Stroke (including cerebral hemorrhage and cerebral infarction) has a high morbidity, mortality and disability rate, and is one of the major diseases that seriously endanger people’s health, and some stroke patients have post-stroke pain (PSP), with an overall incidence of about 8%. The treatment of PSP is very difficult, using analgesic drugs, nerve blocks and other methods, but the efficacy is not satisfactory, has been a clinical treatment problem. In recent years, motor cortex stimulation (MCS) has been gradually used to treat this type of pain, and has achieved more satisfactory analgesic effects.
I. Morbidity characteristics of PSP
Stroke is mainly divided into hemorrhage and infarction, and there has been no definite conclusion as to which of hemorrhage or infarction is more likely to cause PSP. Since infarcts account for about 85% of all strokes, it seems that infarcts are more common in clinical practice. In fact, the key factor causing PSP is not the extent of the stroke, but more importantly the site of the stroke, which commonly causes PSP: the dorsolateral medulla, the thalamus, the posterior limb of the internal capsule, and the cortex or subcortex of the postcentral gyrus, with the dorsolateral medulla and the thalamus being the most common. In 1999, MacGowan et al. reported that the incidence of PSP was as high as 25% in patients with dorsolateral medullary infarction.
Clinical manifestations of PSP
1. Time of occurrence
PSP usually does not appear immediately after stroke, but mostly delayed, with about 50% to 60% occurring within a few days to one month after stroke.
2. Site of pain
PSP usually involves a large area, often involving half of the body, half of the torso or half of the head and face. If the stroke site is in the thalamus or the posterior limb of the internal capsule, PSP may appear in the entire half of the body contralateral to the stroke, including the head, face and trunk; or it may appear only in the contralateral trunk, excluding the head and face; or it may involve only the contralateral head and face, excluding the trunk. If the stroke site is in the dorsolateral aspect of the medulla oblongata, Wallenberg syndrome may occur, manifesting as pain in the ipsilateral head and face and contralateral trunk of the stroke.
3. Nature of pain
The nature of PSP can be burning, cutting, drilling, piercing, throbbing, pins and needles, tearing, and crushing. Among them, burning pain is the most common, with more than 60% of PSP patients presenting with burning pain, sometimes combined with one to two other pains.
The vast majority of PSP persists and tends to worsen progressively with the duration of the disease. In addition, multiple factors can cause paroxysmal pain exacerbation on the background of PSP persistence. For example, emotional changes, muscle contractions, limb movements, hot and cold stimuli, and even touch and wind can induce pain or aggravate pain.
4. Accompanying signs and symptoms of pain
In addition to pain, PSP is almost always accompanied by other positive neurological symptoms and signs, the most common being sensory abnormalities (mainly hypoesthesia and sensory hypersensitivity) and other possible limb paralysis, ataxia, swallowing and choking, hoarseness, diplopia, aphasia, positive cone fasciculations, etc. Leijon et al. reported that 100% of PSP patients had a combination of sensory abnormalities, limb paralysis and ataxia. The prevalence of limb paresis and ataxia was 48% and 58%, respectively.
III. Treatment rationale of MCS
The first case of MCS was reported by Tsubokawa et al. in 1991, who used MCS to treat 12 cases of central pain including PSP and achieved positive results. in 1993, Meyerson et al. reported that MCS was also effective in treating trigeminal nerve-derived pain. Since then, scholars have continued to apply the procedure for the treatment of various intractable pains, especially for PSP with good analgesic effects.
