A new variant of glycogen storage disease. Type IXc.

Three siblings, a body and two girls, had clinical, laboratory, and morphologic findings that were suggestive of glycogen storage disease (GSD) type IXa. Patients of both sexes with phosphorylase kinase (PK) deficiency usually have an excessive glycogen content only in the liver and normal glycogen content and PK activity in muscle. The siblings in this study had an increased glycogen content in the liver but also in muscle and reduced PK activity in liver, muscle, erythrocytes, and leukocytes. This condition should be labeled as GSD type IXc.