Cardiomyopathy, also known as myocardial degeneration, was a disease that caused the deterioration of the function of the cardiac muscle for any reason. People with cardiomyopathy were often at risk for such symptoms as irregular heartbeat, and higher risk for cardiac arrest. It was a terminal condition for Humans during the 1990s.
Ralph Offenhouse, a Human rescued by the USS Enterprise-D in 2364, originally died of the condition, but was revived and cured of it by Doctor Beverly Crusher, after his body was recovered in a cryogenically frozen state. (TNG: "The Neutral Zone")
Following Geordi La Forge's kidnapping and subjection to torture by Dr. Tolian Soran in 2371, Dr. Crusher's initial review of his condition indicated that La Forge's heart had no permanent damage and was perfectly fine, but noted some arterial damage as well as myocardial degeneration. (Star Trek Generations)