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POMPE DISEASE (Glycogen Storage Disease Type II / Acid Maltase Deficiency)

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1. INTRODUCTION

• Goldman-Cecil Medicine, Chapter 391 (GSD)
• Harrison's Principles of Internal Medicine 22e, Chapter on LSDs
• Adams and Victor's Principles of Neurology, 12th ed.

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2. EPIDEMIOLOGY

• Higher prevalence of IOPD in African Americans (carrier frequency ~1/14)
• Higher LOPD prevalence in the Netherlands and certain European populations
• The c.-32-13T>G (IVS1) splice-site variant is the most common LOPD-causing mutation in Caucasians
• Pseudodeficiency alleles (c.[1726A; 2065A]) are MORE common than true Pompe disease (e.g., 1/8,811 in Missouri vs. 1/40,000 for true disease), causing diagnostic challenges on newborn screening

• GeneReviews: Pompe Disease (Updated August 21, 2025) - NCBI Bookshelf NBK1261
• Sperry et al., PMC10035871

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3. ETIOLOGY

• Skeletal muscle
• Cardiac muscle (especially in IOPD)
• Smooth muscle
• Neurons of spinal cord and brainstem (especially IOPD)
• Liver (less prominent)

• Complete absence (<1% residual activity): Classic IOPD with cardiomyopathy
• Partial deficiency (1-10% activity): LOPD, variable severity
• Residual activity correlates inversely with disease severity

• All cells continuously deliver a small amount of cytoplasmic glycogen to lysosomes for degradation via autophagy (autophagic flux). GAA deficiency blocks this pathway, leading to secondary lysosomal and autophagic dysfunction - a key pathomechanism especially in LOPD where autophagy failure drives irreversible muscle damage (Do et al., PMID 38785980).

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4. PHYSIOLOGY AND PATHOPHYSIOLOGY

Normal Lysosomal Glycogen Metabolism

Pathophysiology in Pompe Disease

• Mechanical disruption of myofibrils by lysosomal swelling
• Loss of contractile proteins
• Mitochondrial dysfunction (secondary)
• In IOPD: Glycogen also accumulates in anterior horn cells, brainstem neurons, and neurons of autonomic ganglia

• Heart (IOPD): Glycogen replaces myofibrils -> hypertrophic cardiomyopathy -> outflow tract obstruction, arrhythmias, cardiac failure
• Skeletal muscle: Vacuolation, fibre degeneration -> progressive proximal myopathy, respiratory failure
• Diaphragm: Preferentially affected -> respiratory failure (the leading cause of death in both forms)
• Smooth muscle: Bladder/bowel dysfunction in late LOPD
• CNS (IOPD): Spinal cord and brainstem neuron involvement -> contributes to hypotonia and feeding difficulties
• Vasculature (LOPD): 60% of LOPD patients have intracranial arterial abnormalities (vertebrobasilar dolichoectasia, unruptured aneurysms) - mechanism unclear

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5. GENETICS

Mutational Spectrum

• Most common mutation in LOPD (Caucasians): c.-32-13T>G (IVS1-13T>G) - a leaky splice site mutation that allows some residual GAA protein production (~1-10%), resulting in later onset and milder disease
• Most common in IOPD (non-Caucasians): Missense and nonsense mutations causing complete loss of enzyme activity (e.g., p.Arg854X, p.Trp746X)
• African Americans: Relatively high frequency of c.2560C>T (p.Arg854Cys) and deletion variants
• Dutch founder mutation: c.-32-13T>G present at high frequency in the Netherlands

Genotype-Phenotype Correlation

• Two null alleles (both alleles causing complete loss of function) -> Classic IOPD with cardiomyopathy, <1% GAA activity
• One null + one leaky allele (e.g., IVS1) -> Often IOPD without cardiomyopathy (atypical IOPD) or LOPD
• Two leaky alleles (e.g., IVS1/IVS1) -> LOPD, slowly progressive
• However, genotype does not fully predict phenotype - additional modifying factors (epigenetic, environmental, diet, exercise) influence clinical expression

