The Role of Multimodal Intraoperative Neurophysiological Monitoring in Preventing Neurological Injury During Complex Spinal Deformity Surgeries: A Case-Based Analysis

Case Presentation: A 17-year-old male presented with progressive gait disturbances, lower limb weakness and sensory deficits over the past 6 months. Preoperative MRI revealed severe leftsided scoliosis with convexity at D8 level. Multimodal IONM, including somatosensory evoked potentials (SSEPs), transcranial motor evoked potentials (TcMEPs) and free-run electromyography (EMG), was employed throughout the procedure. Despite preexisting deficits, postoperative assessment showed 85% deformity correction without any new neurological impairment.

Conclusion: Multimodal IONM is indispensable in complex spinal deformity surgeries, particularly in patients with preoperative neurological compromise. The integration of SSEPs, TcMEPs and EMG enhances the efficacy of intraoperative spinal cord monitoring. These minimizes the risk of neurological injury by allowing surgeons to adjust surgical techniques accordingly bestowing them with the flexibility and precision needed to optimize patient outcomes.

Keywords: Idiopathic scoliosis; Neurological complications; Intraoperative neurophysiological monitoring

Introduction
Scoliosis is a structural spinal deformity defined as the curvature of spine in the coronal plane of Cobb angle more than 10 degrees accompanied by a variable degree of rotation of the spinal column. Progression of curvature during periods of rapid growth can result in significant abnormality, which may be even accompanied by cardiopulmonary compromise¹. Idiopathic scoliosis is scoliosis for which there is no definite etiology, unlike neuromuscular, congenital or syndromic types. The prevalence of adolescent idiopathic scoliosis is estimated to be 2-3% but only 10 percent of these patients require treatment^2-5. Surgical Correction for scoliosis is often required for severe cases with significant curvature progression or neurological involvement^6,7. However, the risk of neurological injury during deformity correction is welldocumented, with reported rates ranging from 0.5% to 17%^8,9. The risk of neurological injury during scoliosis correction is multifactorial, arising from mechanical stress, vascular compromise and altered spinal cord dynamics. Overcorrection or excessive distraction may lead to ischemia, while tethering effects can further predispose patients to neurological deficits^10,11. Intraoperative neurophysiological monitoring (IONM) has emerged as a critical adjunct in spine surgery to mitigate spinal cord distress thereby significantly reducing the incidence of postoperative neurological deficits^12,13. IONM helps assess the integrity of neural structures during complex spine surgeries by real time monitoring of spinal cord function employing multiple modalities including somatosensory evoked potentials (SSEPs), transcranial motor evoked potentials (TcMEP) and electromyography (EMG). This allows surgeons to adjust surgical techniques accordingly bestowing them with the flexibility and precision needed to optimize patient outcomes^14,15.

This case report showcases the critical role of multimodal IONM in a 17-year-old male with idiopathic scoliosis and preexisting neurological deficits undergoing deformity correction by Ponte osteotomy and pedicle screw fixation. By continuously assessing intraoperative electrophysiological responses and analyzing postoperative outcomes, we demonstrate the effectiveness of multimodal IONM in enhancing surgical safety, enabling real-time intervention and preventing further neurological deterioration in complex spinal deformity surgeries.

Case report
A 17-year-old male presented with progressive difficulty in walking, lower limb weakness (right > left) and sensory deficits over six months. Neurological examination revealed reduced muscle strength in the lower limbs, with hip flexors graded at 2/5 and distal muscles at 4/5. Reflexes were brisk, with exaggerated knee jerk (3+) and ankle jerks accompanied by well sustained clonus (4+) bilaterally. The Babinski sign was positive bilaterally and sensory deficits were observed corresponding to L3 dermatome and below. Upper limb muscles were normal with normal tone and reflexes. Preoperative MRI revealed severe left-sided scoliosis with a maximum convexity at D8 level. (Figure 1).

