22b: Equine Muscle Disease

Electrolyte abnormality classically associated with synchronous diaphragmatic flutter AKA thumps

Profound hypocalcemia

Treatments for thumps an owner can do at home while waiting for veterinary care

• Commercial electrolyte pastes

• Controlled water intake: 2-3L every 30 mins

• Extra-strength tylenol (NOT NSAIDs)

• Slow cooling

Primary rapid intervention needed by horses with overexertion rhabdomyolysis

Correct volume depletion

Why is fluid therapy so important for horses in a rhabdomyolysis event

Need to diurese to prevent kidney damage

T/F: a horse in a rhabdomyolytic event needs to be cooled as fast as we can

False, don’t want to cool them down too fast

Why is exercise myopathy so significant in the horse compared to other species

Horses are more than half muscle and their muscle gets a lot of the cardiac output

Physiologic changes often associated with chronic exercise associated myopathies

Breakdown in the interplay between electrolytes, mitochondria, and glycogen

What is rhabdomyolysis

Breakdown of muscle due to exercise

Lay terms for rhabdomyolysis

• Tying up

• Set fast

• Azoturia

• Monday morning disease

Pathophysiology of rhabdomyolysis

Damage to sarcoplasmic membrane → high intracellular calcium → apoptosis → protease release → release of leakage enzymes and metabolites

When can muscle regenerate

As long as the basement membrane is intact

What happens to a muscle belly if the basement membrane is damaged

It will heal with a focal defect

Complications associated with severe rhabdomyolysis

• Recumbency

• AKI → CKD

• Laminitis

• Permanent muscle loss

How does myoglobinuria cause renal failure

• Oxidative damage

• Reflex vasoconstriction

• Tubular obstruction from dying epithelial cells and casts

T/F: RER is caused by the same thing as exertional rhabdomyolysis (ER)

False

Cause of RER

Intrinsic abnormality in cellular Ca⁺⁺ levels

Primary demographic for RER

Racing thoroughbreds and standardbreds, ± racing QH, particularly high strung fillies

Management factor that is associated with RER

Diet high in non-structured carbs (NSC) and some degree of extra rest

When do we tend to see RER in standardbreds v thoroughbreds

We tend to see it in standardbreds right after exercise and we tend to see it in thoroughbreds during exercise when they are held back

Primary muscle groups affected by RER

Repulsion muscles: hindlimbs and lumbar muscles

PE findings consistent with RER

Tense, maximally contracted muscles → may look swollen

Gait abnormalities associated with RER

Hypoflexion and baby steps → quickly offloading limbs

Primary muscle groups affected by PSSM

Repulsion muscles

Demographic for PSSM

Younger horses, average is 5 years

Breeds with high prevalence of PSSM

• Percheron

• Belgian

• Paint

• QH

• Appaloosa

Difference between PSSM type 1 and “PSSM type 2”

Type 1 is due to an identified genetic mutation, “PSSM type 2” has a very similar presentation but no identified genetic link (yet) and different biopsy characteristics

Gene mutation that is associated with PSSM type 1

Gain of function mutation in GYS1

PSSM type 1 pathophysiology

Deposition of high quantities of abnormal glycogen in the muscle → compresses adjacent muscle cells → damage

Types of diseases that fell under the term “PSSM type 2”

• PSSM type 2-ER

• Myofibrillar myopathy-ER (MFM-ER)

• MFM-WB

Demographic for PSSM 2-ER

QH that are negative for the known GYS1 mutation

PSSM-2 v myofibrillar myopathy

PSSM-2 is still an abnormal deposition of glycogen in the muscle, but MFM is myofibrillary disarray and ectopic accumulation of the cytoskeletal protein desmin

Demographic for MFM-ER

Middle aged Arabians

Demographic for MFM-WB

Warmbloods

Clinical presentation of PSSM-2

Recurrent exertional rhabdomyolysis

Clinical presentation of MFM-ER

Exertional rhabdomyolysis, often following a period of rest and return to work

Clinical presentation of MFF-WB

Soreness and pain post exercise, not often associated with a rhabdomyolytic crisis

Type of PSSM/MFM that is not associated with high CK/AST

MFM-WB

Common CS of PSSM-1, PSSM-2-ER, and MFM-ER

• Perpetually sore hind and back

• Lameness

• Muscle fasciculations and trembling

• Soreness, tucked up

• Sweating

When might we see PSSM-1 in younger horses

Systemically ill QH/Paint foals <6 mos old

CS of PSSM-1 in a foal

• Recumbent

• Hyperextension (looks like tetanus)

• Stiffness