Diagnostic Workup

A number of tools are available for confirming a clinical diagnosis of Pompe disease, most of which measure GAA enzyme activity in various tissue specimens. Additionally, other tools may be used to assess the functional impact and pathology associated with Pompe disease. More on Assessment Tools >

Pompe Disease Diagnostic Tools

GAA enzyme activity (tested on glycogen or 4-MUG)

Skin fibroblasts
Muscle tissue
Leukocytes
Dried blood spot (not yet commercially available; under investigation in the United States)

GAA Gene Mutation Analysis

Especially for carrier testing (family/sibling)

Prenatal diagnosis

Amniocentesis
Chorionic villi sampling

Newborn screening (emerging technology)

GAA enzyme activity in dried blood spot with analysis by fluorometry or mass spectrometry

Learn more about body system assessment tools used to diagnose and monitor Pompe >

Learn more about motor system assessment tools used to diagnose and monitor Pompe >

Diagnostic Tools: Enzyme Activity Testing

Measurement of residual GAA activity in skin fibroblasts, muscle tissue, and leukocytes using glycogen, maltose, or the artificial substrate 4-MUG are the most common diagnostic methods in current clinical practice. Skin fibroblasts and muscle tissue provide the most reliable results, typically showing:

Undetectable to minimal residual GAA activity in infantile-onset cases
Varying amounts of residual GAA activity in late-onset cases

Researchers report that in skin fibroblasts:

Patients with infantile-onset Pompe disease generally demonstrate less than 1% of normal GAA enzyme activity

Patients with late-onset generally demonstrate less than 40% of normal GAA enzyme activity¹

Diagnostic Tools: Skin Fibroblast

The skin fibroblast assay is currently considered to be the gold standard for the diagnosis of Pompe disease as it is more sensitive and less invasive than other procedures. Cultured fibroblasts are obtained from a skin biopsy which is then grown to confluency prior to the enzyme assay. The biggest challenge with the skin fibroblast assay is the length of time required to culture the cells before the assay is performed. It can take up to 4 to 6 weeks to obtain the test results which can delay the diagnosis significantly. This delay may be detrimental for the patients with infantile onset disease due to the rapidly progressive nature of the disease.

Diagnostic Tools: Leukocyte Assays

Leukocyte-based assays are typically not suitable for definitive diagnosis by simple enzyme assay and are frequently used in combination with lysosomal anti-GAA antibodies to differentiate between lysosomal enzyme activity and activity of nonrelevant isozymes.²

Diagnostic Tools: Genotyping

Genotyping is important in genetic counseling and may be of supportive diagnostic value. It provides a way of determining whether children of carrier parents are also carriers. In cases with a family history of disease, prenatal diagnosis may be made by determining GAA in cultured amniotic cells or chorionic villus biopsies.²

Common Assessment Tools

These qualitative assessment tools can be helpful during the initial diagnosis and long term management of Pompe disease patients. These tools help the physician monitor disease burden and progression. Muscle function testing, imaging studies, and specifically designed Pompe evaluations will help in the diagnosis and management of the disease.

Muscle Function and Pathology

Common tests include:
Quantitative muscle testing (QMT)*
Manual muscle testing (MMT)*
Electromyography (EMG)
Histopathology

*Used in late-onset disease

Muscle function and pathology can be evaluated in a number of ways. Quantitative muscle testing (QMT), in which a strain gauge is used to determine the maximal voluntary isometric contraction, is the preferred method of assessing muscle function.⁶ Muscle function can also be assessed by manual muscle testing (MMT).

Electromyography (EMG) is useful for demonstrating muscle pathology. Evidence of myopathy is seen in most Pompe disease patients, although some muscles may appear normal in patients with late-onset disease.²

Other abnormalities commonly seen on EMG include myotonic discharges (without clinical myotonia), fibrillation potentials, positive sharp waves, and excess electrical irritability. In some cases, a neurology consult is requested in the early stages of diagnosis as a result of clinical suspicion of a neuromuscular disorder.

