3.0 Acquired and Congenital Myopathies Flashcards
What is myopathy?
myopathy is a general term referring to any disease that affects the muscles that control voluntary movement in the body
The lecture details 9 myopathies. What are they?
Equine:
1. equine rhabdomyolysis
2. “the exhausted horse”
3. muscle strain injury (horse)
4. atypical myopathy
Canine:
1. canine rhabdomyolysis
2. coccygeal muscle injury
Other (multi-species):
1. ion channelopathies
2. cell signallinng defects
3. muscular distropies
What is equine rhabdolyolysis syndrome?
literally the dissolution of striated muscle with exercise, seen as muscle cramping or pain that occurs after strenouous activity
also called:
- tying up
- set-fast
- azoturia
- myoglobinuria
- monday morning disease
What are the symptoms of acute equine rhabdomyolysis?
clinical signs:
- stiff movements
- pain
- sweating
- tachycardia
- myoglobinuria (very dark brown in appearance) in severe cases
How do you diagnose equine rhadbomyolysis?
plasma CK and AST activities:
- CK peaks 6 hours post-damage and declines rabidly (12hr half-life) if muscle damage isn’t ongoing
- AST rises slower and lasts for days/weeks at elevated levels
- urine sediment analysis for renal tubular casts (upon damage myoglobin is filtered by the glomeruli and form tubular casts): can obstruct tubules and be nephrotoxic
can also work to rule out other diseases
How do you treat equine rhabdomyolysis?
analgesics:
- opioids best
- can use NSAIDs can be used but only for the MILDest of cases: myoglobin can be nephrotoxic and NSAIDs can exacerbate this
fluids IV/PO + diuretics:
- used to maintain urine output in attempts to prevent or minimize the nephrotoxic effects of myoglobin
How do you test for inherited equine rhabdomyolysis?
firstly, make sure the horse is between episodes and does not presently have CK/AST increases
- exercise test: 20 minutes of lunge exercise trot and canter
- measure pre and 6-hr post-exercise CK levels
- a rise in CK >20% is considered significant
- in a horse with repeted episodes of tying up, inherited rhabdomyolysis is more likely
but note: the amount of CK increase is variable per individual -> ANY rise, when seen alongside rhabdomyolysis symptoms should be investigated as rhabdomyolysis
What is the etiology of inherited exertional equine rhabdomyolysis?
(1) recurrent rhabdomyolysis of Thoroughbreds due to defective calcium regulation (5% of TBs affected, most common in young nervous fillies)
- much is still unknown, though Ca is significant in muscle contraction
(2) polysaccharide storage myopathy 1
- autosomal dominant mutation
- abnormality of glucose metabolism due to mutation in glycogen synthase gene in skeletal muscle
- affects many breeds: QH, warmbloods, draft horses (high prevalence in some), cobs, etc.
- can be DNA tested for PSSM1 ( EDTA blood / hair pluck)
- can also see on biopsy of muscle: internalized nuclei (non-specific) or polysaccharide inclusions on PAS stain
What is the etiology of acquired exertional equine rhabdomyolysis?
- more common in cases of prolonged eccentric contraction (contraction of the muscle while it is lengthening), such as jumping and downhill exercise)
- metabolic exhaustion
- oxidative injury
- electrolyte imbalance
- possible hormonal influence / infectious causes
How do you prevent recurrent exertional rhabdomyolysis?
- oral dantrolene: calcium release channel blocker, give 90mins before exercise (legally cannot race a horse with dantrolene: remove the drug 28 days before a race)
- high fat / low carb diet: though to reduce stress/anxiety which may precipitate an episode but this is not proven (rice bran, vegetable oil)
- regular exercise: a fit horse is less likely to experience rhabdomyolysis, as true muscle exertion takes longer to occur
transition from rest to exercise each day SLOWLY: hand walk first
What is the “exhausted horse syndrome”?
CS, treatment, and prevention
a syndrome usually associated with long rides in hot, humid conditions, most often if the animal is unfit
- combination of muscle glycogen depletion, electrolyte loss from sweat, and hypovolemia
clinical signs:
- depression, dehydration, anorexia
- decreased thirst
- increased RR, HR
- pyrexia
- perfusion abnormalities (decreased CRT)
- synchronous diaphragmatic flutter (“thumps”): phrenic nerve is depolarized by contact with the base of the heart in the thorax, and causes contraction of the diaphragm in time with the heart beat
- muscle pain and stiffness (can be concurrent with rhabdomyolysis)
treatment:
- IV or oral fluids
- electrolyte supplementation
- rapid cooling
- NSAIDs only if also on fluids
- check CK and AST for evidence of rhabdomyolysis
prevention:
- training, heat acclimatization, free access to water, electrolyte supplementation
What is fibrotic myopathy (equine)?
muscle strain injury (over stretching injury) leading to fibrous repair
- common in quarter horses due to the sudden turns they often take during training
- hamstrings, particularly semitendinous, most affected
- fibrotic repair makes subsequent strain/tear more likely
- can be caused initially by muscle tear, IM injection, or neuropathy
- treated with rest, NSAIDs, or surgical resection of fibrotic tissue
associated with the classic gait: hoof is slapped on ground as leg is protracted
What is equine atypical myopathy?
an acute, severe myopathy seen in horses at pasture due to ingestion of hypoglycin A (plant toxin) found in sycamore and acer species
- hypoglycin A is metabolized to MCPA: causes the inhibition of acyl-CoA dehydrogenase enzymes which are responsible for the metabolism of fatty acids before they enter the krebs cycle
- leads to lipid accumulation in skeletal and heart muscle
- severe CK and AST increases
- severe myoglobinuria
diagnosis:
- hypoglycin A and MCPA seerum concentrations
treatments:
- riboflavin (Vit B2): cofactor for acyl-CoA dehydrogenases
- carnitine: protein responsible for moving FAs into the mitochondria
- support carbohydrate metabolism: insulin/glucose?
How does rhabdomyolysis in dogs differ from horses?
very similar, but more associated with prolonged exercise as compared to sprinting
- seen in sled dogs and greyhounds
- same symptoms and treatment as in horses
What is “limber tail”, “cold tail”, “rudder tail”?
coccygeal muscle injury commonly seen in working breeds (labradors, pointers, etc.)
- acutely painful at tail base (essentially rhabsomyolysis of the tail head)
- mild elevation in CK
- predisposed by cold, exercise, swimming, and prolonged caged transport
treat with rest, NSAIDs, and advise less swimming