Pseudodeficiency Alleles

CRIM Status (Cross-Reactive Immunological Material)

• CRIM-positive: Patient produces some GAA protein (even if non-functional) - less likely to mount antibody response to ERT
• CRIM-negative: No GAA protein produced (null/null genotype) - high risk of developing high-sustained antibody titres (HSAT) to ERT, drastically reducing therapeutic efficacy
• Immunomodulation (rituximab + methotrexate before first ERT infusion) is now standard in CRIM-negative patients to prevent HSAT formation - Harrison's 22e
• GeneReviews: Pompe Disease, NBK1261
• Adams and Victor's Neurology, 12th ed.

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6. CLINICAL MANIFESTATIONS

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6a. SYMPTOMS

Infantile-Onset Pompe Disease (IOPD) - Classic Form

Late-Onset Pompe Disease (LOPD)

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6b. SIGNS

IOPD

LOPD

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6c. COMPLICATIONS

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6d. DIFFERENTIAL DIAGNOSIS

For IOPD:

For LOPD:

• GeneReviews, NBK1261 - Differential Diagnosis Table
• Adams and Victor's Neurology, 12th ed.
• Goldman-Cecil Medicine

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7. INVESTIGATIONS OF DISEASE

Biochemical / Enzymatic Investigations

Enzyme Activity Assays (Gold Standard Biochemical Test)

Molecular Genetic Testing

1. Deficient GAA activity in leukocytes OR fibroblasts, AND/OR
2. Identification of biallelic pathogenic GAA variants on molecular genetic testing

Muscle Biopsy

• Adams and Victor's Neurology 12th ed.
• Goldman-Cecil Medicine, Ch. 391

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8. INVESTIGATIONS AT DIAGNOSIS

Cardiac Assessment

Respiratory Assessment

Neuromuscular Assessment

Hearing Assessment

• Audiometry - sensorineural hearing loss documented in IOPD

Brain/Spinal Cord Imaging

• MRI Brain - white matter hyperintensities reported in ERT-treated IOPD survivors; intracranial aneurysm screening in LOPD (MRA/CTA)

Baseline Laboratory Work-up

• CK, ALT, AST, LDH, urinary Hex4
• CRIM status determination (Western blot or genotype-based)
• Pre-ERT antibody baseline

Newborn Screening (NBS)

• DBS GAA enzyme activity -> if low: confirmatory GAA sequencing + CK + echocardiography
• A single abnormal NBS is NOT sufficient for diagnosis - requires confirmation

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9. TREATMENT

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9a. Enzyme Replacement Therapy (ERT) - Disease-Modifying

First-Generation ERT

Second-Generation ERT (Improved Uptake)

• Goldman-Cecil Medicine: "Avalglucosidase alfa, a second-generation recombinant acid α-glucosidase replacement therapy with a significantly higher number of mannose-6-phosphate tags, can provide significant benefit over alglucosidase alfa."
• Harrison's 22e

CRIM-Negative Patients - Immunomodulation Protocol

• Pre-ERT immunomodulation: Rituximab + Methotrexate given before first ERT infusion
• This prevents development of high-sustained antibody titres (HSAT) which otherwise abolish ERT efficacy
• Has greatly improved therapeutic response and long-term survival in CRIM-negative IOPD

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9b. Respiratory Support

• NIV (non-invasive ventilation): BiPAP or CPAP for nocturnal hypoventilation - often the first intervention needed
• Invasive ventilation: Tracheostomy in advanced respiratory failure
• Cough-assist devices / mechanical in-exsufflation (MI-E): To improve cough efficacy and prevent atelectasis
• Chest physiotherapy and airway clearance techniques
• Avoidance of sedatives and respiratory depressants

