Equine Muscle Disease

What are the FOUR types of muscle injury?

Trauma, ischemia, exercise-induced, underlying disease

T/F: Muscle, unlike other soft tissue structures, have good regenerative capacity.

True!

What are FIVE clinical signs that might make you suspect muscle disease?

1. Alterations in muscle tone

2. Muscle stiffness, cramping, pain

3. Muscle fasiculations

4. Muscle atrophy (focal or generalized)

5. Exercise intolerance not associated with other cause

T/F: Diseases like colic, pleuritis, weakness, chronic wasting, lameness, and poor performance can present similar to muscle diseases.

True!

What are some features of a diagnostic workup for muscle disease?

Musculoskeletal exam (static +/- dynamic) -> clin path (chemistry and urine) -> exercise challenge test -> genetic testing -> muscle biopsy -> additional (u/s, bone scan, electromyography, thermography)

What SIX things should be done during a musculoskeletal examination?

1. Observe at rest and in motion

2. Palpation of all muscle groups for heat, pain, muscle tone

3. Palpate and observe for muscle fasiculations/twitching

4. Percussion of muscle groups

5. Lameness evaluation

6. Neurologic exam

What kind of muscle is CK level on bloodwork reflective of?

Skeletal and cardiac muscle

When is peak CK and return to baseline?

Peak: 4-6 hours

Baseline: 3-4 days

When might mild to moderate (800-3000 IU/L) increases in CK be expected (5)?

Trailering, exercise, colic, IM injection, recumbency

What is the nonspecific muscle enzyme that we look at in horses?

AST (aspartate aminotransferase)

When is peak AST and return to baseline?

Peak: 24-48 hours

Baseline: 10-21 days

T/F: AST involves time collection according to expected peaks; serial measurements useful; process quickly (RBC lysis) and keep chilled or frozen if delay in shipping.

True!

T/F: CK features a long return to baseline, while AST has a short return to baseline.

False! CK features a SHORT return to baseline, while AST has a LONG return to baseline.

What muscle enzyme is not specific to muscle and is found in many tissues? (Red-headed stepchild)

LDH

Why is vitamin E important for muscle?

Antioxidant that limits cell membrane free radical injury

Why is selenium important for muscle?

Reduces free radicals via glutathione peroxidase

—- soil content is low in the NE, NW, and Great Lakes.

Selenium

Deficiencies in vitamin E and Selenium show signs of —-.

Rhabdomyolysis

What would you expect to see in severe muscle necrosis in a urine sample?

What must it be differentiated from?

Why is this a problem?

Myoglobinuria

Hemolysis

Toxic to renal tubular cells (diuresis needed)

What is useful in the diagnosis of exertional myopathies, especially in horses with history of exercise intolerance and muscle stiffness?

Exercise test

What is the goal of an exercise test?

Generate a subclinical increase in CK activity

When should you NOT perform an exercise test?

Do not perform when rhabdomyolysis present

When should you measure CK in an exercise test?

What level of increase suggests exertional myopathy?

Baseline and 4-6 hours post-exercise

>5-fold

What is the last diagnostic test you should think about that is more invasive?

Muscle biopsy

What muscle is biopsied using the percutaneous needle biopsy method?

Middle gluteal muscle

T/F: Muscle biopsies should be shipped overnight on icepacks early in the week and not frozen.

True!

What kind of muscle fibers are involved in endurance, aerobic metabolism, and long duration?

Are they fast or slow twitch?

Type I

Slow twitch

What is the major condition that a Type I muscle biopsy is needed for?

Equine Motor Neuron Disease (EMND)

What muscle should be biopsied for a Type I fiber biopsy?

Sacrocaudalis medialis dorsalis

Type II muscle fibers are (slow/fast) twitch.

Fast

What muscle fiber type is oxidative with a short duration?

What kind of use/features does this muscle have?

Type IIa

Speed, endurance, aerobic/anaerobic

What muscle fiber type is glycolytic with a very short duration?

What kind of use/features does this muscle have?

Type IIb

Speed, power, sprinters, anaerobic (QHs)

What muscle should be biopsied for a Type II sample?

Semimembranosis

What THREE conditions is a Type II muscle biopsy needed for?

RSSM, Rhabdomyolysis, neurogenic myopathies

What are the TWO major genetic tests that are actually worth running?

