Synovial Cysts and Myxomas in 16 Cats.

This report describes the clinical and pathologic characteristics of cystic and myxomatous lesions of synovial joints in 16 cats. The average age was 13.4 years. The elbow was most commonly affected (12/16), and all lesions were unilateral. Degenerative joint disease was a frequent concurrent but bilateral condition. The lesions consisted of fluid-filled cysts lined by synoviocytes (3 cases), solid foci of stellate cells in a myxomatous matrix (2 cases), or a combination of the two (11 cases). In some cases there were areas of transition between the cystic and myxomatous lesions. Mitoses and other features of malignancy were rare to nonexistent. In the 13 cats with follow-up information, the lesion gradually increased in size over a period of years. None of the cats died or were euthanized because of this lesion. We propose that some cats with degenerative joint disease develop synovial cysts, which have the potential to transform to a synovial myxoma.