What is the appropriate management for FLAIR (Fluid-Attenuated Inversion Recovery) hyperintensity on MRI (Magnetic Resonance Imaging)?

Management of FLAIR Hyperintensity on MRI

The management of FLAIR hyperintensity depends entirely on the clinical context and anatomical location, requiring immediate correlation with clinical presentation, additional MRI sequences (particularly DWI and contrast-enhanced imaging), and vascular imaging to determine the underlying etiology and guide treatment. 1

Initial Diagnostic Algorithm

Step 1: Determine Clinical Context and Location

Acute neurological symptoms (stroke/TIA):

• FLAIR hyperintensity with corresponding DWI hyperintensity indicates completed infarction requiring immediate stroke protocol 1
• Isolated FLAIR vascular hyperintensities (hyperintense vessels sign) without parenchymal changes suggest proximal arterial occlusion with slow collateral flow—this is tissue at risk and should prompt urgent consideration of revascularization 2, 3, 4
• The hyperintense vessel sign on FLAIR is associated with large vessel occlusion or >90% stenosis and indicates brain tissue at risk before infarction occurs 3
• Same-day MRI with DWI and FLAIR sequences plus vascular imaging (CTA/MRA or duplex ultrasound) is recommended for all suspected stroke/TIA patients 1

Mesial temporal lobe involvement:

• If acute encephalitis is suspected (fever, altered mental status, seizures), immediately start IV aciclovir 10 mg/kg every 8 hours for adults while awaiting CSF PCR results 5
• Bilateral temporal lobe FLAIR hyperintensity is nearly pathognomonic for HSV encephalitis 5
• Perform lumbar puncture for CSF analysis including HSV PCR, but do not delay aciclovir administration 5
• MRI has 90% sensitivity when performed within 48 hours of admission for encephalitis, compared to only 25% for CT 5

Subcortical white matter location:

• Strongly hyperintense FLAIR signal (isointense or hyperintense to CSF) respecting gray-white matter boundaries suggests vasogenic edema 1
• Mildly hyperintense FLAIR signal (hypointense to CSF) disrupting anatomical architecture suggests infiltrative tumor 1
• Multiple large subcortical lesions with ill-defined borders toward white matter but sharp borders toward cortex, affecting U-fibers, suggest PML in immunosuppressed patients 1

Step 2: Obtain Critical Additional Imaging

Essential sequences to clarify FLAIR findings:

• DWI/ADC maps: Distinguish acute infarction (restricted diffusion) from other etiologies 1
• Contrast-enhanced T1: Identify blood-brain barrier breakdown, tumor enhancement, or inflammatory processes 1, 5
• T2*/SWI: Detect hemorrhage (common in melanoma, renal, thyroid metastases) 1
• Vascular imaging (MRA/CTA): Essential when hyperintense vessel sign present or stroke suspected 1, 3

Step 3: Correlate with Clinical Features

Hypertensive emergency (BP >180/120 with symptoms):

• FLAIR hyperintensity in posterior brain regions with headache, visual disturbances, or altered mental status suggests posterior reversible encephalopathy syndrome (PRES) 1
• Confirm with MRI showing increased T2/FLAIR signal in posterior regions, typically reversible with BP control 1
• Initiate controlled BP reduction (avoid precipitous drops) and obtain CT to exclude hemorrhage 1

Known cancer history:

• Multiple rim-enhancing lesions at gray-white junction with surrounding FLAIR hyperintensity (vasogenic edema) suggest metastases 1
• Obtain thin-section volumetric contrast-enhanced T1 imaging (3D sequences like SPACE, CUBE, or VISTA superior to MPRAGE for small metastases) 1
• Distinguish from abscess: metastases typically lack central diffusion restriction (except SCLC), while abscesses show restricted diffusion centrally 1

Immunosuppression (especially natalizumab therapy):

• Large (>3 cm) subcortical FLAIR hyperintense lesions with ill-defined borders, no mass effect, and punctate/patchy enhancement suggest PML 1
• PML lesions affect U-fibers, extend into gyrus, involve cortical gray matter in 50%, and show continuous volume increase 1
• Obtain CSF for JCV PCR; if negative but high clinical suspicion, repeat CSF testing or consider brain biopsy 1

Common Pitfalls to Avoid

• Never delay aciclovir in suspected encephalitis while awaiting imaging or CSF results—mortality increases significantly with treatment delay 5
• Do not assume isolated FLAIR hyperintensity without DWI correlation represents completed infarction—hyperintense vessel sign indicates tissue at risk requiring urgent intervention 2, 3
• Avoid relying on CT alone for encephalitis or early stroke—sensitivity is only 25% for encephalitis and often normal early in stroke 1, 5
• Do not discontinue aciclovir based on single negative CSF PCR if clinical suspicion remains high—false negatives occur, especially early in disease 5
• Recognize that subarachnoid FLAIR hyperintensity is not specific to stroke with reperfusion—it occurs in inflammatory CNS disease, epilepsy, and other conditions without prior contrast administration 6
• In suspected metastases, distinguish cystic from necrotic lesions—cystic metastases show thin smooth ring enhancement with uniform T1 hypointense center, while necrotic lesions have irregular walls 1

Specific Management Pathways

Confirmed acute ischemia (DWI positive):

• FLAIR hyperintensity indicates permanent tissue injury even if symptoms transient 1
• Initiate secondary stroke prevention: antiplatelet therapy, statin, BP control 1
• If carotid stenosis >50% symptomatic or >60-70% asymptomatic, consider revascularization 1

Hyperintense vessel sign without infarction:

• Urgent vascular imaging to identify occlusion/stenosis 3, 4
• Consider revascularization and flow augmentation strategies 3
• Final infarct size typically smaller than area showing hyperintense vessels if treated promptly 3

Suspected glioma:

• Proximity to contrast-enhancing tumor increases probability of high tumor cellularity in FLAIR hyperintense regions 1
• Document precise location with neuronavigation for tissue sampling 1
• Obtain samples from contrast-enhancing, non-enhancing FLAIR hyperintense, and adjacent normal-appearing tissue when feasible 1

Hypertensive encephalopathy/PRES:

• Controlled BP reduction (avoid precipitous drops causing watershed infarction) 1
• FLAIR abnormalities typically fully reversible with timely BP management 1
• Follow-up imaging to confirm resolution 1