Lung bulla

What it means

A lung bulla is an air-filled sac inside the lung, generally defined as larger than one centimetre across, with a thin wall measuring less than a millimetre. It forms when the delicate walls between neighboring air sacs (alveoli) break down and merge, leaving a single larger space that no longer participates in gas exchange the way healthy lung tissue does. A bulla can be a solitary finding or one of several scattered through the lung.

Why it appears on a CT or MRI report

Because bullae are filled with air rather than tissue or fluid, they show up on CT as sharply defined, very dark (low-density) round or oval areas surrounded by a thin, barely visible wall. Radiologists record the size, number, location, and how much of the surrounding lung looks otherwise healthy versus emphysematous. A single small bulla in an otherwise normal-looking lung is read very differently from multiple large bullae crowding out functioning lung tissue, sometimes called bullous lung disease.

What it usually means

The most common cause is emphysema, the lung damage associated with long-term smoking, where destroyed alveolar walls gradually coalesce into larger air spaces. Bullae can also appear in younger, otherwise healthy people, sometimes clustered near the top of the lung, occasionally linked to genetic factors such as alpha-1 antitrypsin deficiency or connective tissue conditions. A small, stable bulla found incidentally on a scan usually causes no symptoms and no functional problem, since the surrounding lung compensates. Larger bullae can compress nearby healthy lung tissue and reduce breathing capacity, and they carry a small but real risk of rupturing, which lets air escape into the chest cavity (a pneumothorax).

When to follow up

A small, incidentally found bulla in someone without lung symptoms usually just gets noted, often with a recommendation to stop smoking if applicable, since that is the single biggest factor in whether bullae progress. Seek urgent medical care for sudden, sharp chest pain or sudden shortness of breath, especially if you already know you have a bulla — these can be signs of a ruptured bulla causing a collapsed lung. Large or enlarging bullae, or ones affecting a significant portion of the lung, are usually followed by a pulmonologist and may occasionally be considered for surgical treatment.

A plain-language way to picture it

Imagine a sponge that has had several of its small pores merge into one larger air pocket after years of wear. The sponge still holds its shape and mostly still works, but that one big pocket doesn't soak up water the way the tiny pores around it do, and if it's stretched too thin, that thin patch is the one most likely to tear. A lung bulla is that worn, merged pocket — usually stable, but the spot to watch if something changes suddenly.