Displacement of fissures is a reliable feature of lobar collapse, and is generally characteristic depending on the affected lobe. The pulmonary vessels and bronchi become crowded together in the affected lobe as the lung loses volume. The sign may be one of the earliest seen in lobar collapse and can often be readily appreciated by comparison with previous radiographs.
Hilar elevation on the posteroanterior (PA) chest radiograph is a well-known sign of upper lobe collapse: the ipsilateral interlobar and lower lobe arteries remain visible as these structures are still outlined by aerated lung. It would seem logical to consider 'hilar depression' to be a sign of lower lobe collapse, but some authorities believe the small hilum to be a more accurate description. This is due to the fact that when a lower lobe collapses, the opaque, collapsed lobe obscures the lower lobe artery that lies within it, and the interlobar artery is usually rotated so the margin is no longer in profile to the frontal X-ray beam. Consequently, it is difficult to recognise the hilum as being depressed and, instead, smaller vascular structures are noted at the expected position of the hilum. Occasionally, confusion with a central hilar mass/adenopathy can arise if the convex margin of an interlobar artery remains visible due to minimal rotation.
As well as vascular reorientation, hilar bronchial alterations also occur. The central large bronchi undergo characteristic changes in position with collapse of either the upper or lower lobe. When either upper lobe collapses significantly, the ipsilateral main bronchus becomes more horizontally orientated than usual; hence the bronchus intermedius and the left lower lobe bronchus swing laterally. Conversely, when either lower lobe collapses, each main bronchus is more vertically orientated than usual, with a medial swing of the bronchus intermedius on the right and the lower lobe bronchus on the left.
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