Evaluation of sympathetic cholinergic response in spinal cord injury: Is a subset of SCI pain sympathetically maintained?

Pain after spinal cord injury (SCI) may be sympathetically maintained. As a preliminary investigation of this hypothesis, we report experiments using Sympathetic Skin Response (SSR) and Skin Conductance Response (SCR) as modalities for testing sympathetic cholinergic integrity in two patients with thoracic spinal cord injury. The presence of a measurable sympathetic response provides useful information about the activity of neuronal tracts in the intermediolateral cell column and the hypothetically undamaged sympathetic chain that supplies specific sympathotomes. SSR and SCR were both elicited and measured using an electrical stimulus at the glabella. SSR was measured at palmar/plantar surfaces to the dorsum of each distal limb. SCR was measured at the index and middle fingers and second and third toes bilaterally. These modalities were used in conjunction with traditional bedside testing and patient medical records. Both study participants are shown to have SCI with complete sensorimotor loss below their injury level (subject A at T7, subject B at T9). Both also report chronic pain, although with markedly different characteristics, suggesting distinct pain processes. Subject A had complete absence of cholinergic sympathetic activity in the distal lower extremities while subject B exhibited a normal sympathetic response in the feet. These preliminary experiments suggest that the preservation of sympathetic efferents may be involved in generating sympathetically maintained pain in a subset of individuals with SCI. The potential pathophysiological implications of these results and the technical challenges of testing sympathetic response in this population will be discussed.