M-M14

Signalment (JPC #1535964): Five-year-old quarter horse mare

HISTORY: This mare showed signs of "tying up" after being exercised. She became recumbent and began to pass coffee-colored urine. The mare was euthanatized 8 days after onset of signs. Clinical pathological findings include: BUN 25 (11-27); AST 1250 (160-412); ALT 1750 (no reference interval given); LDH 7200 (112-456).

HISTOPATHOLOGIC DESCRIPTION: Skeletal muscle: Multifocally and randomly, 75% of myocytes are characterized by one of the following: markedly swollen with pale, vacuolated sarcoplasm (degeneration); hypereosinophilic and hyalinized with fragmented myofibrils, disruption of the endomysium, loss of cross striations, occasional contraction bands, and nuclear pyknosis or karyolysis (necrosis); or lost with replacement by karyorrhectic debris, moderate numbers of macrophages, and few fibroblasts. Multifocally, few myocytes have nuclear internalization and increased sarcoplasmic basophilia with rowing of nuclei that are large with prominent nucleoli (regeneration), and there is moderate proliferation of satellite cells. Fibrin, edema, moderate numbers of histiocytes, lymphocytes, and plasma cells multifocally infiltrate and expand the perimysium and endomysium.

MORPHOLOGIC DIAGNOSIS: Skeletal muscle: Degeneration and necrosis, monophasic, multifocal, random, moderate, with rare myofiber regeneration, and mild histiocytic and lymphoplasmacytic myositis, quarter horse, equine.

CONDITION: Equine exertional rhabdomyolysis

SYNONYMS: Exertional rhabdomyolysis syndrome (ERS), recurrent exertional rhabdomyolysis, exertional myopathy, ‘tying-up’, ‘Monday-morning disease’, ‘set fast’, azoturia, black water, paralytic myoglobinuria

GENERAL DISCUSSION:

"Rhabdomyolysis" means necrosis (lysis) of striated muscle of any cause; exertional myopathy or exertional rhabdomyolysis occurs when myofiber damage is due to exercise stress
Equine exertional rhabdomyolysis is a sporadic disease of many horse breeds, found worldwide, and characterized by muscle injury severe enough to result in myoglobinuria (with resultant renal tubular injury [U-M20]), profound weakness, and recumbency
Many etiologies and predisposing factors have been suggested
1. Affected horses typically have an underlying metabolic myopathy, most commonly equine polysaccharide storage myopathy (M-M20)
2. Strong evidence for an autosomal dominant inheritance for predisposition to exertional rhabdomyolysis in horses with polysaccharide storage myopathy and Thoroughbreds with recurrent exertional rhabdomyolysis
3. Most horses with recurrent exertional rhabdomyolysis respond to a dietary change to high fat and fiber, low starch and sugar
4. Abnormal skeletal muscle calcium homeostasis has been identified in some thoroughbreds
5. High grain feeding and lack of regular exercise have been recognized to be factors leading to exercise-induced muscle injury
6. Vitamin E and selenium deficiency can exacerbate signs
7. Electrolyte imbalance (hypokalemia) is a possible cause
8. Previously suggested etiologies no longer implicated: Excessive muscle lactic acidosis, equine hypothyroidism, vitamin E/selenium deficiency
Affects type 2 (fast twitch, oxidative, glycolytic) muscle fibers [type 2B (2X) glycolytic and 2A oxidative-glycolytic fibers]
1. Note: Nutritional myopathies involve type 1 oxidative fibers
Subclinical exertional myopathy apparently common in horses and sporadic cases may represent recurrent disease

PATHOGENESIS:

Muscle:
1. Inciting cause -> loss of integrity of the myocyte cell membrane (type 2, fast twitch, oxidative) > Myocyte dysfunction and death -> Hypercontraction and coagulative necrosis -> Release of free radicals (local oxidative injury), myoglobin, and enzymes into surrounding tissues and bloodstream
  - Oxygen-derived free radical compounds damage tissues and lead to vascular compromise -> Swelling within muscle sheath -> Ischemia (compartment syndrome), which perpetuates muscle necrosis
2. Muscle necrosis is not painful; pain associated with this condition suspected due to oxidative injury leading to muscle swelling and increased pressure
Kidney (U-M20):
1. Myoglobinemia -> Freely filtered by glomerulus -> Myoglobinuria -> Myoglobin-induced acute tubular injury (myoglobinuric nephropathy) -> Renal failure in severe cases
2. Myoglobinemia causes acute tubular necrosis via two pathways:
  - Myoglobin heme group > lipid peroxidation > release iron > epithelial damage
  - Renal ischemia > secondary to shock > chemical mediators of inflammation
3. Tubular damage leads to effects throughout the rest of the nephron
  - Tubular damage -> Vasoconstriction of preglomerular arterioles (via renin-angiotensin system and other mediators (adenosine, thromboxane, endothelin) -> Decreased GFR
  - Tubular blockage (casts) -> Backpressure -> Oliguria/anuria -> Decreased GFR
  - Tubular leakage -> Interstitial edema -> Increased interstitial pressure -> Further tubular collapse and damage

