C63-31 Not All Atrial Fibrillation Is Cardiac: A Duplication Cyst Masquerading as Chronic Pain and Arrhythmia in a Young Veteran

Esophageal duplication cysts are rare congenital foregut anomalies, typically diagnosed in childhood. In adults, they mimic common thoracic pathologies via mass effect on adjacent structures. We present a case of chronic thoracic back pain, dysphagia and recurrent atrial fibrillation with rapid ventricular response (AF with RVR) ultimately attributed to an esophageal duplication cyst.A 39-year-old male veteran with alcohol use disorder, in utero agent orange exposure, chronic dysphagia and thoracic back pain presented with new AF with RVR, chest pressure and uncontrolled hypertension. He converted to sinus rhythm with maximal rate control and later underwent wide antral circumferential ablation with bilateral pulmonary vein isolation, remaining arrythmia-free off pharmaceutical agents.A month later, he represented palpitations, tachycardia and episodic diarrhea. A 24-hour catecholamine panel was unremarkable. Computed Tomography (CT) revealed a well circumscribed 2.2cm azygoesophageal mass abutting the distal esophagus without contrast enhancement. Positron Emission Tomography-CT showed no uptake, leading differentials included hamartoma or carcinoid.He then underwent endobronchial ultrasound with transbronchial needle aspiration and cryobiopsy confirming an esophageal duplication cyst. Several days post-procedure he developed a fever, leukocytosis and thoracic pain consistent with secondary cyst infection, which resolved with a four-week antibiotic course.Following recovery, he underwent robotic assisted thoracoscopic resection. At multiple follow up visits, he reported complete symptom resolution.This case highlights a rare but important cause of refractory arrythmia with associated GI symptoms due to mass effect on the left atrium and adjacent mediastinal structures from a benign posterior mediastinal lesion in a young adult. Complete resolution following resection underscores the importance of considering mediastinal mass effect in young patients with early-onset arrythmias and overlapping thoracic symptomologies.Esophageal duplication cysts are rare congenital foregut abnormalities that are most often diagnosed during childhood and often located in the posterior mediastinum adjacent to the distal esophagus. These lesions typically do not communicate with the esophageal lumen, as seen in our patient.Prior reports describe similar atrial dysrhythmias attributed to local mass effect seen after esophageal stent placement adjacent to the left atrium, similarly located foregut duplication cysts have also been associated with supraventricular arrythmias in young adults.Infection after EBUS sampling occurs in up to 14% of cases, raising consideration for early surgical referral when malignancy risk is low.This case highlights the importance of considering mediastinal pathology in young patients with atypical arrythmia and multisystem thoracic symptoms and supports early multidisciplinary evaluation to avoid complications.