Pulmonary agenesis refers to complete absence of a lung or lobe and its
bronchi. Aplasia refers to absence of lung tissue,
but rudimentary lobar bronchi are present. Hypoplasia
refers to an underdeveloped lobe which contains both alveoli and bronchi.
Approximately 1 in 15,000 children are born with congenital absence of one
lung and the associated bronchus. Unilateral pulmonary agenesis most likely
results from an in-utero insult during the 4th
week of gestation . Pulmonary agenesis occurs with equal frequency on the
left and right side (other authors report it is more common  or twice as
common on the left ). Right sided agenesis, however, is associated with a
much worse prognosis due to greater anatomic distortion of the airway and great
vessels, recurrent infections, and tracheobronchomalacia.
Concomitant anomalies are frequently found (more than 50% of affected fetuses)
including congenital heart disease, vertebral abnormalities, imperforate anus,
renal agenesis, and tracheoesophageal fistula. [1,10]
Pulmonary agenesis appears as an opaque hemithorax
on the affected side . There is mediastinal shift
into the affected side, and compensatory hyperinflation of the remaining lung
. In right lung agenesis malposition of the
aortic arch and descending aorta are caused by the extreme dextroposition
resulting from the absence of the right lung. The distal trachea is commonly compressed
and bowed posteriorly by the dextroposed
crossing aortic arch. The left mainstem bronchus is
also frequently compressed between the enlarged solitary left oulmonary artery anteriorly, and
the malpositioned descending aorta posteriorly. The ipsilateral lung
parenchyma, blood vessels, and airways are absent at CT .
In pulmonary aplasia there is no lung present,
however, there is a rudimentary bronchus that ends in a blind pouch . In hypoplasia the alveoli and bronchi are present, but the
involved lobe is small. The right upper and right middle lobes are most
commonly involved . The left lung is involved in about 25% of cases .
Although patients with pulmonary hypoplasia typically
have minimal respiratory symptoms, the condition is commonly associated with
other anomalies (seen in up to 60% of patients) including: Spinal anomalies
(scoliosis, hemivertebrae), esophageal or
diaphragmatic hernia, and pulmonary venolobar
syndrome (or Scimitar syndrome). At CXR, the affected lung appears small and
may be hyperlucent due to oligemia
(alternatively, the contralateral lung may appear hyperlucent secondary to compensatory hyperexpansion)
. The ipsilateral pulmonary artery usually
appears diminutive and ipsilateral medistinal shift is typically present .
Interruption of the left or right pulmonary artery is an uncommon anomaly.
In this congenital anomaly the proximal portion of the main pulmonary artery
(arising from the primitive 6th aortic arch) fails to appear during embryologic
development . The term interruption is preferred to absence in view of the
fact that it is usually only the proximal section of the vessel that is absent
while the more peripheral intrapulmonary arterial network remains intact [4,7].
This is explained by the different embryologic origins of the proximal and
distal pulmonary artery branches . The intact intrapulmonary vessels receive
oxygenated blood through systemic collaterals such as the bronchial, intercostal, or internal mammary arteries , or via a PDA
In most cases the interrupted pulmonary artery lies on the side opposite the
aortic arch- therefore, right sided interruption is more common [5,8].
Interruption of the left pulmonary artery is usually associated with a right
sided aortic arch and other cardiovascular anomalies- most commonly tetrology of Fallot . When the
interruption is right sided, there may even be an anomalous artery arising from
the ascending aorta . Patients with right sided interruption have been
grouped into three categories: 1- those having an isolated anomaly (most
common); 2- those having an associated left-to-right shunt (usually a patent ductus arteriosus); and 3- those
having associated pulmonary hypertension (PAH affects 19-25% of patients and is
the most important prognostic indicator) [4,8].
Individuals in groups 2 and 3 are unlikely to survive beyond infancy, while
those in group 1 often present as adults . Recurrent pulmonary infection,
hemorrhage, and mild dyspnea on exertion are the most
common symptoms . Hemoptysis occurs in about 10%
of cases due to rupture of thin-walled, hypertrophied collateral vessels .
The hemoptysis is usually minor and self-limited .
Radiographic findings include an inapparent hilum on the affected side and an enlarged contralateral hilum. The ipsilateral lung is hypoplastic
with volume loss, elevation of the hemidiaphragm, and
mediastinal shift to the affected side .