1. PSSM Type I: two mutation (GYS1 and MH) with associated abnormal polysaccharide

2. HYPP in QH breed, descendants of Impressive

What TWO things can be sent off for genetic testing?

Whole blood (EDTA) or hair with roots (20 mane or tail)

What is an acute, highly fatal form of severe rhabdomyolysis most common in Eastern Canada, Midwest, Texas, and Southern US?

Atypical myopathy (Seasonal Pasture Myopathy)

What causes atypical myopathy?

Ingestion of hypoglycin A in box elder tree seeds

Atypical myopathy is a —- storage myopathy.

Lipid

What is the pathophysiology of atypical myopathy?

Multiple acyl-coA dehydrogenase deficiency (MADD)

What THREE things contribute to MADD?

1. Disruption of mitochondrial enzymatic pathways

2. Accumulation of specific acyl-CoA

3. Increased serum acylcarnitines and urine organic acids

T/F: Antioxidant deficiency may contribute to atypical myopathy.

True!

What are the THREE major risk factors of atypical myopathy?

1. Exposure to box elder tree seeds

2. Pasturing

3. Weather/season

What FIVE pasture thingys can make atypical myopathy more likely?

1. >12 hours daily

2. Pasture for first season (young horse new farm)

3. Sparse pasture, short grass

4. No supplemental hay

5. Abundant trees/dead wood on ground

What TWO weather/season things make atypical myopathy more common?

1. Fall season

2. Recent heavy wind/rain

Name at least FOUR early clinical signs of atypical myopathy.

Reluctance to move, muscular weakness, i hate gina hansen, stiffness, fine muscle tremors, choke, myoglobinuria

Name at least THREE later clinical signs of atypical myopathy.

Depression, sweating, congested mucus membranes, tachycardia, dyspnea

Atypical myopathy can lead to progression to lateral recumbency and death within ———-.

24-72 hours

What is the mortality rate of atypical myopathy with treatment?

75-90%

T/F: Atypical myopathy features a mildly increased CK/AST and myoglobinuria.

False! Atypical myopathy features a MARKEDLY increased CK/AST and myoglobinuria.

What TWO major findings will you see on muscle biopsy with atypical myopathy?

1. Severe myonecrosis (postural/resp muscles, esp. cardiac)

2. Excessive lipid staining (intercostals, diaphragm, postural muscles)

What other diagnostic send out test can confirm MADD in atypical myopathy?

Assays of urine organic acids/serum acylcarnitines

What is the definitive treatment of atypical myopathy?

None- early and aggressive supportive therapy essential

What FOUR ways can you aggressively provide supportive care to a horse with atypical myopathy?

1. Monitor cardiac/renal function and electrolytes

2. IV fluids (glucose, insulin, vitamin B2 and C, electrolytes)

3. Pain management (NSAID, lidocaine, opioids)

4. Oral carnitine/vitamin E

What is the mainstay therapy of atypical myopathy?

IV fluids

What are the FIVE major preventative measures of atypical myopathy?

1. Minimize/prohibit access to box elder trees/seeds in fall

2. Remove all horses from affected pasture until after 3 days frost

3. Monitor CK 1-2 weeks in new cases; consider treating if not WNL

4. Feed supplemental hay while horses on the pasture

5. Consider removing box elder trees that bear seeds

What is an infection of the muscle due to anaerobic clostridial organisms?

Clostridial myositis

Why is clostridial myositis bad news?

Spores replicate rapidly -> toxin production and ischemic necrosis

What is the most common cause of clostridial myositis?

C. Perfringens

What kind of bacteria are Clostridium spp?

Gram + spore-forming anaerobic robs that are uniquitous

What are the FIVE most likely etiologies of clostridial myositis?

1. Direct inoculation (IM injection- most common)

2. Contamination (wounds)

3. Trauma (blunt/local due to injection)

4. GI translocation

5. Latent spores in muscle (cattle mainly)

When do clinical signs usually develop in clostridial myositis?

24-48 hours

What are the THREE major local clinical signs of clostridial myositis?

1. Heat/pain/swelling

2. Stiffness/soreness of affected muscle groups

3. SQ emphysema (50%)

What are the TWO major systemic signs of clostiridal myositis?

1. Lethargy/inappetence/fever/tachycardia

2. SIRS/DIC/laminitis/death

What are the THREE major ways to diagnose clostridial myositis presumptively?

