Juvenile seropositive myasthenia gravis with anti-MuSK antibody after thymectomy.

We report the case of a 17.5-year old girl with generalized myasthenia gravis (MG). When she was 13, she started to complain of episodic diplopia, ptosis and mild fatigability of upper and lower extremity muscles. She was diagnosed with MG 3 years later, after exacerbation of her limb muscle weakness. Acetylcholine receptor antibody was positive (2.56 nmol/l, n < 0.4 nmol/l), anti-MuSK antibodies were not tested at that time. She was treated with a thymectomy. After the operation a gradual worsening of her MG was seen; she reported pirydostygmine intolerance. On retesting, acetylcholine receptor antibodies were negative and she was positive for anti-MuSK antibodies. She received immunosuppressive treatment with prednisone and azathioprine with clinical improvement. She is the third reported patient diagnosed with acetylcholine receptor antibody-positive MG, who became MuSK-positive after thymectomy. Recognition of such clinical patterns may be important for decision making in myasthenia gravis patients.