Glycogen storage disease cardiomyopathies

Article written and reviewed by Dr Luca Moretti

Glycogen storage diseases are inherited disorders of glycogen synthesis, structure, degradation or regulation. The term suggests uniform accumulation, but it groups together different processes: excess normal glycogen, poorly branched material called polyglucosan, impaired energy utilization, and even a shortage of available glycogen. Consequently, the heart may become hypertrophic, dilated, electrically unstable or apparently normal. A type-specific diagnosis is essential, because diet and treatment that are useful in one form may be ineffective or inappropriate in another.

Pompe disease is the paradigmatic lysosomal cardiac glycogen storage disease, whereas PRKAG2 syndrome combines cardiac accumulation, pre-excitation and conduction block through AMPK dysregulation. This monograph focuses mainly on the other glycogen disorders that may involve the heart, particularly GSD III due to AGL, GSD IV due to GBE1, and defects in GYS1, RBCK1, GYG1 and PGM1-CDG. The taxonomic boundary is less important than the biochemical pathway and the organ involved.

Normal glycogen, abnormal glycogen and energy deficiency

Normal glycogen is a branched molecule that makes glucose rapidly available. Enzymes involved in synthesis, branching, phosphorolysis and debranching work sequentially; a defect may lead to accumulation of an incompletely mobilizable molecule or reduce its availability. In the cardiomyocyte, the occupied volume disrupts myofibrils, whereas energy deficiency becomes apparent during stress. The biochemistry of the polymer determines solubility, toxicity and the organs affected.

Polyglucosan has long chains and few branches, precipitates and resists degradation. Variants in GBE1, GYG1 or RBCK1 can cause its accumulation in the liver, muscle, nerves and heart, with very different phenotypes. Simple PAS staining does not distinguish amount, compartment and structure; diastase digestion, microscopy and genetics complete the assessment. Specialized pathology prevents every carbohydrate-containing vacuole from being called Pompe disease.

GSD III due to AGL deficiency

GSD III, or Cori-Forbes disease, results from biallelic AGL variants and deficiency of the debranching enzyme. Type IIIa involves the liver, skeletal muscle and heart; IIIb is mainly hepatic. In childhood, hepatomegaly, ketotic hypoglycemia, hyperlipidemia and impaired growth predominate, whereas myopathy and cardiomyopathy may become more evident with age. The organ transition explains why an adult may have less hypoglycemia but more weakness and hypertrophy.

Hypertrophy in GSD IIIa is often concentric and may be marked, with or without obstruction; function remains preserved for a long time, but fibrosis, dysfunction and arrhythmias have been documented. Wall thickness does not correlate perfectly with symptoms or CK. Echocardiography, ECG, Holter monitoring and CMR track the trajectory together with liver and muscle involvement. IIIa heart disease should be sought even when childhood glycemic control has improved.

Nutrition prevents hypoglycemia and catabolism with meals, uncooked cornstarch and calibrated protein intake; some centers modulate carbohydrates and fats in patients with cardiomyopathy, but evidence comes mainly from small series. A diet that corrects one parameter may worsen another and requires monitoring of growth, lipids, ketones, kidney function and muscle. Nutritional therapy is metabolic pharmacology, not generic high-protein advice.

Diagnosis of GSD III combines debranching enzyme activity and biallelic AGL variants; genetics is now often primary, but the biochemical profile confirms the mechanism. Limit dextrin and glycogen can be measured in selected tissues, whereas liver biopsy is not required merely to establish the name. The IIIa-IIIb distinction requires attention to muscle involvement and variants because it determines cardiac and neuromuscular surveillance.

CK tends to increase with age in type IIIa, and weakness may become distal as well as proximal. Electromyography, strength, respiratory function and activities of daily living describe a domain that does not correlate perfectly with liver disease. Better control of hypoglycemia does not guarantee muscle protection. Neuromuscular progression should be measured alongside cardiac mass when the diet is changed.

Liver disease may progress to fibrosis, cirrhosis, adenomas and hepatocellular carcinoma even when transaminases improve. Ultrasound, elastography, alpha-fetoprotein and imaging are selected according to stage and guidelines. Congestion due to cardiomyopathy can confound stiffness and laboratory findings. Hepatocardiac assessment distinguishes metabolic, portal and congestive injury and informs possible transplantation.

A higher-protein regimen provides gluconeogenic substrates, whereas cornstarch maintains blood glucose; use of ketosis or a higher fat proportion in cardiomyopathy is promising but lacks large trials. Ketones, fatty acids, lipids and growth should be monitored. Nutritional personalization is based on the balance among liver, heart and muscle rather than testimonials of rapid response.

