Cystic bronchiectasis is also associated with an increased bronchoarterial ratio, meaning that the bronchioles are larger than the blood vessels that run alongside them. Infectious causes
Bacterial, mycobacterial, and fungal infections are common causes of lung cavities. Globally, tuberculosis is likely the most common infectious cause of lung cavities. Less commonly, parasitic infections can cause cavities. Viral infections almost never cause lung cavities; in a small study of immunocompromised patients with a lung infection, the presence of a cavity on CT scan essentially ruled out viral infection. In the same study, about one-third of the cavities were caused by a bacterial infection, another third were caused by a mycobacterial infection, and another third were caused by a fungal infection. Bacterial
Bacteria can cause lung cavities in one of two ways; they can either enter the lung through the trachea (windpipe), or they can enter through the bloodstream as septic pulmonary emboli (infected blood clots). Community-acquired pneumonia is an uncommon cause of lung cavities, but cavitary pneumonia is occasionally seen with Streptococcus pneumoniae or Haemophilus influenzae infection. However, since these two species of bacteria are such common causes of pneumonia, they may cause a significant fraction of all cavitary pneumonias. The most common bacterial causes of lung cavities are Streptococcus species and Klebsiella pneumoniae. Less commonly, the bacteria Staphylococcus aureus, Pseudomonas aeruginosa, Acinetobacter, Escherichia coli, and Legionella can cause cavitation.
Generally, cavities have walls that are at least 5 mm thick, while cysts have walls that are 4 mm or less, and often less than 2 mm.The distinction between cysts and cavities is important because the thicker the wall is, the more likely it is to be cancer. Thus, cystic lesions are unlikely to be cancer, while cavitary lesions are often caused by cancer. In a study from 1980 that used chest X-rays to evaluate 65 cases of solitary lung cavities, 0% percent of cavities with walls 1 mm or less were malignant (that is, cancerous), versus 8% of cavities with walls 4 mm or less, 49% of cavities with walls 5 to 15 mm, and 95% of cavities with walls 15 mm or greater. However, a 2007 study that used CT to evaluate lung cavities showed no relationship between wall thickness and the likelihood of malignancy. It did show that malignant cavities are more likely than benign cavities to have an irregular internal wall (49% vs 26%) and have an indentation of the outer wall of the cavity (54% vs 29%).Areas of emphysema are abnormal, air-filled spaces that usually do not have visible walls, and bullae are very thin walled (<1 mm). Cystic bronchiectasis is irreversible bronchial dilation, which is permanent widening of the bronchioles (small airways) in the lung. It can be distinguished on imaging by a lack of bronchial tapering, meaning that the bronchioles do not get narrower as they travel further into the lung.