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Glycogen Storage Diseases for MRCP Part 1

TL;DR

Metab: Glycogen Storage Diseases (Von Gierke/Pompe) is a frequently tested metabolic topic in MRCP Part 1, particularly the differences in enzyme defects, organ involvement, laboratory abnormalities and inheritance patterns. Focus on recognising the characteristic clinical presentations, understanding the underlying biochemistry and avoiding common examination traps. Mastering the distinguishing features between Von Gierke disease and Pompe disease can secure several easy marks in the metabolic medicine section.


Why this topic matters

Metabolic disorders account for a relatively small proportion of the MRCP syllabus, but they frequently appear because they assess integrated knowledge of:

  • Biochemistry

  • Genetics

  • Hepatology

  • Cardiology

  • Neurology

  • Paediatrics

  • Clinical medicine

Rather than expecting detailed biochemical pathways, MRCP focuses on recognising patterns. If you know the hallmark features of the major glycogen storage diseases, these questions become highly scoring.

The two disorders tested most often are:

  1. Von Gierke disease (GSD Type I)

  2. Pompe disease (GSD Type II)

These represent two very different mechanisms of glycogen accumulation:

  • Von Gierke disease results from failure to release glucose from the liver.

  • Pompe disease results from impaired lysosomal glycogen degradation.

Understanding this distinction immediately explains most of the clinical differences between the two diseases.

The Five Most Tested Subtopics

1. Enzyme deficiency

This is one of the highest-yield areas for MRCP.

Disease

Enzyme Defect

Main Site

Consequence

Von Gierke (Type I)

Glucose-6-phosphatase deficiency

Liver and kidney

Inability to convert glucose-6-phosphate into free glucose

Pompe (Type II)

Acid α-glucosidase (acid maltase) deficiency

Lysosomes in all tissues

Progressive lysosomal glycogen accumulation

Exam tip

Do not confuse acid maltase with the enzymes responsible for cytoplasmic glycogen metabolism. Pompe disease is fundamentally a lysosomal storage disorder, making it unique among the classic glycogen storage diseases.

2. Organ involvement

Recognising the affected organs often allows the diagnosis before laboratory results are presented.

Von Gierke disease

Predominantly affects:

  • Liver

  • Kidneys

  • Intestinal mucosa

Clinical manifestations include:

  • Massive hepatomegaly

  • Renomegaly

  • Poor fasting tolerance

  • Growth failure

  • Protuberant abdomen

  • Hypoglycaemic episodes

The heart is usually not the major organ involved.

Pompe disease

The principal organs affected are:

  • Cardiac muscle

  • Skeletal muscle

  • Respiratory muscles

Patients commonly develop:

  • Progressive proximal muscle weakness

  • Hypotonia

  • Respiratory insufficiency

  • Cardiomegaly (especially in infantile disease)

  • Hypertrophic cardiomyopathy

The liver may contain glycogen but significant hypoglycaemia is typically absent.

3. Clinical presentation

Von Gierke disease

Infants usually present after fasting with:

  • Severe hypoglycaemia

  • Tremors

  • Irritability

  • Seizures

  • Hepatomegaly

  • Failure to thrive

Repeated episodes of fasting hypoglycaemia are highly characteristic.

Older patients may develop:

  • Hyperuricaemia

  • Gout

  • Renal impairment

  • Hepatic adenomas

  • Osteopenia

Pompe disease

The presentation depends on age.

Infantile Pompe disease

Typical findings include:

  • Marked hypotonia ("floppy infant")

  • Feeding difficulties

  • Macroglossia

  • Hypertrophic cardiomyopathy

  • Respiratory failure

  • Early death if untreated

Late-onset Pompe disease

Patients develop:

  • Slowly progressive proximal muscle weakness

  • Difficulty climbing stairs

  • Frequent falls

  • Restrictive lung disease

  • No significant cognitive impairment

Unlike Von Gierke disease, fasting hypoglycaemia is not a defining feature.

4. Laboratory findings

Laboratory interpretation is a favourite MRCP testing strategy.

Von Gierke disease

Expect to see:

  • ↓ Blood glucose

  • ↑ Lactate

  • ↑ Triglycerides

  • ↑ Uric acid

  • Metabolic acidosis

  • Elevated liver enzymes

These abnormalities reflect impaired glucose release and diversion of glucose-6-phosphate into alternative metabolic pathways.

