THUR 213 An unusual case of cluster acetylcholine receptor antibody positive myasthenia gravis

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A 73-year-old builder presented with fatigable ptosis and ophthalmoplegia. He was started on pyridostigmine for presumed ocular myasthenia gravis (MG). Despite making an initial improvement he subsequently developed dysarthia, dysphonia and a head drop within 3 weeks. At this time, he also had mild proximal weakness and marked asymmetrical distal weakness of the upper limbs and mild distal lower limb weakness.Conventional serological screening for acetylcholine receptor (AChR) and muscle specific kinase antibodies were negative. However, extended testing using a cell-based cluster assays demonstrated AChR antibody positivity. Repetitive nerve stimulation demonstrated significant decrement more prominent in distal muscles.Despite high dose prednisolone, azathioprine, several courses of IV Immunoglobulins and plasma exchange the patient has had frequent admissions to the high dependency and intensive care setting with respiratory failure.Cluster AChR antibody positive MG has been described only within the last decade and is usually mild and treatment responsive. This is the first case reported to have required intubation and ventilation due to respiratory failure. This case highlights the importance of extending the routine panels for antibody testing in patients with ‘seronegative’ MG and the need to remain vigilant in patients with cluster-AChR positive disease.

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