Light and electron microscopic observations on the synovitis of ankylosing spondylitis.

Light microscopic studies on synovium obtained from seven knees and three hips of patients with ankylosing spondylitis and peripheral arthritis showed surface fibrin, proliferation of synovial lining cells, moderate infiltration with lymphocytes, and sometimes striking numbers of plasma cells. There was some vascular congestion and obliteration, occasional bone and cartilage debris, and a tendency toward increased fibrous tissue. Although the intensity of some findings varies from those in rheumatoid arthritis, there were no consistent distinguishing features. Electron microscopy of eight synovial tissue specimens showed increased type B or synthetic lining cells. Structures that could possibly have been organisms were seen among synovial cells in two patients. Immune complex-like deposits were not seen in vessel walls, although there were other vascular alterations. Synovial fluid studies showed 2,200 to 16,500 white blood cells/mm3 (mean, 8,236), 29% to 93% polymorphonuclear leukocytes (mean, 66%), and 0.5 to 32% lymphocytes (mean, 18%). The presence of at least some activated lymphocytes (lymphoblasts) in seven of nine patients in addition to the above findings in synovium suggest an immunological component to the driven process. No more than 32% lymphocytes was found in any synovial fluid despite the use of nonsteroidal antiinflammatory drugs. Thus no support for synovial fluid lymphocytosis, such as has been described in rheumatoid arthritis patients treated with nonsteroidal antiinflammatory drugs alone, is provided.