9c. Physical and Rehabilitative Therapy

• Regular physiotherapy to maintain strength, flexibility, and prevent contractures
• Aquatic therapy is well tolerated
• High-protein diet may be beneficial (preserves muscle mass; Umpleby et al. data)
• Low-carbohydrate, high-protein diet has been used, particularly in adults
• Occupational therapy: Energy conservation, assistive devices
• Speech therapy: For dysphagia, dysarthria - swallowing assessment and exercises

9d. Cardiac Management (IOPD)

• Medications with caution: Beta-blockers and digoxin may worsen outflow tract obstruction in HCM - use with extreme caution
• Anti-arrhythmic therapy as indicated
• ERT reverses cardiomyopathy in most IOPD patients who start early

9e. Surgical / Orthopaedic Interventions

• Scoliosis surgery if progressive
• Gastrostomy (PEG) for feeding in IOPD with feeding failure
• Hearing aids for sensorineural hearing loss

9f. Emerging Therapies

• George KA et al., "Pompe disease: Unmet needs and emerging therapies," Mol Genet Metab 2024 (PMID 39418752)
• Colella P, "Advances in Pompe Disease Treatment: From Enzyme Replacement to Gene Therapy," Mol Diagn Ther 2024 (PMID 39134822)
• Corsini A, "Improving the treatment of Pompe disease with enzyme replacement therapy," Expert Opin Pharmacother 2025 (PMID 40237692)

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10. COUNSELLING

Genetic Counselling

• Each subsequent sibling: 25% risk of being affected, 50% carrier, 25% unaffected non-carrier
• Affected individual's children: All will be obligate carriers (unless partner is also a carrier)
• If partner is a carrier: Each child has 50% affected, 50% carrier risk

• Cascade screening: Offer molecular genetic testing (GAA variant analysis) to all first-degree relatives once pathogenic variants identified in proband
• Prenatal testing: Chorionic villus sampling (CVS) at 10-12 weeks or amniocentesis at 15-18 weeks - enzyme activity and/or molecular testing of GAA
• Preimplantation Genetic Testing (PGT-M): Available for couples who are both carriers and wish to avoid affected pregnancy
• CRIM status counselling: Explain implications for ERT antibody response and need for pre-ERT immunomodulation in CRIM-negative patients

Disease Counselling

• Explain the spectrum of disease - IOPD is life-threatening within the first year untreated; LOPD is slowly progressive but causes significant disability
• Address prognosis with ERT: Cardiac disease reverses in IOPD; muscle and respiratory involvement may stabilise or improve but may not fully reverse, especially in advanced disease
• Discuss disease monitoring needs (lifelong follow-up)
• Address psychological burden - grief, caregiver stress, impact on schooling/employment
• Connect with patient advocacy groups (e.g., Acid Maltase Deficiency Association - AMDA; International Pompe Association - IPA)
• Address newborn screening for subsequent pregnancies
• Discuss career and lifestyle modifications - activity planning, avoiding activities with fall risk, respiratory precautions during anaesthesia (avoid succinylcholine, careful use of sedatives)
• GeneReviews Pompe Disease, NBK1261

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11. ROUTINE FOLLOW-UP

Monitoring Schedule (Based on MetabERN 2024 CPR, GeneReviews, and Practice Guidelines)

Treatment Targets

• FVC maintained above 50% predicted (ventilatory support threshold)
• Motor function stabilisation or improvement
• Urinary Hex4 within or near normal range
• Antibody titres below HSAT threshold
• Cardiac dimensions normalising on echo (IOPD)
• Parenti G et al., MetabERN Clinical Pathway Recommendations for Pompe disease, Orphanet J Rare Dis 2024 (PMID 39482698)
• GeneReviews: Pompe Disease, NBK1261 (Updated August 21, 2025)

