Clinical Diagnosis

The two general subtypes of glycogen storage disease type II (GSD II), also known as Pompe disease, differ by age of onset and clinical findings.

Infantile-onset Pompe disease is suspected in infants with the following [van den Hout et al 2003, Kishnani et al 2006a]:

• Poor feeding/failure to thrive (44%-97% of cases)
• Motor delay/muscle weakness (20%-63%)
• Respiratory concerns (infections/difficulty) (27%-78%)
• Cardiac problems (shortened PR interval with a broad, wide QRS complex, cardiomegaly, left ventricular outflow obstruction, cardiomyopathy) (50%-92%)

Late-onset (i.e., childhood, juvenile, and adult-onset) Pompe disease is suspected in individuals with proximal muscular weakness and respiratory insufficiency without clinically apparent cardiac involvement.

Testing

Nonspecific tests used to support the diagnosis of Pompe disease

• Serum creatine kinase (CK) concentration is uniformly elevated (as high as 2000 IU/L; normal: 60-305 IU/L) in classic infantile-onset Pompe disease and in the childhood and juvenile variants, but may be normal in adult-onset disease [Laforêt et al 2000, Kishnani et al 2006b]. However, serum CK concentration is elevated in many conditions and must be considered nonspecific.
• Urinary oligosaccharides. Elevation of a specific urinary glucose tetrasaccharide is highly sensitive in Pompe disease but is also seen in other glycogen storage diseases [An et al 2000, Kallwass et al 2007, Young et al 2012].

Testing used to establish the diagnosis of Pompe disease

• Acid alpha-glucosidase (GAA) enzyme activity. GAA enzyme activity analysis can be performed on dried blood spots [Chamoles et al 2004, Zhang et al 2006, Pompe Disease Diagnostic Working Group 2008], thus permitting rapid and sensitive analysis. Standard conditions for assay of GAA in blood samples have been proposed by a Pompe Disease Diagnostic Working Group. Confirmation by measurement of GAA activity in another tissue (e.g., culture skin fibroblasts) or molecular analysis is recommended [Pompe Disease Diagnostic Working Group 2008]. Historically, measurement of GAA enzyme activity has been performed using cultured skin fibroblasts, but it may take four to six weeks to obtain results, and delayed diagnosis and initiation of treatment, particularly in infants, may affect outcome.
  • Complete deficiency (activity <1% of normal controls) of GAA enzyme activity is associated with classic infantile-onset Pompe disease.
  • Partial deficiency (activity 2%-40% of normal controls) of GAA enzyme activity is associated with the non-classic infantile-onset and the late-onset forms [Hirschhorn & Reuser 2001].
    
    Note: (1) As a general rule, the lower the GAA enzyme activity the earlier the age of onset of disease. (2) GAA enzyme activity can be assayed in muscle; however, this invasive procedure usually requires anesthesia, which may not be tolerated in those who have infantile Pompe disease and cardiopulmonary compromise. (3) Peripheral leukocytes have been used to measure GAA enzyme activity but alternate isoenzymes such as maltase-glucoamylase may interfere with the assay. (4) GAA enzyme activity analysis can be performed on dried blood spots [Chamoles et al 2004, Zhang et al 2006], thus permitting rapid and sensitive analysis that is potentially useful for newborn screening [Kishnani et al 2006b].
• Acid alpha-glucosidase protein quantitation can be performed by an antibody-based method in cultured fibroblasts. Such testing may be important in determining if an affected individual produces cross-reactive immunologic material (CRIM). CRIM status affects response to ERT and CRIM-negative individuals need an altered plan for enzyme therapy [Pompe Disease Diagnostic Working Group 2008, Kishnani et al 2010, Messinger et al 2012].
• Muscle biopsy. In contrast to the other glycogen storage disorders, GSD II is also a lysosomal storage disease. In GSD II glycogen storage may be observed in the lysosomes of muscle cells as vacuoles of varying severity that stain positively with periodic acid-Schiff. However, 20%-30% of individuals with late-onset Pompe disease with documented partial enzyme deficiency may not show these muscle-specific changes [Laforêt et al 2000, Winkel et al 2005].

