1 Muscle Notes

Rhabdomyolysis

• Synonymous terms include tying-up, Monday-morning disease, and azoturia (antiquated term).
• The exact cause of exertional rhabdomyolysis in horses is not confirmed for all cases.
• Proposed etiologies include vitamin E/selenium deficiency, electrolyte imbalance, hypothyroidism, and lactic acidosis
• Most cases of exertional rhabdomyolysis in people are due to an enzymatic error in skeletal muscle metabolism.
• The three most common basic pathophysiologic processes associated with recurrent tying-up in horses:
  • Polysaccharide storage myopathy
  • RER / abnormal intracellular calcium regulation
  • Immune-mediated (Streptococcus-associated) myopathy

Polysaccharide Storage Myopathy (PSSM) Pathophysiology

• Glycogen storage involves accumulation of non-bioavailable polysaccharide in type-2 (fast twitch) glycolytic muscle fibers.
• Clinical signs range from muscle atrophy and progressive weakness in draft horses, to muscle soreness/gait abnormalities in Warmbloods, to exertional rhabdomyolysis in Quarter Horses.
• Muscle biopsy histopathology reveals subsarcolemmal vacuoles, glycogen storage, and abnormal periodic acid-Schiff (PAS) positive, amylase-resistant inclusions in fast-twitch fibers.
• Muscle glycogen concentrations are 1.5 times normal, classifying it as a glycogen storage disorder.
• Maximal achievable exercise speed is low
• At equivalent exercise levels, horses use more glycogen during exercise, have similar blood lactate concentrations, but higher muscle lactate concentrations.
• Glycogenolysis and glycolysis are functional
• PSSM is clinically/histologically similar to human glycogenolytic/glycolytic enzyme deficiencies
• Abnormalities in this metabolic pathway are not present.
• Affected horses have more rapid decrease in blood glucose concentrations, classifying them as very insulin sensitive, despite slightly low insulin concentrations.

PSSM Signalment

• Quarter Horses, Paint horses, and Warmbloods are commonly affected.
• Familial inheritance occurs as an autosomal dominant disorder.
• Glycogen synthase mutation (GYS1) causes Type 1 PSSM resulting in a gain of function for glycogen synthase activity that results in a glycogenosis.
• The GYS1 mutation involves an arginine to histidine change at codon 309 leading to increased enzyme activity in PSSM-affected horses.
• Some Quarter Horses have a mutation in their ryanodine receptors (RyR1), leading to malignant hyperthermia (MH).
• Exacerbation of PSSM can occur in individuals with both GYS1 and RYR1 gene mutations, causing fatal complications from exertional rhabdomyolysis.
• Abnormal accumulation of polysaccharide is common in Draft Breed horses, and also is termed Polysaccharide Storage Myopathy (PSSM), indistinguishable from QH PSSM.
• Diagnosing in draft horses is more difficult because an elevation in CK post-exercise does not necessarily occur.
• Muscle biopsy is the most reliable way to diagnose in adult draft horses.
• Abnormal glycogen and complex polysaccharide is only found in type-2 (fast twitch) glycolytic fibers.
• Muscle biopsy reveals subsarcolemmal vacuoles, glycogen storage, and abnormal periodic acid-Schiff (PAS) positive amylase-resistant inclusions in fast-twitch fibers.

PSSM History and Clinical Signs

• Most horses have numerous “tying up” episodes from the beginning of their training.
• Mildly affected horses may only have one or two episodes per year
• Clinical signs range from mild stiffness to severe pain.
• Mild forms: tucked up abdomen, fasciculations in the flank region, and a camped-out stance after exercise.
• Some may present with painful epaxial muscles, resistance to being tacked up, and alteration of stride with exercise; exercise intolerance is the most common complaint.
• A severe tying-up episode in a Quarter-type horse is presumptive evidence of PSSM, muscle biopsy provides definitive evidence.

PSSM Diagnosis

• Clinical signs of tying-up begin when creatine kinase activity is elevated (typically > 20,000-40,000 U/L, and sometimes > 100,000 U/L).
• Due to ongoing muscle damage, CK remains elevated longer compared to other forms of exertional rhabdomyolysis
• Aspartate transaminase (AST) rises more slowly with a prolonged return to baseline.
• Myoglobinuria (dark brown, coffee-colored urine) is present in many cases during a clinical episode.
• Screening exercise test (15-minute trotting on a lunge line) identifies ~80% of affected horses.
• An increase in CK to > 1000 U/L may be observed 4-6 hours after exercise; muscle enzymes remain elevated for long periods, even if the horse is rested.
• Type I PSSM can be determined using a hair sample from a minimum of 25 pulled (not cut) mane hairs to evaluate DNA genotype, which are submitted to the neuromuscular laboratory at the University of Minnesota.
• For horses with PSSM from a non-GYSI mutation (Type 2 PSSM), a muscle biopsy is indicated to determine the presence of PSSM.

