An 11-year-old girl died in her sleep from a heart attack — the result of an undiagnosed and very rare heart condition.

On the morning of April 3, 2024, Matilda Pritchard’s parents found her unresponsive in her bed. Paramedics called to their home in the English town of Pudsey were unable to revive her.

Her father, Steve Pritchard, 43, told South West News Service, via The Daily Mail, that she had gone to sleep “normally” the night before, and was otherwise “healthy.”

But her parents later learned Matilda had an undiagnosed heart condition: arrhythmogenic right ventricular cardiomyopathy (ARVD), a rare disease in which fatty or fibrous tissue replaces damaged heart muscle, causing it to contract weakly and impacting the flow of blood.