Late-Developing Acute Mediastinal Abscess After Sternotomy Demonstrated by Cardiac Computed Tomography
A 77-year-old man presented with a 5-week history of night sweats, intermittent fever, weight loss, and general malaise. He had undergone coronary artery bypass graft surgery 6 years previously in which the left internal mammary artery was grafted onto the left anterior descending artery and reverse saphenous vein conduits were grafted onto the right coronary artery, circumflex coronary artery (Cx), and first obtuse marginal branch (OM1). A tender soft tissue swelling had arisen at the left anterior chest wall. Observations revealed a pyrexia of 39°C, pulse of 102 bpm regular, and blood pressure of 121/62 mm Hg. Clinical examination revealed a prominent, pulsatile, soft tissue swelling overlying the third left intercostal space adjacent to the superior margin of his sternotomy scar (Figure 1 and the Movie in the online Data Supplement). Blood chemistry revealed an elevated white blood cell count (11.6×106/mL) and C-reactive protein (125 mg/L). Chest x-ray revealed widening of the superior mediastinum. Blood cultures failed to grow any pathogenic bacteria.
A bolus-tracked arterial phase contrast–enhanced computed tomogram (CT) of the chest (40–detector row scanner with 100 mL Iomeron 400) revealed a large nonenhancing multilocular fluid collection in the anterior mediastinum extending from the level of the manubrium sterni to the right ventricular outflow tract, consistent with a large superior mediastinal abscess (Figure 2A and 2B). The abscess was more extensive to the left of the midline, where it bulged forward in the second and third left intercostal spaces. The sternotomy wound itself appeared intact with no evidence of osteomyelitis. A cardiac computed tomogram scan was then performed on a 64–detector row scanner to depict the relationship of the abscess to the bypass grafts. This was retrospectively gated and ECG modulated; premedication included 10 mg metoprolol IV and sublingual glyceryl trinitrate. Contrast enhancement was achieved with 100 mL Ultravist 370 IV. The left internal mammary artery graft, although widely patent, passed through the left lateral aspect of the collection; the Cx and OM1 grafts were patent, whereas the right coronary artery graft was rudimentary (Figure 2C and 2D).
Anaerobic Gram-negative bacilli were cultured from a fine-needle aspirate of pus taken from the abscess. Inflammatory indexes failed to settle despite antibiotic therapy with vancomycin and gentamicin, and the patient remained toxic. The anatomic relationship between the abscess and grafts provided by the cardiac computed tomogram allowed a minimally invasive surgical approach. A ministernotomy was performed with drainage of 1.2 L pus. The patient was ultimately discharged after 2 weeks of intravenous antibiotics, with resolution of pyrexia, systemic symptoms, and inflammatory markers.
Mediastinal abscess formation after coronary artery bypass graft is classified under sternal wound complications, which occur with an overall incidence of 1.7%.1 The longest reported dormant period between sternotomy and diagnosis of a mediastinal abscess is 12 years.2 Staphylococci are the pathogens most frequently associated with chronic, poststernotomy mediastinal (encapsulated) abscesses.3 Gram-negative bacteria are recognized pathogens in mediastinal abscesses associated with descending necrotizing mediastinitis, but this condition was not present in our patient. The acute presentation and the presumed rapid expansion of the abscess (and hence its pulsatility) are unusual; metastatic reinfection of a previous abscess cavity as a result of a transient bacteremia remains a possibility. Mediastinal abscesses remote from the coronary artery bypass graft remain serious sequelae of sternotomy; the investigation of unexplained sepsis in patients with prior sternotomy should involve exclusion of this phenomenon, ideally by computed tomogram.
The online-only Data Supplement is available with this article at http://circ.ahajournals.org/cgi/content/full/118/17/e673/DC1.