Other methods of detecting muscle pathology include histopathology of muscle biopsies and magnetic resonance imaging (MRI).² MRI is a noninvasive method for measuring glycogen accumulation in muscle, which may be useful in monitoring disease progression, and possibly the benefit of therapeutic intervention.⁷

A muscle biopsy can provide histopathological information about the level of glycogen storage within the lysosomes of muscle cells and may also return faster results than those of skin fibroblasts. However, muscle biopsy is an invasive procedure and has liabilities. Great care in handling is required so as not to wash out glycogen buildup and alter cell and tissue pathology. With the adult Pompe patient, the site of the tissue sample can directly impact results due to the variability of glycogen accumulation in various muscle tissues. Finally, there are risks associated with anesthesia in Pompe patients if an open biopsy procedure is performed. ¹⁴ These risks should be weighed against the benefit of the glycogen analysis in muscle biopsy, particularly if the patient is already shown to have severe cardiomyopathy and skeletal myopathy.

Histopathologic examination of muscle biopsies -- which is not necessary for a diagnosis but may offer other helpful findings -- can reveal the degree of glycogen deposition within the lysosomes of muscle cells. Vacuoles generally stain positive for glycogen and, in some cases, for the lysosomal enzyme acid phosphatase as well.

The increase of acid phosphatase, which catalyzes the conversion of orthophosphoric monoester and water into alcohol and orthophosphate, may be due to a compensatory effort.⁹ In infantile-onset patients, the increase in glycogen content can be more than tenfold, while the elevation in late-onset patients generally ranges from normal to threefold.²

Motor Function

Common tests include:

Pompe Pediatric Evaluation of Disability Inventory (Pompe PEDI)
Evaluation for achievement of motor milestones
Functional motor testing (timed tests, arm and leg functional testing)*

*Used in late-onset disease

Motor Function: The Pompe Disease Pediatric Evaluation of Disability Inventory (PEDI)

The Pompe PEDI is a standardized, modified version of the PEDI designed for patients with Pompe disease. It has been included in Pompe clinical trials to assist in the interpretation of treatment-related changes in gross and fine motor functioning as they relate to the performance of activities of daily living.

The PEDI is a standardized instrument developed to assess the functional capabilities of children 6 months to 7.5 years of age. Functional Skills (performed independently) and Caregiver Assistance Skills (performed with assistance) are assessed in 3 content domains: Self-care, Mobility, and Social Function. The Pompe PEDI includes all items listed on the PEDI, as well as several additional items to address the specific limitations of patients with Pompe disease. The new items include less challenging tasks involving movement of the head, trunk, and upper extremity items for patients with severe functional loss, and more challenging gross motor tasks for patients with milder symptoms.

The Pompe PEDI has been normed for infants and children from 2 months to 17 years of age. It is important to note that some items in the Caregiver Assistance scale cannot be readily observed in a clinical setting. As a result, clinicians must rely on parent report, which may introduce a degree of subjectivity. An increase in raw score of 1 point indicates the acquisition of 1 new skill for each domain. Scaled scores evaluate intra-patient functional change, without reference to chronological age or same-age peers. In effect, the scaled score shows the extent of the patient’s functionality in each area on a scale of 0 to 100. Normative standard scores, which allow for the comparison of individual patients to normally developing peers from 2 months through 17 years-of-age, were also calculated. Note that a clinically meaningful change in functional status was defined as a gain in scaled score greater than 2 times the standard error (SE) of the Baseline scaled score.⁸

Cardiac

Common tests include:

Echocardiography
Electrocardiography
Chest x-ray
MRI

The heart can be assessed by echocardiography and electrocardiography, MRI, or chest x-ray. These tests are predominately done in patients with infantile-onset Pompe disease. Depending on the patient's individualized presentation, this may occur before or after clinical suspicion of a myopathic disorder is aroused. Infantile-onset patients generally show massive cardiomegaly while late-onset patients rarely ever display hypertrophy of the heart.²

Certain findings are common in Pompe disease. Echocardiography may reveal left ventricular (LV) thickening and/or outflow obstruction in infantile-onset patients, while the ECG exam typically shows a shortening of the PR interval as well as very tall and QRS complexes.^{9, 10} Late-onset patients usually have normal patterns.

Pulmonary

Common tests include:

Pulmonary function tests (spirometry)
Sleep studies

Labs

Common tests include:

Creatine kinase (CK)
Alanine and aspartate aminotransferase (ALT/AST)
Urinary oligosaccharides (under investigation)

Several lab tests may be elevated in Pompe disease, including creatine kinase (CK), the liver function enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST), and urinary excretion of glucose-containing oligosaccharides.^{2, 11}

A 1999 study found that creatine kinase (CK) elevation is a sensitive marker for Pompe disease.¹² Of 18 patients examined, 18 (100%) demonstrated elevated CK levels, while a review of the literature revealed that 94.3% of patients displayed increased levels.

