Tendon Trouble, Ligament Lameness and Muscle Misery: Soft tissue lameness in the horse

We spend a lot of time talking about hoof, bone and joint problems in the horse.  Therefore, this article is devoted to common soft tissue (tendon, ligament, muscle) ailments and their management.

Tendon Trouble

Probably the most common tendon ailment is a “bowed” tendon, named for the bow shape the back side of the lower front limb develops because of the stretching and swelling in the tendon.  There are two tendons that run down the back of the leg, the one nearest the skin (and the one that usually “bows”) is the superficial digital flexor tendon.  The one nearest the cannon bone is the deep digital flexor tendon.  This tendinitis, or, inflammation of the tendon, usually occurs at high speeds or when horses are fatigued.  Poor training, conditioning, conformation, trimming, shoeing and footing are other factors leading to a “bowed” tendon. 
In the acute phase, horses are usually lame, and pain, heat, and swelling are usually present in the affected area.  Ultrasonography can provide measurements of just how severe a “bow” is, and follow-up images are useful in determining how healing is progressing and when a horse can be put back to work.  Initial treatment consists of rest, cold therapy, pressure, bandaging and anti-inflammatories.  Newer therapies include IRAP (interleukin-1 receptor antagonist protein), stem cells and ESWT (extra corporeal shock wave therapy).  Oral silica and gelatin (hydrolyzed collagen) also have evidence supporting their use in the development of healthy connective tissue such as tendons and ligaments.  Depending on the severity of the injury, six months or more may be required for healing, and the horse may not return to its previous performance level.

Ligament Lameness

The suspensory ligament in the horse runs from the knee (or hock) to the ankle, and lies between the deep digital flexor tendon and the cannon bone.  It provides support to the ankle during the weight-bearing phase of the stride.  When this ligament is sprained and inflammation develops, it is called suspensory desmitis, or a “pulled” suspensory.  The causes, diagnosis and management of a “pulled” suspensory are similar to a “bowed” tendon.  In either case, it’s important to contact a veterinarian immediately to obtain a complete diagnosis (including the extent of the injury), begin initial treatment, and develop a rehabilitation plan that includes follow-up examinations and careful reconditioning.  Suspensory ligament injuries can take nine to twelve months or more to heal, and like tendon injuries, may result in the horse being unable to perform at its previous level.

Muscle Misery

“Tying-up” or rhabdomyolysis, is a fairly common condition of horses.  When this form of muscle breakdown is associated with exercise, it is called exertional rhabdomyolysis.  There are three types of exertional rhabdomyolysis: sporadic ER, recurrent ER, and polysaccharide storage myopathy (PSSM).  While a full-blown episode of tying-up looks the same for each type—firm and painful muscles, sweating and anxiety, shortened stride or even inability to move—the reason for the episode and therefore its prevention differ greatly.
Sporadic ER is triggered by external factors that affect muscle function and may occur once or occasionally in any age, breed, or gender of horse or any discipline.  Common triggers include exercise beyond the current level of condition, injury from repetitive motion, heat exhaustion, and imbalances in the diet, especially electrolytes.  Identifying and correcting the trigger factor usually resolves the problem. 
On the other hand, recurrent ER is repeated episodes of tying-up due to an underlying defect in the muscle.  It is thought to be caused by an inherited problem with calcium regulation that is triggered by exercise and/or excitement.  Nearly 5% of racing thoroughbreds are affected by recurrent ER, and an unknown percentage of standardbreds and Arabians.
Horses with PSSM can suffer from repeated episodes of tying-up, but may instead show more subtle signs of muscle dysfunction such as gait abnormalities, reluctance to collect, loss of jumping form, muscle wasting, difficulty backing, difficulty holding up limbs for the farrier and even mild colic.  Clinical signs of PSSM and its diagnosis vary depending on the breed affected.  A recent study estimated the prevalence of PSSM among quarter horses in the US to be between 6 and 12%.  Draft horses, warmbloods and their crosses are also affected. 
Management of both recurrent ER and PSSM involves a low starch/high fat diet, increased turnout and regular, controlled exercise.  There are a few commercial diets specifically designed to contain no more than 20% starch and no less than 10% fat.  Other methods to meet these recommendations are to control starch by replacing grain (especially sweet feed) with a multi-vitamin/mineral supplement or a forage balancer then adding fat separately.  Fat is available in liquid oil, extruded pellets, or granule/powder form.  Experts recommend that when fat is added to the diet, an antioxidant such as Vitamin E should also be added. 

Tendon Trouble, Ligament Lameness and Muscle Misery:
Soft tissue lameness in the horse
By: Lydia F. Gray, DVM, MA
SmartPak Staff Veterinarian and Medical Director
January 2008