Newborn screening. Newborn screening for GSD II, using GAA enzyme activity in dried blood spots as a primary screening tool, has been under study in Taiwan. Variants associated with infantile-onset and late-onset disease have been identified in the Taiwanese population. A nationwide study in Austria using enzymatic activity followed by mutation analysis identified one newborn among 34,736 samples [Mechtler et al 2012]. Similar feasibility studies have occurred in Japan [Oda et al 2011]. Pilot trials in the US are planned in several states.

The rationale for newborn screening is that infants with confirmed infantile-onset GSD II (Pompe disease) who are treated early with ERT have demonstrated improved cardiac status and motor gains, when compared to controls [Chien et al 2009].

Cautions: (1) The pseudodeficiency allele c.1726 G>A (p.Gly576Ser), which is relatively common in Asian populations, complicates the assessment and diagnostic confirmation in these populations. It remains to be seen whether pseudodeficiency is common in other populations [Labrousse et al 2010]. (2) In areas in which mutations that result in CRIM-negative proteins are more common (e.g., the US, the Middle East), newborn screening to facilitate early diagnosis and hence initiation of ERT may not provide much improvement unless alternative therapy protocols using immunomodulation are considered [Messinger et al 2012].

Testing Strategy

To confirm/establish the diagnosis in a proband. Guidelines for the diagnosis of Pompe disease have been put forth by an expert panel from the American College of Medical Genetics [Kishnani et al 2006b] (see ).

Clinical evaluation alone is not sufficient to establish the diagnosis of any of the forms of Pompe disease:

• Assay of acid alpha-glucosidase (GAA) enzyme activity in cultured skin fibroblasts or muscle, considered the gold standard for diagnosis, is the diagnostic test of choice if feasible, but it may take weeks to obtain results.
• Assay of GAA enzyme activity in whole blood or dried bloodspot reliably detects GAA enzyme deficiency; confirmation by a second method is preferred prior to initiation of therapy [Kallwass et al 2007, Pompe Disease Diagnostic Working Group 2008].
• Urinary tetrasaccharide levels are elevated in nearly 100% of individuals with infantile Pompe disease, but may be normal in late-onset disease [Young et al 2009].
  
  Note: Histochemical evidence of glycogen storage in muscle is supportive of a glycogen storage disorder but not specific for Pompe disease.

Identification of two disease-causing GAA alleles using molecular genetic testing provides additional confirmation of the diagnosis.

• Depending on ethnicity and phenotype, an individual could be tested first for one of the three common mutations — p.Asp645Glu, p.Arg854*, and c.336-13T>G — before proceeding to full gene sequence analysis.
• Molecular analysis is an important element in the rapid assessment of cross-reactive immunologic material (CRIM) status in infantile Pompe [Bali et al 2012]. Experience with molecular analysis as the sole diagnostic test is limited, particularly in individuals who are asymptomatic or lacking sentinel clinical findings.

Carrier testing for at-risk relatives using molecular genetic testing requires prior identification of the disease-causing GAA mutations in the family.

Note: Carriers are heterozygotes for this autosomal recessive disorder and are not at risk of developing the disorder.

Predictive testing for at-risk asymptomatic family members using molecular genetic testing requires prior identification of the GAA disease-causing mutations in the family.

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutations in the family.

Genetically Related (Allelic) Disorders

No phenotypes other than those discussed in this GeneReview are known to be associated with mutations in GAA.

Natural History

Glycogen storage disease type II (GSD II; Pompe disease) has been classified based on age of onset, organ involvement, severity, and rate of progression. As a general rule, the earlier the onset of symptoms, the faster the rate of progression.

Although "late-onset" Pompe disease has been divided into childhood, juvenile, and adult-onset forms, many persons with the adult-onset type recall symptoms beginning in childhood and thus, "late-onset" is often the preferred term for those presenting after the first few years of life [Laforêt et al 2000]. Most likely, late-onset Pompe disease represents a clinical continuum in which subtypes cannot be reliably distinguished by age of onset.

Classic infantile-onset Pompe disease may be apparent in utero but more often presents in the first two months of life as hypotonia, generalized muscle weakness, feeding difficulties, failure to thrive, and respiratory distress (see Table 2).

Feeding difficulties may result from facial hypotonia, macroglossia, tongue weakness, and/or poor oromotor skills [van Gelder et al 2012].

