Despite its use for a long time, the way thalamic ventrobasal (VB) stimulation acts to produce pain relief is still unknown. One of the most accepted hypotheses, sponsored by Tsubokawa among others, proposes that VB stimulation excites raphespinal and reticulospinal neurons of the rostroventral medulla which in turn send respectively inhibitory serotonergic and noradrenergic axons through both dorsolateral funiculi (DLF) to the dorsal horn ( DH) nociceptive neurons; this pathway would be the same as is involved in periventricular-periaqueductal gray (PVG-PAG) stimulation induced inhibition of DH nociceptive neurons. This hypothesis implicates the necessity of DLF intactness; in fact, it was showed that section of bilateral DLF inhibits the response of DH nociceptive neurons to VB stimulation. If the above mentioned hypothesis is correct, one could expect that unilateral VB stimulation would produce bilateral pain relief, VB and PVG stimulation would be useful for treating the same modalities of pain and that in patients with central cord-based pain harboring complete cord transection, VB stimulation would not work at all. In order to check these possibilities, the patiens with central cord-based pain admitted to the Division of Neurosurgery, Toronto Hospital between June 1978 and July 1991 to undergo deep brain stimulation (DBS) were reviewed. Sixteen patients were operated on. Based on clinical criteria, four out of these sixteen patients were thought to present complet cord transection (all four were men, with an average age of 48 years and pain secondary to cord injury). The effectiveness of the procedure was evaluated in this subset of patients: 75% of them enjoyed excellent pain relief with VB stimulation; PVG stimulation, however, performed in three out of these four patients, did not produce pain relief. Besides, our clinical experience has demonstrated that VB stimulation is effective in treating only contralateral pain.These results, as well as certain experimental data provided by a review of the literature, seem to provide evidence enough to contest Tsubokawa's hypothesis.
pain; analgesia; electrical stimulation therapy; thalamic nuclei; thalamus; stereotaxis