To describe a case of RBCK1-associated polyglucosan body myopathy presenting with a mild phenotype characterized by myalgia and mild limb weakness.

Glycogen storage diseases are characterized by the abnormal accumulation of glycogen in cells, and can present as myopathy or as a multisystemic disorder. Polyglucosan body disease is characterized by presentations including myopathy, neuropathy and myelopathy with prominent bladder dysfunction. Mutations in RBCK1 have been more recently described as causative of a form of polyglucosan body myopathy, sometimes accompanied by cardiomyopathy and immunodeficiency.

Clinical Case

A 39-year-old female presented for evaluation of lower extremity pain, cramping, and difficulty walking. Her examination was notable for large muscle bulk in the calves, hyporeflexia, and normal muscle power on confrontational motor testing. Her CK level was normal. Multiple prior EMG studies revealed the presence of diffuse abnormal spontaneous activity characterized by myotonic discharges. Family history was significant for a paternal half-sister with weakness and cardiomyopathy, whose genetic testing revealed a homozygous mutation in RBCK1.

A muscle biopsy was performed of the vastus lateralis, revealing a myopathy with polyglucosan bodies. Sequencing of the RBCK1 gene was done, proving two pathogenic mutations in the RBCK1 gene, c.1308G>T and c.513_514delinsA. The second mutation was novel.

This case report broadens the phenotypic description of RBCK1-associated polyglucosan body disease. In this case, the presentation was particularly mild with little objective weakness and a normal CK level. Immunodeficiency was not evident. It has been proposed that mutations in the C-terminus of the protein cause a myopathic phenotype, with mutations closer to the N-terminus presenting as immunodeficiency. Although the patient’s symptoms were fairly mild, the family history of a previously diagnosed RBCK1-related disorder was important in guiding the specific genetic testing.