Hearing abnormalities are common, possibly reflecting a cochlear or conductive pathology or both [Kamphoven et al 2004, van Capelle et al 2010].

Without treatment by enzyme replacement therapy (ERT), the cardiomegaly and hypertrophic cardiomyopathy that are often identified at birth by echocardiography progress to left ventricular outflow obstruction. Enlargement of the heart can also result in diminished lung volumes, atelectasis, and sometimes bronchial compression. Progressive deposition of glycogen results in conduction defects as seen by shortening of the PR interval on ECG.

In untreated infants, death commonly occurs in the first year of life from cardiopulmonary insufficiency [van den Hout et al 2003, Kishnani et al 2006a].

The non-classic variant of infantile-onset Pompe disease usually presents within the first year of life predominantly with motor delays and/or muscle weakness. Muscles are firm and rubbery as a result of glycogen deposition. Pseudohypertrophy of the calf muscles and a Gower sign simulate Duchenne muscular dystrophy (DMD), but these findings typically occur at an earlier age in Pompe disease than in DMD. Muscular weakness progresses more slowly than in classic infantile-onset Pompe disease.

Cardiomegaly can be seen with or without left ventricular outflow obstruction; it is not a major source of clinical morbidity [Slonim et al 2000].

Death from ventilatory failure typically occurs in early childhood.

Late-onset Pompe disease can present at various ages with muscle weakness and respiratory insufficiency. Progression of the disease is often predicted by the age of onset, as progression is more rapid if symptoms present in childhood.

Late-onset Pompe disease presenting in infancy or early childhood may be difficult to differentiate from the non-classic infantile form. Cardiomegaly is not typically seen but progressive muscle weakness resulting in motor delays, swallowing difficulties, and respiratory insufficiency usually occurs as in the infantile form, but at a slower rate.

While initial symptom presentation in late childhood to adolescence is typically not associated with heart complications, some adults with late onset disease have been found to have arteriopathy [El-Gharbawy et al 2011]. Ectasia of the basilar and internal carotid arteries has been noted, and may be associated with clinical signs, such as transient ischemic attacks and 3^rd nerve paralysis [Sacconi et al 2010]. In addition, dilation of the ascending thoracic aorta has been noted. Echocardiography may not always be sufficient to visualize this complication [El-Gharbawy et al 2011].

Progression of skeletal muscle involvement is slower than in the infantile forms and eventually involves the diaphragm and accessory respiratory muscles [Winkel et al 2005]. Affected individuals often become wheelchair dependent because of lower limb weakness. Respiratory failure causes the major morbidity and mortality of this form of the disease [Gungor et al 2011].

Late-onset Pompe disease may present from the second to as late as the seventh decade of life with progressive proximal muscle weakness primarily affecting the lower limbs, as in a limb-girdle muscular dystrophy or polymyositis. Affected adults often describe symptoms beginning in childhood with difficulty participating in sports. Later, fatigue and difficulty with rising from a sitting position, climbing stairs, and walking prompt medical attention. In an untreated cohort of affected individuals with late-onset Pompe, the median age at diagnosis was 38 years, the median survival after diagnosis was 27 years, and the median age at death was 55 years (range 23-77 years) [Gungor et al 2011]

Evidence of advanced osteoporosis in adults with Pompe disease is accumulating; while this is likely in large part secondary to decreased ambulation, other pathologic processes cannot be overlooked [Oktenli 2000, Case et al 2007].

Respiratory failure causes the major morbidity and mortality in this form of the disease [Hagemans et al 2005]. Male gender, severity of skeletal muscle weakness, and duration of disease are all risk factors for severe respiratory insufficiency [van der Beek et al 2011].

Clinical manifestations of late-onset Pompe disease [Hirschhorn & Reuser 2001]

• Progressive proximal muscle weakness (95%) [Winkel et al 2005]
• Respiratory insufficiency
• Exercise intolerance
• Exertional dyspnea
• Orthopnea
• Sleep apnea
• Hyperlordosis and/or scoliosis (childhood and juvenile onset)
• Hepatomegaly (childhood and juvenile onset)
• Macroglossia (childhood onset)
• Difficulty chewing and swallowing
• Increased respiratory infections
• Decreased deep tendon reflexes
• Gower sign
• Joint contractures
• Cardiac hypertrophy (childhood onset)

Electrophysiologic studies. Myopathy can be documented by electromyography (EMG) in all forms of Pompe disease although some muscles may appear normal. In adults, needle EMG of the paraspinal muscles may be required to demonstrate abnormalities [Hobson-Webb et al 2011].

