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.

重症肌无力是一种可治疗的自身免疫性疾病，由针对神经肌肉接头处膜蛋白的自身抗体引起。虽然乙酰胆碱受体抗体是最常见的，但少数患者具有针对肌肉特异性激酶（MuSK 抗体）的抗体。鉴别特征通常包括延髓、呼吸和颈部伸肌的亚急性发作和快速进展，同时保留远端附肢肌肉，最常见于中年女性。在这里，我们展示了一名年轻男性的 MuSK 抗体肌无力综合征的非典型表现，包括渐进性、隐匿性进行性、非疲劳性和非波动性眼部、延髓和食道肌无力，额肌单纤维肌电图正常。该病例在临床上类似于预后不良的线粒体肌病（线粒体神经胃肠道脑病-MNGIE）。由于非典型表现，MuSK 抗体在病程后期才被发现，此时患者对免疫疗法的反应非常好。我们报告了一种不常见但可治疗的疾病的不寻常表现，说明 MuSK 抗体肌无力综合征中可能存在显着的表型异质性。