Motor Cortex Stimulator Placement

Indications

• Motor cortex stimulation (MCS) may be considered for patients with medically refractory deafferentation or neuropathic pain, including central pain syndromes related to stroke or, rarely, trauma or multiple sclerosis; trigeminal neuropathic pain (anesthesia dolorosa and postherpetic neuralgia); glossopharyngeal neuralgia; spinal cord injury; brachial plexus avulsion; and phantom limb or stump pain.

• Overall, efficacy is 40% to 70% for patients with refractory neuropathic pain, but identifying which patients would benefit from MCS is difficult because there is no definitive predictive factor. Likelihood of response to MCS may depend on the pain syndrome being treated or the anatomic location of the pain. Outcomes are better when patients present with no more than mild motor weakness in the region of pain. Patients with trigeminal neuropathic pain, phantom limb pain, and spinal cord injury have shown the most benefit, although at 1 year of follow-up, more than half of all patients, regardless of pain syndrome, responded to MCS.

• A proposed mechanism is that MCS activates descending axons, rather than apical dendrites or cell bodies, which is suggested by blood flow studies showing that the somatosensory cortex is not activated during stimulation. Cerebral blood flow studies show increased regional cerebral blood flow in the ipsilateral ventrolateral thalamus (the site where corticothalamic connections from the stimulated motor and premotor areas predominate) and in the medial thalamus, insula, subgenual cingulate, and brainstem as part of a cascading effect on a series of pain-related structures.

• Preliminary trials of limited duration MCS for poststroke recovery of motor function showed promise for improvement over rehabilitation alone. A phase III study sponsored by Northstar Neuroscience showed no advantage for the combination of concurrent invasive cortical stimulation and rehabilitation over rehabilitation alone. Initial investigations of MCS in patients with Parkinson disease showed promise for improvement in Unified Parkinson Disease Rating Scale III scores at 6 months, but most initial benefits were lost by the end of 12 months, and tremor was poorly controlled by MCS.