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12. APPROACH TO DISEASE - DIAGNOSTIC AND MANAGEMENT FLOWCHART

                          CLINICAL SUSPICION FOR POMPE DISEASE
                                         |
           _____________________________|______________________________
           |                                                          |
    INFANT (< 12 months)                              CHILD/ADULT (≥ 1 year)
    - Floppy infant (hypotonia)                       - Proximal limb weakness
    - Cardiomegaly (on CXR/Echo)                      - Respiratory insufficiency
    - Poor feeding, macroglossia                       - Elevated CK, diaphragm weakness
    - Elevated CK                                      - Sleep-disordered breathing
           |                                                          |
           v                                                          v
    ┌─────────────────────────────────────────────────────────────────────────┐
    │                  STEP 1: INITIAL SCREENING                              │
    │  Dried Blood Spot (DBS) GAA Enzyme Activity Assay                       │
    │  + Serum CK, LDH, urine Hex4                                            │
    │  + ECG + Echocardiogram (IOPD)                                          │
    └─────────────────────────────────────────────────────────────────────────┘
                         |
          _______________|________________
          |                              |
    GAA ACTIVITY LOW               GAA ACTIVITY NORMAL
    (or out-of-range NBS)          → Consider alternative diagnoses
          |
          v
    ┌─────────────────────────────────────────────────────────────┐
    │         STEP 2: CONFIRMATORY TESTING                        │
    │  1. GAA molecular genetic testing (biallelic GAA variants)  │
    │  2. Leukocyte / fibroblast GAA enzyme assay                 │
    │  3. Muscle biopsy if needed (PAS, acid phosphatase, EM)     │
    │  [Rule out pseudodeficiency allele c.[1726A;2065A]]         │
    └─────────────────────────────────────────────────────────────┘
                         |
          _______________|________________
          |                              |
   CONFIRMED POMPE DISEASE         PSEUDODEFICIENCY / CARRIER
                 |                  → No treatment; family counselling
                 v
    ┌─────────────────────────────────────────────────────────────┐
    │              STEP 3: FULL BASELINE EVALUATION               │
    │  - CRIM status (Western blot / genotype)                    │
    │  - Echo, ECG, Holter                                        │
    │  - Spirometry (upright + supine), sleep study               │
    │  - 6MWT, muscle MRI, neurological exam                      │
    │  - Audiometry (IOPD), baseline antibody titres              │
    │  - Genetic counselling; family cascade testing              │
    └─────────────────────────────────────────────────────────────┘
                         |
                         v
    ┌─────────────────────────────────────────────────────────────┐
    │                   STEP 4: TREATMENT                         │
    │                                                             │
    │  CRIM-NEGATIVE IOPD:                                        │
    │   → Rituximab + Methotrexate FIRST (immunomodulation)       │
    │   → Then: Avalglucosidase alfa 20 mg/kg IV q2w              │
    │                                                             │
    │  CRIM-POSITIVE IOPD / LOPD:                                 │
    │   → Avalglucosidase alfa 20 mg/kg IV q2w (1st choice)       │
    │     OR Alglucosidase alfa 20 mg/kg IV q2w                   │
    │     OR Cipaglucosidase + Miglustat (oral chaperone combo)   │
    │                                                             │
    │  SUPPORTIVE:                                                │
    │   → NIV / invasive ventilation as needed                    │
    │   → Physiotherapy, speech therapy, occupational therapy     │
    │   → High-protein diet; cough assist devices                 │
    │   → Cardiac management, hearing aids                        │
    └─────────────────────────────────────────────────────────────┘
                         |
                         v
    ┌─────────────────────────────────────────────────────────────┐
    │              STEP 5: ROUTINE FOLLOW-UP                      │
    │  Every 3-6 months (IOPD) / 6-12 months (LOPD)              │
    │  Monitor: CK, Hex4, spirometry, 6MWT, echo, ERT titres     │
    │  Adjust ERT; escalate respiratory support as needed         │
    │  Genetic counselling; reproductive planning support         │
    └─────────────────────────────────────────────────────────────┘

