Diagnostic imaging features, cytological examination, and treatment of lymphocytic tenosynovitis of the common digital extensor tendon sheath in an eventing horse.

In horses, the structures at the dorsal aspect of the carpus, including the digital extensor tendons, their related tendon sheaths, and bones, are vulnerable to injury because of their superficial location. Injuries to these structures may result in lameness of the affected limb(s) and reduce a horse's athletic performance. A 13-year-old eventing horse that routinely underwent regular exercise exhibited dorsolateral distension of the right carpus. An effusion insensitive to compression was observed in the affected area. No lameness was detected, and the horse exhibited a negative response to the carpal flexion test. Although radiography revealed no abnormal findings in the carpal bones, ultrasonography depicted anechoic fluid and synovial cell proliferation within the common digital extensor tendon sheath. Cytological analysis of the fluid revealed numerous lymphocytes and increased proteinaceous background, suggesting lymphocytic tenosynovitis. The effusion resolved following administration of two intrathecal injections: one injection of corticosteroid combined with hyaluronic acid (HA), and one injection of HA alone. Two weeks after administration of the second injection, daily under-saddle exercise was initiated, consisting of walking and light trotting with a gradual increase in intensity. The horse returned to its habitual intensive training program six weeks following the final injection. In conclusion, the horse was diagnosed with lymphocytic tenosynovitis of the common digital extensor tendon; successful treatment was achieved with administration of corticosteroid and HA. Diagnostic imaging and cytological examination facilitated clinical interpretation and the selection of an appropriate treatment regimen.