Blockage of pain by electrical spinal cord stimulation

BACKGROUND: Electrical spinal cord stimulation (SCS) is an alternative to conventional medication for chronic pain relief. Several hypotheses exist concerning the neurophysiological, vascular, and neurochemical mechanism behind SCS.METHODS: The excitation and blockade effects of the three common SCS waveforms (tonic, burst, and high-frequency stimulation) on the nerve fibers bypassing the region of the electrodes are analyzed in a computational study. The simula-tions are based on the model of Hodgkin and Huxley which is fitted to spike durations of 1 ms. RESULTS: SCS is a FDA approved technique for pain relief, but the mechanisms of action are still under investigation. The first element in the chain of mechanisms is the generation and the block of spikes in nerve fibers close to the stimulating electrode. For these "primary fibers" computer simulations showed that conventional SCS generates sharply synchronized spikes whereas the spread of the spiking times by burst stimulation is expected to cause the suppression of paresthesia. This rather uniform spread of spiking times (in comparison to tonic stimulation) is a consequence of more pulses (5 vs. 1), longer pulses, and increasing intensities within each train of 5 pulses.CONCLUSIONS: High-frequency stimulation can block the conduction of spikes but the distance of the fiber to the lead is a critical factor.