Factors determining pulmonary function in adolescent idiopathic thoracic scoliosis

Adolescent idiopathic thoracic scoliosis may lead to severe pulmonary impairment and early death, but the responsible factors are poorly understood; pulmonary function is only weakly related to the angle of scoliosis. We performed a cross-sectional study using multivariate analysis to identify the individual and additive influence of different features of spinal deformity and nonstructural factors on pulmonary impairment. Pulmonary function was assessed by measuring lung volumes and diffusing capacity, with a priori selection of vital capacity (expressed as percentage of predicted, % VC) as the primary index of pulmonary impairment. Radiologic and physiologic measurements were made independently in 66 subjects who had not previously had spinal surgery. Angle of scoliosis (p = 0.01) was one of four features of spinal deformity associated with reduced % VC; greater number of vertebrae involved (p = 0.007), cephadal location of the curve (p = 0.04), and loss of the normal thoracic kyphosis (p = 0.002) made an equal and additive contribution to pulmonary impairment. Spinal deformity led to reductions in VC, primarily by reducing TLC. Spinal column rotation, respiratory muscle strength, and duration of the curvature were not related to pulmonary function (p > 0.05). We conclude that features of the spinal deformity are the major determinants of pulmonary impairment in idiopathic thoracic scoliosis but that the relationship between deformity and impairment is complex. The severity of pulmonary impairment cannot be inferred to a clinically useful extent from the angle of scoliosis alone.