It
is obvious that all cases of chronic empyema have passed through an acute stage
and that the transition from the acute to the chronic stage is a gradual one,
so that there is no clear line dividing the two conditions. Furthermore, the
conditions under which an empyema may be reasonably defined as " chronic
" are subject to the personal views of individual clinicians; but it is
suggested here that the term, chronic, should be applied to those cases in
which a cavity persists for more than 3 months after the original pleural
effusion.
Aetiology and pathology
If an acute pyogenic empyema is
not drained externally or aborted by aspiration and local chemotherapy, more
and more fibrous tissue is laid down, particu-larly on the parietal
layer of pleura, thus immobilizing the chest wall and preventing expansion of
the lung. It is necessary to emphasize that the fibrous tissue is laid down on
the surface of the pleura and therefore should not be called " pleural
thickening ", although it is often described as such. Sometimes the pus
finds its escape through a broncho-pleural fistula and is expectorated; this is
mentioned above under " Acute Empyema ". In other cases the chest
wall is eroded and some of the pus forms an abscess beneath the overlying skin
and superficial muscles —a condition designated "
empyema necessitatis " (Fig. 14) ;
this is soon followed
(a) (b)
Fig. 14.—Empyema
necessitatis due to actinomycosis : (a) the swelling above and medial to the
left nipple is a subcutaneous abscess communicating with an empyema ; (b)
4J-months later after treatment with systemic penicillin (440 mega-units were
given over a period of 3 months) ; therefore
no operative treatment was employed.
by necrosis of the
tissues covering the abscess, and the pus escapes externally, leaving a sinus
which may heal intermittently or discharge persistently.
The fibrous tissue
formed during this chronic stage gradually contracts so that there is progressive
flattening of the chest wall, which is accompanied by a scoliosis with the
concavity towards the affected side. In addition, the mediastinum, which may
have been displaced towards the healthy side during the acute phase, is slowly
drawn towards the side of the empyema (Fig. 15). The chronic inflammation also
causes an irritation of the periosteum of the ribs overlying the empyema, and
consequently new bone is laid down on the inner surface of the ribs so that
they become triangular in cross-section (Fig. 16).
These changes are not
confined to persistent undrained cavities, for exactly the same series of
events occurs when a drained empyema is not rapidly obliterated by re-expansion
of the lung. The most common cause of persistence of an empyema in spite of
external drainage is premature removal of the tube. A
residual cavity is also
likely to occur if there is any interference with free drainage, as, for
example, when the tube is too long or too short, or when the drainage opening
is not at the bottom of the cavity. Sometimes an empyema fails to heal because the
pleural infection is maintained by the presence of a foreign body, such as a
piece of drainage material or a sequestrated portion of rib; in other cases the
infection may be kept up by an unsuspected aetiological factor, such as
tuberculosis, actinomycosis or pleural carcinomatosis. A chronic cavity may
also persist because the lung is not free to expand. This may be due to the
presence of intrinsic disease, such as carcinoma, or fibrosis associated with a
chronic lung abscess or bronchiectasis. More commonly, however, the lung is
healthy, but is unable to re-expand because it is covered with a rigid layer of
fibrous tissue. The latter may occur as the result of a delay in draining the
empyema or because the surgeon omitted to remove all the fibrin at the time of
operation. The lung is also likely to become covered with a thick layer of scar
tissue if a total empyema has been caused by drainage of a septic pleural
effusion before it has become localized by adhesions. A broncho-pleural fistula
sometimes leads to a delay in re-expansion of the lung but, unless the fistula
is very large, it does not often cause an empyema to become chronic, provided
that the underlying lung is otherwise healthy.
