Definition

Nerve injury following spine surgery appears to be a rare event, but can result in permanent and disabling outcomes. From stabilization of traumatic injuries to the delicate removal of tumors, indications for spinal surgery vary and can lead to different patterns of postoperative neural injury, compromising the spinal cord, individual nerve roots, or even terminal nerves of the brachial plexus. Postoperative complications can range from mild sensory changes to more serious central cord syndrome, Brown-Sequard syndrome, and quadriplegia.

Recognition

Without intraoperative monitoring, nerve injury during spine surgery cannot be identified until the patient is awake after anesthesia. This is often too late for corrective measures. However, intraoperative neurophysiologic monitoring (IONM) allows for real-time assessment and detection of impending neural injury. IONM alerts the surgeon when neural structures are at risk and allows for corrective actions. Overall, IONM attempts to improve the overall safety of spine surgery and limit major, debilitating nerve injury.

IONM consists of various modalities to monitor spinal cord integrity, including SSEPs, tcMEPs, and electromyography (EMG). More specifically, SSEPs allow for the continuous monitoring of the ascending dorsal column–medial lemniscus sensory pathway after the stimulation of a peripheral nerve. However, SSEPs do not monitor the anterior portion of the spinal cord, specifically the corticospinal tracts. To monitor these motor pathways, tcMEPs are intermittently checked at critical moments during the procedure by sending an electrical stimulus through the scalp to stimulate distal muscles groups of the upper and lower extremities. For a more comprehensive survey of spinal cord integrity, tcMEPs can be combined with continuous SSEPs to monitor both the anterior and posterior portions of the spinal cord. Finally, EMG, either spontaneous (free-run) or triggered, allows for more accurate monitoring during surgery around nerve roots and during pedicle screw placement. Although tcMEP can monitor motor function during nerve root surgery, significant overlap in muscle groups does not allow for the isolation of specific nerve roots that can be achieved using EMG.

Although IONM allows for the real-time assessment of neural integrity with the goal of improved safety, standardized recommendations for clinical use do not exist due to a lack of high-quality studies and randomized controlled trials. The use of IONM is common in complex spine surgery, but its overall use is not uniform throughout the spectrum of spine surgery. Although the debate remains about the overall cost/benefit analysis of IONM improving clinical outcomes, this monitoring modality does provide intraoperative feedback of neural integrity in an effort to decrease the incidence of debilitating postoperative nerve injury.

Risk Assessment

Of the varied etiologies of perioperative spinal cord injury, surgical trauma appears to pose the greatest risk for postoperative nerve injury. Mechanisms of injury include direct nerve trauma, compression, traction, insertion of hardware, and major spine geometric changes seen during scoliosis surgery. In addition, other perioperative factors can contribute to neural injury, including improper patient positioning, shoulder taping, hypotension, blood loss, and airway manipulation.

Of great importance is the patient presenting with known cervical spine instability or myelopathy. In this situation, extreme caution must be taken during airway manipulation to prevent pathologic motion of the cervical spine. However, most postoperative injuries to the cervical spine appear to occur in the absence of instability, and instead occur in the patient with cervical spine disease ranging from anatomic abnormalities to inflammatory processes ( Table 50.1 ). These conditions either lower the threshold for cervical spine injury or make airway management more challenging, increasing the risk of intraoperative cervical cord injury. A thorough assessment of these conditions should be ascertained and appropriate imaging studies (i.e., flexion/extension films, computed tomography scan) reviewed before proceeding with spine surgery. Identifying at-risk patients in the preoperative period allows for the implementation of advanced monitoring and airway techniques in an effort to limit and prevent devastating neural injury.

In addition to spinal cord injury, certain conditions increase the risk of perioperative peripheral nerve injury, including diabetes mellitus, renal disease, hypothyroidism, alcohol abuse, vitamin deficiencies, and malnutrition. When choosing the appropriate intraoperative position for these patients, one must consider the risks of peripheral nerve injury versus the benefits of adequate surgical exposure.

Implications

Prevention of perioperative nerve injury during spine surgery influences many aspects of anesthesia practice, including airway management, IONM, and anesthetic choice. Airway management of cervical spine instability, regardless of etiology, requires careful preparation and advanced equipment. These patients are at high risk for perioperative spinal cord injury, and airway manipulation must limit pathologic cervical motion that can lead to critical cord compression. Although there are no definitive standards for the safest approach to successful tracheal intubation, awake fiberoptic intubation remains the safest method. In patients deemed to have a stable cervical spine in the setting of significant pathology, advanced airway techniques should be used based on physical examination findings and pertinent imaging studies. Studies have shown that in patients with critical cervical stenosis there is an increased susceptibility to injury from even minimal neck extension.

When using IONM, the specific type of anesthesia must be carefully planned. Nearly all medications used in current practice interfere with IONM ( Table 50.2 ). Both SSEPs and MEPs are polysynaptic signaling pathways that rely on stable homeostasis for proper function. SSEPs are more robust than tcMEPs, and if used as a sole monitoring technique, a standard balanced general anesthetic with potent volatile agents (approximately 50% minimum alveolar concentration [MAC]) and muscle relaxation can be instituted. When using tcMEPs, TIVA without neuromuscular blockade appears to be the best approach. Both volatile anesthetics and TIVA can be safely used with EMG monitoring, but neuromuscular blocking drugs must be avoided. With all IONM techniques, a stable, background anesthetic should be achieved before critical surgical maneuvers.

TABLE 50.2

Effects of Common Anesthetics on IONM During Spine Surgery

Drug Name	Drug Class	Effect on SSEPs	Effect on MEPs
Propofol	GABA agonist	↓	↓
Etomidate	GABA agonist	↑↑	−
Midazolam	Benzodiazepine	↓	−
Ketamine	NMDA antagonist	↑	−/↓
Fentanyl	Opioid	−/↓	↓
Remifentanil	Opioid	−/↓	−/↓
Sufentanil	Opioid	−/↓	↓
Dexmedetomidine	α 2 -Agonist	−/↓	−/↓
Isoflurane	Inhalational	↓↓	↓↓↓
Sevoflurance	Inhalational	↓↓	↓↓
Desflurane	Inhalational	↓↓	↓↓
N 2 O	Inhalational	↓↓	↓↓

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