PSSM Dietary Management

• Aim to increase the oxidative capacity of skeletal muscle.
• High-fat diet (0.5 to 3.0 kg/day) should comprise 20-25% of total daily (2-3x the fat compared to a typical diet).
• Rice bran, corn oil, or spray dried fat supplements are used to supplement fat.
• Corn oil (1-2 cups/day) is mixed with soaked alfalfa cubes.
• Feed high-quality grass hay.
• Low COH diet: reduce nonfiber, nonstructural carbohydrates (sugars and starches) to ≤ 15% of total daily calories.
• Concentrates with < 33% non-structural carbohydrates are effective when combined with alfalfa hay/pasture roughage; avoid feeding corn (71% nonstructural carbohydrates).
• Sweet feeds contain 47% nonstructural carbohydrate, commercial high fat diets contain 28-39%, and alfalfa pellets contain 2%.
• A high-protein diet (12-17%) is recommended to combat protein use as an energy substrate and build muscle.
• Supplement vitamin E (1000 IU/day) and selenium (1-2 mg/day) due to the increased oxidative potential with a high-fat diet.
• High-fat diets prevent post-exercise elevation in creatine kinase and aspartate transaminase and clinical episodes of exertional rhabdomyolysis after 3-6 months.
• Improvements include attitude, stride, energy, exercise tolerance, and muscling.
• Avoid box stall rest for > 12 hours/day, as it increases the incidence of rhabdomyolysis.
• Dietary change alone is insufficient to prevent rhabdomyolysis - exercise is critical.
• Use a 15-minute exercise test to determine the amount of exercise needed in the initial training stages.
• If the horse fails, 2 weeks of pasture turnout with dietary change is needed.
• If successful, 15 minutes of lunging per day is recommended, increasing a few minutes every few days - once horses can trot for 30 minutes without difficulty, work under saddle can begin.

Recurrent Exertional Rhabdomyolysis (RER)

• Thoroughbreds, Arabians, and Standardbreds are predisposed, with young, nervous fillies being overrepresented.
• Suspected to have a familial basis; genetic transmission not as clear as PSSM.
• Occurs with stress and stall rest before exercise.
• Erroneously believed to be lactic acidosis; treatments are (inappropriately) aimed at resolving lactic acidosis (lactinase, dimethylglycine, bicarbonate).
• RER occurs during aerobic exercise, muscle lactate is low, and is associated with metabolic alkalosis; electrolyte imbalance (sodium, calcium, potassium) has been refuted.
• Most recent studies point to an abnormality in excitation-contraction coupling; affected horses demonstrate prolonged relaxation of muscle after a contractile twitch, suggesting abnormal intracellular calcium regulation causing increased myoplasmic calcium concentrations.
• Clinical signs include muscle fasciculations, stiff-stilted gait, myoglobinuria, and firm painful muscles, especially the epaxial and gluteal mm.
• Diagnosis via muscle biopsy reveals various stages of muscle necrosis and regeneration with centrally located myonuclei.
• Biochemical testing of muscle biopsy samples (intercostals mm = Univ. of Minnesota) identifies sensitivity to halothane and caffeine, as in malignant hyperthermia.

RER Prevention Recommendations

• Minimize stress, standardize daily routine, provide a vitamin-mineral supplement and quality feed, and avoid excess carbohydrate in the diet.
• Administer acepromazine prior to exercise.
• Avoid strict box-stall rest for extended periods - small paddock turnout when the horse can walk freely - return to exercise program when CK returns to normal (more quickly than PSSM).
• Administer Dantrolene (2-4 mg/kg PO) 1 hour before exercise to inhibit calcium release from the calcium release channel (also used to treat malignant hyperthermia).
• Administer Phenytoin (1.4 to 2.7 mg/kg, PO, BID) and monitor levels (8-12 ug/ml) - beware of seizures from toxicity, affects sodium and calcium channels.
• Control clinical signs after 2-6 months of dietary modification in all breeds, including PSSM, and abnormal intracellular calcium regulation.

