What is Tying Up?
‘Tying up’ is a syndrome of muscle disorders with multiple causes.
Some horses are healthy athletes that tie-up sporadically likely due to exercise in excess of their training level, electrolyte depletion, dietary imbalances, or exercise during viral disease (‘Exertional Equine Rhabdomyolysis’).
In certain lines of Thoroughbreds it is postulated that this disease has a genetic susceptibility, and occurs due to an abnormality of muscle contraction (‘Recurrent exertional rhabdomyolysis’ (RER)):
- Research suggests that horses with RER may have an inherent abnormality in intramuscular calcium regulation that is intermittently manifested during exercise.
- Muscle contractions are propagated via electrical impulses; these impulses are stimulated via release of calcium from intracellular membranous storage sites – and muscle relaxation requires energy-dependent pumping of calcium back into storage sites.
- This disease causes an abnormality in the way calcium is regulated by membrane systems in skeletal muscle.
- The disease may lie dormant unless specific factors trigger the calcium regulatory system to malfunction, including stress, excitement, lameness, high grain (sugar/starch) diets, and exercise at submaximal speeds
- Young TB fillies and nervous/anxious horses are more commonly affected.
- The prevalence of this disease in TBs worldwide is ~5%.
Another form of ‘tying-up’ is known as polysaccharide storage myopathy (PSSM)
There are two forms of the disease (PSSM type 1 and 2);
- PSSM Type 1 is caused by a mutation in the glycogen synthase gene (GYS1).
- Type 1 PSSM occurs in Quarter Horses and related breeds, draft breeds and some warmblood breeds (not identified in TBs, Arabs or Standardbreds).
- This mutation causes the horse’s skeletal muscle to make more glycogen (the storage version of sugar), and when exercising there is disruption to the conversion/metabolism of glycogen into glucose for use as an available energy substrate.
- Affected horses develop stiffness, muscle cramping and soreness with light exercise likely due to a deficit of energy generation in their muscles.
- Some horses with this disease also have an increased sensitivity to insulin further increasing storage of sugar as glycogen in the muscle.
- PSSM Type 1 horses are often calm, sedate horses that tie-up when brought back into work after a spell, especially when fed grain/high starch diets.
- This type of disease can now be diagnosed by DNA based blood or hair root test.
- ~6-10% of Quarter horses, American Paint horses, and Appaloosa horses possess the GYS1mutation with the highest frequency in halter Quarter horses (~28% affected).
- PSSM Type 2 has an unknown genetic cause. It occurs in Quarter Horse-related breeds and Warmbloods and is currently only diagnosable via muscle biopsy.
- Horses with severe signs of PSSM should also be tested for a second genetic mutation that causes a muscular disease called Malignant hyperthermia (MH).
- When MH and PSSM occur together, a horse may develop severe episodes of tying up which can be fatal.
- MH occurs in less than 1% of Quarter horses and paint horses.
Clinical Signs of Tying Up
ACUTE Clinical signs include:
- Excessive sweating
- Generalised stiffness (shortened hindlimb stride)
- Reluctance to move
- Sore muscles (firm, painful hindquarter muscles)
- ‘Distress’ and anxiety – elevated respiratory rate and heart rate
- Dark coloured urine (myoglobinuria)
CHRONIC Clinical signs include:
- A lack of energy under saddle
- Reluctance to move forward
- Stopping and stretching out as if to urinate
- A ‘sour attitude’ toward exercise
- Dressage and show jumpers can present with chronic back pain, failure to round over fences, and fasciculations or pain upon palpation of lumbar muscles
What to do in the acute case?
If your horse ‘ties up’ (becomes stiff, sweating excessively and painful):
- Stop exercising the horse and move it to a nearby stable.
- Do not force the horse to walk a long distance.
- Call your veterinarian.
- Rug the horse if the weather is cool or hose down the horse if sweating in warm weather.
- Provide free access to water.
- Remove grain/feed; provide only hay or grass until signs improve.
- Small paddock turnout – once the horse walks freely without pain, usually in 24 -48 hours.
- Discuss ongoing management with your vet – including appropriate diet, exercise and paddock turnout.
- If the problem recurs, consider diagnostic testing.
Diagnosis of the acute case by the veterinarian:
- Clinical signs upon examination (stiffness, muscle pain, excessive sweating)
- Increased muscle enzymes (creatine kinase (CK)and aspartate transaminase (AST)) in the blood.
- A blood test should be repeated prior to returning to demanding exercise to ensure the muscle enzymes have returned to normal (~4 weeks after the acute episode).
Immediate veterinary treatment may include:
- Pain relief:
- NSAIDs (non-steroidal anti-inflammatories such as ‘Bute’ or ‘Flunixin’)
- Sedation (xylazine, detomidine, or Acepromazine (ACP) which is also a good vasodilator and anxiolytic)
- Opioids if severe pain (e.g. Methadone)
- Administration of fluids due to the release of myoglobin from the damaged muscles – myoglobin is nephrotoxic (harmful to the kidneys).
- Oral balanced electrolyte fluid (via a naso-gastric tube)
- Or IV fluid in severe cases
- Box rest for 24 – 48 hours
- After this period the horse should be turned out into a small yard/small paddock for a further week prior to returning to a larger paddock
- Exercise
- The horse should be completely rested for 48 hrs
- Then the horse should start daily – mild/calm/low intensity exercise for ~ 15 mins a day (until CK is <1000 U/L in the blood).
On-going Management of the Horse
DIET
- The horse should be fed an alternative energy source in the form of FATs rather than SUGAR/STARCH
- Eliminate grain and sweet feed completely, alternatives should include:
- Low calorie vitamin and mineral balancerg. Essentials everyday vitamin and mineral balancer
- Rice bran g. Equi-Jewel feed
- Vegetable oils (corn oil, canola oil, soybean oil, 100ml per 100kg – e.g. a 500kg horse can have 500ml of oil – I would split this over two feeds)
- SOAKED hay (soak for 6 hours minimum prior to feeding)
- Electrolyte supplementation is especially important for these horses if sweating during exercise or hot weather.
- Exertional rhabdomyolysis may be exacerbated by inadequate vitamin E and selenium in the diet – it may be worth testing these in the blood or providing a dietary supplement.
- Eliminate grain and sweet feed completely, alternatives should include: