Diagnostic Overlap of Pompe Disease and Myotonic Dystrophy Type 1 in an Adult Patient: A Case Report

Background

Pompe disease and myotonic dystrophy type 1 are inherited neuromuscular disorders that can both cause progressive weakness and respiratory compromise. Their overlapping phenotypes can obscure a second diagnosis, particularly when an initial workup appears to explain the presentation. We report an adult patient in whom findings supporting Pompe disease and myotonic dystrophy type 1 were identified concurrently.

Case Presentation

A 34-year-old woman presented with frequent falls, distal-predominant weakness, upper-limb stiffness, dyspnea, and impaired daily functioning. Examination demonstrated ptosis, myotonia, generalized weakness with severe distal involvement, Gowers sign, and steppage gait. Respiratory testing suggested diaphragmatic weakness. A dried-blood-spot assay screened positive for reduced acid alpha-glucosidase activity. GAA sequencing identified the pathogenic c.-32-13T>G variant in the heterozygous state, and leukocyte testing showed markedly reduced GAA activity. Enzyme replacement therapy with alglucosidase alfa was initiated. Electromyography demonstrated myopathic motor units and myotonic discharges. A brother with a similar phenotype was subsequently diagnosed with myotonic dystrophy type 1 after DMPK testing detected an allele with more than 100 CTG repeats. Given the family history, myotonia, and persistent weakness despite therapy, the patient underwent DMPK testing, which also detected an allele with more than 100 CTG repeats.

Findings

The combined clinical, electrophysiologic, enzymatic, and molecular findings supported diagnoses of Pompe disease and myotonic dystrophy type 1 in the same patient. Alglucosidase alfa improved fatigue, but weakness persisted. Repeat leukocyte testing continued to show reduced GAA activity.

Discussion and Conclusion

This case illustrates how phenotypic overlap between neuromuscular disorders can delay recognition of a coexisting condition. Persistent or discordant findings should prompt reassessment even after an initial diagnosis. Family evaluation and targeted molecular testing were decisive in identifying myotonic dystrophy type 1 and refining the interpretation of ongoing weakness. A systematic approach combining phenotype, family history, electrophysiology, enzyme assays, and genetic testing may reduce diagnostic anchoring in patients with complex neuromuscular presentations.