Miller Fisher Syndrome With Syndrome of Inappropriate Antidiuretic Hormone Secretion: A Case Report

We report the case of a 70-year-old man with an incomplete form of Miller Fisher syndrome (MFS) complicated by syndrome of inappropriate secretion of antidiuretic hormone (SIADH). He presented with diplopia two weeks after a respiratory infection. Neurological examination showed bilateral pupil-sparing third nerve palsy, with normal reflexes, gait, coordination, power, and sensation. Serum sodium was markedly low (112 mmol/L) at presentation, with low serum osmolality, high urine sodium, and elevated urine osmolality. Anti-GQ1b IgG antibodies were positive, confirming the diagnosis of MFS. He was managed supportively with fluid restriction and oral sodium chloride tablets. He did not receive intravenous immunoglobulin. Symptoms resolved completely after eight weeks. This case underscores the importance of recognizing incomplete presentations of MFS and highlights the rare but clinically significant complication of SIADH. Early identification and appropriate management of both conditions are critical to improving patient outcomes and preventing complications.