Syndrome of Inappropriate Antidiuretic Hormone Secretion as the Initial Manifestation of Guillain-Barré Syndrome: A Case Report.

Guillain-Barré syndrome is an acute immune-mediated polyradiculoneuropathy that commonly presents with progressive weakness and areflexia. This report describes euvolemic hypotonic hyponatremia preceding neuromuscular decline in a patient ultimately diagnosed with Guillain-Barré syndrome. We report the case of a 69-year-old man who presented with generalized weakness and was found to have persistent euvolemic hypotonic hyponatremia. Initial evaluation suggested syndrome of inappropriate antidiuretic hormone secretion, and management with fluid restriction and sodium supplementation was initiated. Over the subsequent days, the patient developed rapidly progressive ascending weakness, areflexia, dysphagia, and respiratory failure. Cerebrospinal fluid analysis revealed elevated protein with mild pleocytosis and negative infectious studies, supporting the diagnosis of Guillain-Barré syndrome. Despite treatment with intravenous immunoglobulin, the patient required mechanical ventilation and transfer for plasmapheresis. With intensive supportive care and rehabilitation, gradual neurological recovery was observed over a six-month period. This case highlights the value of considering neurological etiologies in patients presenting with unexplained euvolemic hyponatremia and suggests that early electrolyte abnormalities may precede the development of overt neuromuscular disease.