JPC SYSTEMIC PATHOLOGY

URINARY SYSTEM

December 2023

U-M20

 

Signalment (JPC # 2082446): Quarter horse

 

HISTORY: This 3-year-old quarter horse was recumbent on presentation to the veterinary hospital. The animal's condition continued to deteriorate and was euthanized.  

 

HISTOPATHOLOGIC DESCRIPTION: Skeletal muscle: Diffusely, myocytes exhibit one or more of the following changes: sarcoplasmic swelling, pallor, and vacuolation with loss of cross striations (degeneration), or hypereosinophilic sarcoplasm, fragmentation of myofibrils, and nuclear pyknosis or karyolysis (necrosis). Multifocally the perimysium and endomysium are expanded by abundant hemorrhage, fibrin, edema, and moderate numbers of degenerate neutrophils and fewer lymphocytes. Blood vessels are diffusely congested.

 

Kidney: Diffusely, renal tubular epithelium exhibit one or more of the following changes: hypereosinophilic cytoplasm with a shrunken pyknotic nucleus (necrosis) and multifocal sloughing of tubular epithelium with disruption of the tubular basement membrane (tubulorrhexis); pale, vacuolated, swollen epithelial cells (degeneration); epithelium with abundant basophilic cytoplasm, vesiculate nuclei, and rare mitotic figures (regeneration); and luminal distension and attenuation of epithelium (tubular ectasia). Ectatic tubule lumina often contain one of the following: orange-red, globular, hyaline material (myoglobin); eosinophilic, finely granular material (proteinaceous fluid); or abundant sloughed epithelial cells (cellular cast) and/or karyorrhectic debris (granular casts). Occasional tubules contain granular basophilic mineral. Multifocally, there are perivascular and peritubular interstitial lymphoplasmacytic aggregates admixed with fewer neutrophils and macrophages. Diffusely, there is mild interstitial edema and congestion. Multifocally there is hypertrophy of the parietal epithelium of Bowman’s capsule. 

 

MORPHOLOGIC DIAGNOSIS: 1. Skeletal muscle: Degeneration and necrosis, acute, diffuse, marked, with marked hemorrhage, Quarter horse, equine.

2. Kidney, tubules: Necrosis, degeneration, and regeneration, diffuse, moderate, with myoglobin, cellular and granular casts.

 

ETIOLOGIC DIAGNOSIS: Myoglobin-associated acute tubular injury  

 

CONDITION: Exertional rhabdomyolysis, azoturia, tying-up syndrome, equine paralytic myoglobinuria, and Monday morning disease  

 

GENERAL DISCUSSION:  

• Myoglobin is a nephrotoxic pigment, like hemoglobin, released from myocyte damage or necrosis
• Myoglobinuria secondary to extensive muscle necrosis occurs in the following conditions: Exertional rhabdomyolysis in horses (see M-M14), greyhounds, and wild or exotic animals (capture myopathy); Cassia spp. or Karwinskia spp. toxicity; or severe direct trauma to muscle (e.g. traffic accident)

• Equine recurrent rhabdomyolysis has recently been found to have an underlying myopathy, most often equine polysaccharide storage myopathy

• PBVD: Myoglobinuria can be cytotoxic, reduce renal blood flow, and increase tubular necrosis from renal ischemia
• The role of myoglobin and hemoglobin as primary nephrotoxins is unclear; some sources indicate these are NOT directly nephrotoxic; however, large concentrations in the glomerular filtrate can increase the amount of tubular necrosis; one source states that myoglobin is not nephrotoxic unless the urine is acidic, which results in myoglobin-associated oxidative injury
• Acute kidney injury associated with myoglobinuria is a serious complication of rhabdomyolysis

 

PATHOGENESIS:

• Passes freely through glomerular filtration barrier because of smaller size and lack of binding to carrier protein (as opposed to hemoglobin)
• Generally, can increase the severity of tubular necrosis caused by renal ischemia
• Poorly understood, contributing factors include: Increased hydroxyl free-radical formation concurrently with reduction of ferrous iron compounds and tubular obstruction by myoglobin casts
• Rhabdomyolysis (e.g. equine recurrent rhabdomyolysis [see M-M14], Cassia spp. or Karwinskia spp. toxicity, or severe direct trauma to muscle [e.g. traffic accident]) à release of myoglobin from myocytes à myoglobinemia à myoglobinuria à myoglobin-associated acute tubular injury (also known as myoglobinuric [pigmentary] nephrosis) due to ischemic injury +/- direct toxicosis of myoglobin à death due to acute renal failure (ARF) or problems associated with being down and attempting to rise (esp. horses)
• Renal Injury: The role of myoglobin (and hemoglobin) as a primary nephrotoxin is unclear, but renal failure is likely also impacted by other factors such as anemia/ischemia
• Acute renal failure (ARF) results in oliguria with potentially life-threatening hyperkalemia; pathogenesis of ARF due to ATI is controversial, and includes:

