What is the diagnostic approach for Posterior Reversible Encephalopathy Syndrome (PRES)?

Diagnostic Approach for Posterior Reversible Encephalopathy Syndrome (PRES)

MRI brain without IV contrast is the gold standard for diagnosing PRES, demonstrating characteristic bilateral vasogenic edema predominantly in the parieto-occipital regions on T2-weighted and FLAIR sequences, with diffusion-weighted imaging (DWI) and apparent diffusion coefficient (ADC) maps confirming vasogenic rather than cytotoxic edema. 1, 2

Clinical Recognition

PRES should be suspected in patients presenting with acute neurological symptoms including:

• Altered mental status or encephalopathy 3, 4
• Seizures or status epilepticus 3, 4
• Visual disturbances (including cortical blindness) 3, 5
• Severe headache 3, 4

Key clinical contexts that should trigger diagnostic consideration:

• Acute hypertensive crisis with blood pressure exceeding autoregulatory limits 6, 7
• Recent initiation or dose escalation of immunosuppressive agents (particularly cyclosporine) 3, 7
• Eclampsia or preeclampsia 4, 8
• Renal failure or acute kidney injury 8, 5
• Active chemotherapy or cytotoxic drug administration 3, 7
• Autoimmune disease flares 8

Essential Imaging Protocol

MRI brain without IV contrast is mandatory and should include: 1, 2

• T2-weighted and FLAIR sequences to identify hyperintense vasogenic edema in cortical and subcortical regions 1, 2
• Diffusion-weighted imaging (DWI) with calculated ADC maps to confirm vasogenic edema (increased ADC values) and exclude cytotoxic edema from ischemic stroke 1, 2
• T2 GRE or susceptibility-weighted imaging (SWI)* to detect microhemorrhages, which are compatible with PRES diagnosis 1, 8

Characteristic MRI Findings

Distribution pattern: 2, 4

• Bilateral parieto-occipital regions (most common, predominantly white matter) 2, 4
• Frontal and temporal lobes (frequently involved) 2, 4
• Cerebellum, brainstem, deep white matter, and thalamus (less common but possible) 2

Signal characteristics: 2

• Hyperintensity on T2-weighted and FLAIR images 2
• Isointensity or mild hyperintensity on DWI with increased ADC values (vasogenic edema) 2
• Hemorrhagic transformation may be present and does not exclude diagnosis 8

CT Limitations and Role

CT head without contrast has significant diagnostic limitations: 1

• Low tissue contrast resolution prevents detection of subtle white matter edema 1
• May appear completely normal in early PRES 1

CT should only be used when: 1, 3

• MRI is not immediately available and intracranial hemorrhage must be excluded urgently 1, 3
• Patient has contraindications to MRI 3

Critical Laboratory Workup

Essential blood tests to identify underlying causes and exclude mimics: 1

• Electrolytes (sodium, calcium, magnesium) 1
• Glucose 1
• Complete blood count 1
• Inflammatory markers (CRP, ESR) 1
• Renal function (BUN, creatinine) 1
• Blood pressure monitoring (continuous if possible) 1

Medication history review: 1

• Immunosuppressants (cyclosporine, tacrolimus) 1
• Chemotherapy agents 1
• Recent drug changes or dose escalations 1

Differential Diagnosis to Exclude

The following conditions must be systematically ruled out as they require different management: 1

Vascular Mimics

• Acute ischemic stroke in posterior circulation - distinguished by restricted diffusion (low ADC values) on DWI, unlike PRES which shows increased ADC 1, 2
• Subarachnoid hemorrhage - excluded by CT or FLAIR imaging showing sulcal hyperintensity 1
• Cerebral venous sinus thrombosis - requires MR venography 1

Inflammatory/Infectious Mimics

• Viral encephalitis - requires CSF analysis when infectious etiology suspected 1
• Autoimmune encephalitis - shows different enhancement patterns on contrast-enhanced sequences 1
• CNS vasculitis - demonstrates vessel wall abnormalities on contrast imaging 1

Metabolic Mimics

• Wernicke encephalopathy - different distribution pattern (mammillary bodies, thalamus) 1
• Osmotic demyelination syndrome - central pontine involvement 1

Diagnostic Confirmation Strategy

Follow-up imaging is essential to confirm diagnosis: 2, 7

• Repeat MRI in 7-14 days after treatment initiation to document resolution of vasogenic edema 2, 7
• Clinical and radiological reversibility confirms PRES diagnosis retrospectively 7, 8
• Persistent abnormalities or progression suggest alternative diagnosis (particularly if ADC values decrease, indicating cytotoxic edema and irreversible infarction) 2

Common Diagnostic Pitfalls

Avoid these errors that delay diagnosis:

• Relying on CT alone when MRI is available - CT misses early or subtle PRES in most cases 1
• Failing to obtain DWI/ADC sequences - these are critical to distinguish vasogenic from cytotoxic edema 2
• Dismissing diagnosis when hemorrhage is present - microhemorrhages are compatible with PRES 8
• Assuming normal imaging excludes PRES - rare mild cases may show minimal or no radiographic changes 8
• Delaying imaging in perioperative patients with delayed emergence from anesthesia - PRES should be in the differential 5

Neurology consultation is recommended for comprehensive assessment, particularly when diagnosis is uncertain or atypical features are present 3