Approach to common muscle diseases in horses- week 10 Flashcards
What two things cause sporadic exertional rhabdomylolysis?
- Overexertion
- Exhaustion
How does overexertion cause sporadic exertional rhabdomylolysis?
- Increase in work intensity without foundation of training
- Can affect any horse
Common in polo ponies early in the season - Aetiology unknown
- History of respiratory infection
How does exhaustion cause sporadic exertional rhabdomylolysis?
Racehorses/endurance horses, hot and humid climate
* Weakness, ataxia, tachypnoea, sweating… (collapse)
* May be hyperthermic
* Myoglobinuria and elevated CK, but muscles may palpate normal
What is recurrent exertional rhabdomylolysis?
Also known as Monday morning disease, Azoturia and Setfast
* Syndrome rather than a disease as aetiology and pathogenesis still
unknown
* Female > males
* Nervous/anxious horses
* High prevalence in racing Thoroughbreds (4.9-6.7% of TBs affected)
* Also occurs in Standardbreds and Arabians
* Historically affects horses on high grain diets
What are the clinical signs of recurrent exertional rhabdomylolysis?
Stiff, painful muscles (large muscle groups- gluteals, triceps)
* Myoglobinuria
* Usually present on day following a rest day
How might you diagnose RER?
History and signalment (Trainer often recognises the signs)
* Elevated muscle enzymes
* Muscle biopsy of semimembranosus- often to rule out other
myopathies as no specific changes associated with RER
What is the gold-standard diagnosis for RER?
in vitro contracture studies
on intercostal muscle (not commercially available)
What are the two ways you can manage RER?
- Exercise
- Dantrolene
What is dantrolene?
RYR1 antagonist → inhibits calcium release from sarcoplasmic reticulum
2-4mg/kg orally
Short half-life. Given 1-2 hours before exercise.
Detection time 48hrs (BHA)
What is the clinical presentation for PSSM?
Recurrent episodes
* Muscle pain, weakness, tremors or fasciculations
* Shifting lameness
* Sweating
* Myoglobinuria
* Reluctance to move or in extreme cases recumbency
How might you diagnose PSSM?
Marked elevation of muscle enzymes (CK and AST)
* Myoglobinuria in more severe cases
* Muscle biopsy of semimembranosus
* reveals abnormal polysaccharide in myofibres
* Genotyping for PSSM1
How does PSSM1 occur?
Associated with a single dominant missense mutation in
the equine muscle glycogen synthase gene (GYS1).
* Glycogen synthase catalyses the formation of 1-4
glycosidic bonds in the glycogen molecule
* The ratio between GS and glycogen branching enzyme
should result in normal branched glycogen formation
* The mutant enzyme is unable to be switched off thereby
altering the GS:GBE and making abnormally
polysaccharide
* Pathogenesis not fully understood
What is PSSM 2?
Still a disease of abnormal glycogen
storage.
* Pathogenesis unknown
* Likely to be a group of conditions:
MFM?, MIM?
Abnormal polysaccharide on muscle
biopsy , but absence of GYS1 mutation
What are the dietary changes needed for PSSM 1 management?
- Limit glycogen synthesis (reduce insulin activity)
- Promote breakdown of glycogen
- Low starch, high fat
- Provide 1.5-2% BWT as roughage
- < 10% Digestible energy as non-structural carbohydrates
- > 13 % fat
- Vegetable oil up to 1ml/kg/day
- Supplemental Vitamin E
- Commercial diets
How might you treat rhabdomylolysis?
- Minimise continued muscle damage
- Analgesia
- Correct fluid deficit
What are four ways you can minimise continued muscle damage?
- Rest- deep bed
- Anti-inflammatories (e.g NSAIDS)
- Acepromazine
- Antioxidants
What does post-anaesthetic myopathy look like?
- Localised rather than general myopathy
- Hard, Hot, swollen muscles
What causes compartment syndrome?
inappropriate positioning
How would you treat compartment syndrome?
Symptomatic and supportive treatment
What are the risk factors for atypical myopathy?
- Autumn and spring
- Wet and windy weather
- Presence of Sycamore trees (UK)
- Access to pasture
What are the clinical signs of atypical myopathy?
- Weakness: Horses struggle to walk and stand. May be found recumbent.
- Muscle tremors and painful, firm muscles may be appreciated on palpation
- Myoglobinuria
- Tachycardia, tachypnoea, congested mucous membranes
- Signs of severe colic but normal appetite
- Distended bladder on rectal palpation
How might you diagnose atypical myopathy?
- Elevated muscle enzymes (CK>10,000iu/L)
- Myoglobinuria and acidic urine
- Submit plasma for acylcarnitine profile/serum for hypoglycin A and conjugated MCPA
Confirmation takes 48-72h via RVC Neuromuscular Lab
What does atypical myopathy look like on post-mortem?
necrosis and lipid accumulation in deep postural muscles, necrosis of respiratory and cardiac muscle
What causes hypoglycin A toxicity?
Seeds and seedlings of acer trees