TYPICAL CLINICAL FINDINGS:

Often athletic, heavily muscled, and young (2-5 years old) are affected; females appear to be predisposed
Hindlimb weakness and/or sudden onset of muscle (gluteal, femoral, lumbar muscles) pain with swollen, rigid muscles
Recurrent episodes may manifest with stiff gait during or immediately after exercise, weakness, lameness, pain, and reluctance to move

Signs manifest particularly when worked after a day of rest and full grain ration
Muscle atrophy may occur in some cases
- Clinicopathological findings:
Serum chemistry:
- Increased creatinine and potassium
- Moderate to extreme elevations in:
  - Creatine kinase (CK, muscle specific)
  - Aspartate aminotransferase (AST, not muscle specific)
  - Alanine aminotransferase (ALT, muscle specific in large animals)
  - Lactate dehydrogenase (LDH, not muscle specific)
  - Phosphate, due to shift of PO4 from intracellular fluid to extracellular fluid
- Decreased serum calcium concentration
  - Cause is multifactorial (decreased intake, increased renal loss, increased movement of Ca2+ into damaged cells and profuse sweating)
- +/- increased anion gap (likely due to increased lactate and PO4)
Urinalysis:
- Myoglobinuria: Red-brown urine; does not clear on centrifugation; absence of erythrocytes in sediment; retention of color following addition of saturated ammonium sulfate to the urine; clear normal-colored plasma; no anemia

TYPICAL GROSS FINDINGS:

Skeletal muscle: Moist, swollen, dark, with dry streaks of pallor
1. Similar findings may also be present in the heart/myocardium
Kidney: Swollen with dark brown medullary streaks

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Muscle:
1. Localized or widespread muscle fiber degeneration and necrosis (acute) followed by macrophage infiltration, regeneration, and fibrosis (chronic)
2. Damaged fiber segments generally undergo hypercontraction (with contraction bands) followed by coagulative necrosis
3. Mineralization is usually NOT present
4. Lesions may be monophasic or polyphasic
Kidney: Swollen proximal convoluted tubular epithelial cells; protein casts with orange pigmented (myoglobin) granules

ULTRASTRUCTURAL FINDINGS:

Broadening of the subsarcolemmic space and intermyofibrillar spaces (degeneration) with edema; myofibrillar waving
Dilated sarcoplasmic reticulum (degeneration)
Disassociation and disruption of myofilaments (myofibrillolysis)
Disorganization and streaming of Z bands
Small, disintegrated mitochondria; large electron-dense deposits within mitochondria

ADDITIONAL DIAGNOSTICS:

Periodic acid Schiff (PAS) with diastase may demonstrate abnormal glycogen or polysaccharide accumulation – see equine polysaccharide storage myopathy (M-M20)

DIFFERENTIAL DIAGNOSIS:

