Chemotherapy-induced myasthenia gravis presenting as isolated bulbar palsy in a patient with WHO type B3 thymoma

RATIONALE: Myasthenic crisis rarely manifests exclusively as isolated bulbar palsy in patients with WHO type B3 thymoma undergoing neoadjuvant chemotherapy, often leading to misdiagnosis as chemotherapy toxicity or infection. PATIENT CONCERNS: A 55-year-old man with an invasive anterior mediastinal mass developed progressive dysphagia and dysarthria 5 days after the first cycle of CAP (cyclophosphamide, doxorubicin, cisplatin) chemotherapy, raising concerns for acute neurological deterioration. DIAGNOSES: Neurological examination localized to bulbar involvement. Elevated anti-acetylcholine receptor antibodies (1.03 nmol/L), a positive ice-pack test, and objective improvement with the neostigmine test confirmed a chemotherapy-induced myasthenic crisis. A core-needle biopsy had previously established a WHO type B3 thymoma diagnosis. INTERVENTIONS: The patient was placed nil by mouth with nasogastric decompression and correction of cisplatin-associated hypomagnesemia. Immunomodulation included pulsed methylprednisolone, intravenous immunoglobulin, and oral pyridostigmine. After symptom resolution, he completed a second CAP cycle followed by R0 video-assisted thoracoscopic thymectomy and adjuvant radiotherapy (60 Gy). OUTCOMES: Bulbar symptoms resolved completely within 4 days of immunomodulation. At 29 months of follow-up, neither local tumor recurrence nor myasthenia gravis relapse was observed, with excellent performance status. LESSONS: Isolated bulbar palsy can be the sole inaugural sign of chemotherapy-triggered myasthenic crisis. Clinicians should rapidly screen for acetylcholine receptor antibodies in thymoma patients receiving cytotoxic therapy. Integrating baseline serological risk assessment before chemotherapy may prevent life-threatening delays in care.