GSD IV and polyglucosan diseases

GSD IV results from deficiency of the branching enzyme GBE1 and comprises a continuum including perinatal neuromuscular forms, congenital dilated cardiomyopathy, progressive liver cirrhosis, nonprogressive hepatic forms and adult neurologic disease. Amylopectin-like material damages cells more than glycogen quantity alone. GBE1 heterogeneity prevents prediction of heart and liver involvement from the general diagnostic label.

Liver transplantation treats liver failure but does not correct the gene in the heart, muscle or nerves. Cardiomyopathy and neuromyopathy should be sought before listing, and extrahepatic surveillance continues after transplantation. A severely compromised heart may require combined strategies or separate assessment. Residual systemic disease is central to the transplant risk-benefit assessment.

Biallelic RBCK1 variants may cause myopathy with dilated cardiomyopathy and polyglucosan accumulation, sometimes together with immunodeficiency or autoinflammation. Defects in GYG1, glycogenin-1, cause polyglucosan myopathy and, in some cases, cardiac involvement. An immune-muscle combination or DCM with polyglucosan bodies points toward genes that a restricted HCM panel may not include.

In GSD IV, GBE activity can be measured in fibroblasts, muscle or liver, and genetics confirms biallelic variants. Histology shows PAS-positive, diastase-resistant inclusions and fibrosis, but quantity and distribution vary with phenotype and age. A negative biopsy in a less-involved tissue does not exclude every variant. Multimodal GBE1 confirmation is particularly important before irreversible liver transplantation.

Perinatal neuromuscular forms may present with reduced fetal movements, arthrogryposis, hydrops, hypotonia and respiratory failure; dilated cardiomyopathy contributes to the phenotype but is not universal. Adult forms may resemble adult polyglucosan body disease with motor neuron involvement and neurogenic bladder. The GBE1 continuum links obstetrics, hepatology, neurology and cardiology and makes a rigid age-only classification misleading.

Combined or sequential transplantation is exceptional and depends on which organ limits survival. Subclinical cardiomyopathy may worsen after liver transplantation because of natural progression, medications or stress; conversely, replacing the heart does not halt cirrhosis or neuropathy. The transplant strategy requires multisystem forecasting rather than considering only the severity of the organ that is most clinically apparent at that moment.

Rare defects with cardiac risk

Muscle phosphofructokinase deficiency due to PFKM causes exercise intolerance and hemolysis, whereas cardiomyopathy is rare; PGK1 or ENO3 defects have their own neuromuscular and hematologic combinations. The presence of a glycolytic defect does not justify inferring cardiac accumulation. The precise metabolic pathway determines whether the heart is affected by storage, hemolysis, energy deficiency or an independent cause.

Adult polyglucosan body disease due to GBE1 is predominantly neurologic, whereas cardiomyopathy is more typical at other extremes of the spectrum. Laminins, desmins and autophagy proteins can produce secondary PAS-positive aggregates without being primary glycogen storage diseases. The pathologic differential diagnosis should include myofibrillar myopathies and aggregate disorders before assigning the material to a glycogen defect.

PHKA2 deficiency is X-linked hepatic disease and PHKB deficiency may involve liver and muscle; severe heart disease remains exceptional. In a child with GSD IX and hypertrophy, blood pressure, anemia, coexisting defects and molecular reclassification should be investigated. Cautious causal attribution prevents a rare report from becoming a disproportionate surveillance program for every genotype.

Some defects of lysosomal degradation or autophagy contain glycogen but belong to separate categories: Danon disease due to LAMP2, vacuolar myopathy due to VMA21 and other lysosomal disorders. Deposit localization and manifestations define their biology. Cellular taxonomy is useful because enzyme therapy can reach a lysosome but does not automatically correct an autophagy protein or a cytosolic enzyme.

Synthesis defects and PGM1-CDG

Muscle glycogen synthase deficiency due to GYS1, designated muscle GSD 0, does not produce storage but an inability to store enough glycogen. Patients may have exercise intolerance, syncope, arrhythmias or cardiac arrest, sometimes without evident morphologic cardiomyopathy. This demonstrates that cardiac glycogenosis does not always mean thick walls and that an exertion-related electrical event may be a metabolic manifestation.

PGM1 deficiency alters glucose-1-phosphate/glucose-6-phosphate interconversion and glycosylation; for this reason, the former GSD XIV is now considered PGM1-CDG. Cleft palate or bifid uvula, liver disease, hypoglycemia, endocrinopathies and myopathy may accompany even severe cardiomyopathy. Oral galactose improves several biomarkers and manifestations in specialist protocols. Treatable PGM1 disease is one reason to include glycosylation in the cardiometabolic differential diagnosis.

GSD IX due to phosphorylase kinase deficiency is predominantly hepatic or muscular, and cardiac involvement is not a uniform feature, although specific genotypes and rare reports describe it. Similarly, GSD VI and I require systemic surveillance but should not be presented as common causes of cardiomyopathy. Proportionality of risk prevents exceptional events from being turned into rules for every patient.

