LUNG AGENESIS

lungsDiagram

Pulmonary agenesis is a rare congenital anomaly, the etiology of which is not clearly known. Other systemic comorbidities such as cardiovascular, gastrointestinal, musculoskeletal and urogenital system anomalies can be observed in more than half of the patients.  It is usually diagnosed during childhood. Diagnosis in adulthood  is very rare.

Pulmonary aplasia is a rare congenital pathology in which there is unilateral or bilateral absence of lung tissue. It is distinguished from pulmonary agenesis, although similar, the main difference being that there is a short blind ending bronchus in aplasia.

It is usually unilateral, as bilateral pulmonary aplasia is not viable. It is frequently associated with other congenital abnormalities, mainly cardiovascular, and has been reported to occur with VACTERL syndrome. The acronym VACTERL derives from Vertebratal anomalies (hemivertebrae, congenital scoliosis, caudal regression, spina bifida), Anorectal anomalies (anal atresia), Cardiac anomalies (cleft lip), Tracheo-oesophageal fistula/oesophageal atresia, Renal anomalies (radial ray anomalies), limb anomalies (polydactily, oligodactily). At least three of the above above features (in each category) mis considered necessary for the diagnosis of this condition.

Pulmonary aplasia usually presents with neonatal respiratory distress of variable intensity. In rare cases, it may go unnoticed until later in childhood / adolescence.

Abnormal blood flow in the dorsal aortic arch during the 4th week of gestation had been hypothesized to cause pulmonary agenesis. The contra lateral lung may develop as much as twice more alveoli in response to pulmonary aplasia/agenesis.

On chest x-ray, it can present as an hemithorax white-out or ipsilateral lung volume loss with ipsilateral shift of mediastinal structures and contra lateral lung hyperinflation. A main ipsilateral bronchus is rarely seen, although CT-scan can demonstrate a rudimentary main bronchus.

CT-scan will confirm the absence of lung parenchyma and mediastinal ipsilateral shift. Also, there is an ipsilateral absence of pulmonary artery. It may also show other cardiac congenital malformationsand ipsilateralbronchus remnant.

The main differential diagnosis is pulmonary hypoplasia and complete lung atelectasis.

Pulmonary hypoplasia refers to deficient or incomplete development of parts of the lung. It can develop as a result  of a number of other in-utero anomalies.

The true prevalence of pulmonary hypoplasia is not well known (1,4% of all births according to Knox et.al), but in cases of premature rupture of membranes at 15-28 weeks gestation, the reported prevalence of pulmonary hypoplasia ranges from 9 to 28%.

Pulmonary agenesis means undeveloped pulmonary vessels, bronchi and parenchima. It can be unilateral or bilateral. In unilateral aplasia, the remaining lung contains twice as much alveoli as normal, but has normal bronchi. Although aplasia doesn’t have the same structures, it has a rudimentary bronchus. As agenesis and aplasiaare difficult to distinguish from each other clinically and pathologically, they are often use in the  same sense. On the other hand, pulmonary hypoplasia is characterized by immature lung tissue or by lung tissue decreased at varying degrees. Pulmonary aplasia (agenesis) thought to result from the negative effects that occur on the 4th week of fetal life. Although its etiology is not fully understood. Its incidence in males and females and the occurence of the anomaly in the right or left lung are about the same. Hypoplasia and aplasia are often observed together with other malformations (diaphragma defects, kidney anomalies, extrapulmonary sequestration, muscle or skeleton system defects). Nearly one-third of the patients have congenital heart diseases. Although the most common one is  the atrial septal defect, vebtricular septal defect, patent ductus arteriosus or aorta coarctation can also be observed.  Aplasic patients are usually asymptomatic. Clinical findings change  with the presence of comorbid anomalies and their severity. Recurrent infections can increase the breathing distress. Although the patients with unilateral lung aplasia (agenesis) are believed to die  usually in the neonatal period, there are patients who live up to the adulthood, some of whom live without any symptoms. In aplasia, secretion not cleared from rudimentary bronchus and/or comorbid bronchiectasis  may partially be responsible for the risk of infection. Chronic respiratory failure and eventually respiratory infections can develop in adults.

Pulmonary hypoplasia is characterized by the presence of both bronchi (albeit rudimentary) and alveoly in an under-developed lobe. Both the size and the weight of the lung is reduced.

There are normally four stages of fetal lung development where the pathological process can occur at any level:

# embryonic: from conception – 5th week

# pseudoglandular: from 5th week – 17 week

# canalicular: from 16th week – 24 week (this stage may be more affected by          oligohydramnions)

# terminal sac or alveolar period: from 24th week to term and beyond

There are several key factors required for the adequate development of the lung. These are:

# sufficient amniotic fluid volumes

# adequate thoracic space-size

# normal breathing movement

# normal fluid within the lung

A deficiency in any of these could lead to pulmonary hypoplasia.

Most cases of pulmonary hypoplasia are secondary to other congenital anomalies or pregnancy complications. Some cases however can occur as primary event.

With secondary causes, it can result from factors directly or indirectly compromising the thoracic space available for lung growth.

 

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