Cysts are thin-walled and air-filled (unlike emphysema, which has no wall); their shape and distribution identify the cause — round uniform (LAM), bizarre upper-zone (LCH), basal paramediastinal (Birt–Hogg–Dubé), few perivascular with ground glass (LIP/Sjögren).
Orient first
- LAM: women of reproductive age or tuberous sclerosis; uniform round cysts throughout; renal angiomyolipomas, chylous effusions.
- BHD: lower-zone, paramediastinal, elliptical cysts; renal tumours and skin fibrofolliculomas.
- LIP: few cysts with ground glass and centrilobular nodules in Sjögren or HIV.
Acquire the study
- High-resolution CT chest, thin sections, inspiratory; include the upper abdomen (kidneys).
The manoeuvre
- Lung window: confirm walls (cyst) vs no wall (emphysema).
- Size, shape and number of cysts; zonal distribution (upper, lower, diffuse).
- Associated nodules (LCH), ground glass (LIP), pneumothorax.
- Upper abdomen on the same series: renal angiomyolipomas (LAM/TSC) or renal masses (BHD).
What confirms it
- A cyst pattern with its extrapulmonary association (VEGF-D, renal lesions, skin, serology).
What licenses you to exclude it
- Centrilobular lucencies without walls around a central dot are emphysema.
The classic misread
- Calling cystic bronchiectasis a cystic lung disease — cysts connect to airways.