Introduction: Somatosensory evoked potentials (SSEP) are routinely used intra-operatively to monitor integrity of neural pathways during spinal surgery. The aim of SSEP monitoring is the early identification of surgically induced neurophysiological changes and to allow for their prompt correction, thus reducing the risk of postoperative neurological sequelae.
Methods: In this retrospective study we examine 137 cases of corrective spinal deformities surgery over 5 years. A decrease of 50% in amplitude or an increase of 10% in latency of the SSEP waveform was used as a threshold for intervention. This incidence is referred to as "false positive" if the event is not followed by neurological deficit. In this case it is presumed that either the change was not sufficient for the spinal cord to develop a deficit or that the neurological damage has been reversed through intervention. There is no consistent way to differentiate between the two.
Results: A total of 46 separate incidences of an abnormal SSEP were reported, with surgeons intervening in 32 cases. No postoperative deficit was observed despite the traces of 11 patients in the non-intervened group not returning to acceptable levels. In 14 cases of the intervened group a wakeup test was done and despite all 14 patients having had a successful wakeup test, their SSEP traces remained at below acceptable levels for the rest of the surgery.
Conclusion: None of the 46 patients who had an abnormal SSEP had any post operative deficit. The incidence of false positives (33.6%) was rather high and our experience indicates that this term needs redefinition for intra-operative use. However, the analysis of the cause indicates that while a significant percentage may be attributed to known risks such as reduced segmental cord blood flow, cord distraction, concentration of inhalation anaesthetic, there is a number of unexplained incidents.
Daya A, Webb P, Graham S, Greenough CG, Papastefanou SL. Spinal Cord Monitoring: the term of "false positive" redefined. 13th European Congress of Clinical Neurophysiology. Istanbul, Turkey. May 2008.
Abdul Daya, Cleveland Unit, Middlesbrough. Tel: +44 (0)1642-854-778.
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