The nerve root entry zone is highly vulnerable to intrathecal tetracaine in rabbits

It has been speculated that the nerve root entry zone in the spinal cord, known as the Obersteiner-Redlich zone, may be more sensitive to large concentrations of local anesthetics administered intrathecally. However, there has been no morphological evidence for this. In the present study, we examined morphological changes of nerve fibers at the nerve root entry zone after administration of intrathecal tetracaine in rabbits. Rabbits were assigned to 4 groups (n = 6 in each) and received intrathecal 0.3 mL saline (control), or 1%, 2%, or 4% tetracaine. Neurological and histopathological assessments were performed 1 wk after the administration. Tetracaine 1% selectively injured the myelin sheaths made by oligodendrocytes at the nerve root entry zones of both ventral and dorsal roots, although neurological dysfunction could not be detected. With tetracaine 2% and 4%, histopathological damage extended to the dorsal funiculus, distal part of roots, and cauda equina and neurological dysfunction became apparent. These results demonstrate that the myelin sheaths made by oligodendrocytes at the nerve root entry zone are highly vulnerable to large concentrations of tetracaine given intrathecally.