This paper describes how changes in sports performance helped reveal and track juvenile myasthenia gravis in an elite teenage swimmer. Early symptoms did not appear as obvious muscle weakness. Instead, the first sign was a measurable decline in race times and difficulty breathing during competition. High fitness levels masked classic features and led to delayed diagnosis and initial misdiagnosis as asthma and vocal cord dysfunction.

Once diagnosed, treatment included thymectomy, immunotherapy, and medication adjustments. Swim performance was tracked alongside clinical symptoms and antibody levels. Race times closely reflected disease activity, treatment response, and medication efficacy. Performance decline signaled relapse during steroid tapering, while improvement aligned with clinical remission.

The case shows how objective sports data can support diagnosis, guide treatment decisions, and monitor recovery in pediatric athletes with myasthenia gravis. It also demonstrates that with appropriate treatment, young patients can return to high-level sport and exceed pre-diagnosis performance.