PRES
- PRES stands for Posterior Reversible Encephalopathy Syndrome — a brain that's been pushed past the limit of its blood-pressure thermostat, leaking fluid (mostly vasogenic edema) into the back of the brain.
- The classic look on MRI is symmetric T2/FLAIR-bright swelling in the parieto-occipital lobes — the rear of the brain — usually sparing the very tip of the occipital pole.
- Common triggers: a blood-pressure spike, eclampsia/pre-eclampsia, kidney failure, and certain immunosuppressant or chemo drugs.
- "Reversible" is the hopeful part — fix the cause and it usually melts away — but it can bleed or infarct, so it is not always benign.
- The big mimic to rule out is a posterior circulation stroke; PRES is bilateral and symmetric, edema not following one artery's turf.
Imagine your brain's blood vessels have a built-in thermostat that keeps flow steady whether your blood pressure is sky-high or floor-low. That's cerebral autoregulation, and it's quietly heroic. PRES is what happens when the thermostat gets overwhelmed — usually by a sudden pressure surge — the vessels give up holding the line, and fluid starts weeping out into the brain tissue. The back of the brain takes the worst of it, and that's the whole plot.
Why the back of the brain?
Here's the elegant bit. The arteries that supply the front of the brain come with a generous coating of sympathetic nerves — think of them as little pressure-regulating bodyguards that clamp down when pressure rises. The posterior circulation (the back) is comparatively under-staffed in that department. So when blood pressure spikes, the front holds the door, the back gets overwhelmed first, and fluid leaks out there preferentially.
That fluid is mostly vasogenic edema — water seeping out of leaky vessels into the spaces between cells, like a garden hose with pinholes soaking the surrounding soil. That matters, because vasogenic edema is the "reversible" kind. Patch the hose, the soil dries out, the brain bounces back. Contrast that with the cytotoxic edema of a true stroke, where the cells themselves are dying and swelling — that water doesn't just evaporate when you fix the pressure.
The name oversells two things. It's not always posterior (the frontal lobes, cerebellum, and brainstem can join in), and it's not always reversible (it can progress to hemorrhage or infarction). The name is a useful starting picture, not a promise.
What it looks like on imaging
MRI is the star here, and the hero sequence is FLAIR — a T2-weighted image that mutes the bright spinal fluid so subtle edema next to the ventricles and at the brain's surface jumps out. If you want a refresher on why FLAIR does that, see common MRI sequences.
The picture is fairly recognizable once you've seen it: patchy-to-confluent T2/FLAIR-bright signal in the parieto-occipital white matter, usually on both sides and roughly symmetric. A classic clue is that it tends to spare the very tip of the occipital pole — the calcarine and paramedian occipital cortex — which helps separate it from a posterior cerebral artery stroke, where that exact territory is what dies.
CT is often the first test simply because the patient arrives confused, seizing, or with a screaming headache, and CT is fast. It may show low-density (dark) swelling in the same regions, but it's far less sensitive — a normal CT does not rule out PRES.
The detail that changes the conversation: diffusion
This is where attendings earn their coffee. On DWI (diffusion-weighted imaging), the sequence that lights up acute stroke like a road flare, PRES is usually not restricted — because the water is loose in the interstitium, not trapped in dying cells. So the typical pattern is: bright on FLAIR, but not bright-with-dark-ADC on diffusion. That mismatch is the tell that you're looking at reversible edema rather than a finished infarct.
If a region of PRES does show true diffusion restriction (bright DWI, dark ADC), don't ignore it — that area may have tipped over from reversible edema into actual infarction, and it's less likely to fully recover. PRES and stroke are not mutually exclusive.
Don't get fooled
The job in the reading room is mostly about not calling this a stroke. A posterior circulation ischemic stroke follows one artery's territory, is usually one-sided, and restricts on diffusion. PRES is bilateral, symmetric, spares the occipital poles, and typically doesn't restrict. The other neighbor worth a glance is cerebral venous sinus thrombosis, which can also cause posterior edema — so check the venous sinuses if the story is murky.
| Feature | PRES | Posterior stroke |
|---|---|---|
| Distribution | Bilateral, symmetric | One artery's territory |
| Edema type | Vasogenic (leaky vessels) | Cytotoxic (dying cells) |
| Diffusion (DWI) | Usually no restriction | Restricts |
| Occipital pole | Often spared | Often involved |
| Course | Usually reversible | Permanent tissue loss |
The single most useful question isn't on the scanner — it's the chart. Eclampsia, a hypertensive crisis, renal failure, or a recently started immunosuppressant/chemo drug turns a confusing posterior-edema picture into "oh, that's PRES." Imaging confirms; the clinical context usually diagnoses.
The takeaway
PRES is the brain's pressure thermostat losing a fight, leaking reversible (vasogenic) edema mostly into the back of the brain. Recognize the bilateral, symmetric, FLAIR-bright, diffusion-spared parieto-occipital pattern, anchor it to a trigger like high blood pressure or eclampsia, and the headline is genuinely good: fix the cause, and the brain usually mops up after itself.