Thoracic insufficiency syndrome may result from a primary chest wall problem and/or from scoliosis. This can become so significant at a young age that it can impact normal lung development and growth which can affect the function of the chest in moving air in and out of the lungs.
More specifically, thoracic insufficiency syndrome, a newly characterized disorder, is defined as the inability of the thorax to support normal respiration or lung growth. The thorax, which includes the spine, the rib cage, and the sternum, is the engine of respiration. It must have adequate space for the lungs to grow and it must be able to change that volume for breathing (respiration) to occur. In thoracic insufficiency syndrome, the three-dimensional deformity of the thorax can limit its important role as a respiratory engine. This can affect the spine and rib cage in many different ways, but the common problems are deficiencies in thoracic volume, symmetry, and function.
Most of the work of breathing is done by the diaphragm muscle, which is the thin muscle at the base of the thorax underneath the lungs. When the diaphragm contracts, the volume of the thorax increases with the air brought into the lungs. When the diaphragm muscles relax, air is forced out of the lungs. Another important part of the act of breathing is the outward expansion of the chest wall itself from the many small muscles between the ribs. For the thorax to have optimal efficiency as a respiratory pump, it must have ideal volume for age, the ribs need to have a symmetrical shape in order for them to move properly, and the diaphragm muscle must be properly anchored at the base of the chest. In congenital scoliosis, there can be missing or fused ribs. When there is an absence of ribs of the chest wall, the underlying lung collapses inward with breathing without effectively expanding. In fused ribs and congenital scoliosis, the concave side of the curve usually contains a lung constricted by the fused ribs. Additional loss of thoracic volume may be seen in scoliosis when the curve rotates the spine into the chest on the convexity of the curve, often protruding deeper into the chest in a deformity called lordosis (sway back), flattening the chest with loss of thoracic volume. In this instance, the ribs cannot move well enough to help with respiration, and breathing becomes totally dependent on the diaphragm muscle. Increasing deformity of the thorax results in greater volume loss and may lead to breathing problems to a point that a child may require oxygen or even ventilator support.
There are other conditions that can affect thoracic volume without necessarily causing spine deformity. One example is Jeune syndrome, in which there is a severe narrowing of the chest due to the failure of the ribs to grow. This causes a hypoplastic (small) thorax, constricting the lungs from each side because of the narrow rib cage. Hypoplastic thorax can also be caused by a compromise in the height of the thorax in conditions such as Jarcho-Levine syndrome. Both of these syndromes are rare, but may have a high mortality rate from respiratory problems early in life.