Acid Maltase Deficiency Myopathy Workup
- Author: Stephen Kishner, MD, MHA; Chief Editor: Stephen Kishner, MD, MHA more...
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- Measurement of acid alpha-glucosidase enzyme activity in dried blood specimens is an optimal and reliable diagnostic test for acid maltase deficiency.
- Serum CK usually is elevated in the forms of the disease that affect younger patients, but CK can be within the reference range in the adult variety. CK levels can be as high as 2000 IU/L.
- Serum aspartate aminotransferase and lactic dehydrogenase can also be elevated.
- The tissue concentration level of acid maltase helps to establish a definite diagnosis.
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- In patients with acid maltase deficiency, chest radiographs reveal an enlarged, globular heart associated with pulmonary vascular congestion. Atelectasis can also be seen.
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- Brief, small-amplitude, polyphasic potentials or myopathic features, excessive electrical irritability, and pseudomyotonic discharges usually are seen on EMG.
- A myopathy is demonstrated.
- Neuropathic findings may be revealed.
- Pseudomyotonia may be seen in the absence of myotonia.
- Electrocardiographic findings include extremely tall and broad QRS complexes (representing ventricular depolarization) with a short PR interval (time between P wave and the beginning of the QRS complex), commonly less than 0.009 seconds. The short PR interval may be due to facilitated atrioventricular conduction as a result of myocardial glycogen deposition. A distinct left ventricular trabeculation can be seen on selective angiography.
- Prenatal/postnatal testing
- Prenatal diagnosis using chorionic villi or amniocentesis is available for the fatal infantile form. Decreased serum levels of acid maltase can be detected. Acid phosphatase levels also can be measured and will be elevated.
- Newborn screening for Pompe disease can be performed by determining the total acid alpha-glucosidase in plasma or dried blood spots. The sensitivity of these tests is 82-95%, and the specificity is 100%.
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- The diagnosis of acid maltase deficiency (AMD) usually is made based on the absence or reduction in the levels of alpha-acid maltase in muscle tissue or cultured skin fibroblasts. This deficiency usually is more pronounced in the infantile form than in the juvenile and adult forms.
- Muscle biopsy shows the presence of vacuoles that stain positive for glycogen. The large vacuoles contain material that is positive for the periodic acid-Schiff stain (typical of lysosomal glycogen storage). Acid phosphatase is increased, most likely because of a compensatory increase in the production of lysosomal enzymes. Electron microscopy reveals glycogen accumulation within the vacuoles and in the cytoplasm.
- Brain tissue samples from patients with AMD reveal swelling of the perinuclear space (perikaryal edema) caused by excessive storage of glycogen. These cells usually display absence or dispersion of Nissl substance. The cytoplasm may be foamy or stained irregularly, and the nucleus may not be displaced to the periphery. Storage-induced changes tend to be more diffuse or multifocal rather than localized.
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