Diagnostic and therapeutic role of IONM in recurrent delayed neural compromise during prone positioning for Occipito-Cervical fusion in ankylosing spondylitis.

BACKGROUND

Intraoperative neurophysiological monitoring (IONM) facilitates real-time assessment of neural integrity during high-risk spinal procedures, particularly those involving the craniovertebral junction and cervical spine. Delayed neurophysiological signal changes associated with patient positioning can arise from various etiologies, including mechanical traction, ischemia, and compressive neuropraxia. These risks are further exacerbated in individuals with ankylosing spondylitis (AS) due to inherent spinal rigidity.  CASE PRESENTATION: A 46-year-old male, a known case of AS, with post-traumatic C1-C2 instability and fixed thoracic kyphosis underwent occipito-cervical fusion with continuous multimodal IONM. Baseline and immediate post-positioning signals were normal. However, approximately 30-40 min after prone positioning, a progressive decline in right lowerlimb somatosensory evoked potentials (SSEPs) was observed, followed by attenuation and eventual loss of motor evoked potentials (MEPs). Signals recovered completely upon returning the patient to the supine position, confirming a position-related yet reversible neural conduction compromise. A similar episode recurred upon re-positioning, resolving again after targeted cervical realignment. The procedure concluded without further incident, and the patient emerged neurologically intact.

CONCLUSION

This case highlights the phenomenon of delayed, position-related neurophysiological deterioration during prone spinal surgery. While AS likely contributed to reduced tolerance of the spine to positional stress, other mechanisms such as regional ischemia must also be considered. A near instantaneous reversibility was evidenced despite delayed, protracted and near complete loss of IONM signals. Continuous IONM enabled early recognition and timely intervention, thereby preventing permanent neural injury, emphasizing the importance of vigilant monitoring even during the presumed "non-critical" stages of surgery.