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Drawing on our experience of 16 cases and a review of the English literature, we propose that CSS is under-diagnosed because of exclusive emphasis upon pathologic recognition of the disorder. The classical histological picture comprises a necrotizing vasculitis, eosinophilic tissue infiltration and extravascular granulomas, but it is only found in a minority of cases, and is not pathognomonic of the condition (69, 108). On the other hand, the clinical pattern of the disorder is most distinctive, and CSS can be readily identified on clinical grounds. Typically, it begins with allergic rhinitis, which is often complicated by nasal polyposis and sinusitis. Asthma and peripheral blood eosinophilia are essential features, often accompanied by pulmonary infiltrates. The systemic vasculitis of CSS resembles that of PAN, but severe renal disease is uncommon (the typical renal lesion is a focal segmental glomerulonephritis), and cardiac involvement accounts for 50% of deaths. Diagnostic difficulties arise from the close relationship of CSS to other granulomatous, vasculitic and eosinophilic disorders. CSS is usefully regarded as a point of overlap between these three disease spectrums (Fig. 5). Individual components of each spectrum can occur in the course of CSS; hence cases may be reported as PAN developing as a complication of Löffler syndrome or eosinophilic gastroenteritis (37, 57, 66). The hypereosinophilia of CSS tends to be less severe and more steroid-responsive than in HES, and evidence of eosinophil degranulation was not found in the patients we studied. Complement abnormalities are not a prominent feature of the disorder, and circulating immune complexes were detected in only two cases; both contained IgM. This may be of pathogenetic significance as IgM deposition was a dominant feature in four of the five cases with positive renal immunofluorescence. IgE levels were elevated in all patients studied during the vasculitic phase, and skin-prick tests were positive in 8 of 10 patients tested. CSS responds well to treatment with steroids, although some patients benefit from the addition of immunosuppressive agents. The vasculitic illness is usually of limited duration, but relapses can occur, and should be detected and treated early. Major problems in the post-vasculitic phase stem from hypertension and persisting peripheral nerve damage. Allergic upper and lower respiratory tract disease is an important cause of morbidity in the pre- and post-vasculitic periods.