Treatment Recommendations for Acute Rhabdomyolysis

• Fluid therapy: 40-60 L of isotonic fluids IV will improve muscle blood flow and induce diuresis, preventing renal damage from myoglobinuria.
• Continue IV fluids until urine appears clear; monitor creatinine and urinalyses to assess for renal insufficiency/damage from pigment-associated nephropathy.

Rhabdomyolysis - Analgesic Medication

• NSAID
  • Flunixin meglumine (Banamine®) 1.1 mg/kg IV
  • Butorphanol 10 mg IM (immune-mediated myositis)

Additional Treatment

• Anxiety relief:
  • Xylazine 0.3-0.8 mg/kg IV
  • Detomidine 0.005-0.02 mg/kg IV
  • Acepromazine 0.02-0.06 mg/kg (may improve muscle blood flow)
• Muscular relaxation:
  • Diazepam 15 mg IV (short half-life, administer prior to slinging)
  • Dantrolene 2-10 mg/kg PO

Other Muscle Diseases

• Immune-Mediated (Strep. equi) Myopathy: severe rhabdomyolysis that occurs days to weeks after natural exposure to strangles or vaccination.
  • Pathophysiology: immune-mediated myositis, described in Quarter Horses or Quarter-type breeds.

Myosin Heavy Chain Myopathy (MYHM)

• A heritable disease of quarter horses and related breeds with a codominant mode of inheritance.
• Marked muscle atrophy occurs with vaccination/respiratory illness; affected horses may not completely recover. N = normal My = Myosin heavy chain myopathy Genetic testing should be performed on QH and related breeds
• M protein in streptococcal cell wall has amino acid sequences similar to myosin: muscle biopsy reveals lymphocytic myositis, massive muscle atrophy, and elevated muscle enzymes; acute onset with severe clinical signs.
• Infarctive form is Henoch-Schonlein purpura (highly fatal); muscle biopsy reveals neutrophilic vasculitis throughout muscle mass.
• Extremely painful, firm muscles; horses are often recumbent with extreme elevation in muscle enzymes (CK values > 200,000 U/L); vascular thrombosis may be observed on mucosal surfaces, lung, and intestinal organs.
• MYHM is a potential muscle disease in Quarter Horses that may increase the risk of immune-mediated myositis.

MYHM Treatment

• Standard treatment for acute episode of rhabdomyolysis + aggressive corticosteroid therapy
• Penicillin is administered to treat an apparent or occult Strep equi infection
• Analgesic therapy for longer period of time

Clostridial Myositis (Malignant Edema, Clostridial Myonecrosis)

• Clostridia are Gram+, spore-forming anaerobic bacteria ubiquitous in the environment.
• Intramuscular colonization with Clostridia results in highly pathogenic extracellular exotoxins and enzymes.
• Most cases in horses result from deep penetrating wound or intramuscular injection site; unclear if bacteria are introduced at the time of injection or quiescent spores in muscle germinate in anaerobic environment.
• Bacteria:
  • C. perfringens is most common
  • C. septicum, novyi, sordelli, chauvoei
  • α toxin – hydrolysis phosphatidylcholine and sphingomyelin (cell membranes)
  • ϑ toxin (perfringolysin O) – pore-forming cytolysin → hemolysis
  • κ toxin – collagenase → tissue destruction
• Exotoxins cause extensive tissue destruction and necrosis perpetuating the anaerobic conditions; horses rapidly develop systemic toxemia and die from cardiovascular effects.
• Pathogenesis:
  • Intramuscular injection sites are a common inciting factor (82%)
  • Primarily irritating, non-antibiotic meds: ivermectin, flunixin meglumine, PHF vaccine, antihistamines, vitamin B, mostly cervical injections
  • Site preparation and aseptic injection technique do not have a significant effect on likelihood
  • Castration sites, deep puncture wounds, parturient injuries

Clostridial Myositis Clinical Signs and Diagnosis

• Clinical signs include painful swelling +/- crepitation 48-72 hours after IM injection; overlying skin first warm, then cold; fever, depression, and toxemia; rapid deterioration in clinical signs.
• The speed of progression from clinical signs to death should not be underestimated
• Expect inflammatory leukogram, hyperfibrinogenemia, increased muscle enzymes, and dehydration.
• Diagnosis made via:
  • History of recent IM injection with an irritating compound
  • Rapid progression of clinical signs
  • Impression smears of needle aspirate with large Gram+ rods
  • Ultrasound exam with fluid accumulation in tissue planes +/- gas echoes
  • Anaerobic culture of aspirate

