Download Lobar Collapse (Atelectasis) General Features of Collapse Collapse

Lobar Collapse (Atelectasis)
General Features of Collapse
Collapse may affect a whole lung or a subdivision (lobe, segment or subsegment),
each variant generating different and characteristic radiographic signs. The plain
radiographic signs of lobar collapse may be conveniently divided into those that are
confined to the lobe and its investing pleura (lobar signs) and those that are remote
(extralobar signs).
Lobar Signs
Fissural shift invariably occurs with lobar collapse and is the most frequently detected
and reliable finding. If some air stays in the collapsed lobe the contained blood
vessels remain visible and appear crowded. In lobar collapse a wedge of lung is
involved with its apex attached to the hilum by vessels and airways and its base
usually maintaining peripheral (parietal) pleural contact. The radiographic density of
this wedge depends on: 1. the amount of air or fluid in the alveoli; 2. its vascular
perfusion, which is often reduced by local vasoconstriction secondary to hypoxia; and
3. the nonvascular tissue density, which in part reflects the degree of volume loss.
The wedge-like shadow of a collapsed lobe may be expanded at its apex by an
obstructing hilar mass. This sign may be produced in any lobe but is most easily
appreciated with right upper-lobe collapse caused by a bronchial carcinoma (Golden's
sign).
Extralobar Signs
Most extralobar signs represent compensatory displacements, and the extents of these
are variable and interdependent. In general, shifts are greatest in those structures
nearest to the collapse though the duration of the collapse is also an important
modifying factor. Thus acute changes predominantly affect the mediastinum and
hemidiaphragm and chronic ones the hilum and the adjacent unobstructed lung
(hyperinflation).
Elevation of the hemidiaphragm is most marked with lower lobe collapse, and will be
most marked when there is little hilar shift or compensatory hyperinflation.
Mediastinal shift is more pronounced with left-sided than with right-sided atelectasis
and occurs especially at the level of the collapsed lobe. Thus, in upper-lobe collapse,
tracheal shift and displacement of the anterior junctional line are often major features,
whereas the heart will move little, if at all.
The hilum may show two types of change, consisting of either gross upward or
downward displacement, or rearrangement of individual hilar components leading to
changes in its shape and prominence. The former can be detected by relating the hilar
points to each other, the left normally being higher than the right by 10–20 mm.
Lobes adjacent to the collapsed lobe become hyperinflated and frequently (but not
invariably) demonstrate an increase in transradiancy with a reduction in the number of
vessels per unit area (oligaemia). Hyperinflation often takes time to develop and can
be distinguished from obstructive over-inflation by the lack of air trapping on an
expiratory radiograph.
Collapse of Individual Lobes
Right Upper-Lobe Collapse
On the PA radiograph the minor fissure moves upwards, rotating about the hilum,
and often becomes concave inferiorly. With marked degrees of collapse the flattened
lobe, which by this stage invariably shows increased opacity. The trachea shifts to the
right , The right hilum is elevated.
On the lateral view the minor fissure and the upper half of the major fissure move
towards each other, reducing the intervening angle as they pivot on the hilum. The
anterior margin of the ascending aorta, if previously visible, becomes effaced and a
wedge-like opacity may extend back from the anterior aspect of the lobe to the hilum
(mediastinal wedge).
On CT : the collapsed right upper lobe forms a wedge of soft tissue immediately
adjacent to the mediastinum, extending from the hilum to the anterior chest wall. Its
lateral border (minor fissure) and posterior border (major fissure) are sharp and may
be mildly convex, straight, or concave. In the presence of a hilar mass the lateral
border becomes sigmoid . The carina and right mainstem bronchus are angulated
anteriorly.