Figure 1: Preoperative MRI revealed severe left-sided scoliosis with maximum convexity at D8 

The patient underwent Ponte osteotomy at multiple levels by removing the spinous processes, laminae and facet joints, followed by pedicle screw fixation from D3 to L4. After rod placement, convex compression followed by concave distraction was applied, achieving 85% correction of the scoliotic curve. Multimodal IONM was employed throughout the surgery to monitor spinal cord integrity. Both preoperatively and perioperatively, SSEPs were well-formed in the upper limbs but poorly formed or absent in the lower limbs. TcMEPs were recordable from the abductor pollicis brevis and rectus abdominis but non-recordable from the lower limb muscles (vastus lateralis, tibialis anterior, abductor hallucis longus and anal sphincter), consistent with preexisting deficits. During rod fixation, a transient decrease in rectus abdominis amplitude (<50%) was noted, but no significant intraoperative alarms were triggered. Free-run EMG revealed no neurotronic or abnormal discharges, suggesting the absence of nerve root irritation or injury. Anesthetic induction was done using intravenous agents, Fentanyl 140 mcg and Propofol 140 mg and was maintained with air, oxygen, isoflurane (MAC 0.2-0.3) and propofol infusion. Lighter plane of anesthesia was maintained throughout the surgery by monitoring the train-of-four stimulus (TOF). Hemodynamic parameters remained stable throughout the surgery. 

Postoperative outcome
The patient was monitored closely postoperatively. Clinical examination on POD 7 revealed no new neurological deficits were observed and preoperative symptoms remained stable. Radiographs confirmed 85% deformity correction (Figure 2). The integration of IONM allowed real-time assessment of spinal cord function, enabling proactive intervention and ensuring optimal surgical outcomes.

 Figure 2: Free Run and TcMEP IONM recording during surgery

Discussion
The prevention of neurological injury during spinal deformity correction requires a multimodal approach, particularly in patients with preexisting deficits^9,10. SSEPs assess dorsal column function, TcMEPs evaluate corticospinal tract integrity and EMG detects nerve root irritation or injury¹⁶. In this case, the absence of lower limb SSEPs and TcMEPs preoperatively underscored the severity of neurological compromise, necessitating vigilant intraoperative monitoring¹⁷. The transient decrease in rectus abdominis amplitude during rod fixation was promptly identified and continuously monitored which enabled intraoperative adjustments to prevent potential postoperative deficits (Figure 3).

 
Figure 3: X-Ray Image showing 85% correction of the spinal deformity postoperativel 

Extensive clinical evidence supports the role of multimodal IONM in reducing postoperative neurological deficits in scoliosis surgery^18,19. In a landmark multicenter study, Nuwer, et al²⁰. demonstrated a significant reduction in paralysis risk with the integration of IONM, reinforcing its indispensable role in deformity correction procedures. Additionally, Thirumala, et al²¹. found that the combination of SSEPs and TcMEPs improved diagnostic accuracy in detecting intraoperative spinal cord distress. Our findings align with these reports, reinforcing the necessity of real-time intraoperative monitoring. Notably, in this case, preoperative TcMEPs were absent in the lower limbs, emphasizing the severity of the patient’s neurological compromise. Despite this, careful monitoring and intraoperative adjustments facilitated successful correction without exacerbating neurological deficits, further validating the efficacy of IONM in high-risk cases.

Despite the benefits, IONM has limitations, including variability in interpretation and false- positive or false-negative results²². Factors such as anesthetic variability, patient factors such as positioning and body temperature and underlying pathology may influence signal reliability²³. Anesthetic management is critical for IONM reliability, as high-dose inhalational agents can suppress SSEP and TcMEP signals. TIVA with propofol and remifentanil is preferred for signal preservation, while precise neuromuscular blockade titration maintains EMG responsiveness^24-26. Standardization of monitoring protocols and improved signal-processing algorithms could further refine its diagnostic precision. Also, studies have emphasized the fact that the combined use of SSEPs, TcMEPs and EMG enhances diagnostic precision, mitigating these challenges^27,28. 

Conclusion
This case report underscores the indispensable role of multimodal IONM in mitigating neurological injury during complex spinal deformity surgeries. The integration of somatosensory evoked potentials (SSEPs), transcrania motor evoked potentials (TcMEPs) and electromyography (EMG) facilitates real-time assessment of spinal cord function, enabling early intervention and optimizing patient outcomes. Given its demonstrated efficacy, multimodal IONM should be regarded as a standard of care in scoliosis surgery and should be routinely adopted, particularly in patients with preexisting neurological deficits.

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