Nerve conduction velocity (NCV) studies are normal for both motor and sensory nerves, particularly at the time of diagnosis in infantile-onset disease and in late-onset disease. However, an evolving motor axonal neuropathy has been demonstrated in a child with infantile-onset disease [Burrow et al 2010].

Genotype-Phenotype Correlations

GAA enzyme activity may correlate with age of onset and rate of progression as a "rough" general rule:

• It is assumed that a combination of two mutated alleles that produce essentially no enzyme activity results in infantile-onset Pompe disease. Infants who have infantile Pompe with no cross-reactive material (CRIM-negative) are likely to have two null mutations [Bali et al 2012].
• Various combinations of other alleles resulting in some residual enzyme activity likely cause disease but the age of onset and progression are most likely directly proportional to the residual GAA enzyme activity.

Although a number of mutations seen in homozygosity may suggest a genotype-phenotype correlation, the existence of a number of case reports of both infantile and late-onset Pompe disease in the same family suggests strong caution in extrapolation of these observances [Hoefsloot et al 1990].

Mutations that introduce mRNA instability, such as nonsense mutations, are more commonly seen in the infantile-onset form of Pompe disease as they result in nearly complete absence of GAA enzyme activity.

Missense and splicing mutations may result in either complete or partial absence of GAA enzyme activity and therefore may be seen in both infantile-onset and late-onset Pompe disease [Zampieri et al 2011].

The following are observations about genotype-phenotype correlations with specific mutations (see Table 3):

• p.Glu176Argfs*45 (c.525delT) is an especially common mutation among the Dutch [Van der Kraan et al 1994]. It results in negligible GAA enzyme activity and must be considered one of the more severe alterations. Either in the homozygous state or in the compound heterozygous state with another severe mutation, p.Glu176Argfs*45 predicts the infantile-onset form of Pompe disease, although the correlation is not absolute.
• Deletion of exon 18 (p.Gly828_Asn882del; c.2482_2646del) is also a common mutation, particularly among the Dutch [Van der Kraan et al 1994]. It results in negligible GAA enzyme activity and must be considered one of the more severe mutations. Deletion of exon 18, either in the homozygous state or in the compound heterozygous state with another severe mutation, predicts the infantile-onset form.
• c.336-13T>G is seen in 36% to 90% of late-onset Pompe disease and is not associated with the infantile-onset form [Hermans et al 2004, Montalvo et al 2006]. The mutation leads to a leaky splice site resulting in greatly diminished, but not absent, GAA enzyme activity.
• The mutation p.Asp645Glu, seen among a high proportion (≤80%) of infantile cases in Taiwan and China, is associated with a haplotype, suggesting a founder effect [Shieh & Lin 1998].
• The mutation p.Arg854* is frequently associated with infantile-onset Pompe disease. Found among different ethnicities, the mutation has been observed in up to 60% of individuals of African descent who had a common haplotype, suggesting a founder effect [Becker et al 1998].

Prevalence

The combined incidence of all forms of Pompe disease varies, depending on ethnicity and geographic region, from 1:14,000 in African Americans to 1:100,000 in individuals of European descent (see Table 4).

The mutation p.Asp645Glu, seen among a high proportion (up to 80%) of infantile cases in Taiwan and China, is associated with a haplotype, suggesting a founder effect [Shieh & Lin 1998].

The mutation p.Arg854* is frequently associated with infantile-onset Pompe disease. Found among different ethnicities, the mutation has been observed in up to 60% of individuals of African descent who had a common haplotype, suggesting a founder effect [Becker et al 1998].

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with glycogen storage disease type II (GSD II; Pompe disease), the following guidelines for the initial evaluation of infantile Pompe disease put forth by an expert panel from the American College of Medical Genetics [Kishnani et al 2006b] are recommended (see ):

• Chest radiography. In infantile-onset Pompe disease nearly all affected infants have cardiomegaly on chest x-ray [van den Hout et al 2003]. Further, evaluation of apparent lung volume reduction, areas of atelectasis, and any pulmonary fluid may be helpful in directing other therapies.
  
