As you’re grooming your mare, you notice she has a large, soft swelling on the front of her left knee. She’s not lame, but you call the veterinarian anyway. The vet examines the leg and tells you that your mare has a carpal hygroma. You’ve never heard this term; what does this mean for your horse’s long-term health and soundness?

The vet explains that a hygroma is an accumulation of fluid that follows trauma such as a kick or an impact from falling. Running the knee forcefully into a trailer wall or fence rail can also trigger the buildup of fluid. Falling into the category of “unsightly, but not unsound,” hygromas are most successfully treated in their early stages. The vet may try to drain some of the fluid, use a compression bandage, or infuse the swelling with corticosteroids to reduce its size. However, it may be impossible to return the knee to a normal profile, and fluid may return after it has initially been reduced.

By themselves, hygromas usually don’t cause lameness, although the horse might be sore because of the initial trauma. If lameness is severe or persistent, it may be related to an injury to the hard or soft tissues in and around the joint.