Comparison of sympathetic neurons in orofacial and upper extremity nerves: implications for causalgia

This study compared nerves of the orofacial region with nerves frequently associated with causalgia to determine if there is a significant difference in the proportion of sympathetic neurons within these nerves, which may account for the lower incidence of orofacial causalgia. Three orofacial and two upper extremity nerves were examined. Each nerve was transected and labeled with horseradish peroxidase to identify the cell bodies of neurons contributing axons to the nerve. The study included two trials per nerve, for a total of 10 trials in eight cats. The trigeminal and dorsal root ganglia, containing sensory neurons (SN), and the stellate, middle, and superior cervical ganglia, containing postganglionic sympathetic neurons (PGSN), were sectioned and reacted with tetramethyl benzidine to visualize the labeled neurons. The total number of labeled PGSN and SN were counted and the ratio (PGSN:SN) determined for each of the five nerves. The average PGSN:SN ratio from upper extremity nerves (0.40) is 2 1/2 times greater than the ratio determined for branches of the trigeminal nerve (0.16). The lower proportion of sympathetic neurons within the trigeminal nerves provides an anatomic explanation for the lower incidence of orofacial causalgia consistent with the currently accepted etiology.