Acute synovial fluid eosinophilia associated with delayed pressure urticaria: a role for mast cells?

We report a case of exercise induced joint effusion with synovial fluid (SF) eosinophilia of 9,540/mm3 in a patient with delayed pressure urticaria. The SF eosinophilia was an acute but transient event associated with some evidence of local complement activation. Histologic assessment revealed a normal synovial membrane but with no detectable intact mast cells. These observations suggest that mast cells and eosinophils acting in concert can cause joint inflammation.