Myositis of the Psoas and Longissimus

Dorsi muscles

...and on the eighth day God created the horse in perfect image, to romp, graze, gallop, play and make manure wherever it darn well pleases, in divine grace.

Myositis is inflammation of the muscles. The Longissimus Dorsi muscle extends from the ilium and sacrum up to the neck, which makes it the longest and largest muscle on the body of the animal. There are 2 Psoas muscles, the Psoas Minor and the Psoas Major, and these are subject to injury as well. (1)

Therefore, Myositis of the Psoas and Longissimus Dorsi is inflammation of this particular muscle group.

The Longissimus Dorsi is the most powerful extensor of the loins and back. It helps to extend the neck and flexes the spine in lateral motion.

The Psoas Major helps to rotate the thigh outward and flexes the hip joint. The Psoas Minor helps to flex the pelvis on the loin area at the last rib and hip, and also inclines the area laterally. (2)

Cow kick, anyone? These muscles can help the horse defend itself or rudely tell you to get back in line.

The Myositis, or inflammation, of these muscles is usually due to stress or strain from activities such as fast starts, racing or other disciplines, on the hind legs and hips.

It can also be secondary to ongoing degeneration of joints and bones in the hind legs, in some cases. Physical defects or structural abnormalities of the stifle and tarsus can also cause myositis.

Azoturia can also cause inflammation of the back muscles. (3)

Azoturia, Monday Morning Sickness or tying-up, a condition caused by forced hard exercise after a period of rest longer than 2 days, during which time the amount of feed was not reduced, can also cause myositis. As the animal works, it begins a rapid pulse and sweating, accompanied by stiff gait and difficulty controlling the hind end. Muscles are painful and tense.

It appears that Tying-up is not as severe a syndrome as Azoturia, in that the horse may not sweat too much but will still appear stiff. The muscle atrophy and kidney damage associated with Azoturia are not common with Tying-up, though the Veterinarian may still treat with the same drugs. (4)

Call the Veterinarian, as symptoms of myositits must be carefully diagnosed, so as not to confuse it with “overlapping of the Thoracic or Lumbar Dorsal Spinous,” or with sacroiliac subluxations.

Symptoms of strain in this area may cause the horse to behave as if it has kidney problems.

The horse will arch its back and the stiffness in the hind legs makes the animal carry them up under its body. The abdomen may be tucked up also, and the horse will move with a very short stride. (5)

If the Veteriarian puts pressure over the loin area, of the last rib and hip, this is very painful to the animal and can cause it to rapidly drop its back. The head will raise and if weight is placed in this area, the stride becomes even shorter.

Your Vet will tell you that stall rest is the best remedy for myositis. Anti-inflammatory drugs, such as Phenylbutazone (Bute) or corticosteroids help with pain and inflammation, while intravenous fluids can help remove toxins. (6)

The horse should not be put back to work until all signs of pain are gone for at least 3 weeks.

Remember the Azoturia during this time and take precautions to reduce grain and carb intake while the horse recovers.

The Veterinarian may prescribe Selenium and Vitamin E, but be sure to discuss these, especially the Selenium. These can be helpful in preventing muscle disorders.

Selenium is an essential trace mineral, good for muscles, but too much can lead to overdose or Selenium poisoning. (7)

The horse can go blind, with head pressing, fever and paralysis, even death, in acute cases.

While the horse is on stall rest, the Vet should also be monitoring blood levels of muscle enzymes, in order to evaluate the horse’s progress, recovery and approximate time for return to work.

It’s always one bucking thing after another.

The aforementioned article is meant as a guideline and not Veterinary analysis.

Azoturia is an ugly thing to see an animal go through and it’s never pretty watching a stiff horse try to work.

Always consult your Vet if you think your horse may have Azoturia or Myositis of the Longissimus Dorsi and Psoas Muscles.

Closing with the immortal words of Roy Rogers and Dale Evans, “Happy Trails to You.”

1-7: “Horseman’s Veterinary Encyclopedia,” by Will A. Hadden III, DVM