Pus in the mediastinum.
Usually trauma, post surgical, or esophageal perforation (I had one guy who perforated his esophagitis from a tortilla chip). Neck infections can track down into the mediastinum.
Occasionally actinomycosis will burrow into the mediastinum.
Trauma and post operative: anti staphylococcal therapy.
Esophageal perforation or head&neck infection: (penicillin G OR ampicillin PLUS metronidazole) OR (third generation cephalosporins PLUS metronidazole) OR (quinolone PLUS metronidazole) OR penicillin/beta-lactamase inhibitors OR clindamycin OR cefoxitin OR cefotetan.
Drain the pus, always drain the pus.
Relevant links to my Medscape blog