Indications
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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.
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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.
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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.
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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.
Contraindications
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Patients with severe motor weakness in the affected region, because it is believed that an intact corticospinal tract originating from the motor cortex is necessary for effective pain relief.
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Patients with neuropsychologic limitations to being able to participate fully in the evaluation and treatment process (including adequately communicating with health care workers about the effectiveness of alterations in their stimulation parameters) and patients with affective disorders, including severely depressive or neurotic tendencies.
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Patients with preexisting epilepsy or seizure disorders.
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The implantation of the MCS system is an elective surgical procedure that should be undertaken only when the patient’s health status is stable; any active infection or uncontrolled comorbidities that would increase anesthesia risks (e.g., thrombocytopenia, leukopenia; renal, hepatic, and cardiac failure) should be addressed before considering surgery.
Planning and positioning
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McGill Pain Questionnaire and Visual Analogue Scale scores should be documented preoperatively to provide a baseline for tracking a patient’s postoperative progress.
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Preoperative computed tomography (CT) and magnetic resonance imaging (MRI) of the brain facilitate identification of the central sulcus, sylvian fissure, and inferior and superior frontal sulci. Functional MRI studies may identify the motor cortex area. Frequently after a stroke there is a reorganization of motor areas that does not follow the usual somatotopic organization.
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Routine preoperative laboratory work should be performed, including prothrombin time, partial thromboplastin time, and platelet counts.
Figure 43-1:
The patient is positioned supine or in a lateral position with the side contralateral to the pain upward, with an axillary roll under the same shoulder to prevent venous congestion.
Figure 43-2:
Linear incision is planned to overlie the central sulcus using standard anatomic landmarks on the side contralateral to the patient’s pain. A neuronavigation system is used for planning the incision and to map out the craniotomy to overlie the motor cortex. The incision should be long enough to allow for a circular craniotomy opening with an approximate diameter of at least 5 cm.Related posts:
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