  In late-onset disease, baseline radiographic evaluation of the lungs and heart silhouette is indicated but only rarely reveals cardiomegaly.
• Electrocardiography (ECG). In infantile Pompe disease the majority of affected infants have left ventricular hypertrophy and many have biventricular hypertrophy [van den Hout et al 2003]. Conduction disturbance is often seen.
• Echocardiography. Typically in the infantile forms, echocardiography demonstrates hypertrophic cardiomyopathy with or without left ventricular outflow tract obstruction in the early phases of the disease process. In later stages, dilated cardiomyopathy may be seen.
• Pulmonary assessment. Most infants have varying degrees of respiratory insufficiency. Respiratory status should be established with regard to cough, presence of wheezing or labored breathing, and/or feeding difficulties. Diaphragmatic weakness caused by excessive glycogen deposits results in mild to moderate reduction of vital capacity; however, objective assessment of pulmonary functions in infants is difficult at best. Most infants display respiratory difficulty with feeds or sleep disturbance [Kravitz et al 2005].
  
  Persons with late-onset disease should be evaluated for cough, wheezing, dyspnea, energy level, exercise tolerance, and fatigability. Formal pulmonary function tests in the late-onset forms show pulmonary insufficiency. An attempt to assess ventilatory capacity in the supine position can detect early ventilatory insufficiency. Pulse oximetry, respiratory rate, and venous bicarbonate and/or pCO₂ should be obtained to assess for alveolar hypoventilation [van der Beek et al 2011].
• Nutrition/feeding. Patients should be evaluated for possible feeding difficulties (e.g., facial hypotonia, macroglossia, tongue weakness, and/or poor oromotor skills) [Jones et al 2009, van Gelder et al 2012].
  
  Assessment of growth (i.e., height, weight, head circumference), energy intake, and feeding (including video swallow study) is appropriate.
  
  All infants should be evaluated for gastroesophageal reflux.
• Audiologic. Baseline hearing evaluation including tympanometry is appropriate. See Deafness and Hereditary Hearing Loss Overview for a discussion of age-related methods of hearing evaluation. Sensorineural hearing loss is now documented in children with infantile Pompe disease, and hearing aids may be of benefit [van Capelle et al 2010].
• Disability inventory. All patients should undergo assessment of motor skills and overall functioning to guide subsequent therapies and monitor progression of the disease.
• Medical genetics consultation

Note: Guidelines for the evaluation of individuals with late-onset Pompe disease [Cupler et al 2012] differ in methodology from those detailed above for infantile Pompe disease, but many of the same systems (particularly pulmonary, nutrition, musculoskeletal, and disability inventory) are common to all forms of Pompe disease.

Treatment of Manifestations

Guidelines for the management of Pompe disease have been put forth by an expert panel from the American College of Medical Genetics [Kishnani et al 2006b] (see ):

• Cardiomyopathy. Medical intervention needs to be individualized as use of standard drugs may be contraindicated in certain stages of the disease process (see Agents/Circumstances to Avoid) [Kishnani et al 2006b].
  
  Enzyme replacement therapy (ERT) reduces cardiac mass to varying degrees and improves the ejection fraction, although there may be a transient decrease in the ejection fraction after the first several weeks of ERT [Levine et al 2008].
• Arteriopathy. Treatment does not differ from that in the general population.
• Conduction disturbances. Patients with hypertrophic cardiomyopathy are at high risk for tachyarrhythmia and sudden death [Tabarki et al 2002]. Twenty-four hour Holter monitoring is useful in characterizing the type and severity of rhythm disturbance. Management includes avoidance of stress, infection, fever, dehydration, and anesthesia. Medical therapy, if indicated, often necessitates a careful balance of ventricular function and should be undertaken by a cardiologist familiar with Pompe disease.
  
  ERT results in an increase of the PR interval and a decrease in the left ventricular voltage [Ansong et al 2006].
• Muscle weakness. Physical therapy is appropriate to maintain range of motion and assist in ambulation.
  