Clostridial Myositis Treatment

Antimicrobial therapy (high-dose, frequent administration) using penicillin, metronidazole, oxytetracycline, or chloramphenicol. Also, use NSAIDs for analgesia, antipyretic, and anti-inflammatory effects. Fresh plasma for DIC Supportive care (IV fluids, nasotracheal intubation, nutritional support) Surgical fenestration: open discrete abscesses to heal by second intention or multiple incisions through a diffusely affected area

Hyperkalemic Periodic Paralysis (HYPP)

• Heritable defect of voltage-gated sodium ion channels of muscle cells, disrupting normal ion channel function and causing uncontrolled sodium influx (autosomal dominant genetic defect). Episodic influxes change the voltage current of muscle cells (closer to threshold), causing uncontrolled muscle twitching or profound muscle weakness; high serum potassium concentrations are observed with massive sodium influx, horses appear normal between episodes

HYPP Clinical Signs

• Intermittent episodes of muscular weakness and fasciculations
• Prolapse of the nictitating membrane (3rd eyelid).
• Sweating, anxiety, and tachypnea.
• Episodes vary in severity and frequency.
• Homozygotes suffer more severe, more frequent episodes.
• Episodes may be so mild as to be undetected by the owner.
• Most episodes do not progress to recumbency.
• Some episodes can result in death due to cardiac or respiratory failure.
• Episodes cannot be predicted, but can be induced by stressors.
• Heterozygotes with mild episodes still have a 50% chance of passing the gene.
• Most episodes last 15 minutes to 2 hours.
• High serum K+ concentrations during an episode are the hallmark of HYPP
• Most horses will have [K+] from 8-11 mEq/L.

HYPP Genetic Defect

• Autosomal co-dominant genetic mutation - all affected, homozygote > heterozygote.
• Mutation of the voltage-gated sodium channel.
• Single amino acid substitution
• DNA sequences read TTG instead of TTC, leucine is in the alpha subunit.
• The resting membrane potential is closer to threshold (hyperexcitable) with suboptimal function of Na/K pump.
• Diagnosis via DNA probe for single base substitution within the sequence of the sodium channel using a sample of 25 plucked hairs from mane or tail at the Veterinary Genetics Laboratory.
• AQHA requires HYPP testing of foals to be registered.
• Electromyography provides supportive evidence, but is rarely necessary.
  • Prolonged insertional activity
  • Myotonic discharges
  • Fibrillation potentials
  • Doublets and triplets
• Homozygotes are more severely affected than heterozygotes.
  • Inspiratory noise detected at rest due to dynamic collapse of pharynx and larynx; responds to acetazolamide.
  • Tracheotomy may be necessary during an episode.
  • Neonates may be dysphagic or dyspneic (arytenoid collapse).
  • Often die before 2 years of age
  • Vocalization is often high-pitched.
• As of 2007 the AQHA determined homozygotes were no longer eligible for AQHA registration.

HYPP Emergency Treatment

• Intravenous calcium gluconate diluted in 0.9% saline or 5% dextrose: Ca++ reduces membrane excitability and dextrose stimulates endogenous insulin production, which lowers K+
• Intravenous sodium bicarbonate: alkalosis drives K+ into cells, K+ and H+ ions in exchange transcellularly.
• Intravenous furosemide: increases urinary excretion of K+
• Intramuscular epinephrine increases uptake of K+ by muscle cells.

HYPP Preventative Therapy

• Acetazolamide stabilizes cellular membrane, increases potassium excretion, and affects glucose metabolism.
• Diet should contain less than 1.0% [K+].
• Feed Grass Hay < Brome <<< Alfalfa.
• Avoid molasses and kelp-containing feeds.
• Routine exercise and pasture turnout are ideal.
• Episodes may be observed during anesthesia.

Atypical (Pasture) Myopathy

• Life-threatening toxic myopathy
• Affects Midwestern horses and also those in Europe and Canada
• Affected individuals housed primarily on pasture in fall months
• Classic rhabdomyolysis, potentially recumbency, weakness, and muscle fasciculation without a previous episode of strenuous exercise, as well as significantly elevated muscle enzymes.
• Cardiomyopathy may accompany systemic myositis; examine all organ systems.
• Horses have a lipid storage myopathy involving oxidative postural and respiratory muscles, possibly cardiac muscle, deficiency in skeletal muscle multiple acyl-CoA dehydrogenases (MAD).
• Results from ingestion of hypoglycin A within seeds, specifically from box elder (Acer negundo) tree seeds.
• Clinical signs: cool, wet weather in the fall, poor forage availability, limited concentrate feeding, and box elder trees on pastures.
• Affected pasture has recent climactic alteration and a single or multiple animals on a single property can be affected.
• Can progress rapidly to recumbency causing a rapid death.

