Asphyxiating thoracic dystrophy is also known as Jeune syndrome, after the scientist who described it for the first time in 1855. It is a congenital disorder in which bone growth occurs in an abnormal manner, resulting in severe constriction of the chest, shortening of the ribs, as well as of the arm and leg bones, a short adult stature, and polydactyly.
The outcome is very often fatal especially in infants who are born with extremely narrow or bell-shaped chest cavities. They are unable to breathe properly because of lung hypoplasia, and succumb to respiratory distress, with this often exacerbated by respiratory infections, within a period of two years. In individuals who manage to live beyond childhood, there may be improvement in the thoracic capacity as the child grows, with easier breathing. Such patients usually have less severe skeletal manifestations.
Associated anomalies in Jeune syndrome include cystic renal disease, cardiac defects, and subglottic stenosis which can cause life-threatening airway obstruction. Liver cirrhosis, pancreatic cysts, dental defects and retinal dystrophy can also occur as in other disorders of the ciliopathy group, of which this condition is one.
Prevalence and Inheritance
The prevalence of asphyxiating thoracic dystrophy is about 1 in 100,000 to 1 in 130,000. It is caused by mutations in one or more of 11 genes associated with ciliary structure and function. The first gene to be identified with this syndrome was the IFT80 gene, followed by DYNC2H1, which was then found to be positive for mutation in half of all cases. Genetic mutations may be found in up to 70 percent of individuals with Jeune syndrome.
Jeune syndrome is inherited in an autosomal recessive manner which means that two defective copies of the gene allele responsible for the condition are present in each cell. One copy is inherited from each parent, resulting in the offspring having no normal copy of the gene. If only one copy is inherited, no clinical features occur in the offspring, but a carrier state results (it may be passed on to the offspring).
The basic pathophysiological defect in Jeune syndrome is defective ciliary production. Cilia are cell surface structures which are now known to be involved in many life processes. Ciliary function includes an important transport pathway called intraflagellar transport (IFT) by which synthetic materials are carried in and out of the cilia-associated cells in order to assist them to assemble and maintain cilia. These microscopic structures are essential for regulating many signaling pathways such as the Shh and Wnt pathways which are involved in cell cycles and cell differentiation all over the body, in a diverse range of tissues.
The absence of cilia or presence of abnormal cilia in many cell types leads to dysregulation of the growth and differentiation of bones, the kidney parenchyma, and retinal photoreceptors with their supporting or nutritive cells.
Diagnosis and Treatment
The diagnosis is based on the clinical features, with the typical radiologic findings in which shortened ribs and abnormally short pelvic and limb bones are prominent in a neonate with respiratory difficulty and a short, constricted chest. Genetic testing is performed if required for confirmation.
Treatment is symptomatic, and is aimed at improving respiratory function, treating respiratory infection, and alleviating renal or hepatic symptoms. A prosthetic rib of titanium is now being explored for its role in helping expand the chest cavity in all forms of thoracic insufficiency syndrome, including this condition.
Prognosis depends on the initial severity of bone malformations and the age. If the child manages to survive respiratory infections for the first two years, the chest may grow broader thereafter, leading to improvement of chest capacity and fewer symptoms thereafter.
Reviewed by Afsaneh Khetrapal Bsc (Hons)