Acute effects of nucleus pulposus on blood flow and endoneurial fluid pressure in rat dorsal root ganglia

Study Design. An experimental study to elucidate the initial factors in the pathogenesis of lumbar pain caused by disc herniation. Objective. To evaluate the effects of autologous nucleus pulposus on blood flow and endoneurial fluid pressure in dorsal root ganglia. Summary of Background Data. Human sciatica is known to be associated with c ompression of lumbar nerve roots and dorsal root ganglia by herniated inter vertebral discs. Recently, it has been shown that application of nucleus pu lposus to nerve roots induces injury and pain-related behavior in experimen tal animals. In this study, the authors hypothesized that nucleus pulposus applied to a nerve root would cause increased intraneural edema and reduced blood flow in the corresponding dorsal root ganglia. Studies in peripheral nerves have shown that these initial pathophysiologic disturbances initita te complex events that exacerbate nerve injury and cause pain. Methods. A total of 29 adult female Sprague-Dawley rats weighing 200 to 250 g had their left L5 nerve roots and associated dorsal root ganglia exposed . Autologous nucleus pulposus was harvested from the tail and applied to th e L5 nerve root just proximally to the dorsal root ganglia (nucleus pulposu s group). For control, the same volume of muscle was harvested from the sur gical area in the back and applied similarly to the neural tissue (control group). Blood flow was continuously monitored using a laser Doppler flow pr obe for 3 hours (n = 10) or 4 hours (n = 8) in animals with indwelling cann ulas for measurement of systemic arterial pressure. Endoneurial fluid press ures were recorded with a servonull micropipette system using glass micropi pettes with tip diameters of 4 mu m. Endoneurial fluid pressure in the dors al root ganglia was measured before and 3 hours after application of nucleu s pulposus (n = 7) or muscle (n = 4). After measurement of blood flow and e ndoneurial fluid pressures, the nerve root and dorsal root ganglia were pro cessed for histology and evaluated by light microscope. Results. Blood flow in the nucleus pulposus group was reduced by 10% to 20% from the initial value after 3 to 4 hours. This reduction was statisticall y significant compared with that of the control group (P < 0.01). Endoneuri al fluid pressure was initially 2.6 +/- 1.2 cm H2O in the nucleus pulposus group, and 2.1 +/- 0.6 cm H2O in the control group. Three hours after appli cation, endoneurial fluid pressure was 7.5 +/- 4.6 in the nucleus pulposus group (P < 0.05), and 2.0 +/- 0.8 in the control group (P < 0.05). Edema wa s the principal pathologic finding seen consistently in the nerve roots and in many of the associated dorsal root ganglia from animals treated with nu cleus pulposus.