重症筋無力症と鑑別を要したクッシング病の1例

The patient is a 77-year-old woman with a history of diabetes mellitus that was refractory to the medication and dietary restrictions. Four months prior to the admission, she developed a dropped head and ptosis that worsened in the evening. These symptoms were improved by the edrophonium test and the 3 Hz repetitive nerve stimulation testing was positive ; nevertheless, anti-acetylcholine and anti-muscle-specific tyrosine kinase antibodies were negative. Further examination demonstrated sustained hypokalemia and high levels of cortisol and ACTH. Moreover, CRH and high-dose dexamethasone suppression testings were positive and MRI demonstrated pituitary microadenoma. Based on these findings, she was subsequently diagnosed with Cushing’s disease. After the resection of the pituitary tumor, ptosis improved with an alleviation of systemic edema, suggesting that it was caused by an eyelid edema. This case uniquely illustrates that Cushing’s disease may mimic myasthenia gravis. Differentiation of the two disorders is crucial as treatment with steroids could compromise the interpretation of diagnostic testings for Cushing’s disease and might result in a disease exacerbation. In this case, the history of treatment-refractory diabetes mellitus was helpful cue to differentiate the two disorders.