Interstitial lung disease
|Interstitial lung disease|
|End-stage pulmonary fibrosis of unknown origin, taken from an autopsy in the 1980s|
Interstitial lung disease (ILD), or diffuse parenchymal lung disease (DPLD), is a group of lung diseases affecting the interstitium (the tissue and space around the air sacs of the lungs). It concerns alveolar epithelium, pulmonary capillary endothelium, basement membrane, perivascular and perilymphatic tissues. It may occur when an injury to the lungs triggers an abnormal healing response. Ordinarily, the body generates just the right amount of tissue to repair damage. But in interstitial lung disease, the repair process goes awry and the tissue around the air sacs (alveoli) becomes scarred and thickened. This makes it more difficult for oxygen to pass into the bloodstream. The term ILD is used to distinguish these diseases from obstructive airways diseases.
In children, several unique forms of ILD exist which are specific for the young age groups. The acronym chILD is used for this group of diseases and is derived from the English name, Children’s Interstitial Lung Diseases – chILD.
Prolonged ILD may result in pulmonary fibrosis, but this is not always the case. Idiopathic pulmonary fibrosis is interstitial lung disease for which no obvious cause can be identified (idiopathic), and is associated with typical findings both radiographic (basal and pleural based fibrosis with honeycombing) and pathologic (temporally and spatially heterogeneous fibrosis, histopathologic honeycombing and fibroblastic foci).
ILD may be classified according to the cause. One method of classification is as follows:
- Inhaled substances
- Connective tissue and Autoimmune diseases
- Predominantly in children
Investigation is tailored towards the symptoms and signs. A proper and detailed history looking for the occupational exposures, and for signs of conditions listed above is the first and probably the most important part of the workup in patients with interstitial lung disease. Pulmonary function tests usually show a restrictive defect with decreased diffusion capacity (DLCO).
A lung biopsy is required if the clinical history and imaging are not clearly suggestive of a specific diagnosis or malignancy cannot otherwise be ruled out. In cases where a lung biopsy is indicated, a trans-bronchial biopsy is usually unhelpful, and a surgical lung biopsy is often required.
High resolution CT of the chest is the preferred modality, and differs from routine CT of the chest. Conventional (regular) CT chest examines 7–10 mm slices obtained at 10 mm intervals; high resolution CT examines 1-1.5 mm slices at 10 mm intervals using a high spatial frequency reconstruction algorithm. The HRCT therefore provides approximately 10 times more resolution than the conventional CT chest, allowing the HRCT to elicit details that cannot otherwise be visualized.
Radiologic appearance alone however is not adequate and should be interpreted in the clinical context, keeping in mind the temporal profile of the disease process.
Interstitial lung diseases can be classified according to radiologic patterns.
Pattern of opacities
Acute: Alveolar hemorrhage syndromes, acute eosinophilic pneumonia, acute interstitial pneumonia, cryptogenic organizing pneumonia
Chronic: Chronic eosinophilic pneumonia, cryptogenic organizing pneumonia, lymphoproliferative disorders, pulmonary alveolar proteinosis, sarcoidosis
- Linear or reticular opacities
Acute: Pulmonary edema
Chronic: Idiopathic pulmonary fibrosis, connective tissue associated interstitial lung diseases, asbestosis, sarcoidosis, hypersensitivity pneumonitis, drug-induced lung disease
- Small nodules
Acute: Hypersensitivity pneumonitis
Chronic: Hypersensitivity pneumonitis, sarcoidosis, silicosis, coal workers pneumoconiosis, respiratory bronchiolitis, alveolar microlithiasis
- Cystic airspaces
Chronic: Pulmonary langerhans cell histiocytosis, pulmonary lymphangioleiomyomatosis, honeycomb lung caused by IPF or other diseases
Ground glass opacities
Acute: Alveolar hemorrhage syndromes, pulmonary edema, hypersensitivity pneumonitis, acute inhalational exposures, drug-induced lung diseases, acute interstitial pneumonia
Chronic: Nonspecific interstitial pneumonia, respiratory bronchiolitis associated interstitial lung disease, desquamative interstitial pneumonia, drug-induced lung diseases, pulmonary alveolar proteinosis
- Thickened alveolar septa
Acute: Pulmonary edema
Chronic: Lymphangitic carcinomatosis, pulmonary alveolar proteinosis, sarcoidosis, pulmonary veno occlusive disease
- Upper lung predominance
Pulmonary Langerhans cell histiocytosis, silicosis, coal workers pneumoconiosis, carmustine related pulmonary fibrosis, respiratory broncholitis associated with interstitial lung disease.
- Lower lung predominance
Idiopathic pulmonary fibrosis, pulmonary fibrosis associated with connective tissue diseases, asbestosis, chronic aspiration
- Central predominance (perihilar)
- Peripheral predominance
Idiopathic pulmonary fibrosis, chronic eosinophilic pneumonia, cryptogenic organizing pneumonia
- Pleural effusion or thickening
Pulmonary edema, connective tissue diseases, asbestosis, lymphangitic carcinomatosis, lymphoma, lymphangioleiomyomatosis, drug-induced lung diseases
Sarcoidosis, silicosis, berylliosis, lymphangitic carcinomatosis, lymphoma, lymphocytic interstitial pneumonia
For some types of chILD and few forms adult ILD genetic causes have been identified. These may be identified by blood tests. For a limited number of cases this is a definite advantage, as a precise molecular diagnosis can be done; frequently then there is no need for a lung biopsy. Testing is available for
Surfactant-Protein-B Deficiency (Mutations in SFTPB)
Surfactant-Protein-C Deficiency (Mutations in SFTPC)
ABCA3-Deficiency (Mutations in ABCA3)
Brain Lung Thyroid Syndrome (Mutations in TTF1)
Congenital Pulmonary Alveolar Proteinosis (Mutations in CSFR2A, CSFR2B)
Diffuse developmental disorder
Alveolar Capillary Dysplasia (Mutations in FoxF1)
Idiopathic pulmonary fibrosis
Mutations in telomerase reverse transcriptase (TERT)
Mutations in telomerase RNA component (TERC)
Mutations in the regulator of telomere elongation helicase 1 (RTEL1)
Mutations in poly(A)-specific ribonuclease (PARN)
ILD is not a single disease, but encompasses many different pathological processes. Hence treatment is different for each disease.
If a specific occupational exposure cause is found, the person should avoid that environment. If a drug cause is suspected, that drug should be discontinued.
Many cases due to unknown or connective tissue-based causes are treated with corticosteroids, such as prednisolone. Some people respond to immunosuppressant treatment. Patients with a low level of oxygen in the blood may be given supplemental oxygen.
On October 16, 2014, the Food and Drug Administration approved a new drug for the treatment of Idiopathic Pulmonary Fibrosis (IPF). This drug, Ofev (nintedanib), is marketed by Boehringer Ingelheim Pharmaceuticals, Inc. This drug has been shown to slow the decline of lung function although the drug has not been shown to reduce mortality or improve lung function. The estimated cost of the drug per year is approximately $94,000.
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