Cutaneous mucinous nodules associated with proliferating synovitis of rheumatoid arthritis

Rheumatoid arthritis (RA) is occasionally associated with specific or non-specific cutaneous conditions. In this report, we describe unusual cutaneous nodules arising on the skin overlying the inflamed joints of two patients with RA. The nodules were histopathologically characterized by mucinous granulation tissue associated with neutrophilic infiltration and proliferation of starry-shaped mesenchymal cells. The affected joints of both cases showed synovitis with synovial tissue hyperplasia due to RA. Because hyaluronic mucin, which is the major component of synovial fluids, was deposited in the nodules, intradermal inoculation of synovial cells seemed to be responsible for the nodular development. The nodules of one patient spontaneously resolved in several months. We are unaware of any report describing similar cases. Such nodules should be included as a cutaneous complication of RA, distinct from neoplasias.