By Dr Deepu
The lungs are normally divided into five lobes by three main fissures .
Occasionally, invaginations of the visceral pleura create accessory
fissures that separate individual bronchopulmonary segments into accessory
lobes .
An azygos lobe is found in approximately 0.4% of patients . In
contrast to other accessory lobes, the azygos lobe does not correspond to
a distinct anatomical bronchopulmonary segment .
It forms during
embryogenesis when the precursor of the azygos vein fails to migrate
to its medial position in the mediastinum, where it normally arches
over the origin of the right upper lobe bronchus.
This gives rise
to the following characteristics, which are visible on a standard chest
x-ray
: the laterally displaced azygos vein lies between folds
of parietal pleura, also referred to as the mesoazygos, where it assumes
a characteristic teardrop shape ; the mesoazygos indents the right
upper lobe, thereby creating the accessory (azygos) fissure, which
is similar in shape to an inverted comma; the fissure delineates the
azygos lobe, located superomedially ; laterally, the pleural folds
of the mesoazygos separate before reaching the chest wall, resulting in
a radiopaque triangular area ; and medially, the tracheobronchial
angle appears empty .
An azygos lobe may be confused with a pathological air space such
as a bulla or abscess . In addition, the abnormally located azygos
vein may be mistaken for a pulmonary nodule, while a consolidated
azygos lobe may be confused with a mass . An understanding of the
pathogenesis and characteristic x-ray features of the azygos lobe will
enable an accurate diagnosis in most cases .
If the x-ray findings
are equivocal, computed tomography will be diagnostic .