Muscle glycogen storage diseases without typical cardiomyopathy

McArdle disease due to PYGM causes exercise intolerance, cramps, rhabdomyolysis and the second-wind phenomenon, but the heart uses other isoforms and primary structural cardiomyopathy is not typical. GSD VII due to PFKM is also mainly muscular and hemolytic. Chest pain or dyspnea in these patients deserves ordinary assessment without automatic attribution to glycogen. Tissue specificity protects against unsupported testing and diagnoses.

An elevated CK, conversely, does not distinguish the forms. Exercise, crises, medications and liver injury alter enzymes; AST and ALT may originate from muscle. Glucose, ketones, lactate, uric acid, lipids and liver profile are selected according to the suspected disorder. The biochemical pattern should be collected at baseline and during events when safe, because a crisis may reveal a defect that a short fast does not show.

Cardiometabolic diagnosis

Age and associated findings guide the pathway. Hypotonia, macroglossia and infantile hypertrophy suggest Pompe disease; hepatomegaly and ketotic hypoglycemia with hypertrophy point toward GSD III; cirrhosis and polyglucosan suggest GSD IV; pre-excitation, conduction block and dominant inheritance favor PRKAG2. The phenotypic matrix reduces the number of tests and makes a panel interpretable.

Echocardiography and CMR describe geometry, function, obstruction and fibrosis without identifying the enzyme by themselves. ECG and Holter monitoring look for pre-excitation, tachycardias and conduction abnormalities; exercise testing is adapted to the risk of rhabdomyolysis and metabolic crises. Enzyme assays and genetics confirm the forms for which they are available, whereas muscle, liver or heart biopsy is reserved for unresolved questions. The selected tissue should express the pathway and be processed to preserve glycogen and structures.

Genetic testing uses panels, exome or genome sequencing according to specificity, including copy-number and RNA analyses when needed. A single recessive variant does not confirm the diagnosis, and a VUS should not guide a risky diet. Segregation and enzyme activity can reclassify the result. Orthogonal diagnosis, obtained with two independent types of evidence, is particularly valuable in rare and treatable diseases.

Imaging and arrhythmias by mechanism

The echocardiographic pattern does not reliably distinguish glycogen-related hypertrophy from sarcomeric hypertrophy. Concentric distribution, papillary muscles, a small cavity and obstruction are shared clues; growth rate, liver findings and ECG increase specificity. CMR measures mass and fibrosis, but T1 and LGE do not have a common signature across all glycogen storage diseases. Contextual morphology guides which enzyme to test without replacing the enzyme test.

Pre-excitation is central in PRKAG2 and Danon disease and has been described in Pompe disease, but it does not characterize GSD III or IV. A short PR interval may result from accelerated conduction and a wide QRS from accumulation or block; electrophysiologic study clarifies the pathway. Circuit interpretation prevents every abnormality from being equated with a classic accessory pathway or uniform risk.

Fibrosis, dysfunction and ventricular arrhythmias require the same clinical assessment used for other cardiomyopathies, but sarcomeric risk calculators are not automatically applicable. An ICD treats tachyarrhythmias, not hypoglycemic crises, low output or conduction block unless it provides appropriate pacing. Electrical protection is integrated with metabolic prevention and prognosis of the other organs.

Metabolic crises and procedures

The emergency plan specifies the maximum duration of fasting, tolerated drinks or polymers, access to intravenous glucose and tests to be collected. A normal capillary glucose level does not exclude ketosis and catabolism; conversely, excessive carbohydrate therapy can cause fluctuations or volume overload in a fragile heart. Individual prevention should state the exact diagnosis and center contact, not merely the words glycogen storage disease.

Surgery and endoscopy require planning that minimizes fasting and monitors glucose, lactate, ketones, electrolytes and volume according to the type. Cirrhotic liver disease, respiratory myopathy and cardiomyopathy modify medications and postoperative observation. Substrate continuity is coordinated by the anesthesiologist with the metabolic specialist and is not entrusted to a standard glucose solution alone.

During persistent vomiting, infection or heart failure, hospitalization may be necessary before severe hypoglycemia occurs. CK, urinalysis, renal function and ECG look for rhabdomyolysis and arrhythmias; the heart is reassessed if biomarkers or perfusion change. The systemic crisis is treated simultaneously with its precipitating cause because an antibiotic or antiemetic alone does not stop catabolism.

Growth, pregnancy and family

In children, the diet should support growth and development without excess calories, dyslipidemia or micronutrient deficiency. Schools and caregivers receive instructions on meals and crises, while ensuring that safety does not prevent participation. Puberty changes requirements and may make myopathy or cardiomyopathy evident. Metabolic growth is followed with height velocity, body composition and function, not weight alone.

