Soft tissue swelling and synovial distension associated with the tarsus is very common in horses and may be associated with pain and lameness. In this case, a fluid swelling of synovial origin that initially appeared to be completely separate from any other synovial structure was present in a mare with severe intermittent hind limb lameness. Nuclear scintigraphy, diagnostic analgesia, contrast radiography, and ultrasonography were used to confirm the synovial swelling as the source of lameness. Surgical en-bloc resection of the synovial swelling has been curative. It is hypothesized that an acute trauma caused herniation of the tarsocrural joint synovial membrane. The fistula then sealed but became patent during specific phases of movement, resulting in a sudden influx of synovial fluid and a buildup of pressure. Ultrasonographic examination, contrast radiography, and distension of the tarsocrural joint at surgery all failed to identify this fistula. The associated severe pain and lameness could have been the result of physical distension of the fluid swelling or pressure applied to the surrounding nerves.
Journal of Equine Veterinary Science