Indication: Neuropathic Pain
Motor cortex stimulation has been used for the treatment of central and neuropathic pain syndromes since 1991. It is best indicated for medically unresponsive central and neuropathic pain including that due to thalamic, putaminal, and lateral medullary infarction, traumatic trigeminal neuropathy (not idiopathic trigeminal neuralgia), postherpetic facial neuralgia, brachial plexopathy, and neuropathic pain after a spinal cord injury, and phantom limb pain. There have only been scattered case reports regarding motor cortex stimulation for complex regional pain syndromes. Cortical stimulation is not indicated for patients with a history of seizures. Personality disorders such as severe depression or psychotic disorders need to be screened out prior to using this procedure.
The motor cortex stimulation leads are placed on the dura surgically with the target selected on the primary motor cortex based on somatotopic anatomic landmarks. The optimal stimulation level is that which provides the best pain relief yet does not cause a seizure, pain from dural stimulation, or electromyographic activity. With improved mapping techniques for the epidural stimulation sites in 12 patients with medically intractable neuropathic facial pain, Nguyen reported good to excellent pain relief in 75% of the patients.
The precise mechanism for the motor cortex stimulation in relieving pain remains unknown. Studies have demonstrated that motor cortex stimulation leads to an increase in cerebral blood flow in the ipsilateral thalamus, cingulate gyrus, orbitofrontal cortex, and midbrain. The extent of pain relief correlates best, however, with anterior cingulate gyrus blood flow. Two other hypothesized mechanisms for pain relief after motor cortex stimulation include direct activation of inhibitory interneurons in the spinal cord or indirect inhibition during stimulation.