The thoracic cage is composed of 12 pairs of ribs, the sternum, and 12 thoracic vertebrae. The ribs are interspaced with intercostal muscles. The external intercostals arise from the inferior border of one rib and attach to the superior border of the caudal rib with the external intercostal membrane lying posterior to it. Below this is the internal intercostal which has its fibers running at right angles to the external intercostal and the internal intercostal membrane deeper to the internal intercostal.

The neurovascular structures which are composed of the intercostal vein, artery, and nerve live beneath the internal intercostal muscle. It is important to understand that the vein lies superior to the subcostal groove, whereas the artery and the nerve reside inferior to the subcostal groove. This nerve innervates the adjacent intercostal muscles. Further deeper to the internal intercostal but above the parietal pleura is the innermost intercostal muscle, which becomes the transversus thoracic muscle anteriorly.

A normal thoracic development during childhood is essential for lung growth. Hence, respiratory function will depend on the lung growth and ability of the thorax to act as a dynamic pump to facilitate inhalation and exhalation.

The thoracic spine grows 1.4 cm per year from birth to 5 years of age, 0.6 cm per year from 6 to 10 years of age, and 1.2 cm per year from 11 to 15 years of age.^{1, 3}

Symmetric growth and the correct orientation of the ribs for the age of the child contributes to the width and depth of the rib cage and helps to maximize volume expansion and make respiration more effective. The thoracic cross-sectional volume is directly proportional to the length of the ribs and the degree of rib obliquity. At birth, infants have a square-shaped thoracic cross-section, which becomes a more rectangular-shaped thoracic cross-section in adults.

The ribs are oriented horizontally and the growth of the ribs occurs primarily at the anterior physis. At birth, the thoracic volume is only 6.7% of the adult volume. By 2 years of age, the ribs grows downward more obliquely and the thoracic cross-section changes to an oval shape. By 5 years of age, the thoracic volume increases to 30% of the adult size and to 50% of the adult size by 10 years of age. From age 10 years to skeletal maturity, the thorax grows rapidly and doubles itself to the adult size.

Almost 85% of lung alveolar cells are formed immediately after birth, with only slight increase in the first 2 years of life. The end age of alveolar cell formation is controversial and so is the concept of alveolar cell hypertrophy. The lungs still continue to increase in size even if the alveolar cell formation ceases due to compensatory lung growth triggered by a lung “stretch reflex.” Experimental pneumonectomy in young animals and partial pneumonectomy in children aged 30 months to 5 years showed compensatory lung growth by alveolar cell multiplication.

VEPTR is currently widely accepted treatment for volume depletion deformities of the thorax (VDD). VDD can be classified into three main types:

Thoracic insufficiency may initially present with early occult respiratory deficiency when the child begins to show signs of limited pulmonary capacity. They fatigue easily with play activities and compensate with an increased respiratory rate at rest but do not yet require oxygen support. The final sequela of progressive thoracic insufficiency syndrome is respiratory insufficiency.

An increase in the ventilation requirements reflects an exponential clinical deterioration in respiratory function with important consequences for the family and the child.

Early-onset scoliosis in neuromuscular conditions such as SMA begins in the first decade of life due to truncal weakness. It then progress steadily unless managed surgically and can become lethal. It occurs in 100% of the type I SMA and most of the type II SMA when they become nonambulators.

Physical examination should include careful evaluation of the spinal deformity, pelvic obliquity, sitting balance, and shoulder balance.

Respiratory effort is assessed by determining the rate of respiration, lung auscultation for abnormal breath sounds, anthropomorphic measurements (height, weight, chest circumference at the nipple line, and limb length) looking for failure to thrive, and the thumb excursion test to evaluate thoracic wall expansion during respiration.

The radiographic assessment includes standard posteroanterior (PA) (FIG 1) and lateral views of the spine with rib cage. The Cobb angle is measured for the scoliosis, and the head and truncal decompensation is assessed. The space available for the lungs to expand is measured by the ratio between the concave heights to the convex height.

If cervical spine instability is a possibility, a cervical spine series including flexion and extension views is performed.

A formal consultation with a pediatric pulmonologist, including pulmonary function testing, is very valuable to establish baseline function and counsel the family appropriately regarding risk of postoperative pulmonary complications.

Magnetic resonance imaging (MRI) of the entire spine should be ordered to evaluate the spinal cord and those children who have any physical findings concerning for tethered cord.

Computed tomography lung volumes can be compared with normative values to provide more information but such results may not be available.

Dynamic MRI may be performed to assess the function of the diaphragm, and a screening MRI is routinely performed to look for abnormalities such as cord tethering or syrinx.

Echocardiogram can be done to detect early-onset cor pulmonale.

The primary nonoperative option is observation, with serial radiographs and close monitoring of pulmonary function.

In some cases, braces or serial casting can be offered, although it is often impractical in children with neuromuscular spinal deformity at risk for thoracic insufficiency.

Bilateral rib to pelvis VEPTR technique offers a relatively minimally invasive way to manage early-onset neuromuscular scoliosis associated with thoracic insufficiency. The technique can be performed with relatively low blood loss and morbidity and with much less risk of autofusion as is seen in standard dual growing rod techniques.

The bilateral rib to pelvis VEPTR technique is generally contraindicated in ambulatory children because a recent report shows a significant incidence of crouched gait, likely due to mechanical factors at the lumbosacral junction.⁷