Transient worsening of thyrotoxic myopathy following methimazole and metoprolol initiation in a 12-year-old girl: a case report and literature review.

Objectives: Thyrotoxic myopathy (TM) is a muscle disorder associated with hyperthyroidism. Although TM is common in adults, its incidence in children is unknown due to the limited number of reports. TM usually improves with the treatment of hyperthyroidism. This is the first report of a patient with TM who experienced transient worsening of muscle weakness shortly after administration of methimazole (MMI) and metoprolol tartrate.

Case presentation: A 12-year-old Japanese girl with Graves' disease was administered MMI and metoprolol tartrate. Within 12 h of treatment initiation, the patient experienced difficulty in standing from a chair. Examination revealed proximal lower-limb weakness and reduced grip strength. Based on the patient's clinical course and blood test results, thyrotoxic periodic paralysis, myasthenia gravis, or polymyositis were considered unlikely. While the side effects of MMI and metoprolol tartrate were also considered as differential diagnoses, her history revealed mild pre-existing lower limb muscle weakness for 2 months before treatment, suggesting that the side effects of the medication were unlikely. Given the clinical course, the worsening of TM was the most probable cause, and treatment was continued cautiously. Muscle weakness gradually improved over 3 months as her thyroid hormone levels normalized. Magnetic resonance imaging taken 1 month later revealed gluteus muscle atrophy, which resolved within 10 months.

Conclusions: TM may show transient worsening after MMI and metoprolol tartrate administration, requiring the evaluation of TM, drug side effects, and other possible causes before continuing treatment. This case highlights the importance of recognizing TM in pediatric patients.