Delayed identification of MYBPC1-related myopathy in adolescence: Diagnostic value of resting tongue ultrasonography in myogenic tremor

Background

The MYBPC1 gene, which encodes the slow skeletal myosin binding protein-C, has recently been identified as the causative gene for a rare, mild myopathy associated with myogenic tremor

Case presentation

Our patient was a Japanese girl born full-term via normal delivery. No consanguinity or family history of neuromuscular disorders was reported. The patient's motor developmental milestones were slightly delayed. Independent walking was not achieved until 1 year and 7 months of age. Hand tremors had been observed since 2 years of age. The patient was not physically active and had been experiencing fatigue since 12 years of age. At her first visit to our hospital at 14 years of age, she presented with distal muscle weakness, a high-arched palate, decreased deep tendon reflexes, involuntary tongue movement resembling fasciculations, and mild scoliosis. However, the patient's nerve conduction velocity and electromyogram results were normal. Ultrasonographic examination of the tongue revealed rhythmic movement, consistent with tremors, which disappeared at rest. A heterozygous, missense pathogenic variant (c.[788T>G] (p.Leu263Arg)) in the MYBPC1 gene was identified using the TruSight One Sequencing Panel

Discussion

Muscle weakness is caused by the altered binding of the mutant MYBPC1 protein to myosin. However, the mechanism by which the dysfunction of the MYBPC1 protein, which is not expressed in the central or peripheral nervous system, leads to tremors remains unclear. The diagnosis can only be confirmed via genetic testing. However, tongue tremors may be mistaken for fasciculations. Electromyography and tongue ultrasonography may be useful in achieving an early diagnosis.