Rheumatoid arthritis (RA) is an autoimmune disease characterized with symmetric synovitis and occasional extra articular involvement; also, some neurologic disorders can be observed during the disease course. Myasthenia gravis (MG) is one of them and it is an autoimmune disease of neuromuscular junction. It is caused by autoantibodies against neuromuscular junction proteins: the nicotinic acetylcholine receptor (AChR) and the muscle specific tyrosine kinase (MuSK). Very few studies have reported the associated autoimmune disorders in MuSK-MG. Here, we present the first patient who has MuSK antibody-positive MG gravis and rheumatoid arthritis.

A 53-year-old woman with RA presented with fatigue and fluctuating proximal muscle weakness. Her electroneurophysiological investigation resembled MG. Her AchR antibody level was normal but MuSK antibodies were high. After the acute treatment with plasmapheresis which lead to complete recovery in myasthenic symptoms, she is following with mycophenolate mofetil.

Concomitant autoimmune disorders are common in the population. MG should be considered in patients with an autoimmune disorder and developing new neuromuscular weakness.