When Hypertrophy Hides a Rarer Diagnosis: A Case of Danon Disease.

Danon disease (DD) is a rare X-linked dominant lysosomal storage disorder caused by mutations in the LAMP2 gene. It is typically characterized by a triad of intellectual disability, skeletal myopathy, and cardiomyopathy. Due to the X-linked inheritance pattern, males often present with more severe manifestations. We report an atypical presentation of DD in a young male with isolated cardiomyopathy, incidentally diagnosed during evaluation of an abnormal electrocardiogram showing a pseudo-left bundle branch block in the setting of a Wolff-Parkinson-White pattern. Further workup revealed heart failure with reduced ejection fraction on echocardiography, and cardiac MRI demonstrated late gadolinium enhancement of the left ventricular walls with sparing of the basal septum. Genetic testing confirmed the diagnosis of DD. The patient was managed with guideline-directed medical therapy and received an implantable cardioverter-defibrillator for primary prevention of arrhythmia. He ultimately underwent heart transplantation with a satisfactory clinical outcome. This case underscores the importance of maintaining a high index of suspicion and pursuing comprehensive cardiac and genetic evaluation in young patients with unexplained cardiomyopathy, as early diagnosis of underlying conditions like DD can enable timely and potentially life-saving interventions.