[A case of postural hypoxemia with a final diagnosis of myasthenia gravis].

We present a case of postural hypoxemia with a final diagnosis of myasthenia gravis (MG). A 62-year-old man experienced double vision in his left eye from December 2008 and received a diagnosis of diabetic neuropathy. From mid-December he began to experience breathing difficulties at night when in a supine position and was admitted to our hospital. Bilateral diaphragmatic elevation was observed on a chest X-ray film, and lower lung atelectasis and an anterior mediastinal tumor were observed on chest CT. However, his breathing difficulties only occurred when he was in a supine position. Therefore, we performed blood gas analysis in supine and sitting positions. Hypoxemia, hypercapnia and an increase in A-aDO2 were observed in the supine position, leading to a diagnosis of postural hypoxemia. Due to the exacerbation of his double vision, the patient was referred to the ophthalmology and neurology departments where he tested positive for anti-acetylcholine receptor antibodies and also on a tensilon test, resulting in a final diagnosis of MG. During the tensilon test, the patient's breathing difficulties in the supine position improved, and therefore his postural hypoxemia was thought to have resulted from diaphragmatic muscle weakness as a result of MG. MG respiratory failure is typically of the acute fulminating type and is considered to be a critical condition. However, it should be noted that there are cases, such as the present one, in which MG presents as postural hypoxemia.