Neurophysiologic changes after preganglionic and postganglionic nerve-root constriction: an experimental study in the rat.

STUDY DESIGN: We investigated changes in spinal somatosensory-evoked potential (SSEP) and nerve action potential (NAP), correlated behavior, and associated pathologic observation in experimental radiculopathy. OBJECTIVES: To create a rat model of sacrococcygeal radiculopathy for determining the validity of SSEP and NAP. SUMMARY OF BACKGROUND DATA: We examined the diagnostic sensitivity and value of electrophysiologic tests for evaluating lumbosacral root disease conflict. An appropriate animal model can help verify the value of these tests. METHODS: Preganglionic lesion group rats were given 2 loose ligatures around the cauda equina at the sacrum, and postganglionic lesion group rats were given 2 loose ligatures on the conjunction of the sacrococcygeal nerve roots and the caudalis nerve after they had received a laminectomy. Control group rats received a sham operation. SSEPs and NAPs were recorded preligature and postligature, and 3 times after surgery. These electrophysiologic observations were compared and correlated with tail-flick reflex and histology. RESULTS: All experimental group rats developed thermal hyperalgesia on day 14, as indicated by a significant reduction in TFL (tail-flick latency), which continued for 3 months. Amplitude decreased significantly and latency increased significantly in all SSEP recordings immediately after the operation; these changes persisted for 3 months. There were no significant differences between the experimental groups, but there were significant differences between the control and experimental groups. NAP amplitude and latency from the caudalis nerves did not change in any group in the first 2 postoperative weeks. From the second postoperative week until the 3-month follow-up, amplitude was significantly decreased and latency prolonged in the postganglionic group but unchanged in the others. CONCLUSIONS: Both SSEP and NAP are useful for evaluating electrophysiologic changes after various radiculopathies. The data also suggest that the conductivity of the peripheral nerve (NAP) was affected by the postganglionic compression of the corresponding nerve root, but not by the preganglionic lesion.

Spinal somatosensory evoked potential to evaluate neurophysiologic changes associated with postlaminotomy fibrosis: an experimental study.

STUDY DESIGN: We evaluated electrophysiologic changes of the cauda equina after lumbar laminotomy in rats. OBJECTIVE: To clarify immediate and long-term electrophysiologic and neurologic responses in an experimental postlaminotomy animal fibrosis model. SUMMARY OF BACKGROUND DATA: Postspinal surgery-induced epidural fibrosis is assessed using either Gadolinium- enhanced magnetic resonance imaging (MRI) or intraoperative observations. In experimental animal models mimicking this complication, many approaches are used: advanced imaging (computed tomography, CT; and MRI), functional observations, biomechanical techniques, and histologic examinations. However, no study has reported the substantial neurophysiologic changes of the cauda equina in such a model. METHODS: Rats were given a sham operation (laminar exposure only), a left L5 hemilaminotomy alone, or a left L5 hemilaminotomy with extradural topical collagen. Mixed-nerve-elicited somatosensory-evoked potentials (M-SSEPs) and dermatomal (D)-SSEPs were recorded at the thoracolumbar junction after percutaneous stimulation of the posterior tibial nerve at the bilateral medial ankles and the L5 dermatomal field, respectively. Potentials recorded on the operated and nonoperated sides before surgery and then 30 minutes, 2 weeks, and 1, 2, and 3 months after surgery were compared. Walking track and thermal hyperalgesia test results and a final histologic analysis of perineural fibrosis were correlated. RESULTS: Electrical stimulation yielded reproducible responses in all rats on all tests. Preoperative and postoperative measurements showed no statistically significant differences in M-SSEP or D-SSEP. Postoperative D-SSEPs in both experimental groups showed significant reductions in relative amplitude, but the M-SSEPs of all groups and D-SSEPs of the control groups remained constant. CONCLUSION: SSEP is valuable for detecting electrophysiologic changes after laminotomy fibrosis, but acceptable accuracy requires proper stimulation and recording settings. D-SSEP monitoring provided reliable, useful information about the functional integrity of the cauda equina in this animal model. We recommend D-SSEP monitoring as a supplemental tool for quantifying the effect of postlaminotomy fibrosis on neuropathy.

Value of dermatomal somatosensory evoked potentials in detecting acute nerve root injury: an experimental study with special emphasis on stimulus intensity.

STUDY DESIGN: Dermatomal somatosensory-evoked potentials (D-SSEPs) in rats were recorded at the spinal level after L2-, L4-, and L5-dermatome stimulation. Pre- and post-transection patterns and rates of change of corresponding nerve roots were compared to determine accuracy. OBJECTIVE: To investigate characteristics and normal values of D-SSEP elicited from lower limb dermatomes; to determine specificity, sensitivity, and utility of D-SSEP in detecting single-nerve root injury; and to determine optimal stimulation intensity. SUMMARY OF BACKGROUND DATA: D-SSEP allows assessment of single nerve root-specific pathways, electrodiagnosis of lumbosacral radiculopathy, and intraoperative neuromonitoring. Unacceptably low sensitivity and specificity make its value suspect. D-SSEP is insufficiently documented. METHODS: Eight rats were used to specify a standard D-SSEP waveform and its characteristics, evaluate stimulation sites and strengths, and determine appropriate stimulation and recording techniques. The L4 nerve root was transected in one group of 8 rats and the L5 in another. D-SSEPs were recorded at the thoracolumbar junction following submaximal and supramaximal stimulation at the L2, L4, and L5 dermatomal fields. Potentials recorded before transection, and immediately, 1 hour, and 1 week post-transection were compared. RESULTS: Reproducible spinal responses were obtained in all rats on all tests. Stimulation intensity, but not rates, affected amplitude. Relative amplitude reductions in transected-root D-SSEP were larger using submaximal than supramaximal intensity. D-SSEP elicited by submaximal than supramaximal intensity produced fewer false negatives and false positives. CONCLUSIONS: D-SSEP is valuable for detecting acute single nerve root injury. In clinical settings, submaximal dermatomal stimulation identifies conduction abnormalities more consistently and with fewer false negatives and false positives than does supramaximal stimulation. We recommend submaximal stimulation.