Juvenile dilated cardiomyopathy (JDCM) occurs in Toy Manchester Terriers/English Toy Terriers. It results in sudden cardiac death at a young age, with dogs reported to have passed as early as 10 weeks of age and as late as 1 year of age. Affected dogs usually appear healthy with no signs of heart disease present before the sudden passing. The only external abnormality is that affected male puppies may have unilateral or bilateral cryptorchidism (undescended testicles on one or both sides). For a number of the puppies, their sudden passing occurred within a day of general anesthesia/surgery or exercise. Necropsy (post-mortem examination) may show mild enlargement and dilation of the heart. Microscopic changes include degeneration and scarring of heart tissue and varying degrees of inflammation. Death is presumed to be the result of a sudden fatal arrhythmia (abnormal heart rhythm).