Eosinophilic granulomatosis with polyangiitis as a rare cause of the syndrome of inappropriate antidiuretic hormone secretion.

Eosinophilic granulomatosis with polyangiitis (EGPA, Churg-Strauss syndrome) is a rare multisystem necrotizing vasculitis that involves small- to medium-sized blood vessels. We report a rare case of syndrome of the inappropriate antidiuretic hormone (ADH) secretion (SIADH) secondary to EGPA. A 53-year-old man applied with complaints of pain in the large joints and morning stiffness in knee for 2 months. The patient had the history of impaired fasting glucose, asthma, nasal polyps, and urticaria. Physical examination revealed intrinsic muscle atrophy and weakness in the right hand. Peripheral eosinophil count was 9.78 × 109/L (0.02-0.5), erythrocyte sedimentation rate 39 mm/h (0-20), and C-reactive protein 5.77 mg/dL (0-0.5). Migratory ground-glass pulmonary opacities had been reported in previous chest computed tomography scans. Echocardiography revealed findings compatible with eosinophilic involvement. Electroneuromyographic evaluation showed acute distal axonal neuropathy of right ulnar nerve. EGPA was considered. Oral methylprednisolone treatment was initiated. Intravenous immunoglobulin (IVIG) and cyclophosphamide treatment and gradual tapering of oral steroids were planned. In 24-h urine analysis, sodium was 387 mEq, creatinine was 1156 mg, and volume was 3000 mL. When his medical records were investigated, it was observed that hyponatremia was present for nearly 2 years. While serum osmolality was 270, urine osmolality was 604 mOsm/kg H2O. So, SIADH diagnosis was made. Fluid intake was restricted. Although the patient's sodium level did not return to normal, it rose up to 130 mEq/L. After second cycle of EGPA treatment (cyclophosphamide and IVIG), serum sodium was normal. There is only four other documented cases of SIADH associated with EGPA. We hypothesized that blood supply to the hypothalamus and/or posterior hypophysis might be affected from EGPA vasculitis. Here, in this case, with effective treatment of EGPA, SIADH was resolved which implies a causality between two conditions.