CNS vasculitides

• Multiple bilateral ischemic infarctions in different vascular territories and stages of healing on DWI/MRI
• Multifocal segmental arterial narrowing and/or beading on MR angiography or DSA affecting small and medium vessels
• Concentric vessel wall enhancement and thickening on vessel wall MRI (VW-MRI) distinguishing vasculitis from atherosclerotic disease

• Multiple bilateral ischemic infarctions (60% of cases), variable in size, affecting different vascular territories and in various stages of healing are the most common MRI finding in primary CNS vasculitis.
• Multifocal segmental narrowing and/or beading of both small and medium-sized intracranial blood vessels on MR angiography or digital subtraction angiography, with the middle cerebral artery most commonly affected, followed by anterior and posterior cerebral arteries.
• T2/FLAIR high-signal-intensity white matter lesions appear in approximately 50% of cases but are non-specific and may reflect vasculitis-related ischemia or inflammation.
• Hemorrhagic lesions including cerebral microhemorrhages and intracranial hemorrhage can be identified on GRE/SWI sequences; subarachnoid hemorrhage is rare.
• Meningeal enhancement and/or parenchymal enhancement occurs in up to 40% of cases and is a recognized finding when spinal cord involvement is present.
• Vessel wall MRI demonstrates concentric arterial wall enhancement and thickening in vasculitis, differentiating PACNS from intracranial atherosclerotic disease, reversible cerebral vasoconstriction syndrome, dissection, and moyamoya disease.
• CT angiography may show multifocal intracranial arterial narrowing and/or beading; CT demonstrates areas of hypoattenuation reflecting infarction.
• Majority of patients have brain lesions only; less than 10% have spinal cord-only or combined brain and spinal cord involvement.

• Reversible cerebral vasoconstriction syndrome (RCVS): distinguished by acute thunderclap headaches, vasoconstriction that typically resolves, and segmental narrowing often in larger vessels; absence of infarctions or meningeal enhancement helps differentiate.
• Intracranial atherosclerotic disease (ICAD): eccentric plaque with stenosis in large/medium vessels in high-risk patients; vessel wall MRI shows eccentric rather than concentric enhancement, lacking the diffuse involvement of PACNS.
• Moyamoya disease: progressive cerebral arterial occlusion with characteristic basal ganglia collateral vessels ('puff of smoke'); genetic predisposition and specific imaging pattern distinguish from vasculitis.
• Myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD): demyelinating disorder with white matter lesions; serology and clinical presentation with optic neuritis or myelitis help differentiate.
• Inflammatory cerebral amyloid angiopathy: amyloid-related inflammation; microhemorrhages with predilection for lobar distribution; associated with amyloid PET positivity and older age.
• Systemic vasculitides with CNS involvement (secondary vasculitis): giant cell arteritis, Takayasu arteritis, polyarteritis nodosa, granulomatosis with polyangiitis; systemic manifestations and serologies (ANCA, ESR, CRP) distinguish from primary PACNS.

Report primary CNS vasculitis as follows: 'Multiple bilateral acute and chronic ischemic infarctions in different vascular territories with multifocal segmental narrowing and/or beading of intracranial small and medium-sized vessels on MR angiography, consistent with primary angiitis of the CNS. Vessel wall MRI demonstrates concentric enhancement of affected arterial walls. Clinical correlation with CSF findings and consideration of brain biopsy are recommended to confirm diagnosis and exclude systemic vasculitis or other mimics.'

• Unilateral infarction pattern is rare in PACNS; predominantly unilateral involvement should raise suspicion for alternative diagnoses such as intracranial atherosclerotic disease, dissection, or cardioembolic stroke.
• Normal digital subtraction angiography does not exclude CNS vasculitis; small-vessel disease may be missed on DSA, particularly in the distal cerebral circulation; correlation with MRI and vessel wall imaging is essential.
• T2/FLAIR white matter hyperintensities in 50% of cases are non-specific and can occur in demyelinating diseases, chronic microvascular ischemia, and other inflammatory conditions; ischemic infarction pattern and angiographic narrowing must be present for diagnosis.
• Elevated CSF parameters and non-specific imaging findings may be misattributed to multiple sclerosis, CNS lymphoma, or meningitis; careful exclusion of infectious etiologies and systemic disease is mandatory before attributing findings to primary PACNS.