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13. REFERENCES

Textbooks

1. Goldman L, Schafer AI (eds). Goldman-Cecil Medicine, 27th ed. Philadelphia: Elsevier, 2024. Chapter 391 (Glycogen Storage Diseases) and Chapter 192 (Lysosomal Storage Diseases).
2. Loscalzo J, Fauci A, Kasper D, et al. (eds). Harrison's Principles of Internal Medicine, 22nd ed. New York: McGraw Hill, 2025. Chapter on Lysosomal Storage Diseases and Glycogen Storage Disorders.
3. Ropper AH, Samuels MA, Klein JP, Prasad S. Adams and Victor's Principles of Neurology, 12th ed. New York: McGraw-Hill, 2023. Chapter 45 (Acid Maltase Deficiency / GSD II).
4. Fishman A, Elias J, Fishman J, et al. (eds). Fishman's Pulmonary Diseases and Disorders, 5th ed. New York: McGraw-Hill. Section on Metabolic Myopathies (Acid Maltase Deficiency/Pompe Disease).
5. Emery's Elements of Medical Genetics and Genomics, 16th ed. Edinburgh: Elsevier, 2022. Chapter on Pompe Disease / GSD II.

Online Databases

1. OMIM #232300. Pompe Disease; Glycogen Storage Disease II. Johns Hopkins University. Available at: https://www.omim.org/entry/232300
2. Sperry E, Leslie N, Berry L, Pena L. Pompe Disease. In: Adam MP, et al. (eds). GeneReviews®. Seattle: University of Washington; Initial posting August 31, 2007; Last updated August 21, 2025. Available at: https://www.ncbi.nlm.nih.gov/books/NBK1261

Journal Articles

1. Labella B, Cotti Piccinelli S, Risi B, et al. A Comprehensive Update on Late-Onset Pompe Disease. Biomolecules. 2023 Aug 22. [PMID: 37759679]
2. Do H, Meena NK, Raben N. Failure of Autophagy in Pompe Disease. Biomolecules. 2024 May 13. [PMID: 38785980]
3. George KA, Anding AL, van der Flier A, et al. Pompe disease: Unmet needs and emerging therapies. Mol Genet Metab. 2024 Nov. [PMID: 39418752]
4. Colella P. Advances in Pompe Disease Treatment: From Enzyme Replacement to Gene Therapy. Mol Diagn Ther. 2024 Nov. [PMID: 39134822]
5. Parenti G, Fecarotta S, Alagia M, et al. The European reference network for metabolic diseases (MetabERN) clinical pathway recommendations for Pompe disease. Orphanet J Rare Dis. 2024 Nov 1. [PMID: 39482698]
6. Corsini A. Improving the treatment of Pompe disease with enzyme replacement therapy: current strategies and clinical evidence. Expert Opin Pharmacother. 2025 May. [PMID: 40237692]
7. Sperry E, Leslie N, et al. Clinical features of Pompe disease. Acta Myologica. 2013. [PMC3866902]
8. Diaz-Manera J, Kishnani PS, Kushlaf H, et al. Safety and efficacy of avalglucosidase alfa versus alglucosidase alfa in patients with late-onset Pompe disease (COMET): a phase 3, randomised, multicentre trial. Lancet Neurol. 2021;20:1012-1026.
9. Schoser B, Roberts M, Byrne BJ, et al. Safety and efficacy of cipaglucosidase alfa plus miglustat versus alglucosidase alfa plus placebo in late-onset Pompe disease (PROPEL). Lancet Neurol. 2021;20:1027-1037.

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Key take-home points:

• Pompe disease = GAA deficiency -> lysosomal glycogen accumulation in muscle/heart
• IOPD: fatal in <1 year untreated (cardiomyopathy + hypotonia); LOPD: slow proximal myopathy + diaphragm weakness
• Diagnosis by DBS GAA activity (screen) -> confirmed by leukocyte/fibroblast enzyme assay + GAA biallelic variants
• Beware pseudodeficiency alleles - always confirm with molecular testing
• ERT (avalglucosidase alfa preferred) is standard of care; pre-ERT immunomodulation essential in CRIM-negative patients
• Multidisciplinary lifelong follow-up: respiratory, cardiac, motor, swallowing, hearing, genetic counselling