Original articlesMotor Cortex Stimulation in the Treatment of Central and Neuropathic Pain
Introduction
Motor cortex stimulation has been proposed by Tsubokawa et al. (1) for the treatment of central pain. The study by Meyerson et al. (2) showed that trigeminal neuropathic pain could also be relieved by this technique, but patients presenting with central pain were not improved in that study. The variability of these results 1, 2 could be related to inaccurate positioning of the stimulation electrode. In our preliminary series of patients (3), results were improved in both central and trigeminal neuropathic pain with the use of different stimulation sites. Currently available neuronavigation systems allow a more accurate localization of the central region of the cerebral cortex and would help to further improve the clinical results. We report here on a group of 32 patients treated by chronic stimulation of the motor cortex. In the last 13 cases, neuronavigator guidance was used for localization of the motor cortex.
Thirty-two patients presenting with refractory central and neuropathic pain were treated by chronic stimulation of the motor cortex between May 1993 and January 1997. There were 18 males and 14 females, aged 21–77 years (mean 54 years). The mean follow-up was 27.3 months (3–50 months). Mean history of pain was 7.8 years (4–14 years).
Thirteen patients suffered from central pain secondary to deep brain hematoma (5 cases), thalamic infarct (4 cases), ischemic infarct of the parietal lobe (2 cases), thalamic abscess (1 case), and head trauma (1 case).
Twelve patients suffered from trigeminal neuropathic pain secondary to thermal rhizotomy (7 cases), ENT operation (3 cases), postoperative brainstem lesion (1 case), and skull base trauma (1 case).
Three patients had paraplegic pain and two suffered pain after plexus avulsion, one patient had a peripheral nerve injury (crural nerve), and one, intercostal postherpetic pain. Five patients were reoperated on (a total of 37 operations).
In all patients, pain had previously proved resistant to extensive pharmacotherapy. Thalamic stimulation had been tried but without benefit in three patients. All patients suffered from constant, severe burning pain. Spontaneous paroxystic pain was present in 22 cases and allodynia and dysesthesia in 17 cases.
All patients were evaluated by an independent neurologist specializing in the management of chronic pain and by a psychiatrist. Pain level was evaluated by a visual analog scale (VAS) graduated from 0 to 100 pre-operatively and every 3 months post-operatively. The effects of stimulation were classified into the following three categories: good, reduction of pain level by 70 to 100%; satisfactory, reduction of pain level by 40–69%, and poor, reduction of pain level by less than 40%.
Pre-operatively, all patients had severe pain with a mean score of 85.6 (±7.1) on the VAS. Two-tailed Wilcoxon signed rank tests comparing pre- and post-operative outcome were performed on VAS. In our first 24 patients, the technique initially described by Tsubokawa et al. (4) was used. Under local anesthesia, a four-pole contact electrode (Resume, Medtronics, Inc., Minneapolis, MN, USA) was introduced into the epidural space through a burr hole. The electrode was placed over the central sulcus region identified by a preoperative MRI examination. Somatosensory evoked potentials (SEP) with stimulation of the median nerve were recorded from the four electrode poles. The location of the central sulcus corresponds to the site of a phase reversal of the N20 of the evoked potential. To confirm that the electrode is correctly located, the contact presumably placed over the motor cortex is stimulated (pulse duration: 1 msec, 5 Hz trains) at low frequency (16–20 Hz) and at relatively high intensity (5–15 mA) to induce muscle contractions in the painful area.
Unfortunately, this technique often requires that the electrode be relocated before an optimal position is found. Moreover, the risk of epidural hematoma is considerably increased because the dura is detached over a wide area.
In our last 13 cases (8 new patients and 5 reinterventions), we used a new technique. Anatomical localization of the target was performed using a 3D image-guided navigation system (Elekta Instruments, Stockholm, Sweden). Anatomical reference points can clearly be seen on oblique or curved CT scan reconstructions. On these reconstructions, central, lateral, the interhemispheric, superior, and inferior frontal sulci can be clearly identified. The target can thus be easily located. The use of a flap craniotomy (40–50 mm diameter) allows more extensive electrophysiologic study. The center of the flap should correspond to the target as determined by imaging. SEP are recorded from the 16 contacts of a grid (Cormedica, Vaulx-Milieu, France) applied on the dura. The location of the central sulcus (phase reversal of the N20 wave) can be verified in real time on the CT images by using the pointer or the laser beam of the navigation system. The exact site where the four-pole contact electrode should be placed will depend on results of the electrophysiologic study (SEP and stimulation). We prefer the electrode to be placed perpendicular to the central fissure. The electrode is fixed to the dura with two stitches.
Section snippets
Stimulation parameters
In general, the parameters for chronic stimulation were as follows: frequency 40 Hz (25–55); pulse width 82.5 μsec (60–180), and amplitude 2.1 volts (1.3–4). In most cases, bipolar stimulation was applied with the negative pole placed over the motor cortex and the positive pole over the sensory cortex. Pain relief was often not reported as occurring until several minutes after starting the stimulation. Frequently, there was a long-lasting post-stimulation effect, sometimes amounting to between
Discussion
Stimulation of the motor cortex in the treatment of certain forms of refractory neurogenic pain has in recent years attracted much interest. Studies by Tsubokawa et al. 4, 5 and Meyerson et al. (2) followed by other authors 6, 7, 8, 9, 10, 11 have shown that this treatment modality may be effective in central and neuropathic trigeminal pain. Furthermore, it appears to also be effective in peripheral pain (3). The putative mechanisms of action of cortical stimulation have recently been discussed
Conclusions
Our results confirm that chronic stimulation of the motor cortex is an effective method in treating certain forms of refractory pain. Variability of results appears to be mainly related to the difficulty of identifying the appropriate cortical target area. Neuronavigator localization and guidance technique can help to improve the clinical results.
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