  Proximal motor weakness can result in contractures of the pelvic girdle in infants and children, necessitating aggressive management including surgery [Case et al 2012]. Scoliosis is frequent, particularly in individuals with infantile or childhood onset disease [Roberts et al 2011].
• Nutrition/feeding. Infants may need specialized diets and maximal nutrition, with some requiring gastric feedings. Persons with late-onset disease may also develop feeding concerns and are often managed on a soft diet, with a few requiring gastric or jejunal feedings. Some of these manifestations may improve on ERT [Bernstein et al 2010].
• Respiratory insufficiency. Respiratory support including CPAP and BiPAP may be required. Inspiratory/ expiratory training has improved respiratory muscle strength in adults with late onset Pompe [Jones et al 2011].
  
  Macroglossia and severe respiratory insufficiency in the infantile form may necessitate tracheostomy.

Prevention of Primary Manifestations

Enzyme replacement therapy (ERT) should be initiated as soon as the diagnosis of Pompe disease is established.

The FDA approved the use of Myozyme^® (alglucosidase alfa) for infantile Pompe disease in 2006. Studies on later-onset forms of Pompe disease have been completed, and Lumizyme^® was approved by the FDA in 2010 for use in late-onset Pompe disease. Lumizyme is currently being studied in children with infantile onset Pompe.

Myozyme^® and Lumizyme^® are administered by slow IV infusion at 20-40 mg/kg/dose every two weeks.

In clinical studies, infusion reactions were observed in half of those treated with Myozyme^®. The majority of treated children developed IgG antibodies to Myozyme^® within the first three months of treatment. Infusion reactions appear to be more common in individuals with IgG antibodies. Development of IgE antibodies is less common but may be associated with anaphylactic reaction. Some affected individuals with high sustained IgG titers may have a poor clinical response to treatment (see Testing, Acid alpha-glucosidase protein quantitation). Most infusion-associated reactions can be modified by slowing the rate of infusion or administration of antipyretics, antihistamines, or glucocorticoids. However, anaphylaxis requiring life support measures has been reported. For these reasons – and because many individuals with Pompe disease have preexisting compromise of respiratory and cardiac function – initiation of therapy in centers equipped to provide emergency care is recommended. Transition to home infusion therapy is possible in those who have tolerated several months of infusions without clinical reaction. Home infusion therapy may have benefits to the affected individual and family, but the decision to initiate home infusions requires consideration of multiple factors, including venous access, medical stability, antibody status, and access to skilled home care providers.

Enzyme therapy in infants with known or suspected CRIM negative status requires immunomodulation very early in the treatment course, optimally before the first infusion. There are multiple immunomodulation protocols in use, most of which use rituximab with additional drugs (including mycophenylate mofetil, methotrexate, and sirolimus) [Messinger et al 2012, Elder et al 2013]. Genotyping may play a role in the rapid classification of CRIM status [Bali et al 2012]. However, clinical outcomes are poorer in infants who develop high titer anti-rhGAA antibodies, regardless of CRIM status [Banugararia et al 2011], and additional tools to identify those at risk for this outcome are needed.

In addition to drug-related reactions, children with infantile Pompe disease may have difficulty with anesthesia for procedures related to placement of venous access devices.

The relative resistance of skeletal muscle to effective glycogen depletion with administered alpha glucosidase has been observed in animals and humans, and varies with different enzyme preparations. Type II muscle fibers appear to be quite resistant to therapy. From a clinical viewpoint, the failure of most ventilator-dependent individuals to achieve independence from invasive ventilation is not surprising. Perhaps more disappointing is that five of the 18 infants treated before age six months made no meaningful gains in motor function and six of 18 became dependent on ventilatory assistance. A variety of predictors of poor prognosis were observed, including increase in muscle glycogen during therapy, high IgG titers to alpha glucosidase, and CRIM negativity.

In summary, when compared to an untreated cohort, a majority of those in whom ERT was initiated before age six months and before the need for ventilatory assistance had improved survival, improved ventilator-independent survival, reduced cardiac mass, and significantly improved acquisition of motor skills. Longer-term survivors who underwent early ERT may show sustained improvement in cardiac and motor function [Prater et al 2012]. Factors predicting poor response are incompletely understood and may only be deduced for any given individual by therapeutic trial. While the long-term prognosis is as yet unknown, available studies suggest better cognitive outcomes than had been predicted. Assessment of cognitive abilities is difficult in young children (age <5 years) with infantile Pompe disease, and typical assessment tools frequently underestimate the cognitive abilities of these children [Kishnani et al 2009, Nicolino et al 2009, Ebbink et al 2012].

