Predictive value of somatosensory evoked potentials for long-lasting pain relief after spinal cord stimulation: practical use for patient selection

OBJECTIVE: Spinal cord stimulation (SCS) has been used for more than 30 years in patients with intractable neuropathic pain, and global success rates have varied from 40 to 70%, according to reported series. Patient selection is currently based on a preliminary percutaneous test, which is useful but invasive, increases the risk of infection, and has yielded false-positive and false-negative results. In this study, we evaluated an alternative method of predicting the effectiveness of SCS before deciding whether to implant laminotomy electrodes-specifically, assessment of neural conduction in the dorsal columns with the use of somatosensory evoked potentials (SSEPs). Thus, we examined the value of preoperative central conduction time (CCT) of SSEPs to stimulation at the level of the painful area as a possible predictor of patient outcome after SCS. METHODS: Ninety-five patients were evaluated during a mean follow-up period of 18.8 months. Patients were classified into four categories according to the location of the lesion responsible for pain: 28 patients had lesions of the peripheral nerves, 27 had radicular lesions, 8 had root avulsions, and 32 had cord lesions. The SCS electrode was implanted through an interlaminar opening at the upper part of the painful territory without performing a percutaneous screening test. Clinical and social markers of pain relief (i.e., Visual Analog Scale scores, analgesic drug intake, work status) were evaluated prospectively 2 months after implantation and then annually. RESULTS: The global success rate in our study group, with success defined as at least 50% long-term pain relief, was 54.7% (52 of 95 patients). Statistical analyses showed a clear influence of preoperative CCT on SCS outcome. Thus, the success rate was nil in patients with significantly abnormal CCT, whereas it was 75.4% in patients with normal preoperative SSEPs. Significant differences between the two groups of patients also were observed with regard to medication intake and work status. CONCLUSION: Preoperative SSEPs provide an objective prediction of patient outcome after SCS. We suggest that if a patient's CCT is abolished or significantly altered, the patient should not undergo SCS.