Pompe disease

Routine blood tests may show:

  • Elevated creatine kinase (CK)

  • Elevated AST and ALT due to muscle involvement

  • Reduced acid α-glucosidase activity on enzyme testing

Blood glucose is generally normal.

A question describing severe cardiomyopathy with elevated CK but normal glucose should immediately suggest Pompe disease.

5. Genetics and inheritance

Both disorders are inherited in an autosomal recessive manner.

Important MRCP implications include:

  • Affected siblings may occur despite unaffected parents.

  • Each pregnancy carries a 25% recurrence risk.

  • Both males and females are equally affected.

  • Carrier parents are clinically normal.

When a stem mentions consanguinity together with infantile metabolic disease, always consider inherited enzyme deficiencies.


High-Yield Revision Points (Exam Essentials)

The following points represent the facts most frequently tested in MRCP Part 1. They are worth revising repeatedly because they commonly appear in single best answer (SBA) questions.

  1. Von Gierke disease (GSD Type I) is caused by glucose-6-phosphatase deficiency, preventing the final step of glycogenolysis and gluconeogenesis.

  2. Pompe disease (GSD Type II) is caused by acid α-glucosidase (acid maltase) deficiency, leading to glycogen accumulation within lysosomes.

  3. Severe fasting hypoglycaemia is a hallmark of Von Gierke disease but is not a feature of Pompe disease.

  4. Cardiomyopathy is the classic feature of infantile Pompe disease, whereas the heart is usually spared in Von Gierke disease.

  5. Hyperlactataemia, hyperuricaemia and hypertriglyceridaemia strongly suggest Von Gierke disease.

  6. Progressive proximal muscle weakness and respiratory muscle involvement are characteristic of late-onset Pompe disease.

  7. Both disorders are inherited in an autosomal recessive pattern, making family history and consanguinity important clues.

  8. Enzyme replacement therapy (ERT) is available for Pompe disease and has significantly improved outcomes, especially when started early.

  9. Long-term complications of Von Gierke disease include hepatic adenomas, chronic kidney disease, osteoporosis and gout.

  10. In examination questions, always identify which organ system is predominantly affected—hepatic disease points towards Von Gierke, while cardiac and skeletal muscle disease strongly favours Pompe.


Comparing Von Gierke and Pompe Disease

Feature

Von Gierke Disease (Type I)

Pompe Disease (Type II)

Defective enzyme

Glucose-6-phosphatase

Acid α-glucosidase (acid maltase)

Site of glycogen accumulation

Liver and kidneys

Lysosomes throughout the body

Primary organs affected

Liver, kidneys

Cardiac muscle, skeletal muscle, respiratory muscles

Fasting hypoglycaemia

Yes

No

Hepatomegaly

Marked

Mild or absent

Cardiomyopathy

Rare

Common, especially infantile form

Muscle weakness

Mild

Prominent

Lactate

Increased

Usually normal

Uric acid

Increased

Normal

Triglycerides

Increased

Usually normal

CK

Usually normal

Elevated

Main treatment

Frequent feeds, uncooked cornstarch, metabolic control

Enzyme replacement therapy and supportive care

MRCP Tip: If the stem describes hypoglycaemia plus hepatomegaly, think Von Gierke disease. If it describes hypertrophic cardiomyopathy with hypotonia or progressive limb-girdle weakness, think Pompe disease.


Practical Example (Mini MRCP-Style Case)

Question

A 5-month-old infant is brought to the emergency department because of poor feeding, increasing breathlessness and marked hypotonia. Examination reveals macroglossia, cardiomegaly and signs of hypertrophic cardiomyopathy. Blood glucose is normal, but serum creatine kinase is elevated.

Which enzyme deficiency is the most likely cause?

A. Debranching enzymeB. Glucose-6-phosphataseC. Acid α-glucosidaseD. Muscle glycogen phosphorylaseE. Branching enzyme

Correct answer

C. Acid α-glucosidase

Explanation

The infant has the classic presentation of Pompe disease:

  • Infantile hypotonia

  • Hypertrophic cardiomyopathy

  • Macroglossia

  • Respiratory muscle involvement

  • Raised creatine kinase

  • Normal blood glucose

Acid α-glucosidase deficiency causes glycogen accumulation within lysosomes, particularly affecting cardiac and skeletal muscle. Early diagnosis is important because enzyme replacement therapy can significantly improve survival and quality of life.

Why the other options are incorrect

  • A. Debranching enzyme – Causes Glycogen Storage Disease Type III (Cori disease), which typically presents with hepatomegaly and milder hypoglycaemia.

