Schnitzler's syndrome with prominent neutrophil infiltration misdiagnosed as Sweet's syndrome: a typical example of urticarial neutrophilic dermatosis.

Schnitzler's syndrome is a rare disorder of unknown aetiology characterized by a chronic urticarial eruption, intermittent fever and monoclonal gammopathy. We encountered an interesting patient with this syndrome, who had been misdiagnosed for 10 years as having Sweet's syndrome because of the histopathological picture, which was a prominent perivascular and interstitial neutrophilic infiltrate in the dermis with leucocytoclasia but without vasculitis. An urticarial eruption with this histopathological feature has recently been categorized as neutrophilic urticarial dermatosis, and it is strongly indicative of an associated systemic disease, mainly Schnitzler's syndrome and other inflammatory diseases. We therefore need to be cautious not to confuse Schnitzler's syndrome with Sweet's syndrome. Further, the serum interleukin (IL)-6 levels, but not those of other cytokines and chemokines, correlated with the disease activity in our patient, suggesting that IL-6 may be involved in some of the disease processes, including neutrophil infiltration.