PRES, or posterior reversible encephalopathy syndrome, is a neurological condition in which fluid leaks into brain tissue, causing swelling primarily in the back of the brain. It typically strikes suddenly, triggering seizures, confusion, headaches, and vision problems. The name includes “reversible” because most people recover fully with prompt treatment, but PRES is not always benign. Mortality has been observed in roughly 19% of patients, and functional impairments of varying degree have been reported in up to 44%.
What Happens in the Brain
The core problem in PRES is a breakdown of the blood-brain barrier, the tightly sealed lining of blood vessels that normally keeps fluid and large molecules out of brain tissue. When that barrier fails, fluid seeps into the surrounding brain, creating what doctors call vasogenic edema, essentially waterlogging in areas that shouldn’t have excess fluid.
Two main theories explain why the barrier breaks down. In the first, a sudden spike in blood pressure overwhelms the brain’s ability to regulate its own blood flow. When mean arterial pressure exceeds roughly 160 mm Hg, the small vessels in the brain can no longer constrict enough to protect downstream capillaries. Pressure builds, and fluid is forced through the vessel walls. In the second theory, the blood vessel lining itself is damaged directly by toxins, medications, or immune reactions, leading to the same leaky result even without extreme blood pressure. In practice, both mechanisms likely overlap in many patients.
The back of the brain (the parietal and occipital lobes) is hit hardest because the blood vessels in that region have fewer nerve fibers controlling their ability to tighten, making them more vulnerable to pressure swings. Less commonly, swelling also appears in the brainstem, cerebellum, or deeper brain structures.
Common Triggers
Severe high blood pressure is the most frequent trigger. This can happen during a hypertensive crisis, acute kidney disease, or any condition that causes blood pressure to spike rapidly. People whose blood pressure is normally well controlled may be especially susceptible because their brain vessels haven’t adapted to high pressures.
Preeclampsia and eclampsia during pregnancy are major causes. In one study of 47 eclamptic patients, nearly 98% showed PRES on brain imaging. The combination of high blood pressure, inflammation, and changes in blood vessel function during complicated pregnancies creates a perfect setup for the condition.
Immunosuppressive and chemotherapy drugs are another well-known trigger. These medications can directly damage blood vessel linings, causing barrier breakdown independent of blood pressure. Organ transplant recipients on anti-rejection drugs, cancer patients receiving certain chemotherapy regimens, and people with autoimmune diseases on aggressive treatment are all at elevated risk. Sepsis, severe infections, and autoimmune flares that cause widespread inflammation can also set off the cascade.
Symptoms to Recognize
PRES tends to develop over hours to days, not in an instant. The most common symptoms are:
- Seizures: often the symptom that brings patients to the emergency room, ranging from brief episodes to prolonged or repeated seizures
- Altered mental state: confusion, drowsiness, difficulty concentrating, or in severe cases unresponsiveness
- Visual disturbances: blurred vision, loss of parts of the visual field, or even temporary blindness, reflecting the swelling in the vision-processing areas at the back of the brain
- Severe headache: often sudden and intense, sometimes the earliest symptom before other signs appear
Not everyone gets all four. Some people present mainly with seizures and confusion, while others notice vision changes first. Because these symptoms overlap with stroke, meningitis, and other emergencies, brain imaging is essential to confirm what’s happening.
How PRES Is Diagnosed
An MRI is the gold standard. The hallmark finding is bright signal on FLAIR and T2-weighted sequences (two types of MRI views) in the parietal and occipital regions at the back of the brain. This brightness represents the fluid buildup in brain tissue. A key distinction from stroke is that in PRES, diffusion-weighted imaging usually looks normal or less severely affected, meaning brain cells are swollen with fluid but not yet dying.
Doctors confirm the diagnosis by showing that the abnormalities resolve on follow-up imaging after treatment. If repeat imaging isn’t available, a full return to normal neurological function supports the diagnosis. CT scans can sometimes pick up the swelling, but MRI is far more sensitive and is the preferred test.
Treatment and What to Expect
Treatment targets the underlying trigger. When high blood pressure is the driver, it needs to come down, but carefully. Guidelines call for reducing blood pressure by no more than 20% to 25% in the first few hours. Dropping it too fast can starve the brain of blood flow and cause additional injury. This is done with intravenous medications in a hospital setting, typically in an intensive care or step-down unit where blood pressure can be monitored continuously.
If a medication caused PRES, that drug is stopped or switched. For eclampsia, delivery of the baby is the definitive treatment, alongside magnesium sulfate to control seizures and antihypertensive medications. In the eclampsia study, all mothers recovered, with an average hospital stay of about four days. Seizures during PRES are treated with standard anti-seizure medications, which are often tapered off once the underlying cause is controlled and imaging improves.
Recovery Timeline
Most people see meaningful improvement within days of starting treatment. In a large study tracking recovery, neurological symptoms resolved at a median of 14 days, though the range was enormous, from less than a day to over a year in rare cases. MRI abnormalities take a bit longer to clear. Complete resolution on imaging occurred at a median of 21 days, and partial resolution at about 18 days. In practical terms, many patients feel significantly better within the first one to two weeks, but follow-up imaging a few weeks later is typical to confirm the swelling has resolved.
When PRES Isn’t Fully Reversible
Despite its name, PRES does not always reverse completely. Bleeding within the areas of swelling occurs in 10% to 30% of cases, and hemorrhagic PRES carries a higher risk of lasting damage. Historical reports put mortality in severe or hemorrhagic cases at 16% to 29%, though aggressive modern intensive care has improved these numbers significantly.
Permanent damage is more likely when treatment is delayed, when the underlying cause isn’t identified quickly, or when the edema is severe enough to transition from simple fluid leakage to actual tissue death. The longer brain tissue stays swollen, the greater the risk that the initially reversible injury becomes permanent. This is why rapid recognition and treatment matter so much: what starts as a recoverable problem can become a lasting one if the window for intervention is missed.
Recurrence
PRES can happen more than once if the triggering condition recurs. People who experienced PRES during a hypertensive crisis remain vulnerable if blood pressure spikes again. Those who developed it from a medication may see it return if rechallenged with the same or a similar drug. Awareness of a prior episode helps guide future treatment decisions, particularly for transplant recipients or cancer patients who may need ongoing immunosuppressive therapy. Choosing alternative agents or maintaining tighter blood pressure control can reduce the chance of a repeat episode.