The greatest elevation can be found in infantile-onset patients (as high as 2000 IU/L)⁹, while in some cases, adults may have CK levels within the normal reference range. A blood test including a CK examination may be ordered as an early step to determine whether more invasive testing is warranted.

Patients may demonstrate elevated levels of AST and ALT.

There has been at least one report in which these laboratory findings served as the first clue in a juvenile patient. DiFiore and colleagues in 1993 described a case in which a still asymptomatic juvenile patient presented only with an isolated rise in AST.¹³

Note that Pompe patients typically do not display any abnormalities of glucose metabolism such as hypoglycemia. In addition, Pompe patients usually have normal responses to epinephrine and glucagon administration.²

References

1. Chen YT, Amalfitano A. Towards a molecular therapy for glycogen storage disease type II (Pompe disease). Mol Med Today 2000 Jun; 6(6): 245-251.

2. Hirschhorn, Rochelle and Arnold J. J. Reuser. Glycogen Storage Disease Type II: Acid-Alpha Glucosidase (Acid Maltase) Deficiency. In: Scriver C, Beaudet A, Sly W, Valle D, editors. The Metabolic and Molecular Bases of Inherited Disease. 8th Edition. New York: McGraw-Hill; 2001; 3389-3420.

3. Umapathysivam K, Hopwood JJ, Meikle PJ. Determination of acid α-glucosidase activity in blood spots as a diagnostic test for Pompe disease. Clin Chem. 2001;47:1378-1383.

4. Li Y, Scott R, Chamoles N, Ghavami A, Pinto B, Turecek F, Gelb M. Direct multiplex assay of lysosomal enzymes in dried blood spots for newborn screening. Clin Chem. 2004;50:1785-1796.

5. Chamoles NA, Niizawa G, Blanco M, Gaggioli D, Casentini C. Glycogen storage disease type II: enzymatic screening in dried blood spots on filter paper. Clin Chim Acta 2004;347:97-102.

6. Escolar DM, Henricson EK, Mayhew J, et al. Clinical evaluator reliability for quantitative and manual muscle testing measures of strength in children. Muscle Nerve. 2001;24:787-793.

7. Wary C, Laforêt P, Eymard B, et al. Evaluation of muscle glycogen content by 13C NMR spectroscopy in adult-onset acid maltase deficiency. Neuromusc Disord. 2003;13:545-553.

8. Haley SM, Fragala MA, Skrinar AM. Pompe disease and physical disability. Develop Med Child Neurol. 2003;45:618-623.

9. King, Frank J. Acid Maltase Deficiency Myopathy. eMedicine Specialties. Available at: http://www.emedicine.com/pmr/topic2.htm. Accessed September 7, 2005.

10. Ibrahim, Jennifer. Glycogen Storage Disease Type II. eMedicine Specialties. Available at: http://www.emedicine.com/ped/topic1866.htm. Accessed August 22, 2005.

11. An Y, Young S, Kishnani P, Millington D, Amalfitano A, Corz D, et al. Glucose tetrasaccharide as a biomarker for monitoring the therapeutic response to enzyme replacement therapy for Pompe disease. Mol Genet Metab 2005;85(4):247-254.

12. Ausems MG, Lochman P, van Diggelen OP, et al. A diagnostic protocol for adult-onset glycogen storage disease type II. Neurology 1999 Mar 10; 52(4): 851-853.

13. DiFiore MT, Manfredi R, Marri L, et al. Acid maltase deficiency in childhood. Early diagnosis and clinical follow-up of late-onset glycogen storage disease type II. Acta Neurol (Napoli) 1993 Aug;15(4): 258-267.

14. Ing RJ, Cook DR, Bengur RA,Williams EA, Eck J, Dear Gde L, Ross AK, Kern FH, Kishnani PS. Anaesthetic management of infants with glycogenstorage disease type II: a physiological approach. Paediatr Anaesth 2004;14:514-519.

15. Dreyfus J, Poenaru L. White blood cells and the diagnosis of alpha-glucosidase deficiency. Pediatr Res 1980; 342-344.

16. Niizawa G, Blanco M, Casentini C, Borrajo G, Keutzer J, Pomponio R, Chamoles N. Newborn screening for MPS1 and Pompe disease: a pre-pilot study. Mol Genet Metab 2005;84:202 (abstract, SIMD March 2005).

(Pompe PEDI; Haley, 2003, Dev Med Child Neurol; Haley, 2003, Pediatr Rehabil).