JPC SYSTEMIC PATHOLOGY

CARDIOVASCULAR SYSTEM

February 2025

C-M03 (NP)

 

Signalment (JPC #1713374): 9-week-old female Siamese cat

 

HISTORY: This cat presented with severe dyspnea.

 

HISTOPATHOLOGIC DESCRIPTION: Heart: Diffusely the endocardium is uniformly thickened up to 200 to 250 µm (20x normal) by fibroblasts, dense collagen, and elastic fibers (fibroelastosis). Superficially, fibers are thin, loosely and haphazardly arranged, and separated by increased clear space and scant myxomatous matrix. Fibers within the deeper layers are thicker, more tightly packed, and are parallel. Multifocally the fibroelastic connective tissue extends minimally into the underlying myocardium, rarely surrounding and isolating myofibers and Purkinje fibers.

 

MORPHOLOGIC DIAGNOSIS: Heart, endocardium: Fibroelastosis, diffuse, marked, Siamese, feline.

 

CONDITION: Endocardial fibroelastosis (EFE)

 

GENERAL DISCUSSION:

• Rare, congenital, inherited disease that primarily affects Burmese and Siamese cats leading to early onset of congestive heart failure (CHF) and death; rarely affects dogs; restrictive cardiomyopathy 
• Diffuse endocardial thickening by collagen and elastic fibers without other significant cardiac lesions

 

PATHOGENESIS:

• Unknown
• A developmental reaction characterized by smooth muscle hyperplasia and fibroplasia at the inner 4^th and 5^th layers of the endocardium
• In affected Burmese kittens: Lesions visible microscopically at 1 day of age and grossly by 20 days of age 
  • Initial localized endocardial lymphedema, which suggests obstruction of cardiac lymphatics, with subsequent subendocardial proliferation of fibroblasts; progresses to collagen and elastin deposition
• Degeneration of entrapped Purkinje fibers (left bundle branch) may lead to cardiac conduction abnormalities 

 

TYPICAL CLINICAL FINDINGS:

• Short disease course with dyspnea, cyanosis, pulmonary congestion, hydrothorax, hepatic congestion, ascites, tachycardia, heart murmurs, tachypnea, often sudden death
• Mildly affected animals may reach adulthood and reproduce

 

TYPICAL GROSS FINDINGS:

• Left atrium and ventricle (right heart is rarely involved):
  • Endocardium thickened, silver to white (“porcelain-like”), glistening due to the proliferation of fibroelastic tissue 
  • Dilation of LA and dilation and hypertrophy of LV without any associated cardiac malformation 
• Cardiomegaly, hydropericardium, hydrothorax with chronicity, which is secondary to congestive heart failure 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Endocardium intact and thickened by layers of collagen and elastic fibers
• Endocardial edema with dilated lymphatics; no fibrin or inflammation
• Superficial layers: Fibers are thin and haphazardly oriented
• Deeper layers: Fibers are thick, well organized, and parallel
• Purkinje fibers can become entrapped and undergo degeneration

 

ADDITIONAL DIAGNOSTIC TESTS: 

 

DIFFERENTIAL DIAGNOSIS:

• Secondary EFE (predominantly fibrosis): Prolonged dilatation of any heart chamber will result in diffuse endocardial thickening
  • Gross or histological evidence of a primary cause (congenital, infectious, or inflammatory)
  • Dilated cardiomyopathy of large breed dogs is most common
  • Endocardial fibrosis may be diffuse; or may be focal or multifocal in the atria adjacent to abnormal valves, and is secondary to turbulent blood flow (“jet lesions”)
• Post-infectious myocardial fibrosis
• Feline restrictive cardiomyopathy (left ventricular endocardial fibrosis): Feline cardiomyopathy with severe endomyocardial fibrosis; fibrosis (may resemble granulation tissue) would extend into myocardium, in contrast to EFE; often associated with heart failure and mural thrombi may be present as well as inflammation in the myocardium

 

COMPARATIVE PATHOLOGY:

• EFE has been reported in dogs, horses, cattle, sheep, pigs, and chickens, but primary EFE has only been confirmed in domestic cats and a few dogs; other reports are likely secondary EFE
• Humans: Fibroelastic thickening usually affecting the left ventricular endocardium occurs in patients less than 2 years of age
  • Concomitant problems can occur: aortic valve obstruction or other congenital cardiac anomalies
  • May be the sequela of systemic viral infections (e.g., intrauterine exposure to mumps), gene mutations (e.g., tafazzin)    
• Rats with endocardial spindle cell proliferation:
  • Lesions arise in the endocardium and subendocardial tissue
  • Characterized as fibroproliferative, giving rise to terms including endocardial fibromatosis, fibroelastosis, endocardiosis, and endocardial fibromatous proliferation
  • Possible precursors to schwannomas

 

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Blackwell Publishing; 2016:156.
2. Gal A, Castillo-Alcala F. Cardiovascular System, Pericardial Cavity, and Lymphatic Vessels. Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:666, 673.
3. Robinson WF, Robinson NA. Cardiovascular system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier; 2016:22.
4. Mitchell RN, Andrew AJ. The Heart. In: Kumar V, Abbas AK, Fausto N, Aster J, eds. Robbins and Cotran Pathologic Basis of Disease. 10th ed. Philadelphia, PA: Elsevier; 2021:541, 547.

 

 

 

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