Thoracic Surgeon Encounters Cerebrospinal Fluid? A Rare Case of Mediastinal Arachnoid Cyst

Growth of a spinal arachnoid cyst into the posterior mediastinum is a rare phenomenon not described in the literature. We present the case of a 30-year-old patient with chest pain, cough, fatigue, and occasional dysphagia who was found to have an 8 × 6.5 × 9.5-cm cyst located in the posterior mediastinum on imaging. The patient was taken to the operating room, where clear fluid positive for β₂-microglobulin consistent with cerebrospinal fluid was encountered. Dural communication with the spinal canal was identified, confirming diagnosis of an arachnoid cyst. This report aims to describe this rare thoracic disease and to provide surgical approach and management.