Glycogen storage disease type 6, also called Hers disease, is a condition where glycogenolysis fails to take place due to a deficiency in the enzyme glycogen phosphorylase. Hepatomegaly and hypoglycemia are common consequences, although the symptoms are usually mild.
Glycogen storage disease type 6 (GSD VI) is an inborn error of metabolism that has an autosomal recessive inheritance pattern. Glycogen phosphorylase, liver form (PYGL) gene mutation has been shown to be responsible for the ailment. GSD VI occurs when there is reduced activity of glycogen phosphorylase in the liver, consequently resulting in a buildup of glycogen in addition to systemic hypoglycemia  . GSD VI usually presents in early childhood, as a mild and self-limiting disorder that completely regresses by adolescence or early adulthood. The exception is women who are pregnant, and women who consume alcohol, as they may experience hypoglycemia as adults. Certain mutations allow for enough residual activity of the enzyme that patients experience even milder symptoms and show no biochemical signs .
Typical clinical features include hepatomegaly, impaired growth, hypoglycemia after periods of fasting, and possibly distension of the abdomen. Moreover, ketosis may occur as a result of longer fasts (notably overnight) or vigorous physical activity . Some cases experience no hypoglycemia at all. Elevations in liver enzymes, plasma cholesterol, and triglycerides are a documented finding, although the derangements are not pronounced.
In less common forms of the disease, symptoms (particularly hypoglycemia and hepatomegaly) may be severe and patients may even develop metabolic (lactic) acidosis after meals .
If left untreated, affected individuals may have delayed attainment of motor developmental milestones, and skeletal abnormalities exemplified by continued growth retardation and osteoporosis. The former may also encounter easy fatigability during physical exertion, in addition to decreased muscle tone . Conversely, the disease is not usually associated with intellectual disability. It is further postulated in the literature, that sufferers of GSD VI are at a higher risk of developing hepatic adenoma and liver cirrhosis  . Cardiomyopathy is a rare complication of liver pathology in GSD VI .
Clinical diagnosis of glycogen storage disease type 6 entails the detection of the recognizable features of the medical entity, such as hepatomegaly, delayed growth, hypoglycemia with or without a ketotic state, after fasting or illness.
Biochemical testing confirms the diagnosis. The following laboratory investigations are performed: