By Sharon Martin-Holm
The suspensory ligaments are made of connective tissue and start at the upper, rear part of each cannon bone and travel down the leg, splitting into a “fork” just above the fetlock. Each fork then attaches to a sesamoid bone behind the fetlock. This suspensory ligament, along with the flexor tendons, is an integral part of the sling that holds the fetlock off the ground.
Connective tissue is what “holds stuff together” in the body: the tendons (connect muscle to bone), ligaments (connect bone to bone) and skin are the most obvious examples, but others are more subtle and hidden. For instance, blood vessel walls are largely connective tissue and muscle, and the internal organs are held in their respective places by sheets and strands of connective tissue. Ligaments are made of a particular type of connective tissue called collagen that is arranged in vertical bundles of fibers and is very strong.
DSLD is a process that progressively damages these previously-strong cables, making them painful and weak and resulting in a lame or unrideable horse.
What breeds of horses are affected? The most widely affected breed is the Peruvian Paso, but it is also seen in Standardbreds, some European Warmbloods, Paso Finos, Arabians, Saddlebreds and Quarter horses. It was also recently diagnosed in a Thoroughbred. It has been noted that Peruvians tend to develop DSLD at a younger age, and regardless of previous intense exercise. Most of the other cases in non-Peruvian horses appear to happen due to stresses on the legs – intense training, racing and jumping – and possibly improper training and conditioning methods and poor conformation.
There is much debate over the cause of this disease. There are many who feel that it is genetic; especially in the Peruvian horses this seems a strong possibility. Others, as noted above, say that it happens as a result of trauma to the leg. There are at least 3 centers conducting research on DSLD right now: The University of Georgia, the University of Kentucky and a private firm called DSLD Research, Inc. The injury to the ligament looks the same regardless of which breed of horse has it, however. Some of the collagen fibers get damaged and instead of being replaced by new collagen, the body tries to heal the damaged areas by filling in with cartilage or other disorganized “scar” type material. This happens repeatedly, resulting in inflammation, swelling and a weak ligament that has difficulty bearing the weight of the horse.
The clinical signs usually start with a bilateral (both front legs or both back legs) lameness. At first, there may not be obvious swelling, but as the disease progresses the fetlocks get puffy and sore. The suspensory ligament is often painful upon palpation and it also feels swollen. In late-stage disease, the fetlocks “drop”, becoming more horizontal to the ground. Often the hind legs will look “post-legged” as the angles of the hocks and stifle straighten out. A veterinarian evaluation can often discover the disease at an earlier stage. The fetlock-flexion test (the fetlock is held in extreme flexion for 30-60 seconds, then the horse is trotted or gaited out, looking for lameness) will often be positive in more than one leg. Ultrasound is a very sensitive test for diagnosing DSLD as well. The abnormal fibers and swelling of the diseased ligament show up clearly. One experimental center is researching nuchal ligament biopsy as a way to diagnose it early. The nuchal ligament runs along the spine , in the crest of the neck. This is intriguing, as it implies that we may actually be dealing with a disease that affects the connective tissue in places other than just the legs.
The treatments for DSLD are almost as controversial as the reason why it happens in the first place. Historically, affected horses were kept stalled for several months with supportive leg wraps, and corrective shoes were placed that lifted the heel and relieved some pressure on the suspensory ligament. More recently, it is felt that inactivity is a very bad idea and the horses are recommended to be kept at pasture as much as possible. Anti-inflammatories (such as Bute) are a mainstay of pain control, but of course there are side effects to deal with. MSM (methyl sulfonyl methane) has had encouraging results in many horses. There is no cure, however, and DSLD continues to be a progressive disease that most often results in eventual euthanasia to relieve pain.