In late-onset disease, the major morbidities are motor disability and respiratory insufficiency. In a randomized double-blind placebo-controlled study of 90 affected individuals age eight years and older who were ambulatory and free of invasive ventilatory support at baseline, those receiving the active agent had better preservation of motor function and forced vital capacity at the 78th week evaluation point [van der Ploeg et al 2010]. Similar findings were demonstrated in an open label trial [Strothotte et al 2010].

Prevention of Secondary Complications

Infections need to be aggressively managed.

Immunizations need to be kept current.

Patients and household members should receive annual influenza vaccinations.

Respiratory syncytial virus (RSV) prophylaxis (palivizumab) should be administered in the first two years.

Anesthesia should be used only when absolutely necessary because reduced cardiovascular return and underlying respiratory insufficiency pose significant risks.

Surveillance

Close follow up is indicated. Management and surveillance guidelines have been proposed by the ACMG Work Group on Management of Pompe Disease [Kishnani et al 2006b]:

• Twice-yearly clinical review of development, clinical status, growth, and use of adaptive equipment
• Assessment of respiratory status with each visit with regard to cough, difficulty breathing, wheezing, fatigability, and exercise tolerance:
  • Chest x-rays at regular intervals
  • Pulmonary function tests; yearly or more frequently as indicated
  • Periodic sleep evaluation, which may include regular capnography and pulse oximetry
• Monitoring of overall musculoskeletal and functional status to guide therapies aimed at preventing or minimizing physical impairment and its complications. This may include assessment for scoliosis and bone densitometry.
• Regular nutritional and feeding assessment
• At least annual renal function studies to monitor for secondary complications related to cardiac and/or pulmonary impairment as well as medication effects
• Annual cardiology evaluation in late-onset disease and as needed for infantile-onset disease:
  • Periodic echocardiography. Aortic dilatation has been detected by echocardiography in late-onset Pompe disease [El-Gharbawy et al 2011].
  • Twenty-four-hour ambulatory ECG at baseline and at regular intervals [Cook et al 2006]
  • Screening for cerebral arteriopathy with aneurysmal dilation and rupture leading to cerebral infarcts (strokes) and death, which have also been reported [Laforêt et al 2008, Sacconi et al 2010]. Screening strategies for these findings are being developed, but care teams should have a high index of suspicion for cerebral arteriopathy if an individual with late-onset Pompe disease develops unexplained stroke-like symptoms [Sacconi et al 2010].
• Annual hearing evaluation

Agents/Circumstances to Avoid

Use of standard drugs for treatment of cardiac manifestations may be contraindicated in certain stages of the disease. The use of digoxin, ionotropes, diuretics, and afterload-reducing agents may worsen left ventricular outflow obstruction, although they may be indicated in later stages of the disease.

Hypotension and volume depletion should be avoided.

Exposure to infectious agents is to be avoided.

Evaluation of Relatives at Risk

It is appropriate to offer sibs of a proband either testing of GAA enzyme activity or molecular genetic testing (if the disease-causing mutations have been identified in an affected family member) so that morbidity and mortality can be reduced by early diagnosis and treatment with ERT.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Pregnancy Management

Most individuals with infantile Pompe disease have not reproduced. Many adults with late-onset disease have reproduced. At least one woman treated with ERT during pregnancy and lactation with no adverse effects on the fetus has been reported [de Vries et al 2011]. As would be expected in a woman with a myopathy and respiratory insufficiency, the growing fetus may pose additional complications to the mother’s health. Careful respiratory and cardiac surveillance should be initiated in consultation with maternal fetal medicine specialists.

Therapies Under Investigation

Gene therapy to correct the underlying enzyme defect is under investigation [Raben et al 2002, DeRuisseau et al 2009, Mah et al 2010]. A Phase I/II trial to investigate the ability of AAV- alpha glucosidase to improve ventilation is currently in progress [Byrne et al 2011].

Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.

