MuSK not MNGIE: Atypical MuSK-antibody myasthenia presenting as a genetic disorder.

Myasthenia gravis is a treatable autoimmune disease caused by autoantibodies directed against membrane proteins at the neuromuscular junction. While acetylcholine receptor antibodies are most common, a minority of patients have antibodies directed against muscle-specific kinase (MuSK-antibody). Differentiating features often include subacute onset and rapid progression of bulbar, respiratory and neck extensor muscles, with sparing of distal appendicular muscles, most commonly in middle-aged females. Here we present an atypical presentation of MuSK-antibody myasthenic syndrome in a young male consisting of a gradual-onset, insidiously-progressive, non-fatigable and non-fluctuating ocular, bulbar and oesophageal weakness, with a normal frontalis single fibre EMG. This case clinically resembled a mitochondrial myopathy (Mitochondrial Neurogastrointestinal Encephalopathy-MNGIE) with a poor prognosis. Because of the atypical presentation, MuSK antibodies were identified very late in the disease course, at which point the patient responded very well to immunotherapy. We report an unusual presentation of an uncommon but treatable condition, illustrating significant phenotypic heterogeneity possible in MuSK-antibody myasthenic syndrome.