The specific analgesic mechanism of MCS is not yet fully understood, and the main reason why Tsubokawa et al. attempted MCS for pain treatment was based on their finding in animal experiments that the excitability of neurons in the caudal subnucleus of the trigeminal crestal nucleus was increased after cutting the trigeminal nerve, and stimulation of the motor-sensory cortex could inhibit this excitability, and the stimulation of the motor cortex produced stronger inhibition than that of the sensory cortex. The inhibitory effect was stronger in the motor cortex than in the sensory cortex. Similarly, the excitability of thalamic neurons is enhanced after cutting the crista thalamic tract, and stimulation of the motor cortex is able to inhibit it, and the inhibitory effect is stronger than that produced by stimulation of the sensory cortex. In addition, Lefaucheur et al. selected two patients with neuropathic pain in the upper extremity who were effectively treated with MCS after ineffective treatment by cremasteric electrical stimulation and used the originally implanted cremasteric stimulation electrodes as recording electrodes, and found that downstream specific waveforms could be recorded in the cremasteric plexus when the motor cortex was stimulated by MCS electrodes. With low-intensity anodal monopolar stimulation of the motor cortex, D waves could be recorded in the cremaster, indicating direct activation of cortical cremaster bundle fibers; with low-intensity cathodal monopolar stimulation of the motor cortex, I2 waves could be recorded in the cremaster, indicating indirect activation of cortical cremaster bundles across synapses; and with bipolar stimulation of the motor cortex, which has the best analgesic effect, trans-synaptic I3 waves of cortical cremaster bundles could be recorded in the cremaster. This suggests that the analgesic effect of MCS does not lie in the direct stimulation of the pyramidal tract, but is mainly due to the analgesic effect produced by the downstream inhibition of electrical stimulation in the subcortical transverse fibers or intermediate neuronal conduction.
IV. Surgical method of MCS
MCS is a procedure in which stimulating electrodes are embedded on the surface of the motor cortex to achieve analgesia through chronic electrical stimulation of the motor cortex. Stimulation electrodes are generally chosen to be buried in the contralateral transport cortex of pain, and the specific electrode burial site and method are chosen according to the relationship between the projection of the torso, head and face in the central anterior gyrus. For upper extremity or head and face pain, the electrodes should be placed on the lateral convex part of the contralateral precentral gyrus, and the electrodes should be placed on the epidural. For lower extremity pain, the electrodes should be placed in the contralateral precentral gyrus near the midline. Most of the electrodes need to penetrate deep into the longitudinal fissure to maintain good contact with the motor cortex, so subdural placement is preferable.
The key issue in MCS is how to accurately locate the motor cortex, and the following commonly used methods are generally used in combination to make a comprehensive judgment for localization.
(1) stereotactic frame localization.
(2) Median nerve somatosensory evoked potential N20 recordings, where phase reversal occurs in the central sulcus N20 wave.
(3) Functional MRI localization.
(4) Intraoperative neuroimaging navigation.
(5) intraoperative direct electrical stimulation of the motor cortex. Of these, the latter method is more accurate and practical, being able to induce muscle contraction in the contralateral limb and accurately determine the location of the motor cortex. Intraoperatively, the stimulation electrodes can also be directly connected to the stimulation generator for experimental electrical stimulation, which can determine both the location of the electrodes and the stimulation threshold for inducing muscle spasm or twitching in the contralateral limb, which can be used as the basis for postoperative chronic electrical stimulation treatment parameter tuning. Generally, the pulse generator is implanted at the same time, or the experimental electrical stimulation can be performed for 1~2 weeks first, and then the pulse generator is permanently implanted after it is really effective.
However, there is a wide range of stimulation parameters to choose from, and different scholars are accustomed to using different stimulation parameters, and the effective stimulation parameters for different patients are also different. The commonly used parameters are frequency 30-50 Hz, pulse width 210-300 microseconds, voltage 3-5 volts, and continuous stimulation. The postoperative analgesic efficacy fluctuates in most patients, but after several adjustments of the stimulation parameters, most of them can still obtain the exact analgesic effect, so the timely adjustment of the postoperative stimulation parameters needs to be taken seriously.
V. Therapeutic effect of MCS
The efficacy of MCS in treating PSP and trigeminal neurogenic pain is the most certain, Sindou et al. retrospectively analyzed 127 cases of MCS surgery, and the proportion of patients with PSP and trigeminal neurogenic pain with more than 50% pain relief more than 1 year after surgery was 2/3. This is consistent with the findings of foreign scholars, who concluded that the analgesic satisfaction rate of MCS for patients with pain without or with only mild limb weakness was 73%, while the efficiency of MCS for patients with moderate and severe limb weakness was only 15%.
In conclusion, MCS has the advantages of being reversible, adjustable, less traumatic, and with fewer complications, and is mainly applicable to neuropathic pain such as PSP, which has unique advantages over various destructive analgesic procedures and represents the direction and trend of pain treatment development.