22c: Equine Muscle Disease

Diagnosis of PSSM

• High CK/AST on acute episode

• Genetic testing

• Exercise/stress test

• Biopsy

How does an exercise/stress test evaluate for PSSM/MFM

Pull blood 4-6 hours post exercise and look for a 3 fold increase in serum CK

Biopsy findings associated with PSSM-1

Granular glycogen deposited within the sarcolemma

Biopsy findings associated with RER

Necrosis and regeneration with increased centralized nuclei

Biochemical changes associated with an acute rhabdomyolytic event

• High CK/AST

• Hyponatremia, hypochloremia, hypocalcemia

• Hyperkalemia

• Hyperphosphatemia (significant)

Urine changes associated with an acute rhabdomyolytic event

• Proteinuria

• Hematuria

• Hyposthenuria (severe, kidney damage!)

Treatment for an acute rhabdomyolytic event

• Stop work, don’t move the horse unless you have to

• LOTS of fluids

• Pain management

• Dantrolene

Pain meds of choice for horses in an acute rhabdomyolytic event

Opioids > acetaminophen > other NSAIDs

Why do we want large volumes of fluids for horses with acute rhabdomyolysis

Want to correct volume depletion and then diurese them to protect the kidneys

Dantrolene mechanism of action

Blocks ryanodine receptor → blocks Ca⁺⁺ release

Type of horse that should NOT get dantrolene ever and why

HYPP horses → can cause hyperkalemia

Treatment for milder cases of rhabdomyolysis

Stop working, may get away with oral fluids and oral electrolytes with NSAIDs if not dehydrated

When can you stop giving fluids to a horse with rhabdomyolysis

When the urine runs clear visually and is negative on the urine occult blood test

Why might we add DMSO to fluids for a horse with rhabdomyolysis

It acts as a free radical scavenger

T/F: DMSO works when given PO

False, efficacy not documented

Max concentration of DMSO in fluids

10%

How do we reintroduce exercise for a horse after rhabdomyolysis

SLOWLY. Start with 4 minutes of exercise, recheck enzymes every 3-7 days, then increase by 2 minutes if labs are good

Equine myopathies effectively prevented by dantrolene

• RER

• PSSM

• Malignant hyperthermia

Diet recommendations for a horse with PSSM type 1 or 2

• Low/moderate NSC content (<12%)

• Higher fat

• Adequate not excessive calories

• Adequate vit E and selenium

• Adequate electrolytes, especially if hot

Antioxidant that may be beneficial to add to the diet for MFM horses

CoQ10

Primary source of vitamin E for the horse

Pasture or hay

Diet recommendations for a horse with MFM

• Moderate NSC

• Low fat

• Higher protein

Exercise/rest recommendations for a horse with MFM-ER

Don’t give them full rest after heavy athletic activity, exercise them for 5-10 minutes even on rest days

Exercise/rest recommendations for a horse with MFM-WB

Give them lots of rest and turnout after heavy athletic activity

Types of grasses that are lower in NSC

Warm season grasses are lower in NSC than cool season grasses

How long does it take to clinical improvement for PSSM/MFM from diet modification

4-6 weeks

How does a high fat diet help avoid rhabdomyolysis in PSSM horses

Higher fat → lower glycemic response → increases free fatty acid metabolism by muscles → decreases intramuscular glycogen

Disease caused by HYPP

Muscle fasciculations and weakness

Demographic for HYPP

Young QH

When and how might HYPP present in foals

Very sick foals; pay present for airway obstruction due to paralysis of laryngeal muscles and laryngeal/pharyngeal collapse

Genetic mutation associated with HYPP

Autosomal dominant mutation in SCN4

Pathophysiology of HYPP

Altered sodium channel → failure to close → influx of sodium → RMP much closer to threshold potential → persistent depolarization of muscles

T/F: an HYPP horse is normal between episodes

True

CS of an HYPP episode

• Brief myotonia

• Focal or diffuse fasciculations and sweating

• Third eyelid prolapse

• Severe: airway collapse

• Death

How does HYPP cause death

Hyperkalemia → fatal arrhythmias

Diagnosis for HYPP episode

• CS in a horse with known breeding history

• Hyperkalemia

• Genetic testing

T/F: a horse with HYPP always has high potassium

False, it may be normal by the time you get there

Treatment for HYPP

• Spontaneous recovery

• High NSC/sugar meal or IV dextrose ± insulin if bad

• IV Calcium

• Bicarb

• Tracheostomy for airway obstruction

How does NSC/sugar help a horse with HYPP

Sugar → insulin release → drives potassium into the cells

Why can’t we use molasses as a high NSC meal for HYPP horses

It is high in potassium