Because pneumonia is
the most common precursor of pyogenic infection of the pleural space, there is
a tendency to forget the many other conditions which may lead to suppurative
pleurisy.
Infections derived from the lungs
When the
lung is the source of the infection, it is always possible that some lesion
other than pneumonia—such as bronchogenic carcinoma with
secondary suppuration, lung abscess, bronchiectasis or actinomycosis—may be
responsible for infecting the pleural cavity. Owing to the great increase in
the incidence of
carcinoma of the bronchus, it is particularly important to
remember that this dread disease is commonly complicated by an empyema.
Furthermore, when pneumonia is the cause of the pleural infection, the
pulmonary lesion is often not of the classical pneumococcal or streptococcal
type : for example, an empyema is not an uncommon complication of
staphylococcal pneumonia in infants and small children, and is often preceded
by a spontaneous pneumothorax, due to the rupture of a small subpleural
abscess. The pleural space may also be infected from the lung to which the
bacteria are carried by the blood stream as a result of a generalized pyaemia
or septicaemia, but these cases are now rare owing to the widespread use of
chemotherapy.
Before the introduction
of the antibiotic drugs, pneumonia was frequently complicated by empyema, but
there has been a very remarkable decrease in the incidence of this complication
during recent years.
Infections not derived from the lungs
The
infection causing an empyema, however, is not always derived from the lung.
Bacteria may be carried into the chest by a penetrating wound, but suppuration
is not common unless foreign matter is retained within the thorax, organic
material such as clothing being more likely to lead to infection than are metallic foreign bodies.
However, accidental penetrating wounds of the thorax are not common in
peace-time ; but the enormous increase in transpleural surgical operations,
such as those on the lung, heart, oesophagus and sympathetic chain, have inevitably
led to the surgeon's being responsible for a certain number of cases of
empyema, although, with proper post-operative care, the incidence of pleural
suppuration following such operations is very low. Other sources of infection
of the pleural cavity include subphrenic abscess (Fig. 5), lesions of the
oesophagus, and osteomyelitis of the ribs or spine. According to Harley(1949),
who reviewed 182 cases of subphrenic abscess, empyema occurred in 29 per cent.
Diseases of the oesophagus are not frequently complicated by an empyema,
because the common lesions of this organ, such as carcinoma, are chronic; thus
they allow time for the development of peri-oesophageal adhesions, which form a
barrier to infection of the pleural space. is acute inflammation
of the two pleural surfaces, which is quickly followed by the effusion of a
serous exudate ; this is turbid on account of the presence of polymorphonuclear
cells. Although the inflammatory process is unlikely to be of uniform severity
throughout the pleural space, nevertheless, at this stage, the fluid is free in
the pleural cavity. Subsequently the fluid becomes thicker, as the result of an
increase in the number of dead and living polymorphonuclear cells and of a rise
in the protein content of the fluid. This is usually accompanied by the
deposition of fibrin, sometimes in very large amounts, particularly when
pneumococci are responsible for the infection. In cases due to haemolytic
streptococci, there is usually very little fibrin present, for these organisms
produce a fibrinolysin which breaks down any fibrin deposit. On the other hand,
an empyema originally caused by haemolytic streptococci, but subsequently
sterilized by chemotherapy, may contain as much fibrin as would an empyema due
to pneumococcal infection. Simultaneously with this increase in solid content
of the fluid, the pus usually becomes localized to one part of the pleural
cavity, most commonly the lower posterior part, as a result of fibrinous
adhesion between the two pleural layers. In a few cases the pus may be
subdivided into two or more separate pockets, because the lung becomes adherent
to the parietes between these pockets (Fig. 6). Much more commonly, the fluid
pus is held in a number of loculi, which are separated only by a layer of
fibrin that can easily be broken up with a finger. This loculation by fibrin is
particularly pronounced in cases arising from infection of a haemothorax.
Occasionally
the lung is everywhere in contact with the chest wall, and the pus is entirely
confined to the space between two lobes. However, an interlobar empyema
generally extends to the parietes, which means that a portion of the general
pleural space is also involved, Very rarely the pus becomes isolated to a
pocket lying between the lung and mediastinum (" mediastinal
empyema"), so that the abscess has no contact with the chest wall. It is
not strictly accurate to use the term " mediastinal empyema " for
cases in which the pus, although largely confined to the space between the lung
and mediastinum, also extends to reach part of the chest wall. Exceptionally an
empyema is entirely confined to the space between the inferior surface of the
lower lobe and the diaphragm, and may therefore be described as a diaphragmatic
empyema.
With the
lapse of time, further fibrin is deposited and organization of the deeper
layers into fibrous tissue commences. Occasionally the pus perforates into a
bronchus (broncho-pleural fistula) and is expectorated.
