Compensatory lung hyperinflation
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Compensatory lung hyperinflation (a.k.a. compensatory lung overinflation) is a situation in which due to loss of volume of a lung, unaffected parts of the same lung overinflate as compensation. In more severe cases, the contralateral lung may also overinflate with possible mediastinal shift towards the volume loss 1-4.
When the contralateral lung overinflates to such an extent that it herniates across the midline into the other hemithorax, it is known as massive compensatory hyperinflation or pseudohorseshoe lung (cf. true horseshoe lung) 1,2.
- congenital e.g. pulmonary agenesis 4
Plain radiograph / CT
The commonest finding on a plain radiograph of compensatory hyperinflation of the lung is hyperlucency of the affected hemithorax, often the underlying causal lung pathology is also visible. Relative oligemia is also present in hyperinflation, so that blood vessels are spaced further apart and decreased in number, best appreciated by comparing with the "normal" lung 3.
The presence of compensatory hyperinflation can be helpful as a surrogate marker for the presence of atelectasis, especially on radiographs, when the atelectatic segment cannot be directly visualized 3. A good example of this is in left upper lobe collapse, when the luftsichel sign alerts the reporter to the lobar collapse which may itself be harder to appreciate 3. It is the compensatory overinflated superior segment of the left lower lobe lying between the collapsed left upper lobe and the arch of the aorta that causes this classic sign.
- lung hyperinflation: due to an obstructive process, e.g. COPD, asthma