Motor Cortex Stimulation for Intractable Pain

Richard K. Osenbach, M.D.

Disclosures

Neurosurg Focus. 2006;21(6) 

In This Article

Surgical Technique

Motor cortex stimulation is performed as an elective two-stage procedure. Although the surgical technique for implantation of epidural motor cortex electrodes has evolved over time, there is no single accepted standardized technique. Early on, some surgeons used a simple bur hole technique for introduction of the electrode over the appropriate pain target.[15] In my group our opinion is that a single bur hole has a number of disadvantages, not the least of which is the potential for inaccurate electrode placement. It is difficult if not impossible with the limited exposure of a bur hole to perform the type of intraoperative electrophysiological localization that is critical to accurate electrode placement and success of the procedure. Additionally, because insertion of the electrode through a bur hole results in the dura mater being stripped away from the bone, the risk of an epidural hematoma may be greater because epidural tenting sutures cannot be placed. Consequently, most surgeons prefer a small craniotomy for electrode implantation. Using a linear incision and a small craniotomy results in a minimal increase in operating time and probably carries no more risk than bur hole placement. The significant advantages provided by the additional exposure far and away outweigh any added risks.

Notwithstanding the individual surgeon's preference in technique, the essential requirement is exposure and identification of the area of the motor cortex corresponding to the pain topography. For example, for treatment of facial pain, the electrode should be positioned over the area of the motor cortex where stimulation will elicit contraction of the facial muscles. The electrode is implanted contra lateral to the side of the pain. The best results for facial pain seem to be obtained when the target is identified anterior to the central sulcus just adjacent to or below the inferior frontal sulcus.[3] For treatment of upper-extremity pain, the optimal location is anterior to the central sulcus in the mid- to precentral region. Treatment of pain in the lower extremity and foot is technically more difficult be cause the cortical representation for the leg and foot lies in the interhemispheric fissure and is not directly accessible to epidural stimulation. To achieve lower-extremity stimulation more effectively, some surgeons have used subdural electrodes placed in the interhemispheric fissure. Although our group has had no personal experience with subdural electrodes, it would seem that migration would be more of a problem than with epidural placement. If subdural placement is used, it is important to perform a meticulous, watertight dural closure to minimize the risk of cerebrospinal fluid leakage.

Placement of motor cortex electrodes can be performed after the induction of either general or local anesthesia supplemented by intravenous sedation. General anesthesia may be easier on the patient and can facilitate the use of intraoperative frameless image-guided navigation techniques that are invaluable for anatomical localization. Nevertheless, in our department we have found it more difficult to elicit motor responses with epidural stimulation in patients in whom general anesthesia is induced. Volatile inhalational agents, even in very small amounts, are well known to abolish motor evoked responses. When ever feasible, our preference is to perform implantation of the electrode with the patient receiving monitored anesthesia care in which local anesthesia is used in combination with an intravenous propofol and remifentanyl injection. Despite preoperative counseling and reassurance, however, some patients simply will not tolerate an awake craniotomy or present difficult airway management problems. For these patients, MCS is performed using general anesthesia, and agents known to alter motor responses are carefully avoided.

The craniotomy for electrode implantation is performed with the patient supine and with his or her head secured in Mayfield pins and rotated toward the side of the pain. Just before the skin is incised, we administer a single dose of prophylactic intravenous antibiotics directed against skin flora. Prophylactic anticonvulsant drugs are given, be cause one of the risks of MCS is seizure. Many of these patients have already been receiving long-term therapy with an antiepileptic drug for pharmacological treatment of neuropathic pain, and in these cases whatever anti epileptic medication the patient was taking is continued into the postoperative period. Peripheral stimulating electrodes are placed to record median nerve SSEPs, and electromyography electrodes are inserted into appropriate target muscles to record the responses elicited by motor stimulation.

Three-dimensional frameless stereotactic neuronavigation systems have been increasingly used for surgical planning and anatomical target localization. In our department we use the Stealth neuronavigation system (Medtronic Sofamor-Danek), but any of the currently available frameless stereotactic guidance systems can be used. On the morning of surgery, fiducial markers are applied to the scalp and MR imaging is performed using 1-mm contiguous nonoverlapping slices. The imaging data are transferred into the computer workstation for surgical planning. We use the T2-weighted axial and sagittal MR images to identify the area of the scalp overlying the central sulcus, and a linear incision is outlined parallel to this landmark. The skin is anesthetized with a 50:50 mixture of 1% lidocaine with epinephrine and 0.5% bupivicaine. The incision should be long enough to provide sufficient exposure for a craniotomy with a minimum diameter of 5 to 6 cm (Fig. 3). The navigation system is again used to center the bone flap over the central sulcus, and once the bone flap has been turned, the location of the central sulcus is again confirmed using the neuronavigation system and marked on the dural surface.

Figure 3.

Intraoperative photograph showing a craniotomy measuring 5 to 6 cm in diameter and centered over the central sulcus as determined by frameless stereotactic navigation.

Recently, techniques such as fMR imaging and MEG have been used to improve the accuracy of anatomical target localization (Fig. 4).[17,29] Mogilner and Rezai[17] have used MEG combined with an image-guidance system for anatomical targeting of the precentral gyrus. These investigators have used an fMR imaging motor activation paradigm involving movement of the affected body part followed by MEG mapping of the motor and somatosensory cortices. Somatosensory mapping was performed by re cording and localizing the evoked responses elicited from tactile stimulation of the affected body part. Motor mapping of the upper extremity was performed using a repetitive finger-tapping paradigm with the evoked responses triggered by electromyographic activity. The sensory and motor responses were averaged, filtered, and then mapped onto the corresponding MR image by using standard MEG software. The fMR imaging data were then transferred to a neurosurgical image guidance system by using customized in-house software to perform the coordinate transformation necessary to merge these data with a standard stereotactic MR image, thus allowing the coordinates for the sensory and motor cortices of the affected body to be displayed within the standard surgical planning software.

