Vasculitis is a heterogeneous group of disorders characterized by inflammation of blood vessels. One system of classification of vasculitides is based on the size of the predominant vessels involved (Table 14.1). For example, giant cell arteritis (GCA) involves large-sized blood vessels such as the aorta and its branches, whereas polyarteritis nodosa involves medium-sized vessels containing an internal elastic membrane, muscular media, and adventitia. Small-vessel vasculitis involves capillaries, and postcapillary venules and arterioles. Prior to the 1990s, a formal classification system of vasculitic syndromes did not exist because of a lack of consensus in evidence-based classification of individual patients with vasculitic syndromes (1). This was addressed by the American
College of Rheumatology (ACR) in 1990, which proposed criteria for the classification of seven different vasculitides (2). These criteria are not meant as diagnostic criteria, as they compared patients with different types of vasculitis, but not patients with other systemic or connective tissue diseases.

The ACR classification criteria do not include microscopic polyangiitis (MPA) or consider antineutrophil cytoplasmic antibodies (ANCA) as diagnostic criteria. In 1994, the Chapel Hill Consensus Conference (CHCC) produced definitions for vasculitis (3) and included MPA in its classification criteria. They also recognized that histological data would not be available for all patients, especially when the clinical condition of the patient might preclude obtaining appropriate biopsies. Furthermore, the sample might not be representative or the salient histological features may not be found because of sampling error. The classification scheme in Table 14.1 embodies features of both ACR and CHCC criteria, and is now widely used for epidemiological studies.

The vasculitides affect individuals of all ages, but are predominately seen in the extremes of age groups. Furthermore, the exact etiology is unknown, but it has been demonstrated to be multifactorial with factors such as ethnicity, genes, gender and ultraviolet light, infections, toxins, drugs, smoking, and surgery influencing disease expression (1).

The annual incidence of polyarteritis nodosa (PAN) is 2 to 9 per million, with the highest incidence of 16 per million in Kuwait and 14.8 per million in Japan. Where there has been more attention to hepatitis B vaccination, the prevalence of PAN has decreased.

The prevalence of Behçet’s syndrome is highest in the regions extending from Eastern Asia to the Mediterranean basin and lowest in Western countries. The highest prevalence is seen in Turkey with 80 to 370 cases per 100,000 people. In Asian countries such as Japan, Korea, China, and Middle Eastern countries such as Iran and Saudi Arabia, the prevalence is between 13.5 and 20 cases per 100,000 people. Behçet’s syndrome is more common among females in Asian countries and among males in Middle Eastern countries. It occurs frequently in third and fourth decade of life.

The incidence of antineutrophil cytoplasm antibody (ANCA)-associated (Wegener’s granulomatosis (WG), Churg–Strauss syndrome, and MPA) is approximately 10 to 20 per million per year, with onset peaking between ages 65 and 74 years (5). Wegener’s granulomatosis—a disease with necrotizing granulomata of respiratory tract, necrotizing vasculitis, and focal glomerulonephritis—is more common in men and is rarely seen in children (0.3 per million per year). Various studies have demonstrated that WG is not as common as microscopic angiitis in non-European populations, that is, Japanese and Chinese. Familial cases of WG have been reported, albeit rare. HLA DPB1*0401 has been associated with WG. Environmental factors such as seasonality and drugs have been previously thought to contribute to the development of WG. However, studies conducted by Lane et al. did not confirm the seasonal effect on developing this disease. No single drug has been shown to lead to the development of WG; however, certain drugs—including cocaine—can result in ANCA-associated conditions that mimic primary ANCA-associated vasculitis. Staphylococcus aureus infections have been associated with WG and a higher risk of relapse (5).