Neurophysiologic intraoperative monitoring of scoliosis surgery.

The Scoliosis Research Society defines scoliosis as lateral deviation of the normal vertical line of the spine by at least 10 degrees, accompanied by rotation of the vertebrae within the curve. The degree of curvature is quantified by the Cobb angle, which is the angle formed by the intersection of lines perpendicular to the vertebral endplates at the superior and inferior margins of the curve. Scoliosis may be classified into three broad categories: congenital, neuromuscular, and idiopathic. Congenital scoliosis is due to bony spine anomalies present at birth, resulting in vertebral asymmetry and consequent abnormal curvature of the spine. Neuromuscular scoliosis occurs in patients with underlying neurologic disorders, including cerebral palsy, muscular dystrophy, spinal muscular atrophy, poliomyelitis, as well as spinal tumors and dysraphisms. The most common form, idiopathic scoliosis, in which the etiology is unknown, presents most commonly in adolescence ( 10 years of age), but may also present earlier, in juvenile cases (4–9 years of age) or in infantile cases ( 3 years of age). Idiopathic scoliosis is present in 2% to 4% of children older than 10 years of age (Roach, 1999). Progression of scoliosis may result in impaired mobility, pulmonary compromise, chronic pain, and emotional distress. In patients showing progression of scoliosis, bracing is typically the initial strategy employed to halt progression of the curvature. Surgical intervention is generally warranted in cases in which the curvature is 50%, or 40% in skeletally immature patients, or in patients in whom curvature is progressing despite bracing (Bridwell, 1999). Patients with congenital or neuromuscular causes of scoliosis, such as muscular dystrophy or cerebral palsy, may be at risk for more aggressive progression of scoliosis, and therefore may warrant earlier surgical treatment (Berven and Bradford, 2002; Cambridge and Drennan, 1987). Surgical correction is aimed at partial straightening of the curvature, stabilization of the spine, and cessation of further progression of scoliosis. Hardware is secured segmentally to the vertebral pedicles, followed by partial straightening, termed distraction, of the spine with rigid rods that are manipulated to achieve the desired degree of lesser curvature. Neurologic Risks Associated with Scoliosis Surgery Neurologic impairment, particularly paraparesis or paraplegia, is an infrequent, but potentially devastating complication of scoliosis surgery. As with other types of spine surgery, the spinal cord and nerve roots are at risk for mechanical injury as hardware is affixed to the vertebral pedicles. The likelihood of misdirection of hardware is probably higher in scoliosis surgery due to the abnormal curvature and rotation of the vertebrae, and the risk of spinal cord injury is probably higher because the spinal cord is often situated very close to the concave wall in the scoliotic spine. In addition, certain techniques require passing of sublaminar hardware through the epidural space, further exposing the spinal cord to potential trauma. The greatest risk of spinal cord injury during scoliosis surgery occurs during distraction of the spine. Injury may result from stretching or compression of the cord, or of the anterior spinal artery or its feeding radicular vessels causing spinal cord ischemia and infarction. The anterior spinal circulation supplies the descending corticospinal tracts and the anterior horn, and infarction in this territory may result in paraparesis or paraplegia. Among those undergoing scoliosis surgery, a higher rate of neurologic complications is seen in patients with hyperkyphosis, patients with a high degree of rigid curvature, patients with congenital scoliosis, neuromuscular scoliosis, or cerebral palsy, and patients undergoing combined anterior and posterior approaches (Coe et al., 2006; Forbes et al., 1991; Guigui and Blamoutier, 2005; Hod-Feins et al., 2007; MacEwen et al.,1975; Qiu et al., 2008). Before the development of intraoperative monitoring techniques, MacEwen et al. (1975) reported retrospective surgical outcome data collected by the Scoliosis Research Society. Of 7885 scoliosis surgeries performed from 1965 to 1971, neurologic complications occurred in 0.72%, and the authors suspected that the complication rate was actually somewhat higher due to incomplete reporting. Of a total reported 74 spinal cord injuries, recovery was complete in 22, partial in 28, and there was no recovery in 24 patients.