Additional Investigation After Muscle Biopsy Diagnosis

Given the combination of limb-girdle weakness, prominent respiratory muscle weakness, and the muscle biopsy findings, the patient’s blood sample was sent for dried blood spot assay, which showed a markedly reduced acid α-glucosidase activity at 2.8 pmol/punch/hour (normal: ≥10) with a normal neutral α-glucosidase activity at 110.35 pmol/punch/hour (normal: 23.8–132.6). The subsequent sequencing of the lysosomal acid alpha-glucosidase (GAA) gene revealed a c.-32-13 T > G sequence alteration on one allele and a deletion of exon 18 on the other; both were known pathogenic.

Final Diagnosis

Late-onset Pompe disease

Patient Follow-up

The patient required tracheostomy due to the inability to be weaned off the ventilator. He was started on enzyme replacement therapy (ERT), receiving alglucosidase alfa (20 mg/kg body weight) biweekly. After 12 months, his exercise tolerance, pulmonary function tests, and proximal limb weakness all improved. The patient required assist control ventilation at night but did not require any ventilator support during the day. The ERT was continued. His lowest forced vital capacity (FVC) during the hospitalization was 0.7 Liter (L) which improved to 2.03 L (53% predicted) 2 years after the hospitalization and enzyme replacement therapy.

Discussion

Pompe disease, also known as acid maltase deficiency or glycogenosis type II , is a rare autosomal recessive metabolic disorder caused by a partial or a complete deficiency in lysosomal acid α-1,4-glucosidase activity. The disease has multisystem involvement, including skeletal muscle. The inability to metabolize glycogen leads to intralysosomal and cytoplasmic build-ups of glycogen in skeletal muscle causing glycogen storage myopathy. The two forms of the disease are infantile-onset and late-onset [1, 2].

Late-onset Pompe disease (LOPD) has an estimated frequency of 1:57,000 according to one study [3]. It presents after 1 year of age, most commonly between the second and sixth decade with slowly progressive limb-girdle weakness and respiratory muscle weakness but usually without significant cardiac involvement, although Wolff-Parkinson-White syndrome has been reported [4–6]. It tends to more affect axial muscles and respiratory muscles with more than 70% of patients progressing into respiratory failure, which is the most common cause of death [7]. Although limb-girdle weakness is the most common presenting symptom, respiratory insufficiency can precede the onset of limb weakness [6]. Since the presentation is mild and non-specific at the early stage, the diagnosis can be delayed for years or missed entirely [6, 8], as seen in our case. In one retrospective study, 48% of patients carried wrong diagnoses initially [8].

Diagnosis of LOPD requires a high clinical suspicion based on the clinical presentation and test findings. Serum CK can be mildly elevated or normal in LOPD. NCS is usually normal. Needle EMG often shows myopathic motor unit potentials with increased membrane irritability and myotonia [1, 6], which can be restricted to the paraspinal muscles [9]. Limb muscle EMG can be normal in LOPD as seen in our case, and paraspinal muscles should be examined when LOPD is suspected [10]. Although patients may have EMG myotonia, they do not have clinical myotonia.

Muscle biopsy in Pompe disease typically shows a vacuolar myopathy with increased acid phosphatase activity and increased sarcoplasmic glycogen accumulation. EM often shows glycogen granules accumulated freely in sarcoplasm and/or in membrane bound lysosomes [11, 12]. While infantile-onset Pompe disease usually shows many large sarcoplasmic vacuoles, LOPD often shows a few fibers containing small vacuoles or no vacuoles at all [13–16], which makes the muscle biopsy diagnosis of LOPD quite challenging. It could be due to the lack of significant involvement of the limb muscles biopsied. Glycogen accumulation can be evaluated by PAS stain and EM, but glycogen can be washed out by the preparation in some cases. Therefore, negative muscle biopsy does not exclude the diagnosis of LOPD. It has been shown that even in muscle biopsies of LOPD with no vacuolar changes, sarcoplasmic acid phosphatase-positive globular inclusions can be seen [15]. These inclusions contain lipofuscin pigments [16, 17]. The sarcoplasmic inclusions have been reported and described in all forms of Pompe disease [15, 16, 18–20]. They are different from cytoplasmic bodies as they do not contain Z-band materials and they are acid phosphatase-positive. The acid phosphatase-positive globular inclusion is a useful diagnostic marker especially for LOPD [15].

Diagnosis of LOPD has become easier owing to the development of a fast and reliable screening test, dried blood spot assay [21]. It is the gold standard for diagnosing Pompe disease [22]. The test measures the acid α-glucosidase activity, which is reduced but not absent in LOPD. It is important to also measure the neutral α-glucosidase activity, which should be normal in LOPD, to ensure the quality of the specimen. The common practice now is to order dried blood spot first, which is relatively inexpensive. If it shows reduced acid α-glucosidase activity, then proceed with the GAA gene test. More than 200 mutations have been reported [1]. The GAA mutations discovered in our patient are common. Deletion of exon 18, which leads to a complete loss of the enzyme catalytic function, is commonly seen in the infantile form. The c. -32-13 T > G mutation is seen in over 50% of LOPD. This mutation changes a single nucleotide within the first intron of the GAA gene, which affects the splicing of the exon 2.

Early diagnosis of LOPD is essential because enzyme replacement therapy (ERT) is available to improve or stabilize LOPD [23–26]. Early diagnosis requires the awareness of this disease. It has been advocated that the dried blood spot assay should be ordered for every patient with unexplained limb-girdle weakness but prominent axial and respiratory muscle weakness, especially when EMG shows myotonia in paraspinal muscles [8]. Normal CK, normal limb muscle EMG, and lack of characteristic findings on limb muscle biopsy, however, do not exclude LOPD. Muscle biopsy can be spared as the dried blood spot assay is fast, reliable, inexpensive, and non-invasive. The subsequent GAA gene test is definitive.

Recombinant human acid alpha-glucosidase (rhGAA) was developed in 1990s. Alglucosidase alfa (Myozyme) was approved by the U.S. Food and Drug Administration (FDA) and the European Medicines Agency in 2006 for treating Pompe disease, and Lumizyme was approved in 2010 for treating LOPD in the USA. The randomized Late-Onset Treatment Study (LOTS) showed that biweekly infusion of alglucosidase alfa (20 mg/kg body weight) improved limb and respiratory functions [26]. The open-labeled exploratory muscle biopsy, imaging, and functional assessment (EMBASSY) showed that the muscle tissue glycogen was reduced and the limb and respiratory functions were improved in adult patients with LOPD after 6 months of the alglucosidase alfa treatment [25]. Although both studies only included ambulatory patients without invasive ventilation and no trials have been done in patients with severe LOPD as seen in our case, the consensus committee of the American Association of Neuromuscular & Electrodiagnostic Medicine (AANEM) recommends treating these patients with alglucosidase alfa for at least 1 year, and the ERT should be continued if no decline in symptoms or signs [27]. The side effects of alglucosidase alfa are mild to moderate, and the hypersensitivity should be monitored especially in patients who develop a high alpha-glucosidase antibody titer [27]. A new therapy, reveglucosidase alfa , has been developed to improve the lysosome uptake of the enzyme [28]. Future studies will test whether this new treatment is safe and more effective.

Pearls