stimulus amplitude emerged. The damage threshold was higher for the 'short pulse' waveform. The implications for clinical protocols are discussed.