Definition and incidence

Kyphosis is a deformity marked by an accentuated posterior curvature. Scoliosis is a lateral curvature of the spine. Kyphoscoliosis results when both kyphosis and scoliosis occur concomitantly, causing a lateral bending and rotation of the vertebral column. Scoliosis alone, despite its severity, does not cause sensory or motor impairment. In contrast, kyphosis and kyphoscoliosis may induce spinal cord damage because of the sharp angulation of the spine. Respiratory dysfunction is associated with scoliosis, significant kyphosis, and severe kyphoscoliosis. Scoliosis is the most common spinal deformity, with an incidence of four persons per 1000.

Scoliosis is classified in five categories: idiopathic, congenital, neuropathic (e.g., poliomyelitis, cerebral palsy, syringomyelia, and Friedreich ataxia), myopathic (e.g., muscular dystrophy and amyotonia), and traumatic. Idiopathic scoliosis is the most common deformity, accounting for 80% of all cases. On the basis of the time of onset, idiopathic scoliosis is divided into the following two categories: (1) the rare infantile form (male-to-female ratio is 6:4) and (2) the common adolescent form (male-to-female ratio is 1:9). The children in the adolescent group are born with straight spines; however, at some point during the growth period, their spines begin to bend and deform, with deformation progressively worsening until growth ends. In general, whereas curves associated with adolescent idiopathic scoliosis are convex and deviated to the right, those related to other disease may be deviated to the left. The presence of cervical scoliosis should alert anesthesia personnel to potential difficulties in airway management. Any significant curvature involving the thoracic spine may alter lung function. Unless the deformity is severe, patients with kyphosis are able to maintain normal pulmonary function; in contrast, even mild forms of scoliosis can result in impaired ventilatory function. Severe thoracic deformity may result in respiratory alterations during sleep. Several types of breathing abnormalities have been documented, including obstructive sleep apnea and hypopnea. The lowest HbO₂ saturations occurred during rapid eye movement sleep.

Pathophysiology

Diminution of pulmonary function occurs with curvatures of greater than 60 degrees, and pulmonary symptoms develop with curvatures greater than 70 degrees (as measured by the Cobb technique). Curvatures greater than 100 degrees may be associated with significant gas exchange impairment.

In general, the greater the curvature, the greater the loss of pulmonary function. Because of this, mechanical ventilation becomes inefficient; this inefficiency is the major factor causing respiratory compromise. At the time of diagnosis, it often is possible to document a reduction in lung capacity. The characteristic deformity seen in scoliosis causes one hemithorax to become relatively smaller than the other.

Skeletal chest wall deformity in kyphoscoliosis leads to a reduction in lung volumes and the pulmonary vascular bed. Ventilatory failure associated with severe kyphoscoliosis produces a lung size that is 30% to 65% of normal. As the patient ages, the chest wall becomes less compliant; this increases the work of breathing and leads to hypoventilation and respiratory muscle weakness.

The main features of lung mechanics in the patient with early-stage scoliosis are reduced lung volumes (VC, TLC, FRC, and RV) and reduced chest wall compliance; in the late stages of disease, ventilation/perfusion mismatching with hypoxemia (attributed to alveolar hypoventilation because of a decrease in tidal volume [Vt]), increased pulmonary artery pressure (PAP), hypercapnia, abnormal response to CO₂ stimulation, increased work of breathing, and cor pulmonale occur and eventually lead to cardiorespiratory failure. Reduction in VC to 60% to 80% of the predicted value is a typical finding. The ratio of forced expiratory volume in 1 second to forced vital capacity (FEV₁/FVC) is normal unless other pulmonary diseases are present. Although normocarbia prevails for most of the clinical course, an elevated Paco₂ signifies the onset of respiratory failure. The severity of hypercapnia most closely correlates with the patient’s age and inspiratory muscle strength.