[Three cases of respiratory failure of collagen diseases accompanied by syndrome of inappropriate secretion of antidiuretic hormone (SIADH)].

We experienced three patients who have collagen diseases with respiratory failure accompanied by hyponatremia. They were one systemic lupus erythematosus patient with interstitial pneumonia, one rheumatoid arthritis patient with acute pneumonitis, and one dermatomyositis patient with pulmonary fibrosis and organizing pneumonia. In all 3 patients, hyponatremia appeared along with a decrease in arterial O2 partial pressure (PaO2) and the hyponatremia tended to improve when the PaO2 increased after inhalation of oxygen, even though their respiratory failure were not improved. In dermatomyositis patient, serum Na levels were over-corrected after increase in PaO2. The serum and urine osmolality, serum antidiuretic hormone (ADH) levels and clinical pictures demonstrated a presence of inappropriate secretion of ADH (SIADH) in all 3 cases when hyponatremia and hypoxia appeared. A close association between hyponatremia and hypoxia observed in 3 patients strongly suggested that their SIADH were associated with hypoxia since SIADH could be demonstrated by hypoxia. Therefore, it is important to realize that hypoxia-induced hyponatremia will be promptly corrected to hypernatremia by an oxygen inhalation, which could cause a lethal central pontine myelinolysis.