Page 147 - The Vasculitides, Volume 1: General Considerations and Systemic Vasculitis

Nomenclature and Pathologic Features of Vasculitides 123

IgA1-dominant immune deposits in the walls of small vessels. The alternative name,
Henoch-Schönlein purpura (HSP) was originally based on a set of clinical manifestations that
can be caused by multiple etiologies. Use of the term IgAV emphasizes the need to carefully
consider the specific cause of the clinical manifestations. However, in a clinical setting, as
with other pathologic features, there may be diagnostic criteria that warrant a confident
diagnosis of IgA vasculitis in the absence direct observation of IgA in vessel walls by
immunohistology. The conclusion that a patient has IgAV predicts clinically important
clinical characteristics that guide prognosis and therapy. In patients with similar initial
clinical manifestation of purpura, arthralgia and glomerulonephritis, the prognosis and
appropriate treatment is very different depending on whether or not the clinical findings,
including laboratory testing and pathologic observations are indicative of IgAV,
cryoglobulinemic vasculitis, lupus vasculitis, MPO-ANCA MPA or PR3-ANCA GPA.
However, the etiology and precise pathogenesis of some forms of vasculitis remains
unknown, for example in Kawasaki disease (KD) and Takayasu arteritis (TAK). In this
circumstance, the only defining features are the nature of the vasculitis and associated clinical
features with no indication in the name about the cause.

The CHCC 2012 provides names and definitions for many forms of vasculitis, but does
not provide specific diagnostic criteria or classification criteria [1]. Specific observations that
can practicably be made in a patient for clinical management purposes comprise diagnostic
criteria. Specific observations that can be made in a patient for inclusion in a cohort of
patients for a clinical trial or other research project comprise classification criteria. Although
definitions may remain unchanged, diagnostic criteria and classification criteria evolve over
time as modalities for evaluating patients develop, for example in improved imaging methods
and more sensitive and specific laboratory tests become available).

A widely used first level of categorization of vasculitides divides vasculitides into large
vessel vasculitis (LVV), medium vessel vasculitis (MVV) and small vessel vasculitis (SVV)
(Figure 1) (Table 1) [1, 2]. This is somewhat problematic because it implies that the size or
diameter of the involved vessel alone is the major defining feature for these categories and the
major basis for susceptibility to different forms of vasculitis, however this is definitely not the
case. The different functional and structural characteristic of these categories of vessels may
be more important than the diameter of the vessel in determining their involvement in the
different forms of vasculitis. The relative size of vessels is merely a convenient characteristic
that facilitates reference to these biologically different types of vessels. Another source of
confusion is the fact that there is overlap in the types of vessels that can be involved in
different forms of vasculitis. In particular, medium and small arteries can be involved by
LVV, MVV and SVV.

Large Vessel Vasculitis (LVV)

CHCC 2012 [1] defines LVV as vasculitis affecting large arteries more often than other
vasculitides, and defines large arteries as the aorta and its major branches. However, the point
is made that any size artery may be affected. Importantly, the definition states that LVV
affects large vessels “more often than other vasculitides” rather than LVV has more large

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