Airways Disease & Bronchiectasis
- The airways are the plumbing of the lung — a branching tree of tubes that should taper smoothly and stay thin-walled.
- Bronchiectasis is permanent, abnormal widening of the airways. The cardinal sign: a bronchus wider than the artery running beside it.
- On CT, hunt for the signet ring sign (dilated bronchus + smaller artery) and airways that fail to taper as they head out toward the chest wall.
- Bronchial wall thickening, mucus plugging (the tree-in-bud pattern), and air trapping are the supporting cast that tell you the small airways are also unhappy.
- Bronchiectasis is a morphologic finding, not a single disease — your real job is to suggest why it's there.
Think of the bronchial tree as the household plumbing for air. Big pipe at the top (the trachea), splitting into smaller and smaller pipes, each one a little narrower than its parent, all of it heading out toward the walls. In a healthy lung those pipes are thin-walled and taper so gracefully that by the time they get near the edge of the lung, they've essentially vanished from view. Airways disease is what happens when the plumbing gets thick-walled, clogged, or stretched out like an old garden hose that's been left in the sun too long.
The pairing trick: bronchus vs. its buddy artery
Here's the single most useful idea on this whole page. Throughout the lung, a bronchus travels with a pulmonary artery branch like a pair of friends who refuse to walk anywhere alone. They run side by side, and — this is the key — they should be roughly the same diameter. The artery is the buddy. The bronchus should never be the bigger one.
So when you see a bronchus that's clearly fatter than its companion artery, the plumbing has stretched. That's bronchiectasis — literally "bronchus-stretching." On a CT slice that cuts the pair in cross-section, you get the famous signet ring sign: a fat round bronchus (the band of the ring) with a smaller dot of artery perched on its edge (the gemstone). Once you see it, you can't unsee it.
The second clue is failure to taper. Follow a bronchus from the center of the lung outward — if it stays the same caliber (or, alarmingly, gets wider) as it heads toward the periphery, that's abnormal. Healthy airways should shrink toward the edge, not stubbornly hold their width. And if you can see airways within about a centimeter of the pleural surface at all, they shouldn't be there.
Reading the morphology
Radiologists like to grade how bad the stretching is, mostly to describe it in the report:
| Pattern | What it looks like | Mental picture |
|---|---|---|
| Cylindrical | Uniform mild widening, tram-track parallel walls | A straight pipe, slightly oversized |
| Varicose | Beaded, irregular caliber | A string of sausages |
| Cystic | Clustered round sacs, sometimes with air-fluid levels | A bunch of grapes |
These aren't separate diseases — they're a spectrum of how far the damage has gone, cystic being the most severe. When the airways run roughly in the plane of the slice rather than straight through it, the parallel walls look like a set of tram tracks, which is the same finding seen lengthwise.
Bronchiectasis is best appreciated on a thin-section (high-resolution) chest CT. If you want a refresher on how those acquisitions and windows work, detour through the approach to chest CT.
The supporting cast: walls, mucus, and trapped air
Stretched airways rarely travel alone. The walls usually get thickened (chronic inflammation remodeling the pipe). When mucus and inflammatory gunk fill the smallest peripheral airways, you get the tree-in-bud pattern — clusters of tiny branching dots that genuinely look like a budding twig in springtime. Tree-in-bud screams small airways filled with stuff, classically infection or aspiration.
There's also mucus plugging in the bigger airways — a dilated bronchus packed with mucus instead of air shows up as a tubular soft-tissue density, sometimes branching like fingers of a gloved hand.
Finally, small-airway disease loves to cause air trapping: zones of lung that can't fully exhale, so they stay abnormally dark (lucent) on expiration. This produces mosaic attenuation — a patchwork of lighter and darker lung that looks like someone tiled the lung with two shades of gray. The trick to confirming air trapping is comparing inspiratory and expiratory images: trapped regions stay stubbornly black when the patient breathes out.
Not every dilated-looking airway is true bronchiectasis. Traction bronchiectasis is airways yanked open by surrounding scar in fibrotic lung — the dilation is a consequence of fibrosis, not a primary airway disease. The fix is to look at the lung around it: if it's distorted and fibrotic, you're dealing with the airway changes seen in interstitial lung disease, and that's the headline diagnosis. Also beware reversible "pseudo-bronchiectasis" that can appear with acute infection and resolve later.
So... why is it there?
Finding bronchiectasis is the easy part. The grown-up question is what caused it, and the distribution gives strong hints. Upper-lobe predominant disease nudges you toward cystic fibrosis or prior granulomatous infection like tuberculosis; lower-lobe and basal predominance is more the company kept by recurrent aspiration or post-infectious damage. A central, perihilar pattern with mucus plugging has its own classic associations with allergic airway disease.
When you report bronchiectasis, don't stop at the noun. Note the distribution (which lobes), severity (cylindrical to cystic), and any complications (mucus plugging, signs of active infection). That triad turns a vague observation into something a pulmonologist can actually use.
Airways disease overlaps with the obstructive world of COPD and emphysema — chronic bronchitis is fundamentally an airway-wall problem — and acute airway infection blurs into pneumonia when the infection spills into the air sacs. The bronchial tree doesn't respect our tidy chapter headings.
The single thing to walk away with: when a bronchus is fatter than the artery beside it and refuses to taper toward the chest wall, the plumbing is permanently stretched — that's bronchiectasis, and your next move is to ask the lung why.