Other

Experience with bone marrow transplantation in both humans and cattle with acid alpha-glucosidase deficiency is limited; to date, such treatment is not considered successful [Hirschhorn & Reuser 2001].

Mode of Inheritance

Glycogen storage disease type II (GSD II; Pompe disease) is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

• In most instances, the parents of a proband are heterozygotes and thus carry a single copy of a disease-causing mutation in GAA.
• Heterozygotes (carriers) are asymptomatic.

Sibs of a proband

• At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
• Once an at-risk sib is known to be unaffected, the chance of his/her being a carrier is 2/3.
• Heterozygotes are asymptomatic.

Offspring of a proband

• Children with the more common infantile form of GSD II do not reproduce, although the availability of therapy may alter this expectation through improved fitness of those individuals who respond to enzyme replacement therapy.
• The offspring of an individual with a later-onset form of GSD II are obligate heterozygotes (carriers) for a disease-causing mutation in GAA.

Other family members of a proband. Each sib of an obligate heterozygote is at a 50% risk of being a carrier.

Carrier Detection

Biochemical genetic testing. Measurement of acid alpha-glucosidase enzyme activity in skin fibroblasts, muscle, or peripheral blood leukocytes is unreliable for carrier determination because of significant overlap in residual enzyme activity levels between obligate carriers and the general (non-carrier) population.

Molecular genetic testing. Carrier testing for at-risk family members is possible if the disease-causing mutations in the family have been identified.

Related Genetic Counseling Issues

See Management, Evaluation of Relatives at Risk for information on evaluating at-risk relatives for the purpose of early diagnosis and treatment.

Concordance/discordance of phenotype in family members

• The sib pair concordance in the infantile-onset form of Pompe disease is high in children with null mutations [Hirschhorn & Reuser 2001].
• Because intergenerational phenotypic variation has been reported in several families with Pompe disease, careful genetic counseling is advised [Hoefsloot et al 1990].

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, carriers, or at risk of being carriers.

DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, mutations, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals.

Prenatal Testing

Molecular genetic testing. Prenatal diagnosis for pregnancies at increased risk is possible by analysis of DNA extracted from fetal cells obtained by amniocentesis usually performed at approximately 15 to 18 weeks' gestation or chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation. Both disease-causing alleles of an affected family member must be identified before prenatal testing can be performed. Results of prenatal testing cannot predict the age of onset, clinical course, or degree of disability.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Biochemical genetic testing. Prenatal testing is possible by measuring GAA enzyme activity in uncultured chorionic villi or amniocytes; however, molecular genetic testing is the preferred method if the familial mutations are identified.

Preimplantation genetic diagnosis (PGD) may be an option for some families in which the disease-causing mutations have been identified.

Published Guidelines/Consensus Statements

1. American College of Medical Genetics. Pompe disease diagnosis and management guideline. Available online (pdf). 2006. Accessed 5-3-13.
2. Cupler EJ, Berger KI, Leshner RT, Wolfe GI, Han JJ, Barohn RJ, Kissel JT. AANEM Consensus Committee on Late-onset Pompe Disease.; Consensus treatment recommendations for late-onset Pompe disease. Muscle Nerve. 2012;45:319–33. [PMC free article: PMC3534745] [PubMed: 22173792]

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Suggested Reading

1. American Association of Neuromuscular & Electrodiagnostic Medicine; Diagnostic criteria for late-onset (childhood and adult) Pompe disease. Muscle Nerve. 2009;40:149–60. [PubMed: 19533647]
2. Hirschhorn R, Reuser AJJ. Glycogen storage disease type II: (acid maltase) deficiency. In: Scriver CR, Beaudet AL, Sly WS, Valle D, Vogelstein B, eds. The Online Metabolic and Molecular Bases of Inherited Disease (OMMBID). New York: McGraw-Hill. Chap 135. Available online. Accessed 5-3-13.

Author Notes

Web: STAR Center for Lysosomal Diseases

Revision History

• 9 May 2013 (me) Comprehensive update posted live
• 12 August 2010 (me) Comprehensive update posted live
• 5 August 2008 (cd) Revision: deletion/duplication testing available clinically
• 22 April 2008 (cd) Revision: targeted mutation analysis no longer available clinically
• 31 August 2007 (me) Review posted to live Web site
• 8 January 2007 (btt) Original submission