Idiopathic pulmonary fibrosis can be broken down into idiopathic which means a disease
without a known cause or mechanism, pulmonary which refers to the lungs, and fibrosis which
refers to excess collagen in connective tissue, or interstitial tissue between cells, usually
after tissue damage.
So idiopathic pulmonary fibrosis is the ongoing repair process of having excess collagen or
scar tissue in the interstitial tissue of the lung.
What triggers the repair process is unknown, but it’s a chronic process that leads to
a progressive loss of lung tissue.
Normally, gas exchange happens between the alveoli which carry air and capillaries which
carry blood.
The alveoli are lined by type I and type II alveolar epithelial cells, also called type
I and type II pneumocytes.
Type I pneumocytes make up the majority of cells - they’re simple squamous cells that
form a nearly continuous barrier between the air and underlying connective tissue.
Type II pneumocytes are studded throughout the type I - they’re shaped like cubes,
have microvilli to sweep away invading particles, and secrete surfactant, an oily mixture of
proteins, phospholipids, and neutral lipids which prevent the alveoli from collapsing
during exhalation.
Type II pneumocytes also have the ability to divide to make more type II pneumocytes
and can also divide and become type I pneumocytes as well.
Now, between the type I and type II pneumocytes and the capillaries is interstitial tissue
of the lung, which has macrophages and fibroblasts.
When the alveolar lining is damaged, type I pneumocytes release transforming growth
factor beta1, which gets the type II pneumocytes to stimulate fibroblasts to proliferate and
develop into myofibroblasts, which are fibroblasts with some smooth muscle cell properties.
The myofibroblasts secrete reticular fibers, a type of collagen which provides structural
strength, as well as elastic fibers, which provide the rubber-band like elasticity of
the lungs.
The myofibroblasts then undergo apoptosis, or programmed cell death.
The tissue injury that triggers idiopathic pulmonary fibrosis is unknown, but there are
some known risk factors including old age, being male, and tobacco smoking.
Once the process is triggered, however that happens, the fundamental problem is that type
II pneumocytes over-proliferate during the repair process and it leads to too many myofibroblasts
and too much collagen.
To make matters worse, the myofibroblasts don’t undergo apoptosis normally, and instead
continue to make even more collagen.
As the collagen accumulates, it thickens the interstitial layer between the alveoli and
the capillary and that leads to problems with ventilation - which is carbon dioxide leaving
the body - and oxygenation - oxygen getting into the body.
The excess collagen also causes the lungs to become stiff, making it harder for air
to flow in and out.
This restricted lung expansion from changes to the interstitium means idiopathic pulmonary
fibrosis is categorized as restrictive lung disease, and a particular type called interstitial
lung disease.
It causes a decrease in total lung capacity, or the maximum amount of air the lungs can
hold; also it causes a decrease in the forced vital capacity, or amount of air that can
be expelled with maximal effort after a deep breath; and finally a decrease in the forced
expiratory volume in 1 second, the air exhaled with maximum effort in the first second.
The excess collagen also leads to the loss of alveoli, creating fluid filled spaces,
or cysts, surrounded by thick walls in a pattern called “honeycombing”.
Idiopathic pulmonary fibrosis is a progressive process, so the symptoms generally worsen
over time.
Problems with oxygenation and ventilation can cause coughing and shortness of breath,
as well as cyanosis, which is where the skin turns blue, and digital clubbing, or rounding
of the nail beds.
Over time, it can lead to significant respiratory failure as the lungs lose more and more healthy
functional tissue.
Diagnosis of idiopathic pulmonary fibrosis can be done using chest imaging - for example
a chest CT can reveal honeycomb appearance and thickening of the interstitial walls.
In addition, a pulmonary function test called spirometry can reveal a decrease in total
lung capacity, forced vital capacity, and forced expiratory volume in 1 second.
Which are all characteristic of restrictive lung diseases.
Treatment options for idiopathic pulmonary fibrosis include supplemental oxygen, and
although there are some antifibrotic medications, they generally slow the progression of the
disease, rather than stopping it.
Unfortunately, the only definitive treatment available currently for severe situations
is a lung transplant.
All right, as a quick recap...
Idiopathic pulmonary fibrosis is a restrictive, interstitial lung disease from increased scarring
in the interstitium of the alveoli from an unknown cause of alveolar epithelial cell
damage.
Symptoms, particularly dry cough and shortness of breath, are progressive and the condition
is fatal with few effective treatment options other than lung transplant.