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
What is the pathogenesis of hypoglycin A toxicity?
- Ingested toxin from sycamore tree seeds
- Toxic metabolite
- Binds to FAD to inhibit acyl-coA dehydrogenase
- Beta-oxidation of long chain fatty acids are blocked
- Type-1 muscle fibres are the worst affected
What is the prognosis for atypical myopathy?
- Guarded (potentially 84% mortality)
- If horse survives the first few days of treatment it may be better
What clinical signs may decrease the prognosis for atypical myopathy?
hypoxia, dyspnoea, tachypnoea, sweating, hypothermia, bladder distension, tachycardia, CK >100 000 IU/litre and recumbency
How might you be able to prevent atypical myopathy?
Avoid sycamore trees
* Provide supplementary forage in the autumn.
* Clear fallen sycamore leaves and seeds from grazing area (especially in stormy weather).
* Fence off areas where sycamore leaves/seeds have fallen.
* Bring horses in at night.
* Check neighbouring areas for high-risk plants/seeds.
* Test for the prevalence of HGA in your own horse’s pastures.
How would a horse with a metabolic myopathy present?
Horses present with signs of AM but do not have HGA or MCPA detected on blood tests
What is HYPP?
Gene mutation alters voltage-dependent skeletal muscle sodium
channel
Membrane potential close to firing
Some sodium channels fail to inactivate = ↑ sodium in, ↓ potassium
out
What are the clinical signs of HYPP?
May have daily fasciculations and weakness, may be asymptomatic
May begin with third eyelid prolapse → sweating/fasciculations
Overt cramping
Severe attacks may cause swaying, dog sitting, recumbency
Even paralysis of upper respiratory muscles → respiratory tract
obstruction
Episodes 15-60min long
Fasting, anaesthesia and stress may precipitate signs.
What is malignant hypothermia?
RYR1 (ryanodine receptor) gene mutation
↓ activation and ↑ threshold of receptor → dramatic increase in
intracellular calcium → contraction → muscle heat and necrosis
What are the clinical signs of malignant hypothermia?
- Muscle rigidity
- Abnormally high body temperature
- Tachycardia, tachypnoea
What is nutritional myodegeneration also known as?
White muscle disease
What two nutrients may cause nutritional myodegeneration?
- Selenium
- Vitamin E
What are the main forms of nutritional myodegeneration?
- Cardiac
- Skeletal muscle
How might you diagnose nutritional myodegeneration?
Clinicopathologic abnormalities
* CK/AST elevation
* Myoglobinuria
* Electrolyte derangements
Low selenium (whole blood), low GSH-Px, low α-tocopherol (vit E) (plasma)
Clinical signs
Gross pathology
* Pale, oedematous muscle, calcification
* Hypercontracted muscle fibres on histology
How might you treat nutritional myodegeneration?
Selenium injections IM (irritant, can dilute before injection)
* Oral α-tocopherol supplementation
* Supportive nursing
What is the prognosis for nutitional myodegeneration?
- Cardiorespiratory disease → poor prognosis
- Skeletal muscle disease → better prognosis
What are the clinical signs of vitamin E-Deficient Myopathy?
- Weakness, toe-dragging
- Muscle atrophy
- trembling and muscle fasciculations
- Weight shifting
How would you diagnose vitamin E-Deficient myopathy?
SCDM biopsy: Presence of myopathic not
neuropathic change in sacrocaudalis dorsalis
medialis (SCDM) biopsy
* May have normal serum α-tocopherol
How would you treat Vitamin-E deficient myopathy?
Oral α-tocopherol supplementation
* Not powders, needs to be bioavailable
What is an in-vitro contracture study?
Test that measures the skeletal muscke sreaction
