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STUDY DESIGN Retrospective study. OBJECTIVES To analyze intraoperative neuromonitoring (IONM) alerts in various surgical procedures and clarify incidences and causes of IONM alarms in consecutive adult spinal deformity (ASD) surgeries. SUMMARY OF BACKGROUND DATA ASD surgery has a high neurologic complication rate. IONM may play a role in identifying and preventing neurologic complications. METHODS This study included 275 consecutive ASD patients treated by posterior corrective fusion who had been followed up for more than two years. We divided the patients into 1) the PCO group: multiple posterior column osteotomies; and 2) the 3CO group: three-column osteotomy including pedicle subtraction osteotomy and vertebral column resection. We set a 70% amplitude reduction as the alarm point for transcranial electrical stimulation motor-evoked potentials (Tc-MEPs) using 32-channel IONM. RESULTS The PCO and 3CO groups included 162 and 113 cases, respectively. IONM revealed 32 cases (11.6%) of Tc-MEP alerts, 10.4% in the PCO group, and 13.2% in the 3CO group. Postoperative follow-ups revealed 15 cases (5.5%) of new neurologic deficits, 4.9% in the PCO group, and 6.2% in the 3CO group. Most IONM alarms in the PCO group appeared at the time of rod rotation maneuvers, and 88.9% of alarms were selective for MEP decrease. In contrast, IONM alarms in the 3CO group appeared at the time of spinal shortening, and 80% were global MEP decreases. Immediately after the alarm, neurologic deficits might be rescued by foraminal decompression after rod rotation and by adjusting the length of spinal shortening. Overall, more than 50% of cases with IONM alerts were rescued by intraoperative additional management. CONCLUSION IONM reduced the incidence of neurologic complications in ASD surgery. Spinal surgeons should recognize the type of muscle derivation and respond to such alerts by performing appropriate corrections reflecting the mechanism underlying the neural damage. LEVEL OF EVIDENCE Level IV.