The vasculitides are a range of uncommon disorders characterized by inflammation of blood vessels, leading to organ damage. They can be defined as primary (occurring in isolation) or secondary to another condition (such as systemic lupus erythematosus or rheumatoid arthritis). Pathologically, vascular inflammation leads to narrowing or occlusion, resulting in tissue damage.

Many of the primary vasculitides are defined by the predominant vessel size involved: large-vessel vasculitis (typically giant cell arteritis (GCA) and Takayasu arteritis), medium-vessel vasculitis (polyarteritis nodosa (PAN) and Kawasaki disease (KD)) and small-vessel vasculitis, further grouped into the anti-neutrophil-cytoplasm-antibody (ANCA) associated diseases (granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA) and eosinophilic granulomatosis with polyangiitis (EGPA), the immune-complex-associated diseases (cryoglobulinaemic vasculitis, IgA vasculitis, hypocomplementaemic urticarial vasculitis) and anti-glomerular basement membrane antibody vasculitis.

Other forms of vasculitis do not fit into these patterns; they may be multi-system (e.g. Behcet syndrome), related to infection or affect single organs (e.g. central nervous system vasculitis).

The systemic vasculitides can present with a wide range of manifestations in specific organs. In many forms of vasculitis, constitutional features such as weight loss, fever and malaise are very common. The pattern for each form of vasculitis varies, but there is considerable overlap in some of these conditions. For example, ischaemic abdominal pain, colitis and pancreatitis occur in polyarteritis nodosa (PAN), but also in some of the small-vessel vasculitides. Eye involvement, typically episcleritis, scleritis or uveitis or occasionally retinal haemorrhage, occurs in different forms of small- vessel vasculitis.

Partly due to this overlap of features, but also the fact that we recognize potential cases of vasculitis at a much earlier stage than previously (and sometimes initiate treatment before confirming a diagnosis), for about 20% of cases it may not be possible to give a precise diagnostic label, other than calling it ‘some sort of vasculitis’.

Although it is very tempting to start treatment for patients in whom you suspect a new diagnosis of systemic vasculitis, protocols for definitive therapy are complex. It is more likely that you will do harm than good by administering large doses of glucocorticoid therapy, so don't start these unless advised to do so by a vasculitis specialist.