Immuhistochemically-confirmed mitochondrial cardiomyopathy presenting as a conduction system hamartoma: A case report.

Conduction system hamartoma (CSH) is a rare cardiac lesion characterized by the abnormal proliferation of Purkinje-like myocytes. It predominantly affects female infants and is often associated with sudden cardiac death. Recent studies have linked mitochondrial dysfunction, particularly complex I deficiency, with CSH. We report an autopsy case of an eight-month-old female infant who died suddenly following mild gastrointestinal symptoms. A gross examination revealed mild cardiac hypertrophy without nodular lesions. Histological analysis identified multifocal aggregates of Purkinje-like cells with clear or foamy cytoplasm, some forming well-circumscribed nodules in the non-compacted myocardium. Immunohistochemistry demonstrated a marked reduction in complex I expression, supporting mitochondrial dysfunction. Although prominent trabeculations and deep recesses suggestive of left ventricular noncompaction were observed, they did not meet the strict diagnostic criteria. This case supports the potential role of mitochondrial complex I deficiency as a key pathogenic mechanism in CSH and highlights the significance of detailed histopathological and immunohistochemical analyses for an accurate diagnosis, especially in cases of sudden unexplained infant death.