Muscle Biopsy as a Diagnostic Tool in Pompe Disease

Abstract

Today, myopathological, biochemical and genetic characteristics of Pompe disease are well established, and many reports have been published following the first very precisely documented myopatholgical description of adultonset Pompe disease in 1968. The clinical awareness and need for diagnostic precision of Pompe disease has increased since enzyme replacement therapy (ERT) has become widely available. Primary characteristic myopathological findings of Pompe disease are rounded vacuoles that are irregular in shape and size. All infantile biopsies show pronounced vacuolisation of almost all fibres. In juvenile biopsies, the percentage of vacuolated fibres decreases to 75%, and in most adult patients only 10–50% of the fibres reveal vacuolisation. In infantile cases, both fibre types are equally affected, while in juvenile and adult cases a predominant vacuolisation of type 2 fibres occurs. The second myopathological hallmark is periodic acid-Schiff (PAS)– positive material packed in these vacuoles, sometimes lost due to fixation procedures and therefore sometimes only notable as optically empty vacuolar spaces. Thirdly, acid phosphates histochemistry reveals positive staining in vacuolated fibres, indicating lysosomal dysfunction. In addition to these classic findings, autophagic vacuoles, sometimes containing heterogeneous material including cytoplasmic degradation products and myeloid structures, are seen at all ages, especially in type 2 fibres. Ultra-structurally, these autophagic vacuoles contained particulate glycogen and/or autophagic debris, including myelin-like figures. Almost all samples show lipofuscin accumulation in the area of lysosomal residual bodies and rarely lipid droplets as a sign of lysosomal ageing. Infrequently, single rimmed vacuoles are notable. Fibre splitting is an uncommon aspect. Due to excessive fibre vacuolisation, a subsarcolemmal packing of mitochondria is notable in most cases. Single fibre necrosis, basophilia and phagocytosis may be present in biopsies of all ages. Furthermore, mild fibrosis and fatty degeneration may be found in about 10% of the biopsies. Immunohistochemistry does not normally present signs of an inflammatory myopathy. In selected biopsies, some macrophages may be noticed, and vacuoles may contain caveolin-3 and dysferlin positive membrane debris. Although all classic hallmarks of Pompe disease are well documented over several decades, Pompe disease in adult-onset patients may still be missed histologically, especially when a clinically unaffected muscle is chosen for biopsy. Consequently, in modern diagnostics, dry blood spot (DBS) testing followed by -glucosidase (GAA) gene nalysis helps to overcome this muscle biopsy neglect. However, in some adult patients presenting with a blurred imb girdle weakness and atrophy, combined muscle MRI, light and electron microscopy, and biochemical analysis ay be still essential for making an accurate diagnosis of Pompe disease.