• Obstruction of tubular flow by cellular debris and casts as well as interstitial edema
• Preglomerular vasoconstriction (due to RAAS activation)
  • Fluid sequestration from injured muscle induces volume depletionà activation of sympathetic nervous system, antidiuretic hormone, and renin-angiotensin aldosterone systemà vasoconstrictionà ischemic injury
• Leakage of tubular fluid into the interstitium
• Impaired glomerular permeability 
• Vascular reactivity and/or 
• Vasoconstriction of afferent arterioles (tubuloglomerular feedback following renal tubular epithelial degeneration)

 

TYPICAL CLINICAL FINDINGS:

• Equine recurrent rhabdomyolysis: Variable, sudden onset weakness and/or pain of the hindlimbs, unable or reluctant to move (“tying up”), +/- sweating and generalized tremors, recumbency
• Myoglobinuria: Dark red-brown discoloration of the urine that does not clear with centrifugation (does clear for hematuria, does not clear for hemoglobinuria), positive reaction for urine occult blood (also positive hematuria and hemoglobinuria), and proteinuria
  • Myoglobin concentration must exceed 40mg/100mL
  • Will cause positive reaction for blood on urinalysis
• Muscle necrosis: Markedly increased aspartate aminotransferase (AST) and creatine kinase (CK), as well as myoglobinemia, hyperkalemia (release of potassium by damaged muscle), and hyperphosphatemia
  • Myoglobinemia results in normal, clear colored serum (hemoglobinemia due to intravascular hemolysis results in pink serum)
• Renal failure: Azotemia (prerenal, renal, and post-renal causes), low urine specific gravity, hyperphosphatemia, granular casts

 

TYPICAL GROSS FINDINGS:

• May be inapparent
• Kidney: Swollen, red-brown to blue-black cortex with brown-red streaks in the medulla  
• Skeletal muscle: Gluteal, femoral, and lumbar muscles are most severely affected; may be swollen and board-like (rigid), moist, and dark or have pale streaks or blotchy to linear hemorrhage if ischemic complications occur; gluteal muscle atrophy may be a feature of recovery in moderate to severe cases
• Myocardium is occasionally involved in rhabdomyolysis

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:              

• Kidney:   
  1. Tubular ischemic necrosis (tubulorrhexis, coagulative necrosis), degeneration, and regeneration; may have red granules in tubular epithelial cells
  2. Intratubular orange-red, refractile pigment casts (indistinguishable from hemoglobin casts)
  3. Interstitial edema 
• Muscle: Multifocal degeneration and necrosis of muscle fibers with little or no inflammation; mineralization is NOT usually seen with equine recurrent rhabdomyolysis (versus mineralization commonly seen with Vitamin E/Selenium deficiency); lesions are typically monophasic but may be polyphasic in horses with repeated bouts of necrosis or ongoing injury

• In horses with polysaccharide storage myopathy, abnormal inclusions of pale pink glycogen and blue-grey complex polysaccharide (in severe cases) may be seen in myofibers

 

ADDITIONAL DIAGNOSTIC TESTS:

• Urine ammonium sulfate (saturated): Unreliable test
• Urinalysis occult blood positive reaction (measure of peroxidase activity that occurs with erythrocytes, hemoglobin, and myoglobin)
• Immunohistochemical staining:
  1. Anti-myoglobin antibody
  2. HSP60- intracellular marker of acute tubular injury (AKI)
• PAS staining should be performed on skeletal muscle on all cases of exertional rhabdomyolysis or postanesthetic myopathy of horses for evidence of chronic myopathy and abnormal glycogen and complex polysaccharide storage
• Clin path:
  1. Urine sediment: Hemoglobin and myoglobin casts both orange to red-brown and cannot be differentiated microscopically; hemoglobin casts occur with intravascular hemolysis, whereas myoglobin casts occur with severe myolysis
  2. Plasma is clear with myoglobinuria due to rapid clearance, as opposed to pink to red plasma with hemoglobinuria

 

DIFFERENTIAL DIAGNOSIS:

