glycogen storage disease

Liver GSDs (Types I, III, IV, VI)

Primary symptom is fasting hypoglycemia and hepatomegaly (because the liver’s job is to maintain blood glucose).

Muscle GSDs (Types II, V, VII)

Primary symptoms are weakness, exercise intolerance, and cramping (because the muscle’s job is to use glycogen for its own energy).

The Classic Board Mnemonic (Types I, II, III, V)

Very Poor Carbohydrate Metabolism $\rightarrow$ Von Gierke, Pompe, Cori, McArdle.

Von Gierke (type I)

Deficient Enzyme: Glucose-6-phosphatase

Primary Organ: Liver, Kidney

Key Clinical Hallmark: Severe fasting hypoglycemia, massive hepatomegaly, lactic acidosis.

Pompe (type II)

Deficient Enzyme: Lysosomal acid alpha-1,4-glucosidase

Primary Organ: Heart, Muscle

Key Clinical Hallmark: Cardiomegaly, hypertrophic cardiomyopathy, "floppy baby."

Cori (type III)

Deficient Enzyme: Debranching enzyme (alpha-1,6-glucosidase)

Primary Organ: Liver, Muscle

Key Clinical Hallmark: Mild hypoglycemia, hepatomegaly, normal blood lactate.

Andersen (type IV)

Deficient Enzyme: Branching enzyme

Primary Organ: Liver, Spleen

Key Clinical Hallmark: Cirrhosis, portal hypertension, early childhood death.

McArdle (type V)

Deficient Enzyme: Muscle Glycogen Phosphorylase

Primary Organ: Skeletal Muscle

Key Clinical Hallmark: Exercise cramps, myoglobinuria (red urine), "Second Wind."

Hers (type VI)

Deficient Enzyme: Hepatic Glycogen Phosphorylase

Primary Organ: Liver

Key Clinical Hallmark: Mild fasting hypoglycemia, hepatomegaly (milder Von Gierke).

Type I: Von Gierke Disease (The Liver Trap)

Lacks Glucose-6-phosphatase. This is the final enzyme required for the liver to release free glucose into the blood from both glycogenolysis (glycogen breakdown) and gluconeogenesis (making new glucose).

• _{^{The Consequence: The liver is trapped with glucose-6-phosphate. It cannot release it, leading to profound fasting hypoglycemia. The trapped glucose-6-phosphate forces the liver to store massive amounts of glycogen, causing massive hepatomegaly and renomegaly.}}

Von Gierke Disease: The Board Traps (The 3 Hyper-s)

Hyperlacticacidemia (Lactic Acidosis)

Hyperuricemia (Gout)

Hyperlipidemia

Von Gierke Disease: treatment pearl

Frequent oral glucose or uncooked cornstarch (provides a slow, steady release of glucose). Avoid dietary fructose and galactose (they feed into the blocked pathway).

Type II: Pompe Disease (The Heart Trap)

The Defect: Lacks Lysosomal acid $\alpha$-1,4-glucosidase (also called acid maltase).

• _{^{The Consequence: This is the only GSD that is also classified as a Lysosomal Storage Disease. A small amount of glycogen is normally degraded in lysosomes. Without this enzyme, glycogen builds up and destroys lysosomal structure, particularly in muscle tissue.}}

• Clinical Presentation: Painful muscle cramps and weakness starting immediately upon exercising.

• Board Mnemonic: "Pompe trashes the Pump (the heart)."

Pompe Disease: Pharmacotherapy

Enzyme Replacement Therapy (ERT) is available: Alglucosidase alfa (Myozyme/Lumizyme) given via IV infusion.

Type V: McArdle Disease (The Muscle Trap)

• The Defect: Lacks Muscle Glycogen Phosphorylase (Myophosphorylase).

• _{^{The Consequence: Skeletal muscle cannot break down its own glycogen stores for energy during the early stages of vigorous exercise. (Note: Blood glucose levels remain perfectly normal because liver phosphorylase is intact).}}

• _{^{Clinical Presentation: Painful muscle cramps and weakness starting immediately upon exercising.}}

McArdle Disease: The Board Diagnostic Clues

Myoglobinuria

Flat Venous Lactate Curve

The "Second Wind" Phenomenon

Myoglobinuria

Muscle damage during exercise releases myoglobin, causing dark/red urine.

Flat Venous Lactate Curve

Normally, exercise causes blood lactate to rise. In McArdle's, because muscle glycogen isn't breaking down, glycolysis isn't happening in the muscle, so venous lactate does not rise during the ischemic exercise test.

The "Second Wind" Phenomenon

If the patient pushes through the initial painful cramps, the pain suddenly disappears after 10–15 minutes. Why? Vasodilation occurs, allowing circulating free fatty acids and hepatic blood glucose to finally reach and fuel the muscle.

Von Gierke (Type I) vs. Cori (Type III)

Both present with fasting hypoglycemia and enlarged livers.

• The Key Difference: Blood Lactate. Von Gierke has severe lactic acidosis. Cori disease has normal blood lactate levels (because gluconeogenesis is fully intact in Cori). Furthermore, Cori disease will show the accumulation of abnormal, short, highly branched glycogen molecules called "limit dextrins."

McArdle (Type V) vs. Pompe (Type II)

Both affect muscles.

• The Key Difference: Age and Organ. Pompe kills infants via heart failure. McArdle presents in adolescents/adults as exercise-induced cramps with no heart involvement.