Increased Risk of Postoperative Neurologic Deficit for Spinal Surgery Patients with Unobtainable Intraoperative Evoked Potential Data

Introduction: Multi-modality evoked potential monitoring has become a standard of care during spinal surgery at many institutions. Patients with absent or unobtainable intraoperative data despite functional neurologic status may increase the risk for undetected neurologic injury and are not easily predicted. To date, a comprehensive review of this patient population has not been reported.

Purpose: To examine incidence and potential causality of absent SSEP and NMEP data & postoperative neurologic deficits in spinal surgery patients with primarily intact but not totally normal neurologic status (functional ambulators).

Methods: 4,402 consecutive orthopaedic spinal surgeries at one institution from 01/94 through 12/03 were reviewed. Cases lacking sufficient monitoring data, despite functional neural integrity (ambulators, intact sensation) were identified. Diagnoses were divided into 6 general categories: Neuromuscular scoliosis, pathologic spine disease, fracture, congenital deformity, idiopathic deformity & degenerative disease. Relationship between absent EP data and associated neurologic and/or medical pathology was evaluated.

Results: S: 59 out of 4402 cases (1.34%) had absent SSEP and/or NMEP intraoperative data despite functional neural integrity (46 ambulators/ 13 non-ambulators). Diagnostic prevalence was as follows: Neuromuscular scoliosis (Charcot-Marie Tooth, Freiderich’s ataxia, cerebral palsy; 22/324, 6.80%), congenital (congenital spine dislocation, achondroplastic dwarfism, hemivertebrae; n=11 /idiopathic AIS n=3 deformity 14/1099, 1.27%), pathologic spine disease (spinal column tumor, syringomyelia, epidural lesions; 9/141, 6.4%), degenerative spondylosis, stenosis; 9/2367, 0.38%), fracture (5/379, 1.32%). Some degree of pre-existing neurologic compromise was identified in 56/59 of cases (95%). The remaining 3 cases had totally normal neurologic examinations. The group was categorized by extent of missing data and ambulatory status (see table).

5.08% of study patients awoke with increased neurologic deficit (3/59); 2 global, paraplegic deficits (3.5%), 1 nerve root deficit (1.7%). The incidence of postoperative neurologic deficit in the entire surgical population was 0.75% (33/4402); 8 global deficits (0.18%), 25 nerve root deficits (0.57%). A Fisher Exact Test demonstrated a statistically significant difference between this incidence in the two populations, 59 vs. 4402, as a whole (P<.012) and the incidence of global paraplegic deficits (P< .006).

Discussion: Although we found only a small percentage of cases in whom we were unable to obtain data (0.68%), the incidence of post-operative neurologic deficit is significantly higher in this group (P <.012) than the population with intraoperative evoked potential data.

Conclusion : Unobtainable SSEP and/or NMEP data during spinal surgery is infrequent (1.34%). Certain underlying conditions significantly increase the chances for absent EP data (i.e. Charcot-Marie Tooth). Most significantly, patients with unobtainable data pose a much higher risk (P<.012) for post-operative neurologic deficits. Multiple Stagnara wake-up tests are strongly recommended when evoked potential data cannot be obtained.