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A 69 year old woman was referred for cardiac assessment with dyspnoea (New York Heart Association functional class III). Her ECG showed sinus rhythm with right bundle branch block and T wave inversion in inferior leads II, III, and AVF (upper panels, A) An echocardiogram revealed presence of hypertrophic cardiomyopathy (HCM). Interventricular septum was severely thickened at 2.9 cm (lower panel, A). There was severe inducible left ventricular (LV) outflow tract obstruction (gradient > 70 mm Hg, post-intravenous dobutamine infusion). She underwent successful percutaneous ethanol septal ablation. Her monozygotic twin sister (sister B), confirmed by genetic examination, was asymptomatic. Her ECG was different to sister A. There was regression of the R waves in precordial leads and borderline non-specific ST segment changes (upper panels, B). Both sisters had normal blood pressure. An echocardiogram showed no signs of LV outflow tract obstruction and the interventricular septum was only moderately thickened at 1.5 cm (lower panel, B). LV diameters were also different in both sisters (3.8 cm and 4.5 cm, A and B, respectively). However, left atrial size was similar (4.1 cm for both). Both sisters had abnormal LV diastolic indices. Mitral A wave and pulmonary venous flow atrial reversal difference was negative (sister A −60 ms, sister B −35 ms). In both sisters early diastolic tissue Doppler echocardiographic indices were abnormal. Unfortunately, we have not been able to identify the mutation of sarcomeric protein.
These monozygotic twins showed a different phenotypic expression of HCM. Both twins lived far away from each other and had different habits (such as exercise, diet). Sister A had severe LV septal hypertrophy which required treatment while sister B was asymptomatic with only moderate LV hypertrophy. However, both twins had diastolic LV dysfunction. This unique observation on the phenotypic expression in monozygotic twins with HCM supports the proposal that environmental factors may have a significant impact on both the morphological expression and clinical presentation of HCM.