Abstract
Objective: To illustrate the diagnostic value of electromyography (EMG) in late-onset Pompe disease (LOPD) — a rare, treatable disease that can be difficult to distinguish from other neuromuscular disorders such as spinal muscular atrophy (SMA) and limb-girdle muscular dystrophy (LGMD). Case report: A 14-year-old girl presented with dyspnea, fatigue, flaccid quadriparesis, and respiratory failure requiring ventilation. History revealed exertional dyspnea since age 12, progressive worsening, foot drop, and recurrent pneumonia in early childhood. Examination showed distal-greater-than-proximal limb weakness, areflexia, and no clinical myotonia. The first needle EMG showed spontaneous activity with large, long-duration motor unit action potentials (MUAPs), initially suggesting SMA, but genetic testing for SMA was negative. A second EMG one month later revealed myotonic discharges alternating with complex repetitive discharges (CRDs) without clinical myotonia, raising suspicion of acid maltase deficiency (Pompe disease). Dried blood spot testing and genetic analysis confirmed compound heterozygous GAA gene variants, establishing the diagnosis of LOPD. The patient was treated with enzyme replacement therapy (Myozyme) and showed marked improvement: weaned off the ventilator after 4 months, discharged after 8 months, regained ambulation, and now requires ventilation only during sleep. Conclusion: LOPD, though rare, should be suspected in patients with progressive proximal muscle weakness, respiratory insufficiency, and hyperCKemia. EMG—less invasive and less costly than muscle biopsy—is a useful tool for suggesting LOPD when myotonic discharges are found without clinical myotonia. However, an initial EMG pattern of large, long-duration MUAPs may lead to misdiagnosis as SMA.
