Thoracic Imaging 2003 - Society of Thoracic Radiology

The Idiopathic Interstitial Pneumonias
Jeffrey R. Galvin, M.D.
Chief, Pulmonary and Mediastinal Radiology
Armed Forces Institute of Pathology and Clinical Professor Department of Radiology
University of Maryland
Learning Objectives
1. Review the current classification of the idiopathic interstitial
pneumonias
2. Enumerate the characteristic radiologic patterns and underlying
pathology
3. Discuss the postulated pathogenesis for these diseases
Introduction
The idiopathic interstitial pneumonias (IIPs) are a group of
nonneoplastic disorders that were originally described by Dr.
Averill A. Leibow in the mid 70’s. The patients usually present
with prolonged symptoms (months) of dyspnea and the lung
biopsy usually demonstrates varying patterns of interstitial
fibrosis and inflammation. The IIPs are currently separated into
the following diagnostic categories:
Idiopathic Pulmonary Fibrosis (Usual Interstitial Pneumonitis)
Respiratory Bronchiolitis-Associated Interstitial Lung Diseae
Desquamative Interstitial Pneumonitis
Acute Interstitial Pneumonitis (Hammon Rich)
Cryptogenic Organizing Pneumonia
Nonspecific Interstitial Pneumonia
The plain radiograph usually demonstrates areas of abnormally
increased density and the lungs tend to be small.
However, the process of describing a chest film with “chronic
diffuse lung disease” is often frustrating. These diseases display
a limited range of findings on chest radiographs, and the differential
diagnosis list often becomes lengthy. In addition, the radiographic
concept of “alveolar vs. interstitial” infiltrates has limitations.
Dr. Felson, the radiologist who extended the concept of
interstitial lung disease to the reading of chest radiographs, recognized
that it was difficult to predict the microscopic distribution
of lung disease based on the imaging. In fact, there are few
lung diseases that involve the interstitium alone. Even
Dr.Leibow, recognized that exudation into the alveolar spaces
was common. We now accept that cells within the alveoli may
be a dominant finding within the alveoli and respiratory bronchioles
in diseases such as respiratory bronchiolitis and desquamative
interstitial pneumonia.
The information now available from high resolution computed
tomographic (HRCT) scans of the lung has changed our
approach to the assessment of patients with the idiopathic interstitial
pneumonias. The process of improving scanner resolution
has blurred the distinction between a gross anatomic lung specimen
and the in vivo image of that specimen. As we shall see the
anatomic distribution of abnormalities in the IIPs plays an
important diagnostic role. This is true at both at the microscopic
level and grossly.
The majority of patients who are suspected of having a
chronic infiltrative process in the lungs will undergo an HRCT
study during the initial investigation. When combined with clinical
information the imaging data are often adequate, and no
further diagnostic work-up is necessary. When a biopsy is
required, images provide key information that helps determine
the biopsy site and the type of procedure to be used. This is
especially important when the patient is a cigarette smoker, and
the symptoms may be related totally or in part to cigarette
smoke. The HRCT study has become an integral part of the prebiopsy
workup. The area to biopsy is better delineated, and
since these processes do not involve the lung uniformly, multiple
biopsies from one lung are preferable. Computed tomography
is critical to proper site selection.
Experience with these diseases dictates close cooperation
between the pulmonogist, radiologist and pathologist in order to
improve diagnostic accuracy. A recent Joint Statement of the
American Thoracic Society and the European Respiratory
Society was published January 15, 2002. This paper provides a
comprehensive review of the current state of our knowledge
regarding the idiopathic interstitial pneumonias.
The Idiopathic Interstitial Pneumonias
Idiopathic Pulmonary Fibrosis (IPF) has come to have a
more limited definition. This disease currently encompasses a
group of patients with progressive fibrosing interstitial pneumonia.
The patients usually present in the 5 th decade and relate a
history of 6 months or more of increasing dyspnea on exertion.
The chest radiograph demonstrates low lung volumes with
peripheral reticular opacities that are found predominantly in the
lower lung fields. The chest CT is characterized by reticular
opacities and honeycombing in a strikingly peripheral and lower
lobe distribution. These changes are often associated with traction
bronchiectasis and architectural distortion. Ground glass
may also be found to a lesser extent. When ground glass is associated
with reticulation and bronchiectasis then it usually correlates
with the presence of fibrosis on biopsy. A confident diagnosis
of IPF based on typical clinical features of IPF obviates
the need for biopsy. However, typical HRCT features of IPF are
found in only 50% of patients with biopsy proven disease.
Respiratory Bronchiolitis-Associated Interstitial Lung
Disease (RB-ILD) and Desquamative Interstitial Pneumonia
(DIP) are best thought of as a spectrum of smoking related
fibrotic and inflammatory reactions. Respiratory bronchiolitis is
found in the lung biopsies of essentially all smokers although it
rarely causes symptoms. It is characterized by the presence of
pigmented macrophages within and around respiratory bronchioles.
Mild peribrochiolar fibrosis may occur. A small percentage
of the smokers with respiratory bronchiolitis will present with
significant dypnea and hypoxemia. The patients are almost
always heavy smokers in their fourth or fifth decade who
improve after cessation of smoking. The chest radiograph
demonstrates airway thickening in the majority of patients but
may be normal. The chest CT reveals centrilobular nodules and
ground glass with an upper lobe predilection. Mild upper lobe
emphysema is also common. Evidence of air trapping may be
present on expiratory CT scans. The differential diagnosis for
this appearance includes hypersensitivity pneumonitis and
Langerhans Cell Histiocytosis.
DIP is now considered to be a more extensive form of respiratory
bronchiolitis in which the macrophages are found within
the alveolar spaces in addition to those found in the peribronchiolar
region. Rarely, the disease occurs in non-smokers. The
patients present with dypnea and low lung volumes. The chest
radiograph is usually characterized by patchy lower lobe ground
glass. However, the chest radiograph is normal in up to 25% of
cases. The chest CT demonstrates ground glass opacity in a
peripheral and lower lobe distribution in the majority of cases.
Reticular opacities and irregular lines are usually found in the
lung bases and honeycombing is found in less than one third.
Acute Interstitial Pneumonia (AIP) is a rapidly progressive
process with a subacute (2-3 weeks) presentation. Most patients
relate a history suggestive of a prior viral illness and the histologic
pattern is indistinguishable from adult respiratory distress
syndrome. The patients develop severe hypoxemia and mechani-
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