Gilbert Gottfried died after a long illness. What is myotonic dystrophy type II, the rare disease that reportedly caused his death?

Iconic comedian Gilbert Gottfried died on Tuesday after what his family said was "a long illness." His representative said Gottfried's death was caused by a heart rhythm issue called recurrent ventricular tachycardia, which resulted from myotonic dystrophy type II. What is this rare disease? 

There are two types of myotonic dystrophy, a disease that affects the muscles and other body systems, according to the National Institutes of Health (NIH). This disease is the most common form of muscular dystrophy that begins in adulthood.

Myotonic dystrophy type II, which Gottfried reportedly had, is inherited, NIH explains. It causes muscle weakness, pain and stiffness, and the symptoms usually develop during a person's 20s or 30s.