Figure 4.

Illustrative fMR image. The data can be used for localization of specific motor areas, fused with frameless stereotactic anatomical data, and then used for direct targeting.

Although frameless neuronavigation techniques, either alone or combined with fMR imaging data, provide an elegant method of target localization, they do not completely supplant the use of electrophysiological target confirmation. Once the location of the central sulcus has been marked on the dural surface, electrophysiological testing is performed using SSEPs and motor stimulation mapping. First, median nerve SSEPs are recorded from a 4 3 4-electrode grid aligned parallel to the central sulcus, making certain the grid covers both the pre- and postcentral gyri. The central sulcus is localized by the phase reversal of the N20/P20 wave (Figs. 5 and 6). This confirms the location of the representation of the hand region in the precentral gyrus; the facial region is located more inferiorly. More specific localization of the facial region may be possible if trigeminal evoked potentials are used, although achieving clear responses from stimulation of the trigeminal system is technically more difficult. After the central sulcus has been identified, stimulation mapping is performed to identify the appropriate area of the precentral gyrus for electrode placement. Motor stimulation can be performed through the grid used for SSEP recording, with a bipolar stimulator, or through the electrode to be implanted. Stimulation is performed using monophasic pulses of 200- to 400-msec duration. The minimum amplitude required to produce motor activation is noted. We have occasionally required amplitudes of 20 to 30 mA to elicit motor contraction. In general, the amplitude needed to produce motor responses is higher when using epidural stimulation. Motor contraction can be elicited at relatively lower amplitudes when the procedure is performed after induction of local anesthesia. It has been suggested that stimulation be performed at a relatively low frequency (1 Hz) because motor responses elicited by high-frequency stimulation are susceptible to habituation, and high-frequency stimulation at intensities exceeding the motor threshold may be more likely to produce a seizure.[31] Compound muscle action potentials are recorded using needle or surface electrodes applied over the area of interest and muscle contractions are observed. Successful MCS depends to a large degree on placement of the electrode over the area from which muscle contractions can be obtained. In some cases, particularly in patients with severe hemiparesis or phantom pain, it may not be possible to elicit muscle contraction. In such cases, the electrode should be placed in the best possible location based on the available anatomical and physiological data that can be obtained.

Figure 5.

Tracings in which the typical N20/P22 phase reversal across the central sulcus is easily seen.

Figure 6.

A 4 × 4 electrode grid is used to record SSEPs elicited from contralateral median nerve stimulation.

After confirmation of the target, a four-contact plate electrode is placed on the dura mater and positioned perpendicular to and bisecting the central sulcus (Fig. 7). The electrode is sutured to the dura mater to prevent migration. Some authors[3] have advocated implantation of two electrodes to cover a wider area of the motor cortex in selected cases in which the region of pain may be more extensive. Although most surgeons orient the electrode perpendicular to the motor cortex, the optimal orientation is not known (Fig. 8). Indeed, some surgeons have implanted the electrode parallel to the orientation of the motor cortex, with good results. The electrode is connected to a temporary extension wire that is then tunneled out through a separate stab incision. The exit site of the temporary wires is kept well away from the potential location of the permanent extension cable in the event the trial is successful and the patient goes on to receive a permanent implant.

Figure 7.

Intraoperative photograph showing the permanent electrodes, which have been secured to the dura mater.

Figure 8.

Postoperative x-ray film showing the epidural electrodes.

Screening for effectiveness can begin once the patient is fully alert. There is no standardized protocol for screening, and the duration of the trial, frequency of stimulation, and stimulation parameters are highly variable from one investigator to another. At our institution we customarily perform a 1-week in-hospital trial. The anode and cathode are selected to provide optimal stimulation based on the results of intraoperative motor mapping. Stimulation is performed at a rate of 50 Hz and using a pulse width of 200 to 400 msec. The amplitude is adjusted to a level that is approximately 50 to 65% of the motor threshold. Stimulation is performed for 2 hours on and 2 hours off, and no stimulation is performed at night. The stimulation parameters are adjusted throughout the testing based on the analgesic response. Generally, at least a 50% reduction in baseline pain is desirable, although some patients obtain less than 50% pain reduction but clearly appear to benefit from the therapy. Unfortunately, the "50% benchmark" has traditionally been used for evaluating the effectiveness not only of MCS, but of nearly all implantable pain therapy devices. However, there are certainly patients who may attain functional improvement despite a somewhat less than 50% reduction in their baseline pain. Obviously, the decision to go forward with permanent implantation in this subgroup of patients needs to be carefully weighed on a case-by-case basis.

Assuming the trial is successful, the patient is returned to the operating room for internalization of the permanent system after induction of general anesthesia. The patient is placed supine with the head turned and the neck slightly extended, and the previous incision, neck, and anterior chest are isolated and draped. A portion of the prior incision is reopened, the connection of the electrode to the temporary extension wire is disconnected, and the latter is discarded. An incision for the IPG is made two fingerbreadths below and parallel to the clavicle, and a subcutaneous pocket large enough to accommodate the device is fashioned just above the pectoralis fascia. An extension wire with a low-profile connector is tunneled from the infraclavicular pocket to a small intermediate incision just behind the ear and connected to the intracranial electrode. It is important to locate the connector over the bone in the retromastoid region. If the connector is pulled more inferiorly into the soft tissues of the neck, it will probably break or erode through the skin with time. The extension wire is then connected to an IPG capable of accommodating either four or eight electrodes.

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