Dark bronchus sign | Radiology Signs

Answer:

The dark bronchus sign is the illusory appearance on HRCT of the chest of a normal, air-filled bronchus standing out as conspicuously darker than the lung parenchyma around it, produced when that parenchyma is diffusely and fairly uniformly affected by ground-glass opacification. Diffuse ground-glass change of this kind is easy to miss, particularly when the chest radiograph is normal or equivocal, so a bronchus that appears unexpectedly lucent against its background is a useful trigger to look harder for subtle, widespread disease rather than dismiss the scan as normal. The sign was first described in Pneumocystis jirovecii (formerly carinii) pneumonia, in which early, pre-consolidative alveolar involvement can produce exactly this uniform ground-glass pattern in an immunocompromised patient whose radiograph looks deceptively clear, but it is not specific to that organism and has since been reported with pulmonary oedema, diffuse alveolar haemorrhage and drug-induced pneumonitis, so it should prompt a search for the cause of the ground-glass opacity rather than a single diagnosis. It is distinguished from the air bronchogram, in which a bronchus is made conspicuous by denser, fluid-filled alveolar consolidation around it rather than by ground-glass change; if the ground-glass opacity progresses to frank consolidation, the appearance converts from a dark bronchus sign to a classical air bronchogram.

Why is it called so?

The bronchus looks conspicuously dark because ground-glass opacification has raised the attenuation of the surrounding lung while the air-filled lumen stays unchanged, an optical contrast effect rather than a true change in the airway itself.

Pathophysiology

Ground-glass opacity arises when alveolar airspaces are partially filled with fluid, cells or organisms, or when the interstitium thickens, without the airspaces becoming completely opacified; the result is a hazy increase in lung attenuation through which vessels and airway walls can still be seen. Because the bronchial lumen remains air-filled and its density is unchanged, the contrast between the near-zero attenuation of the airway and the modestly raised attenuation of the surrounding parenchyma widens, and the eye registers this as the bronchus standing out, darker than expected. In Pneumocystis jirovecii pneumonia, organisms and a foamy intra-alveolar exudate accumulate within the airspaces early in the illness, producing exactly this uniform, diffuse pattern before frank consolidation fills the alveoli completely and replaces the sign with an air bronchogram instead.

Alternative names: Black bronchus sign

Other associated named signs: Air bronchogram of alveolar consolidation and the crazy-paving pattern of ground-glass opacity with septal thickening seen as Pneumocystis pneumonia evolves

References

1. Yadav P, Seith A, Sood R. The ‘dark bronchus’ sign: HRCT diagnosis of Pneumocystis carinii pneumonia. Ann Thorac Med. 2007;2(1):26-7. PMID: 19724672.