JPC SYSTEMIC PATHOLOGY

CARDIOVASCULAR SYSTEM

February 2025

C-M02 (NP)

 

Signalment (JPC #1806696): Age and gender not specified guinea pig

 

HISTORY: None

 

HISTOPATHOLOGIC DESCRIPTION: Heart: Within the subendothelial aspect of the left side of the interventricular septum, extending into and expanding the underlying superficial 1/3 of the myocardium, there is a focally extensive, well-circumscribed, unencapsulated, 4 x 2 mm, subendocardial nodule composed of myocytes with sarcoplasm markedly distended by a single, up to 120 µm diameter vacuole which contains a small amount of granular, lacy material (glycogen). The affected nuclei are often compressed and displaced to the periphery. Rarely, the affected myocyte nucleus is centrally located and surrounded by radiating finely fibrillar eosinophilic processes (spider cells). Within the nodule are few small, thin bands of normal appearing cardiac myocytes, surrounded and separated by the glycogen-distended cells. 

 

MORPHOLOGIC DIAGNOSIS: Heart, myofibers: Vacuolar change, glycogen-type, multifocal, moderate (rhabdomyomatosis), guinea pig, rodent

 

CONDITION: Rhabdomyomatosis

 

SYNONYM: Cardiac rhabdomyoma; cardiac glycogenosis

 

GENERAL DISCUSSION:

• Incidental finding in the heart
• May occur in any region of the heart; most common in the left ventricle
• Animals of all ages, as young as 3-weeks, suggesting congenital condition

 

PATHOGENESIS:

• Etiology and pathogenesis unknown
• Thought to be a hamartoma or malformation rather than a true neoplasm
• Congenital disorder of glycogen metabolism is proposed as a pathogenesis
• Other possible etiologies include myocardial degeneration and scurvy
• Ultrastructural and immunohistochemical studies in swine suggest it is a congenital dysplasia of cardiac muscle and/or Purkinje cells

 

TYPICAL CLINICAL FINDINGS:

• Usually asymptomatic
• Chylopericardium and right-sided congestive heart failure have been reported in an affected dog

 

TYPICAL GROSS FINDINGS:

• Irregular, pale, white, pink, or tan myocardial foci or streaks occurring in various regions of the heart including ventricles, atria, interventricular septa, and papillary muscles
• Size varies from barely visible to 3 cm in swine
• May be observed in any location in the heart, but most commonly located in the left ventricle 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Well-circumscribed, unencapsulated
• Spongy network of myofibers containing large, clear vacuoles and a small amount of fibrillar to granular, eosinophilic cytoplasm
• Occasional flattening and displacement of the myocyte nucleus to the periphery
• “Spider cells” - myocytes with centrally located nuclei and radiating fibrillar sarcoplasmic processes
• Mitotic figures are rare or absent

 

ADDITIONAL DIAGNOSTIC TESTS:

 

DIFFERENTIAL DIAGNOSIS:

• Mesenchymoma
• Rare benign cardiac neoplasm in guinea pigs, most often in the right atrium
• Composed of mesenchymal tissue (adipose, angiomatous, cartilaginous, osseous, hematopoietic, myxomatous tissue, and smooth muscle)

 

COMPARATIVE PATHOLOGY:

• Humans: Typically occur the subcutaneous tissue in the head and neck of young male children < 1 year old
• Genital rhabdomyoma is a rare tumor occurring in the vagina or vulva of young to middle-aged women
• Swine: Typically an incidental finding at slaughter; red wattle pigs predisposed
• Ultrastructural and immunohistochemical studies in swine suggest it is a congenital dysplasia of cardiac muscle and/or Purkinje cells
• Report in 4 Gottingen minipigs; left ventricular papillary muscle and left ventricular wall (Feller, Toxicol Pathol, 2023)
• Rare reports in cattle, sheep, and dogs

 

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents & Rabbits. 4th ed. Ames, IA: John Wiley & Sons, Inc; 2016:241-242.
2. Cooper BJ, Valentine BA. Tumors of muscle. In: Meuten DJ, ed. Tumors in Domestic Animals. 5th ed. Ames, IA: John Wiley & Sons, Inc; 2017:445. 
3. Feller LE, Sargeant A, Ehrhart EJ, Balmer B, Nelson K, Lamoureux J. Cardiac Rhabdomyoma in Four Göttingen Minipigs. Toxicol Pathol. 2023;51(1-2):61-66.
4. Gal A, Castillo-Alcala F. Cardiovascular System, Pericardial Cavity, and Lymphatic Vessels. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St Louis, MO: Elsevier Inc.; 2022:667. 
5. Mansfield CS, Callanan JJ, McAllister H. Intra-atrial rhabdomyoma causing chylopericardium and right-sided congestive heart failure in a dog. Vet Rec. 2000;147(10):246-247.
6. Robinson WF, Robinson NA. Cardiovascular system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. Philadelphia, PA: Elsevier Inc; 2016:52.

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