Introduction:
Posterior Reversible Encephalopathy Syndrome (PRES) is a neurotoxic state characterized by vasogenic edema predominantly involving the posterior circulation territories of the brain. It is a reversible condition if promptly identified and treated. Imaging, particularly MRI, plays a vital role in the diagnosis, assessment, and follow-up of PRES.
Etiologies & Risk Factors:
- Hypertensive encephalopathy
- Eclampsia/preeclampsia
- Renal failure
- Autoimmune diseases
- Cytotoxic/immunosuppressive drugs (e.g., cyclosporine, tacrolimus)
- Sepsis
Clinical Presentation:
- Headache
- Altered mental status
- Seizures
- Visual disturbances (e.g., cortical blindness)
- Focal neurological deficits
Imaging Modalities Used:
1. MRI Brain – Modality of Choice
T2W/FLAIR Sequences:
- Symmetrical hyperintensities in:
- Parieto-occipital lobes (most common)
- Also involves frontal lobes, temporal lobes, cerebellum, and brainstem
- Hyperintensities represent vasogenic edema
DWI/ADC:
- Increased ADC values → vasogenic edema (no restricted diffusion)
- Helps differentiate from cytotoxic edema (e.g., infarcts)
T1W with contrast:
- Usually no enhancement, but patchy or leptomeningeal enhancement may be seen in some cases
GRE/SWI:
- May show microhemorrhages, especially in chronic or severe cases
2. CT Brain (Non-contrast):
- May show hypodense areas in posterior white matter
- Less sensitive than MRI, especially in early/mild cases
Distribution Patterns:
- Parieto-occipital pattern (classical)
- Frontal pattern
- Brainstem or cerebellar involvement
- Basal ganglia or thalamic involvement (less common)
- Spinal cord involvement (rare)
Differential Diagnosis:
- Acute infarct (shows restricted diffusion on DWI)
- Hypoglycemia
- Hypoxic-ischemic injury
- Encephalitis
- Demyelinating disorders
Prognosis:
- PRES is reversible with prompt control of blood pressure or discontinuation of offending agents.
- Follow-up imaging shows resolution of edema in most cases within 1–2 weeks.
Summary / Key Points:
- PRES is best detected using MRI, particularly T2/FLAIR and DWI/ADC.
- Typical findings are bilateral posterior vasogenic edema without restricted diffusion.
- DWI and ADC sequences are crucial to distinguish PRES from infarction.
- Early recognition on imaging allows timely management and reversibility of symptoms.
