Disorders of the Thoracic Spine

Thuy K. Lin and Justin C. Reis

The thoracic volume is determined by the height of the thoracic spine and the width and depth of the rib cage. Disorders that distort the chest wall or cause chest wall restriction can compromise respiratory function. The most common disorders are scoliosis, kyphosis, kyphoscoliosis, and ankylosing spondylitis.

SCOLIOSIS/KYPHOSIS/KYPHOSCOLIOSIS

Scoliosis is characterized by a lateral curvature of the spinal column with rotation of the vertebrae. Its severity is defined by the angle between the tangents of the most inclined vertebral plateau, also known as the Cobb angle. The greater the Cobb angle, the shorter the hemithorax on the concave side of the curve. Kyphosis is defined by anteroposterior angulation of the spine. Scoliosis is usually associated with a component of kyphosis, but kyphosis can occur in isolation. Kyphoscoliosis is characterized by distortion of the thoracic cage, which can cause impairment of lung growth. Respiratory symptoms are usually mild unless: (1) the Cobb angle is greater than 100°, (2) scoliosis occurs prior to age 9, or (3) the patient has kyphoscoliosis. Congenital kyphosis, kyphoscoliosis, and scoliosis are all part of a spectrum of spinal deformities due to developmental vertebral anomalies.

Eighty percent of kyphoscoliosis begins in childhood and is idiopathic; the rest are due to neuromuscular diseases (e.g., poliomyelitis, syringomyelia, neurofibromatosis), congenital defects of the spine, vertebral disease (e.g., tuberculosis, tumor, osteomalacia), and thoracic disease (e.g., emphysema, thoracoplasty). Idiopathic kyphoscoliosis is more common in women (4:1) and is usually not as severe compared to the deformities of poliomyelitis, tuberculosis, and congenital spine defects. Infantile scoliosis (onset 0 to 3 years) and juvenile scoliosis (onset 4 to 9 years) are more likely to be associated with Cobb angles greater than 100°, fused or absent ribs, rotation of the spine with secondary rib deformity, or restriction of rib motion. Pulmonary development involves growth of new alveoli until 5 to 8 years of age. Any bony abnormality that reduces thoracic volume during this time may affect lung size. Individuals who reach skeletal maturity with a vital capacity of less than 45% of predicted (using arm span to determine predicted height) have an increased risk of respiratory failure when their lung function starts to decline from the age of 35 onward. However, studies of patients followed for 50 to 60 years demonstrate that the more common adolescent scoliosis is rarely a cause of mortality.

Patients with kyphoscoliosis are subject to a variety of mechanical factors, which eventually contribute to alveolar hypoventilation. With progressive disease, total lung capacity (TLC), vital capacity (VC), and functional residual capacity (FRC) can become reduced, mainly due to reductions in chest wall compliance and alterations in the mid-position of the thoracic cage. The degree of pulmonary restriction and gas exchange impairment is highly correlated with the angle of scoliosis. In 1975, Kafer established a correlation between the degree of scoliosis and TLC, VC, FRC, and residual volume. The degree of scoliosis (higher Cobb angle) was negatively correlated with alveolar ventilation (VA) and VC and positively correlated with the physiological dead space/tidal volume ratio (VD/VT). Deformity above T10 is associated with a greater degree of respiratory impairment. At rest, minute ventilation is reduced due to diminished tidal volume. With exercise, the mechanical effects of the chest wall deformity are of greater significance and hypoventilation is more marked.

Mild kyphoscoliosis has a good prognosis. Pulmonary rehabilitation may improve pulmonary function and exercise capacity. Jones et al. performed 6-minute walk tests on six patients with moderate-to-severe kyphoscoliosis (mean Cobb angle 79°) and reported that oxygen therapy relieved symptoms of dyspnea and improved desaturation, but did not increase the 6-minute walk distance, in contrast to patients with chronic obstructive pulmonary disease (COPD). Supplemental oxygen may alleviate vasoconstriction associated with pulmonary hypertension secondary to regional or global alveolar hypoventilation. Patients with respiratory failure can benefit from noninvasive positive pressure ventilation, which increases lung compliance, decreases the work of breathing, and allows fatigued respiratory muscles to rest.

Surgery in adults with established kyphoscoliosis has questionable benefits and can result in significant complications. Gitelman et al. performed a retrospective review of 49 patients with adolescent idiopathic scoliosis who underwent corrective surgical procedures. Over 10 years, pulmonary function tests showed that the group who underwent corrective chest wall surgery had no change in forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV₁), but demonstrated a significant decrease in percent-predicted FVC. The group that underwent posterior spinal fusion/instrumentation with iliac crest bone graft and no thoracic cage disruption had a significant increase in both FVC and FEV₁, but no change in percent-predicted values.

Indications for surgery in young patients include (1) progression of disease despite good external brace care, (2) deformity that is too advanced to respond to external bracing, (3) scoliosis greater than 50°, (4) intractable pain, (5) nonalignment of occiput over sacrum, and (6) psychiatric disturbances. Surgical treatment for progressive congenital kyphosis or kyphoscoliosis is indicated at an early age, not only to prevent severe spinal deformity and possible neurologic complications but also to prevent the adverse effects on lung development and function caused by constriction of the thoracic cage and impairment of diaphragmatic movement.

ANKYLOSING SPONDYLITIS

Ankylosing spondylitis (AS) is a chronic inflammatory disease affecting joints of the axial skeleton with secondary fibrosis and ossification of the ligamentous structures of the spine, sacroiliac joints, and rib cage. It predominately affects males aged 20 to 40 years. Approximately 90% of patients with AS are positive for the HLA-B27 antigen. Ankylosing spondylitis can affect the tracheobronchial tree and pulmonary parenchyma, and is associated with several unique pulmonary manifestations, including fibrobullous, fibrocystic, and pleural chest wall disease.

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