Peripherally the ipsilateral lung is oligemic, but there is a reticular network of vascular
markings which reflect the collateral arterial supply . There is hyperinflation
of the contralateral lung which herniates
across midline [4,8]. Rib notching may be seen on the
affected side if the collateral supply has developed from the intercostal arteries . In addition to the other findings
already described, CT will demonstrate a fine reticular increased attenuation
within the periphery of the affected lung and serrated thickening of the pleura
that represents trans-pleural collateral vessels .
(PVS), Hypogenetic lung syndrome, or Scimitar
PVS is also known as Scimitar syndrome or hypogenetic
lung syndrome. The disorder is most frequently characterized by unilateral
pulmonary aplasia/ hypoplasia
and ipsilateral total or partial anomalous pulmonary
venous return (from the lower lobe) which drains into the inferior vena cava
below the diaphragm or at the junction of the inferior vena cava at the right
atrium (or less commonly to the hepatic, azygous, or
portal vein, or into the right atrium). The draining vein produces a
curvilinear vascular shadow which courses towards the hemidiaphragm
and has the appearance of a Turkish sword (or scimitar). Since the anomalous
vein drains blood from the lung into the inferior vena cava, a left-to-right
shunt is established. Scimitar syndrome occurs almost exclusively on the right,
with only one left-sided case report . Other findings include: ipsilateral hypoplastic or absent
central pulmonary artery (these patients will have a systemic arterial supply
to peripheral pulmonary arteries of the affected lung from the descending
thoracic or upper abdominal aorta), abnormal lobulation
(bilateral left bronchial branching patterns), horseshoe lung, pulmonary
sequestration, accessory hemidiaphragm, and cardiac dextropositioning due to the right lung hypoplasia.
An accessory hemidiaphragm is a thin membrane in
the right hemithorax that is fused anteriorly with the hemidiaphragm
and courses postero-superiorly to join the posterior
chest wall. The membrane separates the right hemithorax
into two parts- trapping all or part of the right middle or lower lobes beneath
it. If the trapped lung is not aerated, the accessory hemidiaphragm
and trapped lung appear as a solid mass along the right hemidiaphragm.
If the trapped lung is aerated, an accessory fissure-like oblique line can be
seen on the lateral chest radiograph. 
About 25% of affected patients have associated congenital heart disease - most commonly a secundum type atrial septal defect.
Patients with PVS are usually asymptomatic, but may have recurrent pulmonary
infections. Surgical correction is reserved for patients with a substantial
In hypoplasia the heart and mediastinum
are shifted towards the involved side (dextropositioning
of the heart if right sided). There is compensatory overexpansion of the
opposite lung. The thoracic cage is smaller on the affected side with
approximation of the ribs, although this may not be evident at birth. The
pulmonary vascularity on the involved side is usually
diminished. A retrosternal
opacity may be seen. This density has previously been erroneously ascribed to extrapleural areolar tissue
anterior to the visceral pleura of the right lung. In actuality, this density
is produced by the interface of the displaced mediastinum
with the aerated, small right lung (Proto, ARRS 1996). In PVS a scimitar vein
can be identified. On angiography there is an absent or hypoplastic
ipsilateral pulmonary artery, and there may be a
systemic arterial supply to the abnormal segment of lung.
In horseshoe lung the right and left lungs are fused posteriorly
by an ishmus of pulmonary tissue.
Horse shoe lung is an uncommon malformation that is typically associated
with hypogenetic lung syndrome or lobar agenesis-aplasia complexes . In horse-shoe lung a portion of the
right lower lobe crosses the midline and is fused with the left lower lobe .
Typically, the fused segments are sheathed in a continuous layer of parietal
pleura which forms a communication between the right and left pleural cavities
. Anatomically, this isthmus of pulmonary tissue is located posterior to the
heart and anterior to the esophagus and descending aorta . In most cases the
arterial supply to the isthmus is from an anomalous branch of the right
pulmonary artery . The bronchi arise from the right bronchial tree .
Asymptomatic patients are managed conservatively . The diagnosis can be
confirmed at angiography which demonstrates a branch arising from the inferior
aspect of the proximal right pulmonary artery crossing behind the heart to
supply a part of the left lung .
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