Background: Hypertrophic cardiomyopathy in identical twins is rare. Cases of hypertrophic cardiomyopathy with homogenous and heterogeneous phenotypes have been described in the literature.

Case summary: We report a pair of monozygotic twins (Twin A and Twin B) with identical morphological expression of hypertrophic cardiomyopathy. On initial evaluation, both twins had resting left ventricular outflow tract obstruction, Grade II diastolic dysfunction, and New York Heart Association (NYHA) Class II symptoms, but they had a different clinical course afterward. Twin A progressed from NYHA Class II to Class III with a high left ventricular outflow tract pressure gradient that was unresponsive to medical treatment and required alcohol septal ablation. Twin B responded very well to medical treatment. Both patients had no risk factors for sudden cardiac death, and neither required an implantable cardioverter defibrillator.

Discussion: The morphology of hypertrophic cardiomyopathy has a strong genetic basis, but epigenetic factors may affect disease expression.