Right Middle-Lobe Collapse
Collapse of the middle lobe is usually more obvious on a lateral than on a frontal
radiograph, and its signs on the latter may be subtle and easily missed. In addition, in
the frontal projection the signs often become less obvious as the degree of collapse
increases and this may mistakenly be interpreted as improvement. In the frontal
projection the minor fissure moves down, pivoting about the hilum, however increase
in radio-opacity may be minimal and difficult to appreciate, it is nevertheless usually
sufficient to blur the normally sharp right-heart border (silhouette sign) and this is the
best sign of middle-lobe collapse on a PA view if the minor fissure is not visible. The
significance of such a finding must be assessed with a lateral view, on which the signs
of a middle-lobe collapse are usually more obvious, the collapsed lobe having a
triangular configuration with its apex at the hilum . The lobe is angulated caudally and
usually makes contact anteriorly with the lower sternum — a finding that helps to
differentiate middle-lobe collapse from fluid encysted in the major fissure. A lordotic
view, like the lateral, orientates the collapsed lobe so that one long axis is tangential
to the X-ray beam making the increased radio-opacity more readily appreciated. In the
lordotic projection the collapsed middle lobe is triangular, with its apex pointing
laterally.
On CT the collapsed right middle lobe adopts a triangular shape with a laterally
pointing apex that is retracted from the chest wall, and a base against the mediastinum
. The posterior border (the major fissure) is usually quite sharp whereas the anterior
(the minor fissure) is often less well defined.
Collapse of the Right or Left Lower Lobe
Both these lobes collapse backwards, downwards and medially. With minor volume
loss and maintained aeration, the reorientated major fissure may become visible on
the frontal view as an oblique line passing downwards and outwards from the hilar
region. The medial end of such a re-orientated fissure passes through the hilar shadow
to the spine, distinguishing it from the minor fissure which stops at the interlobar
pulmonary artery.
As collapse progresses, a triangular opacity develops, with its apex in the hilar region
and its base on the diaphragm . This triangular opacity may be difficult to appreciate
on the left where it lies behind the heart. The lateral border of the collapsed lower
lobe is usually well defined on the frontal view, but exceptions are not uncommon.
With further loss of volume the lower lobe becomes a thin slab hugging the
mediastinum. On the right, however, the (aerated) middle lobe usually makes contact
with the border forming part of the hemidiaphragm, which consequently remains
sharp. Hilar depression, medial shift of its inferior components (making the hilum
appear small), and hyperinflation of the ipsilateral upper lobe are common and useful
signs . Diaphragmatic and mediastinal shift are variable in degree; the latter is largely
confined to the lower mediastinum where the heart shifts to the ipsilateral side.
In the lateral view the oblique fissure moves downwards and backwards in an
approximately parallel fashion and the posterior part of the hemidiaphragm becomes
effaced. With marked volume loss the main finding is an increase in the radio-opacity
of the posterior costophrenic angle which may be difficult to identify and which may
closely resemble a small pleural effusion.
On CT lower-lobe collapse produces a wedge of tissue abutting the mediastinum and
spine that is sharply marginated by the oblique fissure which faces laterally or
anterolaterally. The wedge maintains contact with the mediastinum and diaphragm.
Collapse of the Left Upper Lobe
The presence of the lingula prevents collapse of the left upper lobe being a mirror
image of right upper-lobe collapse. The main finding on the frontal radiograph is a
veil-like opacity with a hazy margin, spreading outwards, upwards and downwards
from the left hilum . The outlines of the aortic knuckle, left hilum, and left-heart
border are ill defined and the upper mediastinum shifts towards the left. As volume
loss increases there is compensatory hyperinflation of the left lower lobe which
intrudes medially and at the lung apex. The hilum is elevated and reorientated so that
the left mainstem bronchus becomes more horizontal. Because the volume of the left
upper lobe exceeds that of the right, collapse generates greater compensatory changes.
On the lateral view the oblique fissure moves upwards and forwards, remaining
relatively straight and roughly parallel to the anterior chest wall.
On CT the collapsed left upper lobe adopts a triangular configuration with its base
anterolaterally and its apex directed posteriorly towards the hilum. Contact with the
apex of the thorax may be lost.