Nous rapportons le cas clinique d’un adolescent âgé de 17ans ayant consulté au service des urgences pour une douleur axillaire gauche, d’apparition brutale, associée à une tuméfaction. L’examen clinique avait révélé une masse souple et douloureuse de la paroi thoracique latérale gauche, sans anomalie respiratoire, ni cardiovasculaire. Le reste de l’examen était sans particularité. Le bilan biologique avait mis en évidence des signes de cytolyse avec une élévation importante des enzymes musculaires (créatine-phospho-kinases) et des transaminases, sans syndrome inflammatoire, ni anomalie rénale. L’échographie du creux axillaire avait montré une masse hypoéchogène liquidienne au niveau axillaire et l’imagerie par résonance magnétique un hyper-signal du muscle dentelé antérieur gauche. Ces signes cliniques et biologiques avaient disparu rapidement et spontanément. Il s’agissait donc d’une myosite focale aiguë. La cause de cette entité n’est pas connue, mais il semble qu’elle soit bénigne et ne nécessite pas d’investigations particulières.

We describe the case of a 17-year-old teenager who consulted in an emergency unit for acute lateral chest pain with tumefaction. The boy was healthy and had no medical problems or allergies. Upon awakening, he felt faintness and a left lateral chest pain. Movement of the left shoulder was difficult. In the emergency department, the clinical examination showed a painful left axillary mass, normal respiratory and heart function, and normal clinical parameters. Biological assessment revealed substantial cytolysis with major elevation of creatine phosphokinase (CPK) (41 times the normal value) and moderately elevated liver enzymes (2.5 times the normal value), with no inflammatory syndrome, hematology signs, or kidney failure. Axillary ultrasound found a low echogenic and well-limited mass, 6.2cm long, compatible with lipoma. MRI showed a T2-weighted hypersignal of the left serratus anterior muscle, with intramuscular infiltrate compatible with inflammatory disease of this muscle. Clinical symptoms ended in a few days with no after-effects and CPK gradually decreased. This study reports a case of benign acute myositis of the serratus anterior muscle in a healthy 17-year-old teenager. Although no infectious etiology was found, it seems that this semiological entity is a form of benign acute myositis (BAM), a benign muscular inflammation, arising from viral infection and occurring in school-age children, more frequently in males, between 6 and 12 years of age. On average, the symptoms begin 7 days after infection, more often the flu (Influenza virus B or A, parainfluenza virus). It is important to note that this pathology is acute and benign and there is no need for any invasive investigations. The family should be reassured because BAM is a self-limited disease and in the great majority of cases has no after-effects.