Nutritional Muscular Dystrophy (White Muscle Disease)

• Noninflammatory degenerative disease of skeletal and cardiac muscles caused by selenium and/or vitamin E deficiency.
• Clinical course may be acute/fulminant or subacute/insidious in onset.
• With acute disease, death may be immediate (arrhythmias) or within hours (exhaustion/circulatory collapse).
• The subacute form has profound muscular weakness, usually from birth to 60 days.
• Clinical signs: Stiff gait and tense, painful muscles with intermittent episodes of muscular weakness/fasciculations.
• Lingual and pharyngeal muscle involvement is common, resulting in dysphagia/poor suckle reflex.
• Dysphagic foals may have oral/nasal regurgitation of milk, and aspiration pneumonia/starvation can result.
• Myocardial involvement indicates poor prognosis for survival.
• Clinicopathologic findings:
  • CK ranges are 1,500-50,000, and AST ranges 1,500-4,000.
  • Myoglobinuria may be observed.
  • Hyponatremia, hypochloremia, and hyperkalemia.
• Pathophysiology:
  • Selenium is a vital component of glutathione peroxidase, protecting cell membranes from lipid peroxidation; glutathione peroxidase and vitamin E work synergistically.

Diagnosis and Treatment of White Muscle Disease

• Diagnosis:
  • Whole blood selenium concentration
  • Glutathione peroxidase activity
  • Biopsy - obtained from the gluteal, semimembranosus, semitendinosus and lingual muscles, looking for hyaline degeneration, fragmentation, lysis, and mineralization.
• Treatment
  • Administer E-Se® to clinically affected foals at 1 ml/100 lb deep IM.
  • Intravenous use can cause fatal anaphylactoid reactions.
  • Provide supportive care for electrolyte disturbances, acid-base abnormalities, and dehydration, and place a feeding tube in dysphagic, anorectic, or recumbent foals to prevent aspiration pneumonia.
  • Broad-spectrum antibiotics for aspiration pneumonia/immunosuppression.
  • Nonsteroidal anti-inflammatories for muscle pain, inflammation, and swelling.
  • Restrict exercise to prevent further muscle damage.
  • Foals responsive to treatment should regain control within two to six days.
• Prevention: Supplement selenium in deficient areas including the Pacific Northwest, Great Lakes region, New England states, and southeastern Atlantic coast.
  • Supplement mares at 0.10-0.20 ppm.
  • Can administer parenteral vitamin E and selenium to mare 3 weeks to 3 months before.
  • Administer vitamin E and selenium at a dose of 1 ml/100 lbs prophylactically to foals.
  • Well-balanced ration of properly cured and stored hay or green forage and a concentrate is the best source of vitamin E; dietary supplementation is more effective than intramuscular injection.

Glycogen Branching Enzyme Deficiency

• Glycogen branching enzyme (GBE) activity is absent in cardiac, skeletal muscle, liver, and peripheral WBC. Glycogen consists of straight chain α 1,4 linkages every 7-9 glucose molecules.
• Glycogen synthase synthesizes the straight-chain linkages and GBE catalyzes the branch points.
• Mutation of the GBE gene results in a failure to produce this enzyme.
• GBE deficiency in humans is one of the rarest glycogenoses
• Genetic testing for GBE available through University of California genetics testing laboratory.
• Clinical Signs:
  • Stillborn, weak, or failure to stand
  • Flexural limb deformities
  • Seizures
  • Respiratory failure is rapidly corrected with ventilation
  • Cardiac failure may occur
  • Sudden death may occur with exercise (fatal arrhythmia)
• Clinical Pathology:
  • Leukopenia may develop
  • Muscle enzymes may elevate
  • GGT activity may elevate
  • Intermittent, sever hypoglycemia may occur
• Pathology:
  • There are no consistent gross post-mortem changes
  • PAS stain identifies globular or crystalline intracellular inclusions
  • Inadequate polysaccharide accumulation may interfere with diagnosis in very young foals
  • Molecular techniques are necessary for diagnosis
• GBE activity in dams is around 50%