CNS Vasculitis & Moyamoya
- CNS vasculitis is inflammation of the brain's own blood vessels — the imaging clue is strokes in odd, non-vascular-territory patterns, often in someone too young for plain old atherosclerosis.
- The catheter angiogram look is alternating narrowing and widening ("beading") in multiple vessels — but a normal angiogram does not rule it out, because the smallest vessels are below its resolution.
- Moyamoya is a separate, slow disease: the internal carotid arteries silently strangle themselves shut at the top, and the brain grows a tangle of tiny collateral vessels to survive.
- That collateral tangle looks like a faint "puff of smoke" on angiography — which is literally what moyamoya means in Japanese.
- Both are the answer to the same exam question: young patient, recurrent strokes, no obvious cardiac or atherosclerotic cause.
Most strokes are boring in a reassuring way: a clot floats up from a furred-up carotid or a fibrillating heart, lodges in a brain artery, and kills the territory downstream. But every so often you get a stroke that makes no sense — a young person, a clean heart, clean neck vessels, and yet little infarcts scattered around the brain like someone flicked paint at the wall. That's when you start thinking the vessels themselves are the problem. This page is about two ways the brain's plumbing turns on itself.
Vasculitis: the pipes get inflamed
Imagine the blood vessels in your brain as garden hoses. Normally they're smooth and supple. In vasculitis, the wall of the hose gets inflamed, swollen, and irritated — so the channel inside narrows in patches, scars, and in places balloons back out. The result is a hose that's lumpy: pinched here, bulging there.
That lumpiness is the whole imaging story. On a catheter angiogram the classic look is beading — alternating segments of narrowing and dilation along a vessel, like a string of sausages. Multiple vessels, in multiple territories, is what nudges you toward inflammation rather than a single clot.
The downstream consequence is what the patient actually feels: small infarcts in scattered, non-territorial distributions, often hitting both gray and white matter, sometimes with a few microbleeds. On MRI you may also see vessel-wall thickening that lights up after contrast — the inflamed hose wall enhancing.
A crucial humility check: a normal angiogram does not exclude CNS vasculitis. When the disease lives in the truly tiny vessels, the angiogram — which can only resolve the bigger pipes — looks fine. Diagnosis then leans on the clinical picture, spinal fluid, and sometimes a brain biopsy. Imaging supports the story; it rarely closes the case alone.
CNS vasculitis comes in two broad flavors: primary (the inflammation is confined to the brain and cord, with no systemic disease) and secondary (the brain is caught up in a body-wide process — infection, autoimmune disease, drugs). You don't need to memorize the list; you need to recognize the pattern and say the word "vasculitis" out loud.
The great mimic is RCVS (reversible cerebral vasoconstriction syndrome). It also produces beaded vessels and thunderclap headache, but the narrowing is spasm, not inflammation — and it reverses within weeks. Treatment is opposite (vasculitis gets immunosuppression; RCVS does not), so the distinction matters. The tell is largely time: if the beading resolves on follow-up imaging, it was probably RCVS all along.
Moyamoya: a slow self-strangulation
Moyamoya is a completely different mechanism, even though it lands in the same "young person, weird strokes" bin. Here the internal carotid arteries — the big trunk lines feeding the front of the brain — progressively thicken and clamp themselves shut right at the top, where they branch. No inflammation, no clot. Just a slow, idiopathic narrowing that strangles the supply over years.
The brain, being a survivalist, refuses to die quietly. Starved of flow, it sprouts a frantic mesh of tiny collateral vessels at the base of the brain to smuggle blood around the blockage. On an angiogram that mesh is faint, fuzzy, and hazy — which is exactly why it was named moyamoya, Japanese for "puff of smoke." A wispy gray cloud where crisp arteries should be.
You can catch the same thing non-invasively. On MR angiography and routine MRI, look for the carotid tops dropping out and an odd cluster of tiny flow voids deep in the brain — the collaterals, caught in cross-section. There's even a soft MRI sign where sulci over the surface enhance like a faint vine — the dilated pial collaterals straining to help.
Because moyamoya is a problem of too little flow, its strokes are often watershed or borderzone in pattern — the last fields the failing arteries can reach — and kids are prone to ischemia while adults more often bleed when the fragile collaterals rupture.
How to keep them straight
| Feature | CNS vasculitis | Moyamoya |
|---|---|---|
| Core problem | Vessel wall inflammation | Slow ICA stenosis/occlusion, no inflammation |
| Angiogram clue | Multifocal beading (narrow–wide–narrow) | Carotid dropout + "puff of smoke" collaterals |
| Vessel wall on MRI | Often enhances (inflamed) | No inflammatory enhancement |
| Stroke pattern | Scattered, non-territorial; ± microbleeds | Watershed ischemia (kids), hemorrhage (adults) |
| The big mimic | RCVS (reversible, spasm) | Moyamoya syndrome from another disease (e.g. sickle cell) |
One last bit of vocabulary that trips people up: when that puff-of-smoke pattern shows up because of another condition — sickle cell disease, prior radiation, certain syndromes — it's called moyamoya syndrome, while idiopathic primary disease is moyamoya disease. Same picture, different backstory.
If you only carry one thing away: when the strokes don't respect the usual vascular territories and the patient is too young or too healthy for ordinary stroke, stop blaming clots and start interrogating the vessel walls themselves. Also rule out the sneakier structural causes like a dissection before you commit to "the immune system did it."