Reversible very late onset myasthenia gravis following intravesical Bacillus Calmette-Guerine treatment: a case report

Myasthenia gravis (MG) is an autoimmune neuromuscular junction disorder occasionally triggered by immunomodulatory therapies. While immune checkpoint inhibitors are well-recognized culprits, traditional antineoplastic treatments like intravesical Bacillus Calmette-Guerine (BCG) can also precipitate severe immune-related adverse events. We describe a rare presentation of BCG-induced MG, emphasizing the crucial diagnostic role of clinical neurophysiology in evaluating atypical focal onset. A 76-year-old male with high-grade urothelial carcinoma underwent transurethral resection followed by a standard six-week induction of intravesical BCG. Twelve days after the final instillation, he developed isolated, severe neck extensor weakness resulting in dropped head syndrome (antecollis). Following an unrevealing cervical magnetic resonance imaging scan and the failure of empirical conservative management, a comprehensive neurophysiological evaluation was strictly performed. Concentric needle electromyography of the splenius capitis muscle revealed focal myopathic alterations, including early recruitment of small motor unit potentials, positive sharp waves, and fibrillation potentials. Crucially, low-frequency repetitive nerve stimulation (RNS) of the spinal accessory nerve, recording from the trapezius, demonstrated a significant, diagnostic decremental response of the compound muscle action potential amplitude. Serological assays confirmed highly elevated anti-acetylcholine receptor antibodies. Chest computed tomography excluded thymoma, establishing the diagnosis of very late-onset generalized MG (MGFA class IIb). Initial symptomatic monotherapy with pyridostigmine lacked efficacy. However, targeted therapy combining oral prednisone (0.5 mg/kg/day) and pyridostigmine induced a dramatic, rapid resolution of the antecollis and subsequent dysphagia. Corticosteroids were progressively tapered and definitively discontinued after six months, achieving sustained clinical remission without relapses. We report the third formally documented case of very late-onset generalized MG triggered by intravesical BCG immunotherapy, hypothetically mediated by pathogenic epitope sharing between mycobacterial and self-antigens. This case distinctly highlights the pivotal diagnostic utility of RNS in differentiating acute atypical axial weakness. Furthermore, it demonstrates that BCG-induced MG exhibits a highly favourable, fully reversible prognosis following prompt, targeted corticosteroid intervention.