Les malformations costales et la conduite à tenir devant leur découverte souvent fortuite restent peu connues. Nous rapportons le cas d’une fillette de 4ans et 4mois, prise en charge à l’unité des urgences pédiatriques pour la découverte fortuite d’une tuméfaction pariétale thoracique antérieure gauche dans un contexte fébrile. Le diagnostic de malformation costale congénitale a été posé par la radiographie du gril costal, l’échographie et l’imagerie par résonance magnétique (IRM) qui montraient une bifidité chondrale de la jonction sterno-costale du 4^e arc costal gauche. L’exploration d’une tuméfaction costale nécessite de bien définir sa localisation antérieure, latérale ou postérieure, de connaître les variations anatomiques, de pratiquer un interrogatoire minutieux avec un examen clinique complet à la recherche d’anomalies associées, de réaliser une radiographie pulmonaire avec gril costal et une échographie en première intention avant de poser le diagnostic de variante anatomique.

Rib malformation and anatomical variations are not well known and are still often underdiagnosed. Usually, rib malformations are fortuitously discovered. We describe here the case of a girl, 4years and 4months old, who presented at the emergency unit for fever and an anterior tumefaction of the ribcage, without any other symptoms. She was eupneic with a normal pulmonary auscultation and viral tonsillitis with a negative streptococcus test. The thoracic tumefaction was parasternal, painless, and fixed and measured approximately 2.5×2cm. Ultrasound findings consisted of a duplicated and hypoechogenic hypertrophy of the sterno-costal cartilage of the 4th left rib. Magnetic resonance imaging (MRI) confirmed the diagnosis of chondral bifidity of the sterno-costal junction of the 4th left rib. Fever, due to the viral tonsillitis, disappeared after 4days. Rib malformations are rare, often anterior, unilateral, and preferentially located on the 3rd or the 4th rib. The main malformative rib lesions are bifid ribs, rib spurs, and widened ribs. Very rarely, they can be associated with Gorlin-Goltz syndrome or with other malformations such as VATER complex. The main differential diagnoses of these rib malformations are traumatic, tumoral, and infectious etiologies. In case of tumoral diseases, the topography of the lesion focuses the etiologic diagnosis: whereas an anterior and cartilaginous lesion is always benign, a lateral or posterior lesion can be an Ewing sarcoma. Rib malformation investigation consists in meticulous questioning, a complete clinical examination looking for any associated anomaly, completed by basic imaging explorations such as plain thoracic radiography focused on the ribcage and ultrasound. Finally, complementary computerized tomography or preferably MRI, depending on the anatomic location of the lesion, confirms the final diagnosis, as presented in our case report, and removes any uncertainty.