Recurrent Mastoiditis Mimics IgG4 Related Disease: A Potential Diagnostic Pitfall.

IgG4-related disease (IgG4-RD) is a recently recognized entity that causes progressive fibrosis and formation of mass lesions. IgG4-RD can be diagnosed histologically by its hallmarks of storiform fibrosis, prominent lymphoplasmacytic infiltrate, and obliterative phlebitis, accompanied by the infiltration of excessive numbers of IgG4-positive plasma cells as well as elevations in serum IgG4 concentrations. A recent publication reported a case of IgG4-RD in the mastoid sinus, representing a new anatomic location for this disease. We report two additional cases of IgG4-RD occurring in the mastoid and causing clinical mastoiditis. The presenting symptoms were varied-tinnitus, hearing loss, and cranial nerve palsies. All three cases showed a dense lymphoplasmacytic infiltrate, storiform type fibrosis as well as elevated numbers of IgG4 positive plasma cells. The three patients responded to immunosuppressive therapy that included steroids and Rituximab. We further investigated 162 consecutive mastoiditis cases at our institution in order to determine the frequency of IgG4-RD as a previously unrecognized cause of mastoiditis. Within this latter cohort we identified nine cases of mastoiditis that had two of the histologic features of IgG4-RD, specifically storiform fibrosis and a dense lymphoplasmacytic infiltrate. Two of these cases showed >50 IgG4-positive plasma cells per high-power field with IgG4-IgG ratio of >40 %, thus fulfilling histological criteria for IgG4-RD. However, both were due to severe acute or chronic infection. In conclusion, we reaffirm IgG4 related mastoiditis as a distinct but uncommon cause of recurrent mastoiditis. The diagnosis of IgG4-related mastoiditis should be rendered with caution, and only after the exclusion of potential mimickers, particularly infection.