Mastocytoma in the common carpal sheath of the digital flexor tendons of a horse.

The study involved the diagnosis and treatment of a horse with a mast cell tumor, identified as the cause of right forelimb lameness and inflammation of the common carpal sheath of the digital flexor tendons. Though the tumor was surgically removed, it returned 22 months later.

Research Context

• The subject of the study was a 13-year-old Morgan gelding, or male horse, which was brought in due to right forelimb lameness and tenosynovitis (inflammation of the tendon sheath) in the right common carpal sheath of the digital flexor tendons.
• The tenosynovitis was acute, showing visible swelling or effusion.
• The horse’s condition suggested an underlying health problem, catalyzing a deeper investigation.

Investigation and Diagnosis

• An ultrasonographic examination revealed a soft tissue mass within the tendon sheath, close to the lower part (distal physis) of the radius – one of the major bones in the foreleg of the horse.
• Cytology (study of cells) and culture of the fluid from the inflamed region indicated a sterile, eosinophilic tenosynovitis, suggesting that inflammation was not due to a bacterial infection but might involve an immune response, as eosinophils are types of white blood cells that combat certain infections and are often associated with allergic reactions and certain types of cancers.
• Further examination via tenoscopy (inspection of a tendon sheath using an endoscope) confirmed a capsulated soft tissue mass, which was then surgically removed through a procedure called thecotomy.

Postoperation Findings

• Upon examining the surgically removed mass with histology (the study of microscopic anatomy of cells and tissues), it was revealed to be a mast cell tumor, or mastocytoma.
• Despite initial successful removal of the mass, the horse developed a recurrence of the lameness and eosinophilic tenosynovitis 22 months postoperation due to the mastocytoma returning.

Conclusion

• The researchers concluded that mastocytosis (presence of excessive mast cells) should be considered as a potential diagnosis in horses presenting with lameness associated with tenosynovitis.
• They suggested that surgical treatment coupled with rest and postoperative treatment (intrasynovial and systemic corticosteroids) could potentially provide relief, but acknowledged that it may not entirely prevent a recurrence of the tumor.