The spine normally curves posteriorly in the thoracic region and anteriorly in the lumbar region. These physiologic curves are the thoracic kyphosis and the lumbar lordosis, respectively. The spine is not normally curved when viewed from the front or back. Scoliosis refers to a lateral curvature of the spine. Curves are classified as structural or nonstructural. A nonstructural curve, such as lumbar scoliosis from a leg length discrepancy, will resolve when the patient is supine or uses a shoe lift and does not require surgical correction. In contrast, structural scoliosis lacks normal flexibility and does not correct with bending or lying supine. In addition to the lateral curvature of the spine, the vertebrae are rotated and the rib cage may be markedly deformed (Fig. 58.1). As demonstrated in the computed tomography scan image in Figure 58.2, this thoracic deformity may lead to a significant decrease in total lung volume. Particularly, note the decrease in left lung volume relative to the right.

Idiopathic scoliosis is the most common type of scoliosis (70% of all cases) and occurs in infantile, juvenile, and adolescent forms. As the name implies, the cause is unknown but appears to be multifactorial, including abnormalities of collagen, brainstem function, equilibrium, hormones, and growth. Genetic factors are important in its development, as evidenced by an increased incidence of scoliosis in relatives of affected patients. Evidence suggests that pronounced forms of scoliosis (curve greater than 11 degrees) occur only in the carriers of a mutant allele with incomplete penetrance (30% of men, 50% of women).

The prevalence of idiopathic scoliosis in large screening studies depends on the definition of scoliosis and the population screened. The prevalence of spinal curves greater than 10 degrees is 1.5% to 3%; greater than 20 degrees, it is 0.3% to 0.5%; and greater than 30 degrees, it is 0.2% to 0.3%. The adolescent form of idiopathic scoliosis is by far the most

common in the United States. The male-to-female prevalence ratio depends in part on the age of the patient, but scoliosis requiring surgical correction is more common in women. The ratio increases with the severity of the curve, with a ratio of girls to boys at 2:1 for curves of 10 degrees, and increasing to 10:1 for curves over 30 degrees.

Many etiologic classifications exist for structural scoliosis. Neuromuscular scoliosis (paralytic scoliosis) may occur as a result of diseases such as cerebral palsy, muscular dystrophy, poliomyelitis, familial dysautonomia, and so on. This type of scoliosis is associated with significantly increased intraoperative blood loss compared with idiopathic scoliosis. Congenital scoliosis is the result of congenital anomalies such as hemivertebrae and fused vertebrae or ribs. Neurofibromatosis and Marfan syndrome are also associated with scoliosis. These underlying conditions may have a major impact on the anesthetic plan. The classification of structural scoliosis is as follows:

In 1966, the Scoliosis Research Society standardized the method for assessing the severity of scoliosis. The most common measure of severity is Cobb angle. Figure 58.3 illustrates how Cobb angle is measured on a spine radiograph. A perpendicular (2) is constructed from the bottom of the lowest vertebrae (1) whose bottom tilts toward the concavity of the curve, and another perpendicular (4) from the top of the highest vertebrae (3) whose top tilts toward the concavity. The angle (5) at which these perpendiculars intersect is Cobb angle. Numerous studies have documented that the more severe the thoracic curve (greater Cobb angle), the
more profound the disturbance in pulmonary function. Surgical treatment is usually recommended for curves greater than 45 to 50 degrees. Curves greater than 60 degrees are usually associated with decreases in pulmonary function.

In a series of 79 patients with thoracic scoliosis, the mean Cobb angle was 45 degrees and vital capacity was decreased by an average of 22%. Figure 58.4 demonstrates that forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV₁) decrease with increasing thoracic curve severity.

Scoliosis severity and impairment of pulmonary function also increase with greater number of vertebrae involved, more cephalad location of the curve, and loss of the normal thoracic kyphosis. Severe curves have a worse prognosis because they tend to progress and if long-standing can cause permanent damage of the lung parenchyma, respiratory failure, cor pulmonale, and death. It is important to note that patients with neuromuscular types of scoliosis may have a much more profound decrease in pulmonary function for any given curve severity.

A decrease in lung volumes, a restrictive pattern, is most commonly seen in thoracic scoliosis. The greatest reduction occurs in vital capacity, which is typically reduced to 60% to 80% of predicted. Total lung capacity, functional residual capacity, inspiratory capacity, and expiratory reserve volume are also decreased. An increase in residual volume has been reported in patients with congenital scoliosis and in patients with idiopathic scoliosis 3 years following corrective spine fusion.

During exercise, ventilation is adequate, but tidal volume is reduced and respiratory rate is increased. Maximal work capacity may also be decreased. Unless there is coexisting obstructive airway disease, the ratio of FEV₁/FVC is normal. Impaired respiratory muscle function also occurs in scoliosis, as evidenced by a decrease in inspiratory force to 70% of normal values. The decrease in inspiratory force is probably due to the inspiratory muscles working at a mechanical disadvantage because of the chest wall deformity.

These abnormalities in pulmonary function are usually the result of abnormal thoracic cage geometry producing a marked decrease in chest wall compliance rather than any abnormality in the lungs or respiratory muscles themselves. In dynamic magnetic resonance imaging, adolescent girls with idiopathic scoliosis and healthy controls showed no difference in diaphragmatic motion. The changes in chest wall compliance can be mimicked in normal volunteers by chest strapping. Exceptions include congenital and infantile scoliosis in which growth of the lungs may be impaired early in development by the thoracic deformity.

