Introduction

Myasthenia gravis (MG) is characterized by muscle weakness with diurnal fluctuation. Usually, MG is caused by autoimmune mechanism and most of them have acetylcholine receptor (AChR) antibody. Recently other autoantibodies, such as muscle-specific receptor tyrosine kinase (MuSK) antibody and lipoprotein receptor-related protein 4 (LRP4) antibody, have been found in MG. The case with anti-MuSK antibody usually reported to occur in young female adults or female infants or toddlers, and to have prominent oculobulbar symptoms.

Case

A 13-year-old girl was referred to the department of pediatric neurology due to recently developed bilateral ptosis. The symptom gradually worsened over time and accompanied with diplopia and dysarthria. On neurologic examination, muscle tone and deep tendon reflexes were intact, but extraocular movement was limited, especially in lateral gaze. Magnetic resonance imaging of the brain showed no abnormality. Neostigmine test showed positive response, and repetitive nerve stimulation test revealed a decrement of compound muscle action potential. Serum creatine kinase was normal, and anti-AChR antibody was negative. But, anti-MuSK antibody was positive (65.1nmol/L, reference value ≤0.02). Initially, pyridostigmine was prescribed, and steroid was added at day 8, and then azathioprin was added at day 20. She showed complete recovery at day 28 on medication.

Conclusion

We report a case of anti-Musk-antibody-positive ocular MG in a 13-year-old-girl who showed complete improvement with immunosuppressant.