Intraoperative Neuromonitoring for Spinal Surgery

wake-up test is rarely performed unless suspicion for significant neurologic injury is high, and intraoperative evaluation is deemed to be absolutely necessary.4 IONM began in the 1970s as direct evaluation of the dorsal column through SSEP assessment.6 Nuwer et al7 established the clinical efficacy of intraoperative SSEP monitoring during scoliosis surgery in a multicenter survey of 51,263 spine surgeries. The authors concluded that SSEP detection of postoperative neurologic deficits had an overall sensitivity of 92.0% and specificity of 98.9%.7 In addition, neuromonitoring was associated with a decreased rate of neurologic deficits. However, no definitive data supported this assertion. Merton and Morton8 were the first to excite cortical neurons with high-voltage transcranial electrical stimulation to activate contralateral motor activity. Establishing TcMEPs facilitated direct intraoperative monitoring of anterior column motor activity.8 The development of TcMEPs enabled evaluation of the corticospinal tract, spinal cord interneurons, anterior horn cells, and peripheral nerves. IONM modalities allow spine surgeons to monitor the integrity of the central and peripheral nervous systems continuously in real time to prevent, minimize, or reverse neurologic injury. The aim of this article is to provide an overview of current IONM modalities and their applications in cervical, thoracic, and lumbar spine surgery. potentials (SSEPs), and transcranial motorevoked potentials (TcMEPs) to assess the integrity of the central and peripheral nervous systems.1 IONM has a variety of applications in other surgical subspecialties such as urology, otolaryngology, endocrinology, intracranial neurosurgery, interventional neuroradiology, vascular surgery, and orthopedic surgery, including pelvic fracture internal fixation.1 The Stagnara wake-up test was one of the earliest IONM tests to be used in spine surgery.2 This intraoperative test requires a gradual reduction of anesthesia until the patient is able to move both the upper and lower extremities voluntarily. Assessment of the primary motor cortex, anterior motor pathways of the spinal cord, nerve roots, and peripheral nerves through the wake-up test allows detection of gross intraoperative motor changes.3 However, fine motor changes or abnormalities are not readily identified using this test. Furthermore, proper assessment requires optimal patient and anesthesiologist participation and evaluation by a physician who is not participating in the procedure.4 Repeated wake-up tests have been associated with decreased interperformance reliability, increased risk of air embolism, self-extubation, patient recall of events, sterile field contamination, and patient positional changes that may lead to neural compression.5 The limitations of the Stagnara wake-up test led to the development of other modalities. Currently, the LEARNING OBJECTIVES: After participating in this activity, the spine surgeon should be better able to: 1. Understand a few of the various modalities available in neuromonitoring as it relates to spine surgery. 2. Explain the uses and limitations of spine surgery intraoperative neuromonitoring. 3. Describe general management and evaluation of patients with intraoperative neuromonitoring abnormalities.