It has been documented that patients with thoracic scoliosis have arterial oxygen desaturation compared to normal controls. Both arterial PCO₂ and pH are usually normal. Several studies
have failed to show a correlation between the severity of the spinal curve and the degree of arterial oxygen desaturation.

Arterial hypoxemia is probably caused by ventilation/perfusion ([V with dot above]/[Q with dot above]) inequalities. Decreased diffusing capacity and alveolar hypoventilation may also play a role. Nevertheless, the diffusing capacity is not decreased sufficiently to be the sole cause of the hypoxemia. Similarly, alveolar ventilation at rest and during exercise is usually normal; therefore, arterial PCO₂ is usually normal. It has been reported that some patients have a closing capacity higher than functional residual capacity, resulting in premature airway closure during normal tidal breathing. Other studies have failed to demonstrate this finding. Some authors have reported an increase in the ratio of dead space to tidal volume (V_D/V_T), whereas other recent series have found V_D/V_T to be normal. This discrepancy may be related to the patient population studied, the severity of the scoliosis, or both. Severe and long-standing scoliosis can produce severe [V with dot above]/[Q with dot above] abnormalities, alveolar hypoventilation, carbon dioxide retention, and more severe hypoxemia. If not surgically treated, severe scoliosis increases the risk of premature death from respiratory failure after 40 years of age. A survey of patients with respiratory failure in Sweden found that a vital capacity less than 50% of predicted and a Cobb angle greater than 100 degrees indicated an increased risk of respiratory failure.

The slope of the ventilatory response to carbon dioxide may be decreased in patients with scoliosis. This is probably not specific to scoliosis because this response is known to be reduced in situations in which the work of breathing is increased even in the absence of a chest wall deformity. Patients with mild scoliosis have been reported to exhibit abnormal ventilatory patterns in response to hypoxemia and hypercarbia. This pattern tends to minimize the work of breathing, a higher respiratory rate, and lower tidal volume.

It has also been demonstrated that during maximal exercise, patients with mild-tomoderate scoliosis exhibit a significantly decreased ventilatory capacity, reduced exercise tolerance, and oxygen consumption despite normal pulmonary function testing at rest.

Patients with scoliosis may develop elevated pulmonary vascular resistance and pulmonary hypertension. This may result in right ventricular hypertrophy and eventually right ventricular failure. A 50-year study of untreated scoliosis demonstrated that the mortality rate of these patients was twice that of the general population and respiratory failure or right-sided heart failure accounted for 60% of the deaths.

The increase in pulmonary vascular resistance is probably due to several factors. Hypoxemia produces pulmonary vasoconstriction, an increase in pulmonary vascular resistance, and hence an increase in pulmonary arterial pressure. Chronic hypoxemia will produce hypertensive vascular changes, and pulmonary hypertension may become irreversible. It has also been proposed that the chest wall deformity compresses some lung regions, increasing vascular resistance in those regions. Finally, if scoliosis develops in the first 6 years of life, the growth of the pulmonary vascular bed may be impaired by the chest wall deformity.
Supporting this concept are reports of a decrease in the number of vascular units per lung volume in patients with scoliosis.

The most common cardiovascular abnormality in patients with scoliosis is mitral valve prolapse. Some conditions that are associated with scoliosis also affect the cardiovascular system. Patients with Duchenne muscular dystrophy develop a cardiomyopathy in the second decade of life that may not be appreciated on the basis of clinical symptoms because these patients are unable to exercise. The electrocardiogram (ECG) may reveal tachycardia, prolonged PR and QRS intervals, ST abnormalities, bundle branch block, Q waves in the left precordial leads, and tall R waves in the right precordial leads. Ejection fraction may be decreased on echocardiogram.

Patients with Marfan syndrome may have mitral and aortic insufficiency, aneurysm of the proximal ascending aorta, and abnormalities of the conduction system.

The association of scoliosis and congenital heart disease has been well established. Although no specific cardiac lesion has been identified, some series have suggested that scoliosis is more common in patients with cyanotic heart disease.

In the past 40 years, there has been no supportive evidence of a direct relationship. However, some scoliosis patients may have an associated or underlying muscle disorders such as central core disease or multiminicore disease (also see Chapter 54, section A.3). These muscular dystrophies are associated with an increased risk of malignant hyperthermia.

It is important to identify the location of the curve, the age of onset, its severity, the direction of the curve, and the etiology of the scoliosis. The location of the curve is important because thoracic scoliosis is associated with pulmonary function abnormalities. Cervical scoliosis may cause difficulties in airway management and may be associated with other congenital anomalies.

The age of onset of scoliosis is of critical importance because the lung continues to grow and develop from birth until 8 years of age. The number of alveoli increases from approximately 20 million at birth to 250 million at 4 years of age. The development of significant thoracic scoliosis during this phase of rapid growth impairs lung development. A significant reduction in alveolar number has been demonstrated in patients with early-onset thoracic scoliosis, predisposing these patients to impaired gas exchange and pulmonary hypertension.

The severity of the curve is important because thoracic curves greater than 60 degrees generally produce significant decreases in pulmonary function. Curves greater than 100 degrees may be associated with significant impairment in gas exchange. Most curves in adolescent idiopathic scoliosis are convex to the right, just as most people are right-handed. A left thoracic convexity should raise the index of suspicion to look for other underlying conditions and congenital anomalies. Finally, an understanding of the etiology of the scoliosis is important because underlying conditions such as muscular dystrophy or cerebral palsy will influence anesthetic management.