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Intraoperative neurologic monitoring (IONM) for adolescent idiopathic scoliosis (AIS) is recommended in order to decrease the risk of neurologic deficit. While combined use of somatosensory (SSEP) and motor (MEP) evoked potentials increases sensitivity and reliability of signals in the AIS population and thereby increases patient safety, the absence of preoperative signals can pose a challenge to any surgeon.
In case of the absence or weakness of MEP/SSEP signals at any time in the procedure, technical issues should be ruled out first. Specifically, patient positioning should be checked to ensure that all pressure points are padded and that the upper extremities are not hyper-abducted. Additionally, patient temperature, and blood pressure should be evaluated to rule out hypothermia and hypotension, respectively.
If technical troubleshooting is unsuccessful and satisfactory preoperative signals are unable to be obtained in scenarios where gas is being utilized for anesthesia, we recommend converting to TIVA. If there is still no signal with TIVA, a Stagnara wake-up test may result in improved signals as the sedation decreases. Continued absence of signals, however, may lead to termination of the procedure and necessitate further preoperative testing.
Intraoperatively, decreased signals after exposure prior to instrumentation may signal an accumulation of anesthetic agents such as propofol, or may be secondary to intraoperative blood loss. In cases where weakened signals are seen after instrumentation, this is likely a true neurological event. For cases in which decreased or absent neuromonitoring signals are seen during any point in the procedure, the patient should be admitted to the surgical floor or PICU postoperatively.