• Dark brown urine:  
  • Myoglobinuria: Brownish urine; absence of erythrocytes in the sediment; clear normal-colored plasma; no anemia
  • Hemoglobinuria: Red to brown; absence of erythrocytes in sediment; reddish discoloration of plasma (hemoglobinemia); evidence of anemia
  • Hematuria: Red, cloudy urine; clear plasma, erythrocytes in the sediment; possibly anemia
• Hemoglobinuria due to severe intravascular hemolysis: 

• Horses: Red maple (Acer rubrum) toxicity
• Sheep: Chronic copper toxicity; “gun metal” (blue-black) kidney
• Cattle: Leptospirosis sp. or Babesia bovis
• Dogs: Autoimmune hemolytic anemia or babesiosis

• Muscle necrosis (skeletal and cardiac): Skeletal lesions are similar in all of these cases; clinical history and feed analysis are essential for diagnosis 

• Vitamin E/selenium deficiency (nutritional myopathy, white muscle disease): calcification is often a feature; affected muscles are pale and may be gritty; horses can have profound involvement of the temporal and masseter muscles 
• Ionophore toxicity (e.g. monensin)
• Toxic plant ingestion (Cassia sp.)
• Cantharidin toxicity (blister beetle)
• Endotoxic injury

 

COMPARATIVE PATHOLOGY:  

Rhabomyolysis:

• Capture myopathy (CM): Massive muscle injury caused by overexertion or associated with restraint and/or transportation; occurs in wild animals, marine mammals; resembles exertional rhabdomyolysis
  • Cetaceans have higher myoglobin content in skeletal muscle (Nueangphuet et. al, 2022, J Vet Diagn Invest)
• Malignant hyperthermia (pigs, dogs, humans, horses):  Genetic defect of the calcium-release channel (ryanodine receptor – ryr1) of skeletal muscle sarcoplasmic reticulum in lean, heavily muscled pig breeds; halothane anesthesia may precipitate condition; also termed porcine stress syndrome or hertztod; autosomal dominant condition EXCEPT in pig (autosomal recessive)
• Canine exertional rhabdomyolysis:  A syndrome affecting racing greyhounds and racing sled dogs of likely different etiologies due to the difference in type of exercise (sprint vs. endurance racing)
  • Racing greyhounds: Proposed predisposing factors include excitable nature, lack of physical fitness, hot and humid conditions, and overexertion of physically fit dogs; longissimus, quadriceps, and biceps femoris muscles most often affected; myoglobinuria can lead to death by renal failure; clinically and histologically similar to equine exertional rhabdomyolysis  
  • Sled dogs: Sudden death with massive rhabdomyolysis; there is often selective muscle involvement without gross lesions 

 

REFERENCES:  

1. Cooper BJ, Valentine BA. Muscle and tendon. In: Maxie MG, ed. Jubb, Kennedy and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:221-224.
2. Duncan M.  Perissodactyls. In: Terio KA, McAloose D, St. Leger J, ed. Pathology of Wildlife and Zoo Animals, Cambridge, MA Academic Press; 2018: 437-438.
3. Higgins D, Rose K, Spratt D. Monotremes and Marsupials.  In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals, Cambridge, MA Academic Press; 2018: 457, 460.
4. Howerth EW, Nemeth NM, Ryser-Degiorgis, MP. Cervidae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals, Cambridge, MA Academic Press; 2018: 151.
5. Nueangphuet P, Hamano T, Hirai T, Sakaguchi Y, Sonoda H, Otsuka M, Yamato O, Hobo S, Ikeda M, Yamaguchi R. Rhabdomyolysis, myoglobinuric nephrosis, and crystalline nephropathy in a captive bottlenose dolphin. J Vet Diagn Invest. 2022 Jul;34(4):668-673.
6. Stidworthy MF, Denk D. Sphenisciformes, Gaviiformes, Podicipediformes, Procellariiformes, and Pelecaniformes.  In: Terio KA, McAloose D, St. Leger J, ed. Pathology of Wildlife and Zoo Animals, Cambridge, MA Academic Press; 2018: 655.
7. Stockham SL, Scott MA. Erythrocytes. In: Fundamentals of Veterinary Clinical Pathology. 2nd ed. Hoboken, NJ: Wiley; 2013: 176.
8. Stockham SL, Scott MA. Urinary System. In: Fundamentals of Veterinary Clinical Pathology. 2nd ed. Hoboken, NJ: Wiley; 2013: 466.
9. Sula MM, Lane LV. The Urinary System. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:735, 746.
10. Valentine BA. Skeletal Muscle. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1017, 1019-1020, 1022.
11. Wamsley HL. Examination of the Urine Sediment. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:384.

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