Small vessel vasculitis
There are several vasculitides that affect small vessels. Pauci-immune
Granulomatosis with polyangiitis. Systemic vasculitis of medium and small arteries, including venules and arterioles. Produces granulomatous inflammation of the respiratory tracts and necrotizing, pauci-immune glomerulonephritis. Most common cause of saddle nose deformity in USA (nose flattened due to destruction of nasal septum by granulomatous inflammation). Almost all patients with granulomatosis with polyangiitis have c-antineutrophil cytoplasmic antibody, but not vice versa. Current treatment of choice is cyclophosphamide. At least two out of four criteria yields sensitivity and specificity of 88 and 92%. Nasal or oral inflammation (oral ulcers or purulent/bloody nasal discharge, may be painful)
Abnormal chest X-ray with showing nodules, infiltrates, or cavities
Microscopic hematuria or red blood cell casts
Vessel biopsy shows granulomatous inflammation
Peak incidence: ages 40–60, males > femalesEosinophilic granulomatosis with polyangiitis (formerly known as Churg–Strauss syndrome). Affects medium and small vessels with vascular and extravascular granulomatosis. Classically involves arteries of lungs and skin, but may be generalized. At least four criteria yields sensitivity and specificity of 85 and 99.7%. Asthma (history of wheezing or presently wheezing)
Eosinophilia > 10% on complete blood count
Mononeuropathy or polyneuropathy
Migratory or transient pulmonary opacities on chest x-ray
Paranasal sinus abnormalities
Vessel biopsy showing eosinophils in extravascular areasMicroscopic polyarteritis/polyangiitis. Affects capillaries, venules, or arterioles. Thought to be part of a group that includes granulomatosis with polyangiitis since both are associated with antineutrophil cytoplasmic antibody and similar extrapulmonary manifestations. Patients do not usually have symptomatic or histologic respiratory involvement. Immune complex
Hypersensitivity vasculitis (allergic vasculitis). Usually due to a hypersensitivity reaction to a known drug.
At least three out of five criteria yields sensitivity and specificity of 95 and 91%: 

Age at onset ≥ 50 years
New onset headache with localized tenderness
Temporal artery tenderness or decreased pulsation
Elevated ESR ≥ 50 mm/hour Westergren
Temporal artery biopsy showing vasculitis with mononuclear cell infiltrate or granulomatous inflammation, usually with multinucleated giant cells

Medium vessel vasculitis
These conditions are sometimes considered together with the small vessel vasculitides.Polyarteritis nodosa (PAN). Systemic necrotizing vasculitis and aneurysm formation affecting both medium and small arteries. If only small vessels are affected, it is called microscopic polyangiitis, although it is more associated with granulomatosis with polyangiitis than to classic PAN. At least 3 out of 10 criteria yields sensitivity and specificity of 82 and 87%:

Unexplained weight loss > 4 kg
Livedo reticularis
Testicular pain
Myalgias, weakness
Abdominal pain, diarrhea, and GI bleeding
Mononeuropathy or polyneuropathy
New onset diastolic blood pressure > 90 mmHg
Elevated serum blood urea nitrogen (> 40 mg/dL) or serum creatinine (> 1.5 mg/dL)
Hepatitis B infection
Arteriographic abnormalities
Arterial biopsy showing polymorphonuclear cellsKawasaki disease. Usually in children (age<4), it affects large, medium, and small vessels, prominently the coronary arteries. Associated with a mucocutaneous lymph node syndrome. Diagnosis requires fever lasting five days or more with at least four out of five criteria:

Bilateral conjunctival injection
Injected or fissured lips, injected pharynx, or strawberry tongue
Erythema of palms/soles, edema of hands/feet, periungual desquamation
Polymorphous rash
Cervical lymphadenopathy (at least one node > 1.5 cm)Isolated cerebral vasculitis. Affects medium and small arteries over a diffuse CNS area, without symptomatic extracranial vessel involvement. Patients have CNS symptoms as well as cerebral vasculitis by angiography and leptomeningeal biopsy.