Introduction Sympathetic system injury is a known but rare complication in scoliosis deformity correction. It is not common following posterior correction. We report a case of diastematomyelia with neuromuscular scoliosis with unusual complication of sympathetic outflow disturbance, after posterior instrumented correction. Case Description A 13-year-old girl presented with complaints of deformity in the back first noticed 4 years ago. Roentgenogram revealed a right thoracolumbar kyphoscoliotic deformity of 105° with apex at T8 with non-structural lumbar and cervicothoracic curves with positive sagittal alignment. Magnetic resonance imaging showed split-cord malformation with bony crest near the apex of the curve. Detethering followed by removal of the bony crest and restoration of the dual dural sleeves of the split cord into single neural tube was done in the first stage. In the second stage, pedicle screw fixation with was done from D3 to L3. Deformity correction was achieved using multilevel Smith Peterson osteotomy and concave rib osteotomy. On the second postoperative day, intensive care unit staff noticed persistent sinus tachycardia and profuse sweating in both upper limbs, chest, and upper-back. Twenty-four-hour Holter monitoring did not reveal any abnormality. Patient improved gradually and was discharged on postoperative day 9 when both sinus tachycardia and hyperhidrosis resolved. Conclusions Sympathetic chain disturbances after surgery recover with time. The exact time duration needed for recovery is not yet defined, however. Spine surgeons should be aware of this postsurgical complication and identify it so that management can be initiated. The symptoms may be long and drawn out, thus the roles of communication with and counseling of the patient as cannot be underemphasized.