Interstitial lung disease
Interstitial lung disease (ILD), also known as diffuse parenchymal lung disease (DPLD), refers to 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.
The term ILD is used to distinguish these diseases from obstructive airways diseases.
Prolonged ILD may result in pulmonary fibrosis, but this is not always the case. The term Idiopathic pulmonary fibrosis is used to describe interstitial lung disease for which no obvious cause can be identified (idiopathic), and is associated with typical radiographic (basal and pleural based fibrosis with honeycombing) and pathologic (temporally and spatially heterogeneous fibrosis, histopathologic honeycombing and fibroblastic foci) findings.

Causes
ILD may be classified according to the cause. One method of classification is as follows:
1. Inhaled substances
Inorganic
•    Silicosis
•    Asbestosis
•    Berylliosis
Organic
•    Hypersensitivity pneumonitis
2. drug-induced
Antibiotics
Chemotherapeutic drugs
Antiarrhythmic agents
Statins
3. Connective tissue disease
•    Systemic sclerosis
•    Polymyositis
•    Dermatomyositis
•    Systemic lupus erythematosus
•    Rheumatoid arthritis
4. Infection
•    Atypical pneumonia
•    Pneumocystis pneumonia (PCP)
•    Tuberculosis
•    Chlamydia trachomatis
•    Respiratory Syncytial Virus
5. Idiopathic
•    Sarcoidosis
•    Idiopathic pulmonary fibrosis
•    Hamman-Rich syndrome
•    Antisynthetase syndrome
6. Malignancy
•    Lymphangitic carcinomatosis

Diagnosis
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.
X-rays
Chest radiography is usually the first test to detect interstitial lung diseases, but the chest radiograph can be normal in up to 10% of patients, especially early on the disease process.
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
Consolidation
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

 

Distribution
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)
Sarcoidosis, berylliosis

Peripheral predominance
Idiopathic pulmonary fibrosis, chronic eosinophilic pneumonia, cryptogenic organizing pneumonia

 

Treatment
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.
Pulmonary rehabilitation appears to be useful.  Lung transplantation is an option if the ILD progresses despite therapy in appropriately selected patients with no other contraindications.