Polysaccharide storage myopathy (PSSM; M-M20): Myopathy of quarter horse and draft-related breeds; abnormal accumulation of intracytoplasmic PAS-positive, amylase-resistant material within type 2A oxidative-glycolytic and type 2B glycolytic fibers; there are two forms of PSSM:
1. Glycogen synthase 1 gene (GYS1) positive-amylase resistant type
2. GYS1 negative animals-amylase sensitive; accumulation of glycogen, glucose-6-phosphate and polysaccharide inclusions; autosomal recessive inheritance suggested
Malignant edema (gas gangrene): Ruminants, horses, and swine very susceptible; rarely carnivores; etiologies include Clostridium septicum, Clostridium perfringens, Clostridium novyi, Clostridium sordelli and Clostridium chauvoei (M-B01); cellulitis > serohemorrhagic myositis, large gaseous and/or edematous areas adjacent to wounds
Hyperkalemic periodic paralysis (HYPP): Myotonic disorder of quarter horse; often well-muscled; laryngeal muscle dysfunction > laryngospasms; no gross findings or histopathological lesions
Ischemic myopathy: Degenerative myopathy secondary to muscle hypoxia after prolonged anesthesia; from systemic hypotension, improper padding, prolonged recumbency; degenerative and/or regenerative changes within affected muscles
Nutritional myopathy (M-M11): Foals; associated with vitamin E or selenium deficiencies; affects temporal and masseter muscles; bilaterally symmetric atrophy; pale muscles with yellow to white streaks; myocardial lesions with nutritional myopathy, distinguishes from exertional or ischemic myopathy
Ionophore toxicity (monensin) (M-T01): Cardiac, skeletal muscle necrosis; pale streaks
Plant toxicity (Cassia occidentalis, coffee weed, box elder weed) (M-T02): Pale areas within the skeletal, cardiac muscle; multifocal areas of necrosis with zero to minimal mineralization
Protozoal myopathy (Sarcocystis sp.) (M-P04): Common incidental finding in skeletal muscle, cardiac myofibers; lacks inflammatory response, unless cysts rupture
Malignant hyperthermia (pigs, dogs, humans, horses):
1. Genetic defect of the calcium-release channel (ryanodine receptor – ryr1) of skeletal muscle sarcoplasmic reticulum
2. Autosomal dominant condition EXCEPT in pig (autosomal recessive)
3. Halothane anesthesia may precipitate condition
4. In pigs, also termed porcine stress syndrome or hertztod; affects lean, heavily muscled pig breeds
Atypical myopathy (AM): An acute seasonal rhabdomyolysis seen primarily in equids (also reported in 2 Bactrian camels (Camelus bactrianus)) due to ingestion of material from sycamore maples
1. Hypoglycin A (HGA) and methylenecyclopropyl-glycine (MCPG) (toxic metabolites) inhibit acyl-CoA dehydrogenases, leading to hyaline degeneration of type 1 muscle fibers (intercostal muscles, neck, tongue, masticatory) (Hirz, J Vet Diagn Invest. 2021)

Differentials for pigmenturia

Myoglobinuria: Red-brown urine; does not clear on centrifugation; absence of erythrocytes in sediment; retention of color following addition of saturated ammonium sulfate to the urine; clear normal-colored plasma; no anemia
Hematuria: Red, cloudy urine; clears on centrifugation; erythrocytes in sediment; possibly anemia
Hemoglobinuria: Red-brown urine; does not clear on centrifugation; absence of erythrocytes in sediment; color clears following addition of saturated ammonium sulfate to the urine; reddish discoloration of plasma; evidence of anemia (hemoglobinemia)

COMPARATIVE PATHOLOGY:

Exertional rhabdomyolysis: Dogs, nonhuman primates, cattle
1. Canine exertional rhabdomyolysis: Racing greyhounds and racing sled dogs
  - Racing greyhounds: Proposed predisposing factors include an excitable nature, lack of physical fitness, hot and humid conditions, and overexertion of physically fit dogs; gross lesions typically absent; histopathologic findings include acute degenerative changes found in susceptible muscles (longissimus, quadriceps, and biceps femoris muscles)
  - Racing sled dogs: Sudden death may occur during racing; gross lesions not often seen
Capture myopathy (CM):
1. Massive muscle injury caused by overexertion (i.e. exertional rhabdomyolysis) associated with restraint and/or transportation, but can be seen with any prolonged exertion (including mating)
2. Reported in many species of mammal, avian, and reptiles, including wild and captive animals, domestic animals (transport myopathy in bovids), marine mammals, chelonids,
  - Common in macropods (e.g., kangaroos)
  - Avians: May be leading causes of death in ratites and other long-legged birds
  - Recent report in a captive bottlenose dolphin with multifocal myonecrosis, renal tubular injury, crystalline nephropathy (Nueangphuet, J Vet Diagn Invest. 2022)
3. Pathogenesis: Circulating catecholamines may play a role; dehydration and hyperthermia are predisposing factors
4. Clinical signs: May manifest days to weeks after event; dyspnea, weakness, muscle tremors or muscle rigidity, hyperthermia, collapse, and often death
5. Multifocal, monophasic skeletal muscle lesions; prominent muscle edema, indistinct pale streaks and hemorrhagic streaking of skeletal muscle
Deep pectoral myopathy (“Green muscle disease”)
1. In heavy breeds of chickens and turkeys, an exertional compartment syndrome of the supracoracoidius (deep pectoral) muscle characterized by ischemic necrosis
2. The deep pectoral muscles (which flaps the wings) are covered by a thin, tight fascia; mild inflammation/swelling -> compression -> occlusion of blood supply -> ischemia and pressure necrosis -> release of myoglobin (imparts a green color to the tissue)
3. Similar to “march gangrene” in humans

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