Root Ganglia and Chronically Injured Axons: a Physiological Basis for The: R a D I C U L a R Pain of N E R V E Root Compression

The radicular pain of sciatica was ascribed by Mixter and Barr to compression of thespinal root by a hernia~d intervertebral disc. It was assumed that root compression produced prolonged firing in the injured sensory fibers and led to pain perceived in tho peripheral distribution of those fibers. This concept has been challenged on the basis that acute peripheral nerve compression neuropathies are usually painless. Furthermore, animal experiments have rarely shown more than several seconds of repetitive firing in acutely compressed nerves or nerve roots. It has been suggested that "radicular pain" is actually pain referred to the extremity through activation of deep spinal and paraspinal nociceptors. Our ex.periments on cat lumbar dorsal roots and rabblt sural nerves have confirmed~that . . . . . . . acutecompression of the root or nerve does not produce more':than: severM seconds of repet~.~ive firing. However, long periods of repetitive firing (5--25 rain): follow minimal acute compression cf the normal dorsal foot, ganglion. Chronic injury of dorsal roots or sural nerce produces a: marked"incre~e in mechanical sensitivity; several minutes of repetitive f ir ingmay f011owacute compression of such chronically injured sites. Such prolonged responses could be evoked repeatedly in a population of both rapidiyand sl0wly conducting fibers. Since mechanical compression of either the dorsal root ganglion or of chronically injured roots can induce prolo aged repetitive firing in sensory axons, we conclude that radicular pain is due to activity in the fibers appropriate to the area of perceived pain.