This issue of Annals of the Rheumatic Diseases includes two articles describing the proposed paediatric-specific classification criteria for childhood polyarteritis nodosa (c-PAN), Wegener granulomatosis (c-WG), Takayasu arteritis (c-TA) and Henoch–Schönlein purpura (HSP) which resulted from an ambitious multicentre, international collaborative project that was initiated in Vienna in 2005 and culminated in the 2008 Ankara Consensus Conference (see articles on pages 790 and 798).1 2

The vasculitides are a heterogeneous collection of multisystem disorders that manifest as the inflammation of blood vessels and may occur as a primary process or may be secondary to another disorder, most commonly infection. Despite this commonality, the vasculitides differ significantly in their clinical, radiographic, laboratory and histopathological features. Much of this heterogeneity arises from the different sizes of the blood vessels that are primarily affected, the involvement of arteries, veins, or both, and from the varying patterns of organ involvement, all of which have important implications for the optimal medical treatment and prognosis of the disorders. Perhaps some of the most interesting and significant variations within the vasculitides are those associated with the age of onset. For example, the prevalence of certain vasculitides varies considerably by age of onset. Kawasaki disease (KD), one of the most common paediatric vasculitides, only rarely manifests in adulthood, while other vasculitides, such as giant cell arteritis, are seen only in adulthood. Other vasculitides have different patterns of organ involvement and differing prognoses depending on age of onset, such as in HSP, for which the prevalence of arthritis and the prognosis associated with kidney involvement both vary significantly according to the age of onset.3 4

Development of classification criteria for the vasculitides has been particularly challenging given these variable disease presentations and the evolution of laboratory and radiographic techniques that have changed the way in which the …