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© BMJ Publishing Group Limited 2022. No commercial reuse. See rights and permissions. Published by BMJ. DESCRIPTION Sternoclavicular joint septic arthritis is a relatively rare but serious joint infection, which is sometimes complicated by chest wall abscess and mediastinitis. It can spread by direct extension into a neighbouring structure through the space formed by the cervical fascia. Common risk factors include an immunocompromised host, diabetes mellitus, trauma, infection at a distant site and intravenous drug use. We present a case of sternoclavicular joint septic arthritis during regular steroid use, which resulted in mediastinitis through the pretracheal space despite early puncture drainage. A woman in her early 80s with right shoulder pain presented to the emergency department. She had been taking prednisolone (5 mg daily) for 3 years to address nonspecific numbness in both upper limbs. She had no other comorbidities. She had stable vital signs, but had a fever of 38.8°C and had redness and tenderness around the right sternoclavicular joint. Her C reactive protein (CRP) level was 25.6 mg/dL. The differential diagnosis at this point was sternoclavicular joint septic arthritis; crystalline arthritis as it was an acuteonset monarthritis. There was also the possibility of synovitis, acne, pustulosis, hyperostosis and osteitis (SAPHO) syndrome or cellulitis, but SAPHO syndrome did not meet the diagnostic criteria. CT revealed an abscess localised to the sternoclavicular joint (figure 1), leading to the patient being diagnosed with sternoclavicular joint septic arthritis. Emergency CTguided puncture drainage was performed on the same day (figure 2), and the patient was admitted. Ampicillinsulbactam treatment was initiated. On day 4, Proteus mirabilis was cultured from her blood and abscess samples and by day 5, her general condition had not deteriorated, but her fever persisted. CT was reperformed, and the findings showed that the sternoclavicular joint abscess had increased in size and had extended into the mediastinum and the right extrapleural