Hamartoma of mature cardiac myocytes

Hamartomas of mature cardiac myocytes (HMCM) are benign tumors arising from mature striated cardiac myocytes.

Terminology

Terms that are not recommended include ‘cardiac hamartoma’ or ‘hamartoma of adult cardiac myocytes’ ¹.

Epidemiology

A hamartoma of mature cardiac myocytes is a very rare cardiac tumor, which has been found in a wide age range but more commonly in younger patients. There is a clear male predilection ¹.

Diagnosis

The final diagnosis is based on histology.

Most tumors are incidentally detected on imaging studies or autopsy ¹.

Diagnostic criteria

An essential diagnostic criterion according to the WHO classification of bone tumors (2020 - blue book) is a cardiac mass formed by nodular collections of disorganized cardiac myocytes.

A further desirable criterion is the admixture of adipose tissue and nerves ¹.

Clinical presentation

Clinical signs and symptoms if present revolve around arrhythmia including palpitations, dizziness, syncope, dyspnea, chest pain and variable abnormalities on ECG ¹.

Complications

Hamartomas of mature cardiac myocytes might be complicated by the following ¹:

• ventricular tachycardia, ventricular fibrillation
• cardiac arrest
• sudden cardiac death

Pathology

Hamartomas of mature cardiac myocytes are benign cardiac tumors made up of disorganized differentiated mature cardiomyocytes ¹.

Etiology

The exact etiology of hamartomas of mature cardiac myocytes is unknown ¹.

Location

Hamartomas of mature cardiac myocytes are usually found within the walls of the ventricles and rarely in the atria. They can occur as multiple lesions ¹.

Macroscopic appearance

Macroscopically hamartomas of mature cardiac myocytes are poorly defined tumors that are closely interlinked to the adjacent myocardium with a pale white-tannish grey appearance and a firm consistency ^1,2.

Microscopic appearance

Histologically hamartomas of mature cardiac myocytes are characterized by the following features ^1,2:

• a disorganized pattern of enlarged or hypertrophied striated cardiac myocytes
• sarcoplasmic vacuolization
• bizarre nuclei
• myocyte disarray with herringbone, whorled or haphazard arrangement
• associated with interstitial fibrosis

Immunophenotype

Immunohistochemistry stains are usually positive for muscular markers such as desmin, troponin, muscle-specific actin and/or myosin ¹.

Radiographic features

Echocardiography

Echocardiography might show a homogeneous intramyocardial mass ³.

CT

Thoracic CT or cardiac CT might show an enhancing well-circumscribed mass ⁴.

MRI

On cardiac MRI the appearance has been described as well-delineated homogeneous and intramural ^3,4.

Signal characteristics

• T1: isointense to mildly hyperintense
• T2: mildly hypointense to hyperintense
• STIR: hyperintense
• T1C+ (Gd): early avid enhancement

Radiology report

The radiological report should include a description of the following:

• location and extent
• relation to other chambers and valves
• relation to adjacent structures
• associated myocardial fibrosis

Treatment and prognosis

Hamartomas of mature cardiac myocytes are benign but can cause arrhythmias and sudden cardiac death ¹. Management includes surgical excision in symptomatic tumors, which has been proven to be effective in treating arrhythmia and obstruction ¹.

History and etymology

Hamartomas of mature cardiac myocytes were first reported by a Japanese pathologist, Akira Tanimura, and his colleagues in 1988 ^2,6.

Differential diagnosis

Conditions or tumors that might mimic the appearance of hamartomas of mature cardiac myocytes include ¹:

• cardiac rhabdomyoma
• cardiac fibroma
• conduction system hamartoma
• mass-like hypertrophic cardiomyopathy (more diffuse myocardial involvement, less vascularized)