Developmental Abnormalities of the Lung

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Bronchial atresia leads to severe narrowing of a bronchus supplying an area of lung.
Overexpansion of the lung segment may occur from air trapping.

Pulmonary hypoplasia reflects incomplete development of the lung, which is smaller than
normal. This is often seen with other congenital abnormalities and is associated with lung
compression by other abnormal masses and oligohydramnios.

Bronchogenic cysts are usually attached to the trachea and represent accessory bronchial
buds, which become sealed off from the rest of the main airways. They are lined by a
respiratory epithelium and contain mucinous fluid.
Bronchopulmonary sequestration is due to development of a portion of lung that does
not communicate with the normal bronchial tree.
Patients develop an abnormal lung mass and the abnormal areas develop dilated airways. There
are recurrent infections, with frequent development of an abscess.

OTHER LUNG DISEASES IN CHILDREN

Immotile cilia syndrome leads to recurrent chest infections.
Recurrent chest infections in childhood may be due to defective cilial function, the immotile
cilia syndrome. Imaging studies show reduced clearance of tracer substances inhaled into the
lungs, and biopsy of cilia-bearing nasal mucosa shows unco-ordinated beating with time-lapse
photography. In some individuals, cilia are seen to have an abnormal structure, with absence of
structural components such as the dynein arms.

A syndrome of absent frontal sinuses, bronchiectasis and situs inversus (organs on wrong side), termed
Kartagener's syndrome, is due to defective cilial motility.

Cystic fibrosis is characterized by abnormally thick mucus
Cystic fibrosis is the most common autosomal recessive disorder in the Caucasian population,
with an incidence of 1:2500 newborns, 1:25 adults being a heterozygous carrier of the CF gene. It is
a multisystem disease and, most importantly, causes recurrent chest infections in childhood, associated
with pancreatic exocrine dysfunction. The disease is caused by the production of an abnormally viscid
mucus that cannot be cleared from the lungs and causes blockage of the main pancreatic ducts.
40% of cases present with respiratory disease.
30% present as failure to thrive and malabsorption.
20% present at birth with meconium ileus (due to abnormally viscid intestinal mucus).
10% present with liver disease, recurrent nasal polyps or sinusitis.
The molecular pathogenesis of the disorder is defective function in a membrane chloride channel of epithelial cells, which causes decreased release of sodium and water to liquefy mucus.
In the respiratory tract, bronchi and bronchioles become obstructed by abnormally viscid mucus, which
leads to four main problems.
The obstruction and stagnation of secretions leads to repeated bouts of infection, particularly with
Staphylococcus aureus and the mucoid form of Pseudomonas.
Bronchiectasis is a frequent complication leading to haemoptysis.
Hyperinflation of lungs due to air trapping behind mucin plugs increases the risk of developing pneumothorax .
Hypoxia, scarring and destruction of the pulmonary vascular bed lead to pulmonary hypertension and cor
pulmonale.
The median age of survival is just over 30 years.
The gene for cystic fibrosis (CF gene) is on the long arm of chromosome 7. It encodes a protein termed the
cystic fibrosis transmembrane conductance regulator (CFTR).
In normal mucus-secreting epithelia (a) chloride channels open in response to increased concentration
of cAMP (generated by stimulation of cell surface receptors), which activates a protein kinase (PKA).
Phosphorylation of the CFTR protein opens the channels,
and secretion of chloride, water and sodium is facilitated.
In the main mutation causing CF (b) (70% of cases), the protein is not present in the cell surface,
stimulated chloride secretion cannot occur, cells do not secrete water and sodium, and mucus is extremely
viscid. Such cases have severe clinical disease.
Over 100 other different mutations of the CF gene lead to expression of an abnormal CFTR in the cell
membrane, with only partial activity in response to cAMP {\B (c).} Such cases have moderately viscid
mucus and milder clinical disease.
The detection of the CF gene now allows for fetal diagnosis and carrier detection. Gene therapy to
place a normal CF gene in respiratory tract epithelial cells is a real possibility, using an inhaled viral vector.
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