1. Clinical signs/history

2. Ultrasound

3. Cytology/FNA/impression smear

What are the TWO major clinical signs/history that can help diagnose clostridial myositis?

1. Swelling that expands

2. History of recent injection/trauma/wound

What are TWO ultrasound findings to make a presumptive diagnosis of clostridial myositis?

1. Gas production in tissues

2. Fluid accumulation between muscle planes

What are TWO cytology/FNA/impression smear findings that can lead to a presumptive diagnosis of clostridial myositis?

1. Presence of G+ rod-shaped bacteria

2. With/without evidence of spores

What is the major definitive diagnostic for clostridial myositis?

What TWO samples can this be done on?

ANAEROBIC bacterial culture

Aspirated fluid and tissue samples

T/F: You should collect a sample for clostridial myositis in a low oxygen tension container.

True!

What are the SIX major treatments of clostridial myositis?

1. Tissue exposure (oxygenation)

2. Drainage

3. Lavage

4. Antimicrobial therapy

5. Analgesics

6. Supportive care/monitoring

What is the most common muscle disorder in horses?

Exertional myopathies

What are some of the common presenting signs of exertional myopathy?

Stiff gait, excessive sweating, tachypnea, tachycardia, reluctance to move, anxiety, myoglobinuria, exercise intolerance

How do muscle feel on palpation with exertional myopathy?

Firm and painful (back/hindquarters)

What is the common name for exertional rhabdomyolysis?

Tying up

What are the TWO major forms of ER?

Sporadic and chronic

What are the THREE major forms of chronic ER?

MH, RER, and PSSM

Sporadic ER is an (intrinsic/extrinsic) muscle condition.

Extrinsic

What are FOUR situations that can cause sporadic ER?

1. Exercised beyond level of condition

2. Diet high in NSC and low in forage

3. Hot, humid environments

4. Recent respiratory infection

What are some of the major treatments for sporadic ER?

Rest, fluids, sedatives (alpha-2), NSAIDs, muscle relaxants, dantrolene

What causes recurrent ER (RER)?

Abnormality in intracellular Ca2+ regulation

What kind of horses are most likely to get RER?

Racing breeds (TB > STB > Arab)

What muscle fibers are typically affected by RER?

Type II

What are SIX predisposing factors for RER?

1. Temperament (nervous/hot)

2. Female

3. Diet (concentrates)

4. Exercise duration/intensity

5. Rest before exercise or if held back

T/F: RER typically involves an abnormality in muscle contraction and relaxation with a sudden onset during exercise.

True!

In RER, dysregulation of intracellular calcium from the —- results in excessive ———————— and ——

SR

Myoplasmic calcium

Myodegeneration

An autosomal dominant inheritance for RER is proposed in what horse breed?

Thoroughbred

What are some of the major clinical signs of RER?

Reluctance to move, stilted gait, rigid/painful muscles (esp. hind), sweating, tachycardia, tachypnea, pain, anxiety, colic, recumbency

What might you see in the urine and muscle enzymes with RER?

Urine: severe myoglobinuria

Enzymes: increased CK (>50,000)

The value of what diagnostic is restricted to recurrent and unmanageable episodes to rule in or out causes of ER because findings are often nonspecific?

Muscle biopsy

What are TWO histologic findings of RER?

1. Abnormal polysaccharide

2. Central nuclei

What is resting CK/activity between episodes of RER like?

Variable (age/sex dependent- usually normal or slight increase and increase with exercise)

What are the TWO major goals of RER management?

1. Alleviate anxiety and muscle pain (ace, dorm/torb, lidocaine CRI)

2. Replace fluid loss and correct electrolyte abnormalities to prevent renal damage

What are the TWO most common electrolyte imbalances in RER?

Hyperkalemia and hypocalcemia

T/F: Corticosteroids, muscle relaxants, and diuretics are used to manage RER, but have an unproven benefit.

True!

A horse with RER should rest until CK is less than —-, then return to exercise gradually.

Exercise

What are TWO environmental ways to manage RER?

1. Identify and avoid stressors

2. Tailor needs to individual

What is important for horses with RER, especially if in training?

Daily exercise

What are TWO major dietary means to manage RER?

1. Low starch and high fat (

2. Salt/electrolyte supplements

T/F: Supplements to reduce lactic acid build-up in horses with RER is a beneficial tool to support them inexpensively.

False! Supplements to reduce lactic acid buildup are NOT useful in RER horses.