Reorganization of sensory modalities evoked by microstimulation in region of the thalamic principal sensory nucleus in patients with pain due to nervous system injury.

Stimulation of the somatosensory system is more likely to evoke pain in patients with chronic pain after nervous system injury than in patients without somatosensory abnormalities. We now describe results of stimulation through a microelectrode at microampere thresholds (threshold microstimulation; TMIS) in the region of the human thalamic principal sensory nucleus (ventral caudal; Vc) during operations for treatment of movement disorders or of chronic pain. Patients were trained preoperatively to use a standard questionnaire to describe the location (projected field) and quality of sensations evoked by TMIS intraoperatively. The region of Vc was divided on the basis of projected fields into areas representing the part of the body where the patients experienced chronic pain (pain affected) or did not experience chronic pain (pain unaffected) and into a control area located in the thalamus of patients with movement disorders and no experience of chronic pain. The region of the Vc was also divided into a core region and a posterior-inferior region. The core was defined as the region above a standard radiologic horizontal line (anterior commissure-posterior commissure line; ACPC line) where the majority of cells responded to innocuous somatosensory stimulation. The posterior-inferior area was a cellular area posterior and inferior to the core. In both the core and the posterior-inferior regions, the proportion of sites where TMIS evoked pain was larger in pain-affected and unaffected areas than in control areas. The number of sites where thermal (warm or cold) sensations were evoked was correspondingly smaller, so that the total of pain-plus-thermal (sensation of warmth or cold) sites was the same in all areas. Therefore, sites pain where stimulation evoked pain in patients with neuropathic pain (i.e., pain following an injury to the nervous system) may correspond to sites where thermal sensations were evoked by stimulation in patients without somatosensory abnormality.