Motor Cortex Stimulation for Intractable Pain

Conclusions

There is little doubt that MCS provides excellent relief in carefully selected patients with a variety of neuropathic pain syndromes. Nevertheless, despite the knowledge that has been developed regarding this technique, many unanswered questions remain ( Table 5 ). The mechanism(s) of action of MCS are far from clear. Studies in which positron emission tomography was used in patients undergoing MCS have provided some insight into potential mechanisms, and several theories have been proposed. It has been possible to study the mechanisms of vagal nerve stimulation by using interleaved blood oxygen level-dependent fMR imaging, and it may become feasible to apply this technique to MCS in the near future. Certainly continued advancements in our understanding of the effects of MCS will undoubtedly lead to improvements in the clinical application of this technique in the future.

So far, MCS has been applied mostly for central poststroke and trigeminal neuropathic pain. However, many other indications have emerged and continue to evolve. Larger numbers of patients will be needed to determine the ideal clinical indications for this therapy. There is some suggestion that preoperative pharmacological testing may have a predictive value relating to who will respond to MCS, and it would therefore seem relevant for anyone performing MCS to consider pharmacological testing. The application of TMS preoperatively may also prove useful in predicting outcome after MCS.

One of the major unanswered questions regarding long-term MCS concerns selection of optimal stimulation parameters. As can be seen from the foregoing discussion, there is wide variability in stimulation parameters. It will be necessary to test multiple different parameters systematically in a large number of patients to determine if in fact a set of standard stimulation parameters even exists. Is there an optimal stimulation frequency? Should the stimulation amplitude be set at a percentage of motor threshold or at a relatively lower level? How often should stimulation be performed and for what duration? We may well discover that a standard set of stimulation parameters will not be effective in a majority of patients and that optimal parameters will need to be determined on an individual basis. It will be interesting to determine if there is any correlation between the stimulation parameters and the underlying pain condition. So far, most patients have undergone MCS in which a single quadripolar electrode is used. It may be that multiple electrodes will prove to be more effective, particularly for patients with relatively large areas of pain. Also, the question remains whether there is a better way to determine optimal electrode location, perhaps using a multicontact electrode grid for testing and then placing the permanent electrode(s) in the area(s) from which stimulation produced the best analgesic response.

Despite the many unanswered questions, it would seem that MCS will become an increasingly important tool in the armamentarium of the neurosurgeon who treats patients with difficult chronic pain problems.

Neurosurg Focus. 2006;21(6) © 2006  American Association of Neurological Surgeons