Clinical characteristics: The clinical manifestations of glycogen storage disease type IV (GSD IV) discussed in this entry span a continuum of different subtypes with variable ages of onset, severity, and clinical features. Clinical findings vary extensively both within and between families.
The fatal perinatal neuromuscular subtype presents in utero with fetal akinesia deformation sequence, including decreased fetal movements, polyhydramnios, and fetal hydrops. Death usually occurs in the neonatal period.
The congenital neuromuscular subtype presents in the newborn period with profound hypotonia, respiratory distress, and dilated cardiomyopathy. Death usually occurs in early infancy.
Infants with the classic (progressive) hepatic subtype may appear normal at birth, but rapidly develop failure to thrive; hepatomegaly, liver dysfunction, and progressive liver cirrhosis; hypotonia; and cardiomyopathy. Without liver transplantation, death from liver failure usually occurs by age five years.
Children with the non-progressive hepatic subtype tend to present with hepatomegaly, liver dysfunction, myopathy, and hypotonia; however, they are likely to survive without progression of the liver disease and may not show cardiac, skeletal muscle, or neurologic involvement.
The childhood neuromuscular subtype is rare and the course is variable, ranging from onset in the second decade with a mild disease course to a more severe, progressive course resulting in death in the third decade.
Diagnosis/testing: The diagnosis is established in a proband by the demonstration of glycogen branching enzyme (GBE) deficiency in liver, muscle, or skin fibroblasts or the identification of biallelic pathogenic variants in GBE1 on molecular genetic testing.
Management: Treatment of manifestations: Management should involve a multidisciplinary team including specialists in hepatology, neurology, nutrition, medical or biochemical genetics, and child development. Liver transplantation is the only treatment option for individuals with the progressive hepatic subtype of GSD IV who develop liver failure; however, the risk for morbidity and mortality is high, in part because of the extrahepatic manifestations of GSD type IV, especially cardiomyopathy. Children with skeletal myopathy and/or hypotonia warrant developmental evaluation and physical therapy as needed. Those with cardiomyopathy warrant care by a cardiologist. Heart transplant may be an option in individuals with severe cardiac involvement.
Prevention of secondary complications: Prevent nutritional deficiencies (e.g., of fat-soluble vitamins) by ensuring adequate dietary intake; prevent perioperative bleeding by assessment of a coagulation profile and use of fresh frozen plasma as needed.
Surveillance: No clinical guidelines for surveillance are available. The following evaluations are suggested (with frequency varying according to disease severity): liver function tests including liver transaminases, albumin, and coagulation profile (PT and PTT); abdominal ultrasound examination; echocardiogram; neurologic assessment; nutritional assessment. If cardiomyopathy was not observed on baseline screening echocardiogram at the time of initial diagnosis, repeat echocardiograms every three months during infancy, every six months during early childhood, and annually thereafter.
Evaluation of relatives at risk: If the GBE1 pathogenic variants have been identified in an affected family member, test at-risk relatives to allow for early diagnosis and management of disease manifestations.
Genetic counseling: GSD IV is inherited in an autosomal recessive manner. Each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Although affected sibs are expected to manifest the same subtype of GSD IV, the age of onset and presentation may differ. Carrier testing for at-risk family members and prenatal testing for pregnancies at increased risk are possible based on molecular testing if the pathogenic variants in the family have been identified. If the pathogenic variants have not been identified, GBE testing on cultured amniocytes can be performed for prenatal diagnosis.
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