During pregnancy, glucose consumption, nausea, blood volume and cardiac workload increase. Women with GSD III or other hepatic forms require a plan for fasting, delivery and the postpartum period; cardiomyopathy and liver disease determine the level of care required. Obstetric-metabolic management anticipates hypoglycemia and congestion and ensures glucose even during procedures without imposing excessive volumes.

Most of the forms discussed are recessive, but PRKAG2 is dominant and some phosphorylase kinase deficiencies are X-linked. Counseling therefore does not use a single recurrence risk. Targeted testing in siblings can identify a presymptomatic person, whereas partner testing and reproductive options depend on the variant. Pathway-specific genetics replaces the incorrect assumption that all glycogen storage diseases are inherited in the same way.

Treatment, crises and follow-up

Treatment depends on the type: enzyme therapy and immunomodulation in Pompe disease, nutrition and fasting prevention in GSD III, galactose in PGM1-CDG, and supportive treatment and selected transplantation in GSD IV. There is no single glycogen storage disease diet. During fever, vomiting or surgery, an emergency plan prevents hypoglycemia and catabolism with specific substrates and monitoring. The etiologic prescription is more important than any universal supplement.

Heart failure, arrhythmias and obstruction are treated according to physiology, taking into account blood pressure, a small cavity, liver disease and metabolic risk. Medications, procedural fasting and anesthesia are planned with the metabolic center; cardiac transplantation takes into account progression in the liver and muscle. Perioperative safety comes from advance communication and continuous access to glucose and monitoring when appropriate for the specific defect.

Follow-up combines growth, nutrition, metabolic episodes, CK, liver assessment, strength, respiration, ECG, echocardiography and CMR. A familial variant enables cascade testing according to the mode of inheritance; most forms are recessive, whereas PRKAG2 is dominant. Prognosis ranges from cardiac regression with enzyme therapy to multiorgan progression and should be formulated by gene and stage. Coordinated surveillance transforms a heterogeneous category into specific clinical plans.

Follow-up endpoints should be reproducible: the same laboratory for CK and metabolites when possible, the same method for mass, and documentation of diet and episodes. Apparent improvement during growth may depend on how mass is indexed. Longitudinal quality makes it possible to attribute a change to therapy rather than to age, technique or different adherence before testing.

Apps and glucose sensors can describe fasting exposure and variability, but accuracy at low concentrations and interference require capillary or plasma confirmation. Continuous data are useful for adapting the diet, not for replacing clinical assessment. Metabolic technology becomes effective when the center interprets patterns rather than reacting to every individual alarm.

In cardiomyopathy with hypoglycemia, an arrhythmia may be precipitated by catecholamines, electrolytes or energy deficiency as well as scar. Simultaneous monitoring of rhythm and glucose during symptoms may clarify the relationship. Temporal correlation allows treatment of the metabolic precipitant without overlooking a substrate that requires a device or its own arrhythmia therapy.

Drug interpretation takes the liver, kidney and muscle into account. Statins may worsen symptoms in some patients with myopathy but are not automatically contraindicated when vascular risk is high; dose, CK and alternatives are discussed. Individual pharmacology avoids both indiscriminate withdrawal of useful therapies and attribution of worsening muscle symptoms solely to the glycogen storage disease.

Psychological support addresses nighttime feeding, fear of hypoglycemia, limitations and family uncertainty. Eating disorders may be concealed by a prescribed diet and should be recognized. Metabolic mental health affects adherence and quality of life and deserves different tools for children, adolescents and caregivers.

International registries are essential for defining natural history and response in ultrarare forms, but they require confirmed genotype and updated nomenclature. Grouping all defects under GSD dilutes signals. Harmonized phenotyping links material, compartment, organs and therapy and allows one family's experience to contribute to verifiable future decisions.

Final communication should name the defect and what it does not involve. Saying glycogen storage disease without specifying the type does not indicate the risk of hypoglycemia, cardiomyopathy or the required diet and may cause errors in an emergency. The operational diagnosis states the gene, variants, mode of inheritance, organs observed and crisis plan, transforming a biochemical category into usable clinical instructions.

Research on gene and RNA therapy faces the difficulty of reaching the liver, muscle and heart simultaneously and maintaining expression during growth. A hepatic endpoint does not guarantee myocardial benefit. Tissue distribution must therefore be demonstrated together with immunologic safety and durability before a preclinical result is interpreted as a systemic cure.

Daily activity is also an outcome: frequency of nighttime meals, school days missed, falls, ventilation and exercise tolerance describe the burden of disease. A person-centered assessment integrates these data with glucose and cardiac mass and prevents an improved laboratory result from masking worsening function.

The plan is updated after every new genetic classification because a more precise diagnosis may change diet and surveillance. Etiologic reassessment is particularly important in patients diagnosed decades ago only by biopsy or with terminology that is now obsolete.