= PACNS = idiopathic inflammatory disease of medium-sized to small arteries affecting CNS / peripheral nervous system WITHOUT evidence of generalized inflammation

Age: 5–6^th decade

• acute / subacute confusion, hallucinations, headache
• paresis, cranial neuropathy, loss of consciousness
• elevated inflammatory markers: ↑ ESR
• CSF analysis: ↑ opening pressure, ↑ protein level

Location: spinal cord abnormalities (5%)

• Nonspecific CT and MR imaging findings!

CT:

• areas of low attenuation ← suggestive of ischemic events

MR:

• discrete / diffuse supra- and infratentorial lesions involving deep + superficial white matter
• ± vessel irregularities
• ± areas of infarct + hemorrhage
• lesion enhancement in 90%

DSA (supports diagnosis in spite of negative biopsies):

• focal / multifocal segmental narrowing / occlusion / irregularities in both hemispheres
• collateral vessel formation
• prolonged circulation time
• microaneurysm (unusual)

Subsets:

1. Hemorrhagic form (11–12%)
2. Cause: hemorrhagic transformation of recent infarction / focal necrosis of intracerebral blood vessel
   • intracerebral >subarachnoid hemorrhage
3. Pseudotumoral form (15%)
   • nonspecific mass lesions characterized by central necrosis, surrounding edema, infiltration of adjacent structures, mass effect, variable contrast enhancement
     N.B.: often misinterpreted as malignant neoplasms
4. Rapidly progressive primary vasculitis of CNS
   • numerous bilateral lesions of large cerebral vessels
   • several bilateral cerebral infarctions
     Prognosis: poor, often fatal
5. Angiography-negative, biopsy-positive PACNS
   Site: small arteries / arterioles
   • cognitive dysfunction, ⇑ CSF protein
   • meningeal / parenchymal enhancing lesions at MR
   
   Prognosis: good with favorable response to treatment
6. Childhood PACNS
   Site: unilateral, proximal, multifocal, supratentorial involving gray + white matter with preference for basal ganglia / lateral lenticulostriate vasculature; frequently terminal segment of ICA and proximal segments of ACA + MCA

Dx: leptomeningeal + brain parenchymal biopsies

Rx: high-dose steroids, cytotoxic agents