The big question with pleural
fluid is whether the patient has a transudate
or an exudate. Now, the difference is that in
a transudate, the pleura is normal, and what
you’re getting is excessive formation of
pleural fluid by a passive transfer across
from the blood. So this occurs with increased
venous pressure such as heart failure or a
low oncotic pressure of the blood such as
decreased blood protein content. These may
be bilateral but can be unilateral,
and it can be associated with peripheral oedema
because the same mechanisms would cause pleural
fluid in these patients will also be caused
of peripheral oedema. Exudates, in contrast,
the pleura is the abnormality. The problem
is that the pleura is abnormal and driving
fluid formation. And that occurs with tumour
of the pleura, inflammation of the pleura,
or an infection. And these tend to be unilateral
and won’t have associated peripheral oedema
because this is a specific pleura abnormality.
So, what are the causes of pleural effusions?
Well, with transudates, you can have an increased
venous pressure and that might be due to cardiac
failure, mainly right side of cardiac failure,
but also left side of cardiac failure.
It could be fluid overload. If somebody has got
renal failure and they have an excess amount
of fluid in the system, then you may get pleural
fluid forming. And pericardial disease also
causes pleural effusion. It’s often forgotten
as a cause of this.
The protein content of the blood is required
for retaining fluid within the blood.
And that with a low albumin content, you end up with
the fluid seeping out into the pleural space,
into ascites, and into the peripheral oedema
as well. So patients with very low protein
content of the blood will present with pleural
effusions. These include liver cirrhosis,
nephrotic syndrome, and very rarely, protein
losing gut enteropathies or malabsorbtion.
But the commonest cause there would probably
be the renal and the liver problems.
Exudates, in contrast, are due to abnormalities
of the pleura as I mentioned before. So these
occur in tumours, and usually, there’s a
secondary pleural metastases, either from
a lung cancer, from a breast cancer, or from
a GI tract malignancies. Those are the common
causes, but in fact, multiple different types
of cancers can spread to the pleura to cause
pleural effusions. And the presence of a pleural
effusion with somebody with no malignancy
is very suggestive. They’re going to have
metastatic disease affecting the pleura.
Then again, there are primary pleural cancers
as well. Actually, these are very rare in
general apart from mesothelioma which is a
primary cancer of the pleura that is associated
with asbestos exposure that I’ve discussed
in the lung oncology lecture. There are other
causes of exudate pleural effusions. These
basically involve pleural inflammation.
Probably the common cause might be a pulmonary embolus
where you often get small effusions formed
with that. But then there are connective tissue
diseases. Rheumatoid arthritis, for example,
will often have patients with pleural effusions,
and systemic lupus erythematosus is another
cause of inflammatory pleural effusions.
Benign asbestos effusions occur in people
who have been exposed to asbestos where the
asbestos has inflamed the pleura and caused
an exudated pleural effusion but hasn’t gone on
to cause a mesothelioma. Occasionally,
drugs can cause pleural effusions. The classic
example would be practolol and then actually,
many exudate pleural effusions we cannot identify
the cause and can’t characterize as idiopathic.
The last major category of exudated pleural
effusions is those due to infection.
It’s an incredibly important category, and these
occur in three main circumstances. One is
a para pneumonic effusion which in effusion
is associated with pneumonia. The second is
the empyema which is a pleural infection without
associated pneumonia. And the third very important
is tuberculosis. There are a whole range of
very rare causes of pleural effusions needing
chylothorax, we have chyle forming in the
pleural space, ovarian tumours, or Meig’s
syndrome. Pancreatitis can cause inflammation
of the pleura, and of course, trauma itself
can cause the leaking of blood into the pleura
and that’s called haemothorax. But those are
all, in general, relatively rare or obvious
So, how would you investigate somebody with
a pleural effusion? Well, first off, you need
to confirm there’s a pleural effusion by
doing a chest X-ray, and a pleural ultrasound
will be also very helpful indeed. This is an
ultrasound picture showing that they clearly
are very black area which is the fluid with
a lung separated from the chest wall by that
black area. If somebody has bilateral pleural
effusions, then they are likely to be transudated
just because they’re bilateral. So if somebody
has bilateral effusions, there’s an obvious
cause for transudate, such as known heart
failure or low albumin due to nephrotic syndrome.
Actually, you don’t need any further tests.
The answer to why the patient has pleural
effusions is there. It’s due to the cause
of the transudates.