The diagnosis of primary CNS vasculitis / childhood Primary Angiitis of the CNS (cPACNS) requires:

• A newly acquired focal or diffuse neurological deficit and/or psychiatric symptom in a child > 1 month and <19 years of age plus 
• Angiographic and/or brain biopsy evidence of CNS vasculitis in the absence of an underlying systemic condition known to cause or mimic the findings 

CNS vasculitis is subdivided based on the modality that confirms the diagnosis. Children with evidence of vasculitis on angiography are considered to have angiography-positive, large-medium vessel CNS vasculitis. Those who have a negative angiography and evidence of vasculitis solely on brain biopsy are commonly classified as angiography-negative, small vessel CNS vasculitis. Children with evidence of inflammation and suspected CNS vasculitis who have not had a confirmatory test should be classified as "inflammatory brain disease, suspected CNS vasculitis". 

• Inflammation is targeting the large and medium cerebral vessels.
• Diagnosis is confirmed by angiography, which may show evidence of cerebral vessel stenosis, beading, tortuosity, and/or occlusion (see References Aviv, 2006; Aviv 2007; Eleftheriou, 2010).
• Two subtypes of angiography-positive cPACNS are recognized based on disease course: 
  • Non-Progressive cPACNS is a monophasic inflammatory vessel wall disease affecting unilaterally the proximal vessel segments of the anterior and/or middle cerebral artery and/or the distal internal carotid artery. This monophasic inflammatory disease is also known as Transient Cerebral Arteriopathy (TCA), Focal Cerebral Arteriopathy (FCA), and Post-Varicella Angiopathy (PVA).
  • Progressive cPACNS is an ongoing inflammatory disease of the CNS vessels frequently affecting both proximal and distal vessel segments. It may present as a unilateral or bilateral vessel disease. Diagnosis is confirmed on angiography when children either have both proximal and distal vessel stenoses in ≥ 1 vascular territory on initial angiography or have new vessel segments affected on repeat vascular imaging at three months of illness. The latter commonly occurs despite corticosteroid therapy. Fortunately, children with progressive cPACNS on corticosteroid therapy commonly do not acquire new neurological deficits. 
• Children with angiography-positive CNS vasculitis do not require a brain biopsy; in fact biopsies commonly demonstrate tissue damage due to ischemia.

• Inflammation is solely targeting the small cerebral vessels. 
• Diagnosis is confirmed by brain biopsy, which typically demonstrates evidence of intramural and perivascular inflammation. The characteristic inflammatory phenotype is a lymphocytic infiltrate of primarily T-cells, some additional B-cells, macrophages and eosinophils may be found . Fibrinoid necrosis or granulomas are not commonly seen (see References Elbers, 2010; Twilt 2011).
• By definition, children with small vessel CNS vasculitis have a normal angiography.
• Different subtypes of small vessel vasculitis may exist. This needs to be further studied.
• To date, no specific MRI pattern was found to be associated with SV-cPACNS. However, leptomeningeal enhancement appears to be specific for SV-cPACNS, in particular considering demyelinating diseases as a differential diagnoses. Of note: Spinal cord inflammation and optic nerve inflammation are commonly seen in children with SV-cPACNS.