Multimodal intraoperative monitoring techniques are, at the current level of care and technology, considered mainstay. They allow ongoing evaluation of the functional nature of various neural pathways as well as clear identification of vital neural structures. This approach enhances the likelihood of a more favorable postoperative outcome for various neurosurgical, orthopedic, vascular, and other respective or ablative procedures. Intraoperative monitoring involves a multidisciplinary effort with coordinated input from anesthesiology, neurophysiology, and the operating surgical staff. Different modalities are available to monitor, continuously, important anatomic pathways and to assure the proper identification of eloquent neural tissue, which will be discussed in detail within this article.
Historical note and terminology
Intraoperative neurophysiological assessment has become an integral part of certain surgical procedures. It can be divided into two basic activities: (1) monitoring is the continuous “on-line” assessment of the functional integrity of neural pathways, and (2) mapping is the functional identification and preservation of neural structures (110). Among other modalities, somatosensory-evoked potentials (SSEPs) and transcranial motor-evoked potentials (TcMEPs; either magnetic or electrical) are of distinct clinical benefit and additively advantageous.
Somatosensory-evoked potentials are obtained by averaging the electrical signals generated by multiple electrical stimulations of peripheral nerves. These measurements are made over the scalp and skin on the neck and the back (over the spinous processes of the vertebrae) to monitor these signals in the cortex and spinal cord. Median nerve stimulation at the wrist or posterior tibial nerve stimulation at the popliteal fossa or medial malleolus (ankle) is commonly used. This convention is in accord with upper extremity somatosensory-evoked potentials (UESSEPs) and lower extremity somatosensory-evoked potentials (LESSEPs), respectively. Intraoperative somatosensory-evoked potentials are obtained, by repetitive measurement, via the elicitation of time-locked waveforms in order to monitor functionality of the posterior column-medial lemniscus (PCML) pathway, resulting in detection of possible impending spinal cord damage during surgery and in prevention of peri- and post-operative neurologic deficits.
Classically, the most common procedure for which intraoperative somatosensory-evoked potential monitoring is utilized is during the spinal deformity corrective surgery, primarily for scoliosis or kyphosis (44). Historically, before the use of intraoperative somatosensory-evoked potentials, the Stagnara “wake up” test was employed in patients undergoing spinal surgery (139). This test, still employed, involves lifting the patient's anesthesia level during the procedure and verifying intact lower and/or upper extremity motor function (ie, digit/phalangeal movements) before deepening the anesthesia level repeatedly, once again. Limitations include the lack of sensory information, the inability to obtain continuous information, inadvertent extubation, loss of patient positioning with the risk for injury, destabilization of vital signs, and the potential for psychologically unfavorable postoperative memory recall (101). The wake-up test can at most be administered a few times throughout the length of the surgical intervention. Uncooperative patients are also unable to participate in the wake-up test. Further, the critical time window for reversal of a deficit could be lost when waiting for the patient to awaken. Before intraoperative somatosensory-evoked potential monitoring was widely available, the incidence of postoperative paraplegia consequent to Harrington rod placement or other spinal instrumentation or distraction was 0.5% to 1.6% (77).
Intraoperative somatosensory-evoked potential monitoring has been performed in adults for many years, whereas in children, it became more widely used in the 1980s and early 1990s. One of the limitations of intraoperative somatosensory-evoked potential monitoring is that mainly the posterior-column somatosensory system is monitored; the dorsal spinocerebellar tract is monitored to a lesser extent. Experts in the field have looked for a means of evaluating the motor system, especially the corticospinal tracts, cytoarchitecturally situated more anterolaterally within the spinal cord parenchyma, during surgery. In the 1990s, two techniques had effectively been studied that attempted to monitor the motor system during central nervous system surgery: (1) magnetic stimulation and (2) electrical stimulation of the motor cortex or spinal cord (motor-evoked potentials, or MEPs) (79). Further, the combined approach (motor and sensory versus single modality, ie, motor or sensory) provides rapid detection of cord ischemia and other risk factors for postoperative neurologic sequelae during orthopedic spinal surgery or during thoracoabdominal aorta surgery. The neurosurgical team at University of Aachen, after reviewing their intra- and postoperative data, deduced that motor-evoked potentials monitoring was superior to somatosensory-evoked potentials monitoring in detecting impending impairment of the functional integrity of cerebral and spinal cord motor pathways during surgery. In the clinical operative setting whereby the SSEPs and the MEPs demonstrate stable signal activity over time, one can be reasonably reassured that the pyramidal tract function has remained, equally, intact. Another advantage of combined monitoring is that in the event one modality is rendered non-recordable, another remains functional and available (141). To recap, combined SSEP/TcMEP “multi-modality” monitoring provides a higher positive/negative predictive value than single-modality monitoring techniques (51).
The level of care for patients is enhanced, and indications toward improved outcomes have broadened. Intraoperative neuromonitoring was beneficial in guiding resection of “even” thalamic neoplasms, an otherwise presumed surgically inaccessible, deep-seated, crucial structure (16). Regarding descending or thoracoabdominal aortic repair, intraoperative neuromonitoring was found, surgically, to be possibly instrumental in the prevention of postoperative paraplegia (70).