Abstract
It has been reported that stimulation of the midbrain periaqueductal gray (PAG) is effective for relief of intractable pain. However, this procedure appears to have some disadvantages; i.e., technical difficulty of inserting electrodes exactly, side effects such as abnormal occular movement, and limitation of candidates because of the poor effect on pain in those with morphine-tolerance. Previously the authors reported that stimulation of the thalamic relay nucleus as well as of PAG produced long lasting, profound excitation of raphe-spinal neurons which then inhibit nociceptive dorsal horn neurons. Furthermore, naloxone (which is a specific antagonist of morphine-like substances) was shown to avoid the excitation induced by PAG stimulation, but not that by induced thalamic relay nucleus stimulation. Based on these findings, attempts were made to treat intractable pain with morphine-tolerance by stimulation of thalamic relay nucleus. Prevention of stimulation-tolerance was attempted by administration of monoamine precusors, i.e., l-DOPA and l-tryptophan, on the basis of the experimental observation reported previously. Chronic implantation of a stimulating electrode in the thalamic relay nucleus was performed in five cases which suffered from cancer pain (2 cases), thalamic pain (1 case), and paraplegic pain (2 cases). All of these cases enjoyed excellent relief of pain at least during the initial period. Only one case, which suffered from paraplegic pain developed stimulation-tolerance. Experiences in the other four cases suggested that administration of l-DOPA prevent the stimulation-tolerance. On the other hand, there was no evidence that administration of l-tryptophan interfered with progress of stimulation-tolerance. There were no operative complications and no side effects with this stimulating treatment.
In summary, stimulation of the thalamic relay nucleus, which is more safe and acurate when electrode insertion using electrophysiological monitoring is done during operation, is effective in relief of intractable pain, even with morphine-tolerance. Administration of l-DOPA seems to prevent stimulation-tolerance.
