What is the brush sign on brain MRI, and what are its diagnostic and management implications in acute ischemic stroke?

Brush Sign on Brain MRI

The brush sign on brain MRI refers to prominent hypointensity of deep medullary veins and subependymal veins visible on T2-weighted or susceptibility-weighted imaging (SWI), most commonly seen in acute ischemic stroke with major cerebral artery occlusion, and serves as an important predictor of hemorrhagic transformation and worse clinical outcomes.*

What the Brush Sign Represents

The brush sign appears as enlarged, prominent medullary veins on susceptibility-weighted sequences, creating a brush-like appearance radiating from the ventricles toward the cortex. This finding reflects:

• Severe oxygen metabolic impairment in the ischemic penumbra, with more profound reduction in cerebral metabolic rate of oxygen (CMRO2) compared to patients without the sign 1
• Venous congestion due to impaired venous drainage in the setting of major arterial occlusion
• Increased oxygen extraction as compensatory mechanism in severely hypoperfused tissue

The sign is most frequently observed with M1 or M2 middle cerebral artery occlusions 2.

Diagnostic Implications

Imaging Characteristics

The brush sign can be graded by severity using an asymmetry index (AI) comparing affected to normal hemisphere 3:

• BS=0: No visible asymmetry
• BS=1: Mild asymmetry
• BS=2: Marked asymmetry

Two morphological patterns exist 4:

• Isolated type: Discrete focal venous prominence
• Continuous type: Extensive, confluent venous prominence

Associated Findings

Patients with brush sign demonstrate 1:

• Larger ischemic core volumes (median 24.5 mL vs 12.4 mL without sign)
• Larger ischemic penumbra volumes (median 15.0 mL vs 8.9 mL)
• Higher baseline NIHSS scores (median 15.5 vs 13.0)
• More severe CMRO2 impairment in both core (-52.9% vs -44.1%) and penumbra (-31.0% vs -13.8%)

Management Implications

Risk Stratification for Hemorrhagic Transformation

The presence of brush sign independently predicts hemorrhagic transformation after intravenous thrombolysis 2, 3:

• Hemorrhagic transformation occurs in 64% of brush sign-positive patients vs 18% without the sign
• Multivariate analysis shows odds ratio of 9.08 (95% CI: 1.4-59.8) for hemorrhagic transformation 2
• Risk is particularly elevated in patients without successful reperfusion 3

Treatment Considerations

While current AHA/ASA guidelines do not specifically address the brush sign, the imaging workflow recommendations are relevant 5:

1. Do not delay IV tPA administration to obtain advanced MRI sequences including SWI if patient is within treatment window (Class I, Level A)

2. Obtain vascular imaging (CTA or MRA) during initial evaluation if endovascular therapy is contemplated, but without delaying IV tPA (Class I, Level A) 5

3. Consider advanced perfusion imaging (CT or MR perfusion) for patient selection beyond standard time windows (Class IIb, Level B) 5

Prognostic Value

Brush sign presence correlates with poorer functional outcomes 1:

• Higher risk of not achieving functional independence at 3 months (OR = 1.8,95% CI: 1.3-3.4)
• Good outcome (mRS 0-1) at discharge: 24% with brush sign vs 45% without 2

Clinical Algorithm

When brush sign is identified on admission MRI:

1. Proceed with standard reperfusion therapy (IV tPA and/or EVT) per guideline indications—the sign does not contraindicate treatment 5

2. Heighten monitoring for hemorrhagic transformation with:

   • More frequent neurological assessments
   • Lower threshold for repeat imaging if clinical deterioration occurs
   • Strict blood pressure control per stroke protocols

3. Consider more aggressive neuroprotective measures:

   • Optimize cerebral perfusion pressure
   • Avoid hypotension
   • Maintain normoglycemia

4. Counsel families regarding increased risk of complications and potentially worse prognosis

Important Caveats

• The brush sign is not a contraindication to thrombolysis or thrombectomy—it is a risk marker, not an absolute predictor
• Detection requires susceptibility-weighted imaging or T2 sequences*, which may not be part of all acute stroke protocols
• The sign has higher specificity than sensitivity for hemorrhagic transformation—absence does not guarantee safety
• Most evidence comes from 3T MRI; detection may be less reliable at lower field strengths 2
• The sign can occur in other conditions beyond acute stroke (moyamoya disease, cerebral venous thrombosis, Sturge-Weber syndrome) 4