A rare glycogen storage disease with fetal or neonatal onset of severe cardiomyopathy with non-lysosomal glycogen accumulation and fatal outcome in infancy. Patients present with massive cardiomegaly, severe cardiac and respiratory complications and failure to thrive. Non-specific facial dysmorphism, bilateral cataracts, macroglossia, hydrocephalus, enlarged kidneys and skeletal muscle involvement have been reported in some cases.

Hypertrophic cardiomyopathy-29 (CMH29) is characterized by recurrent syncope, dyspnea on exertion, and palpitations. The clinical phenotype is associated with a poor prognosis due to lethal arrhythmias and cardiac failure. Cardiac muscle biopsies show intermyofibrillar accumulation of glycogen and polyglucosan bodies within cardiomyocytes, and skeletal muscle accumulation of glycogen has also been observed (Hedberg-Oldfors et al., 2019). For a general phenotypic description and discussion of genetic heterogeneity of hypertrophic cardiomyopathy, see CMH1 (192600).