Defining Pulmonary Fibrosis

PULMONARY FIBROSIS (PF) DESCRIBES A CONDITION in which the lung tissue becomes thickened, stiff, and scarred.1 The medical terminology used to describe this scar tissue is fibrosis. The alveoli (air sacs) and the blood vessels within the lungs are responsible for delivering oxygen to the body, including the brain, heart, and other organs. All of the body’s functions depend upon delivery of a steady supply of oxygen. As lung tissue becomes scarred and thicker, it is more difficult for the lungs to transfer oxygen into the bloodstream. As a result, the brain, heart, and other organs do not get the oxygen they need to function properly.1 In some cases, doctors can determine the cause of the fibrosis (scarring), but in many cases the cause remains unknown. When there is no known cause for the development of pulmonary fibrosis (and certain radiographic and/or pathologic criteria are met), the disease is called idiopathic pulmonary fibrosis or IPF. More specifically, consensus treatment guidelines from international lung societies define IPF as “a specific form of chronic, progressive fibrosing interstitial pneumonia of unknown cause, occurring primarily in older adults, limited to the lungs, and associated with the histopathologic and/or radiologic pattern of UIP [usual interstitial pneumonia].”2

There are more than 200 related diseases of the lung known as interstitial lung diseases (ILD), which are also referred to as diffuse parenchymal lung diseases or DPLD. Because these diseases affect the interstitium — the space around the alveoli — ILDs are classified as a group. However, ILDs may also affect other parts of the lungs. Many ILDs have similar characteristics to IPF and most result in lung fibrosis.

There is a subgroup of ILDs called idiopathic interstitial pneumonias (IIP), where the lung tissue becomes inflamed and scarring can also occur. The word pneumonia is used to describe inflammation and not an infection such as bacterial pneumonia. IIP is further broken down into a number of pathological subtypes. These subtypes include usual interstitial pneumonia (UIP), non-specific interstitial pneumonia (NSIP), desquamative interstitial pneumonia (DIP), respiratory bronchiolitis-associated interstitial lung disease (RB-ILD), acute interstitial pneumonia (AIP), cryptogenic organizing pneumonia (COP), and lymphocytic interstitial pneumonia (LIP).2 IPF is a subtype of IIP, the pathological pattern seen in IPF is UIP. It is important for your care providers to differentiate the specific subtype of interstitial disease, since treatment, management, and prognosis can vary quite dramatically. If there is a clear association with another illness or the lung scarring (fibrosis) is the result of a side effect from a medication or an exposure to an agent known to cause PF, then the cause of the disease is no longer considered idiopathic. PF clearly associated with another disease, such as scleroderma or rheumatoid arthritis, would be referred to as pulmonary fibrosis secondary to scleroderma or secondary to rheumatoid arthritis. As we learn more about interstitial lung diseases, the terminology may evolve.

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Updated April 2013
This information has been approved by Gregory P. Cosgrove, MD (September 2012)