TOPIC: Pediatrics TYPE: Fellow Case Reports INTRODUCTION: Hereditary multiple exostoses (HME) is a syndrome characterized by presence of two or more osteochondromas (exostoses) in the appendicular or axial skeleton. Here, we present a case of an adolescent patient presenting with chest pain as the only symptom of an intrathoracic exostosis leading to the diagnosis of HME. CASE PRESENTATION: A previously healthy 15-year-old male presented to the pediatric pulmonary clinic for months-long history of recurrent left-sided chest pain, without any associated symptoms. Chest x-ray showed an ill-defined nodular opacity in the left upper lobe, with central calcification identified. Additional work-up with spirometry, complete blood count, fungal antibody panel, and Histoplasma urine antigen testing was normal. Family history was positive for pulmonary embolism and "blood clotting issues" with no specific syndromes identified. A chest computed tomography angiography (CTA) with contrast was ordered for further evaluation.Chest CTA revealed a large bony exostosis arising from the anterior aspect of the left third rib, with two areas of surrounding nodular soft tissue measuring 3 cm x 1.3 cm x 2.6 cm. There was extension into the lung abutting the pleura and lateral aspect of the mediastinum. Additional bony protuberances were noted along the left fifth rib, right second rib, and bilateral scapulae. A diagnosis of HME was made because of the presence of multiple exostoses. Due to continued chest pain he was referred to cardiothoracic surgery. The decision was made to monitor symptoms and obtain repeat imaging in six months to monitor growth or worsening lung impingement. DISCUSSION: HME is characterized by the presence of two or more exostoses in the appendicular or axial skeleton. The most common location of exostoses is in the appendicular skeleton. Though rare, involvement of the ribs can occur [1]. Previous case reports have identified presence of multiple costal exostoses in patients with known HME [1,2]. The patient presented here represents a case with chest pain as the only presenting complaint prior to diagnosis of HME being made. The presence of a costal exostosis should be considered in adolescents presenting with chest pain if other more common causes are ruled out. The presence of multiple exostoses supports the diagnosis of HME. Cases of HME with costal exostoses can be associated with intrathoracic complications, including hemothorax, pneumothorax, and pericardial effusion [3]. Supportive care and observation is the usual plan of care. However, if complications develop, surgical resection may be necessary and cardiothoracic surgery consultation is recommended. CONCLUSIONS: Chest pain may be the only presenting symptom in pediatric patients with HME. REFERENCE #1: Mazza D, Fabbri M, Calderaro C, Iorio C, Labianca L, Poggi C, Turturro F, Montanaro A, Ferretti A. Chest pain caused by multiple exostoses of the ribs: A case report and a review of literature. World J Orthop. 2017 May 18;8(5):436-440 REFERENCE #2: Khosla A, Parry RL. Costal osteochondroma causing pneumothorax in an adolescent: a case report and review of the literature. J Pediatr Surg. 2010 Nov;45(11):2250-3. REFERENCE #3: Yan G, Littlewood A, Latimer MD. Unusual cause of pleuritic chest pain in a child. BMJ Case Rep. 2016 Nov 28;2016:bcr2016217307. DISCLOSURES: No relevant relationships by Brooke Gustafson, source=Web Response No relevant relationships by Lisa Ulrich, source=Web Response