Page 153 - The Vasculitides, Volume 1: General Considerations and Systemic Vasculitis

Nomenclature and Pathologic Features of Vasculitides 129

node syndrome. The absence of ANCA helps distinguish the necrotizing arteritis of PAN
from the histopathologically indistinguishable necrotizing arteritis of ANCA-associated
vasculitis (AAV), including MPA [9, 10]. The absence of mucocutaneous lymph node
syndrome distinguishes PAN from the necrotizing arteritis of KD [1]. Further, necrotizing
arteritis that is indistinguishable from PAN also can secondarily occur in systemic lupus
erythematous (SLE), rheumatoid arthritis (RA) and other systemic autoimmune and
autoinflammatory conditions; secondary to infection notably with the hepatitis B virus
(HBV); and as a component of variable vessel vasculitis (VVV) in BD and CD; or as a SOV
such as cutaneous arteritis.

Thus, a diagnosis of PAN is a diagnosis of exclusion, that is, all other forms of vasculitis
that can cause necrotizing arteritis must be excluded before a diagnosis of PAN is
appropriate. In a clinical setting, this can be difficult and may require that a patient initially
have a generic diagnosis of necrotizing arteries until a more specific diagnosis is warranted.

KD is defined as arteritis associated with the mucocutaneous lymph node syndrome and
predominantly affecting medium and small arteries [1]. Coronary arteries are often involved
although the aorta and other large arteries may be involved. KD usually occurs in infants and
young children, with onset after 5 years old very uncommon. The acute necrotizing arteritis of
KD can resemble the acute lesion of PAN, which is why this disease was once called infantile
PAN by some investigators. However, in general, compared to PAN the acute arteritis of KD
has more monocytes, less neutrophils, and more vessel wall edema (medial cell dehiscence)
and less fibrinoid necrosis (Figure 3B). Coronary arteritis can occur anywhere along the
course of coronary arteries in the epicardium and myocardium, but occurs most often in the
origin of the coronary arteries adjacent to the aorta. Inflammatory aneurysms form at the sites
of inflammation and necrosis, and may induce thrombosis resulting in myocardial infarction,
which is the major cause for mortality in KD.

Small Vessel Vasculitis (SVV)

CHCC 2012 [1] defines SVV as vasculitis predominantly affecting small vessels, defined
as small intraparenchymal arteries, arterioles, capillaries and venules. Medium arteries and
veins may be affected. As noted already, LVV and MVV also can affect arteries, and MVV
can cause acute necrotizing inflammatory lesions in arteries that are indistinguishable from
the lesions of SVV necrotizing arteritis. Both MVV and LVV cause necrotizing arteritis with
conspicuous fibrinoid necrosis (Figure 3A and 4A). Thus, identifying arteritis does not
distinguish between MVV and SVV. Whether or not arterioles are involved also is not an
effective means of distinguishing between MVV and SVV because identifying arterioles is
problematic. The most workable definition of arterioles is “the smallest branches of arteries.”
Definitions of arterioles based on structural features such as layers of muscle cells in the
media and absence on internal elastica; do not consistently identify a distinct vessel type.

Thus using arteriolar involvement is not a reliable approach to distinguishing between
MVV and SVV. The best approach for identifying a SVV is to identify involvement of
venules and capillaries, for example dermal venulitis (Figure 4B), glomerular capillaritis
(glomerulonephritis) (Figure 4C) or pulmonary alveolar capillaritis (Figure 4D).

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