Epidemiology, Risk Factors, and Pathophysiology

Spinal curvature is the most common cause of chest wall deformity. The causes of chest wall deformity are shown in Box 63-1. By far, the most frequently found scoliosis is the idiopathic variety, which accounts for approximately 80% of cases. Idiopathic scoliosis is defined as lateral curvature for which no cause can be identified; congenital forms of scoliosis are related to a developmental abnormality of the spine, as in failure of segmentation (e.g., fused vertebrae), failure of formation (e.g., hemivertebrae), or genetic syndromes (e.g., spondylocostal dysostosis or Klippel-Feil or Goldenhar syndrome).

Scoliotic curves of more than 35 degrees are present in 1 in 1000 population, and those that exceed 70 degrees are estimated to occur at a rate of 0.1 in 1000. A gender predilection for more pronounced deformity has been recognized: Females are at greater risk for severe curvature. It has been estimated that approximately 500,000 persons with a scoliotic curve of greater than 30 degrees are living in the United States. Approximately 3 or 4 children per 1000 will require specialist supervision for their spinal curvature, and a third of these will require intervention (e.g., corrective surgery or bracing). Idiopathic scoliosis occurs more often with increasing maternal age and in higher socioeconomic groups, but no association has been found between the incidence of scoliosis and birth order or season of birth. A subclassification of idiopathic scoliosis is based on age at onset of the spinal changes resulting in curvature—infantile (birth to age 3 years), juvenile (3 to 11 years), and adolescent (11 years and older).

Scoliosis is associated with a variety of congenital syndromes. Marfan syndrome affects 1 in 5000 of the population, and approximately 70% of these patients develop a spinal deformity. Diagnosis can be confirmed by linkage to the Marfan syndrome gene MFS1, the protein product of which produces fibrillin. Marfan genotype-phenotype correlations show association with severe mutations in 25% of persons with the syndrome, with 50% of those in the latter half of the exon (exons 33 to 63). Related syndromes may result from mutations in microfibrils that interact with fibrillin in the extracellular matrix. Congenital contractual arachnodactyly (Beals syndrome), in which scoliosis is common, also has been shown to be caused by fibrillin deficiency.

Neurofibromatosis type 1 (NF) is a multisystem disease, with scoliosis being the most common bone manifestation, occurring in 10% to 30% of patients. Genome-wide scans have identified the likely chromosomal locus on 17q11.

Genetics of Idiopathic Scoliosis

The genetic basis of idiopathic scoliosis remains unclear, and causation may be multifactorial in that particular growth patterns may exacerbate a genetic predisposition. Support for an underlying genetic cause comes from data showing an incidence of idiopathic scoliosis in 6.94%, 3.69%, and 1.55% in first-, second-, and third-degree relatives, respectively, of 114 affected persons. These findings are consistent with either an autosomal dominant or a multifactorial mode of inheritance. A large kindred with autosomal dominant idiopathic scoliosis has been identified with a chromosomal locus on 17p11. By contrast, congenital scoliosis is relatively common among congenital malformations and is associated with congenital heart and renal tract anomalies. An autosomal recessive form of congenital scoliosis has been found in male and female sibships with consanguineous parents, associated with lack of vertebral segmentation and fused ribs. Mouse models for idiopathic scoliosis have been developed, and the list of candidate genes continues to grow, indicating the underlying etiologic complexity and probable interaction of genetic, environmental, and developmental factors.

Spinal curvature is acquired in neuromuscular disorders (Figure 63-3) that involve the chest wall and thoracic musculature before skeletal maturity occurs. Scoliosis develops in more than 50% of boys with Duchenne muscular dystrophy (DMD), and spinal curvature is common in many of the other congenital muscular dystrophies, myopathies, and conditions such as type I and type II spinal muscular atrophy. The introduction of steroid therapy in childhood in DMD may lessen the severity of scoliosis by reducing the rate of loss of muscle strength, such that wheelchair dependency occurs later in adolescence and peak vital capacity is increased, although the number of prospective randomized controlled trials has been limited.

Figure 63-3 Radiograph showing extensive severe neuromuscular scoliosis in a patient with type II spinal muscular atrophy.

A scoliotic deformity often develops as a sequela of thoracotomy carried out in childhood or young adulthood.

Kyphosis

Idiopathic kyphosis is rare. An increase in thoracic kyphosis occurs with age and is exacerbated by factors that increase a tendency to osteoporosis, such as oral corticosteroid therapy. Pott’s disease, or tuberculosis (TB) of the spine, is still a common cause of acquired kyphosis in TB-endemic geographic regions.

Effects of Chest Wall Deformity on Respiratory and Cardiac Function

Chest wall disorders affect respiratory function and cause a restrictive ventilatory defect. Any significant scoliosis or kyphosis results in a loss of height, so arm span can be used to predict normal lung volumes. As a general rule, patients who have a thoracic curve of greater than 70 degrees are subject to significant ventilatory limitation.

Lung Volumes

Although both scoliosis and kyphosis diminish lung volumes, which results in a restrictive ventilatory defect, lateral curvature has a more profound effect on chest wall mechanics. Total lung capacity is reduced in all chest wall disorders. In a pure scoliosis, both vital capacity (VC) and expiratory reserve volume are decreased, with relative preservation of residual volume (Table 63-1). An obstructive ventilatory defect is rare in scoliosis and kyphosis, unless the individual is a smoker, has coexistent asthma, or the scoliosis results in bronchial torsion.

Table 63-1 Typical Results on Pulmonary Function Testing in Patients With Idiopathic Thoracic Scoliosis

Parameter	Effect
Forced expiratory volume in 1 second (FEV1)	Reduced
Forced vital capacity (FVC)	Reduced
FEV1/FVC	Normal
Residual volume	Normal
Total lung capacity	Reduced
Transfer factor for carbon monoxide—diffusion capacity (DLCO)	Reduced
Transfer coefficient (KCO)—DLCO/accessible alveolar volume	Supranormal*

* Transfer coefficient usually is supranormal, but it is reduced in the presence of pulmonary hypertension.

The relationship between pulmonary impairment and the deformity is complex and cannot be predicted accurately from the Cobb angle alone. The four major determinants of a reduced VC are the number of vertebrae involved in the curve, position of the curve closer to the head, Cobb angle, and the degree of loss of normal thoracic kyphosis.

In paralytic scoliosis, lung volumes are reduced not only by chest wall restriction but also by inspiratory muscle weakness.

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