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We report the case of an 80-year-old male patient with a history of thymoma incidentally identified on follow-up thoracic imaging after a respiratory infection. Complete thymectomy revealed a WHO type B2 thymoma, Masaoka stage IIa.  One month postoperatively, the patient developed binocular diplopia. After six months, he reported dysphagia, and at nine months he was referred to neurology for dysarthria and fluctuating nasal speech. Neurological examination revealed bilateral ptosis, diplopia in primary gaze, and nasal voice exacerbated by fatigability maneuvers, with no additional relevant findings. Laboratory studies showed elevated TSH levels; brain and orbital MRI revealed no significant abnormalities. AChR antibodies were negative (<0.02 nMol/L) on two separate radioimmunoassays. Single-fiber electromyography of the orbicularis oculi muscle showed increased jitter (97.3 µs) and blocking, consistent with impaired neuromuscular transmission. A trial of cholinesterase inhibitors was poorly tolerated due to gastrointestinal adverse effects and was discontinued. Given the high clinical suspicion of myasthenic syndrome, oral methylprednisolone was initiated and titrated to 20 mg/day, resulting in marked improvement in approximately six weeks. Subsequently, anti–MuSK antibodies were tested and found to be positive at 1.53 nMol/L (positive >0.05 nMol/L) using radioimmunoassay. 

To the best of the authors’ knowledge, only five cases of thymoma-associated myasthenia gravis seronegative for AChR antibodies but positive for anti-MuSK antibodies have been reported in the literature to date.  Further studies are warranted to clarify the underlying mechanisms of this association.