of the lung. So the pathology of the ILD is
dictated by the underlying cause. Pulmonary
fibrosis is the commonest cause of interstitial
lung disease, and the most important clinically.
As mentioned already, it is due to an infiltration
of interstitium and alveoli with inflammatory
material and increased extracellular matrix
and collagen and fibroblasts. This is normally
in a bibasal distribution, usually subpleural,
usually relatively symmetrical, and importantly,
former fibrosis of the idiopathics type
is generally a progressive disease.
This infiltration of the alveoli and interstitium
with extra material makes oxygen diffusion
from the alveolus into the pulmonary capillary
much slower. So uptake of oxygen as measured
by a transfer factor will be reduced in pulmonary
fibrosis. There are multiple underlying causes
of pulmonary fibrosis, but idiopathic is by
far the commonest. And there's really no
idea exactly why these patients have pulmonary
fibrosis. It is commoner in patients who smoke
and in patients who have had occupations where
there has been dust exposure, suggesting there
is some form of environmental insult that
underlies why these patients might develop
disease. In the United Kingdom, the incidence
pulmonary fibrosis is five people per 100,000.
This table describes the differences between
the different causes of pulmonary fibrosis.
At the top, we have idiopathic disease, which
is as we mentioned, is a bibasal symmetrical
progressive disease tends to affect men.
The median age of onset is 70 years. There is
a very similar disease but that's associated
with connective tissue diseases such as rheumatoid
arthritis, systemic sclerosis, and dermatomyositis.
This is a more even distribution between men
and women, and tends to affect younger patients.
But again, it's a bibasal symmetrical disease
and can be progressive, although overall,
the prognosis of connective tissue associated
pulmonary fibrosis is much better than idiopathic
pulmonary fibrosis. Then you may get diseases
such as end-stage
of sarcoidosis or hypersensitivity pneumonitis
where the disease is not bibasal distribution
but tends to be upper lobe disease. As I have
already mentioned, both sarcoid and hypersensitivity
pneumonitis may cause pulmonary fibrosis, but
this tends to be an upper lobe distribution
rather than a basal. Then there are some causes
of pulmonary fibrosis which often start with
an acute onset, such as drug toxicity, radiotherapy,
and after, an episode of adult respiratory
distress syndrome. So, how does pulmonary
fibrosis present? Well,
usually, it presents with exertion of dyspnoea.
The patient would say, “I have been getting
breathless and more breathless over the past
few weeks or few months.” Sometimes the
patient may have a cough as well, which is
usually derived and can be productive of a clear
phlegm. When you talk to the patient and discuss
their background history and their social
circumstances, you may identify potential
cause for pulmonary fibrosis such as a history
of rheumatoid arthritis, somebody who maybe
a farmer that have farmer’s lung, one of
the forms of hypersensitivity pneumonitis
or they could have worked with asbestos in
the past, and therefore, have pulmonary fibrosis
due to asbestosis. In addition, they could
have had radiotherapy or chemotherapy, and
that might be identified when you take their
drug history in their past medical history.
And other questions you may want to ask in
this always is a little unusual when you ask
the patients whether they have any pets such
as birds or any specific hobbies, because hypersensitivity
pneumonitis tends to be dictated by exposure
to pets or hobbies or specific occupations.
When you examine the patient, they may be