Empyema may be defined as the presence of pus in any portion of either pleural cavity. It is the result of infection, which may be introduced in three different ways: first, though rarely, through the blood stream; second, directly through a wound; third, and most frequently, through the spread of an inflammatory or septic process from the lung or other adjacent structure.
Whatever the source or cause of pleural infection may be, it will result either in a local walled-off abscess or in purulent infection of the entire pleural cavity. If the site of infection is in an adherent area or in an area that is in great part surrounded by pleural adhesions, or if the process is slow so as to permit adhesions to form, and especially if it is situated in an interlobar area where there is little movement between the two surfaces—if one or more of these
DANNA JA. THE TREATMENT OF EMPYEMA BY ASPIRATION AND AIR REPLACEMENT WITHOUT DRAINAGEREVIEW OF THIRTY-FIVE CASES. JAMA. 1931;96(18):1453–1456. doi:10.1001/jama.1931.02720440001001