  • B. Glucose-6-phosphatase – Causes Von Gierke disease, characterised by severe fasting hypoglycaemia, lactic acidosis and hyperuricaemia rather than cardiomyopathy.

  • D. Muscle glycogen phosphorylase – Causes McArdle disease (Type V), presenting with exercise intolerance, muscle cramps and myoglobinuria in adolescents or adults.

  • E. Branching enzyme – Causes Andersen disease (Type IV), associated with progressive liver cirrhosis rather than hypertrophic cardiomyopathy.


Practical Study Checklist

Before sitting MRCP Part 1, ensure you can confidently answer the following questions:

  • □ Can you identify the enzyme deficiency for Von Gierke and Pompe disease?

  • □ Can you explain why hypoglycaemia occurs in Von Gierke disease?

  • □ Can you recognise the infant with Pompe disease from a clinical vignette?

  • □ Can you distinguish hepatic glycogen storage disorders from lysosomal storage disorders?

  • □ Can you recall the characteristic laboratory abnormalities in Von Gierke disease?

  • □ Can you identify the major organs affected in each condition?

  • □ Can you remember which disease is associated with hypertrophic cardiomyopathy?

  • □ Can you recall which disease has an effective enzyme replacement therapy?

  • □ Can you differentiate Pompe disease from McArdle and Cori disease in SBA questions?

  • □ Can you identify the common long-term complications of Von Gierke disease?

Students who can answer each of these confidently are well prepared for the metabolic medicine questions encountered in MRCP Part 1.


Medical student studying glycogen storage diseases including Von Gierke and Pompe disease for MRCP Part 1 revision.

Common Pitfalls (5 Exam Traps)

  • Confusing Pompe disease with other glycogen storage diseases. Remember that Pompe is also a lysosomal storage disorder, unlike the other classic GSDs.

  • Assuming all glycogen storage diseases cause hypoglycaemia. Significant fasting hypoglycaemia is typical of Von Gierke disease but not Pompe disease.

  • Forgetting the importance of cardiomyopathy. Hypertrophic cardiomyopathy is one of the strongest diagnostic clues for infantile Pompe disease.

  • Ignoring biochemical clues. Hyperlactataemia, hyperuricaemia and hypertriglyceridaemia together strongly point towards Von Gierke disease.

  • Overlooking available treatment. Pompe disease has disease-specific enzyme replacement therapy, making early diagnosis clinically important.


FAQs

1. What is the easiest way to distinguish Von Gierke disease from Pompe disease in MRCP Part 1 questions?

Think about the primary organ involved. Von Gierke disease mainly affects the liver, causing fasting hypoglycaemia, hepatomegaly and metabolic abnormalities. Pompe disease primarily affects cardiac and skeletal muscle, leading to hypotonia, cardiomyopathy and respiratory muscle weakness with normal blood glucose.

2. Which enzyme deficiencies should I memorise for MRCP Part 1?

The two essential enzyme deficiencies are:

  • Von Gierke disease (Type I): Glucose-6-phosphatase deficiency

  • Pompe disease (Type II): Acid α-glucosidase (acid maltase) deficiency

These enzyme defects are among the most frequently tested metabolic facts in the examination.

3. Why does Von Gierke disease cause hyperuricaemia?

Because glucose-6-phosphate cannot be converted into free glucose, it is diverted into alternative metabolic pathways. Increased lactate competes with uric acid for renal excretion, resulting in hyperuricaemia, which may eventually lead to gout.

4. Is Pompe disease a glycogen storage disease or a lysosomal storage disease?

It is both. Pompe disease is classified as Glycogen Storage Disease Type II, but it is unique because glycogen accumulates within lysosomes due to acid α-glucosidase deficiency. This distinction is frequently tested in MRCP Part 1.

5. How should I revise glycogen storage diseases for MRCP Part 1?

Focus on pattern recognition rather than memorising every glycogen storage disease. Learn the enzyme defect, affected organs, key biochemical abnormalities and hallmark clinical features of Von Gierke, Pompe, Cori and McArdle diseases, then reinforce your knowledge with regular SBA practice and timed mock examinations.


Ready to start

Metabolic disorders are often perceived as difficult because they involve biochemistry and genetics. However, MRCP Part 1 questions usually reward candidates who can recognise classic clinical patterns rather than recall complex metabolic pathways.

After reviewing this topic:


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