Syndrome of Inappropriate Antidiuretic Hormone Associated with Eosinophilic Granulomatosis with Polyangiitis

A 78-year-old woman with a history of bronchial asthma presented with distal dominant sensory disturbance and weakness in the upper and lower extremities. A biopsy of the left peroneus brevis muscle showed active vasculitis with inflammation extending into muscle fascicles and fibrinoid necrosis of the vessel wall, consistent with eosinophilic granulomatosis with polyangiitis (EGPA). Despite her decreased serum osmolarity, her serum antidiuretic hormone level was not reduced, consistent with the syndrome of inappropriate antidiuretic hormone (SIADH). Intravenous and oral steroid therapy improved her neurological symptoms. Clinicians should consider EGPA as a concurrent, and potentially causative, disorder in cases of SIADH.