Patients with interstitial lung diseases (ILDs), also called diffuse parenchymal lung diseases, generally present with breathlessness due to impaired gas exchange as a consequence of widespread inflammation and/or fibrosis of the alveolar walls.

There are more than 300 different conditions included among the total number of ILDs. For epidemiological purposes, a practical, and therefore appealing, classification distinguishes ILDs of known cause from those of unknown aetiology (table 1). Some of these diseases, such as sarcoidosis and ILD associated with connective tissue disease (CTD), also affect other organs and this may determine the prognosis to a greater extent than the lung dysfunction.

ILDs usually have a gradual onset but can also present an acute course. Chest radiography and thoracic computed tomography (CT) typically show widespread nodular and/or fine linear (‘reticular’) shadowing with, at a later stage, fibrotic distortion and sometimes ‘honeycombing’ of the lungs. Pulmonary function testing shows a ‘restrictive’ (and, much less often, an ‘obstructive’) ventilatory defect and hypoxaemia (low blood oxygen), which is particularly seen during exercise. Diagnosis is often made using a combination of the clinical, pathophysiological, immunological and imaging (especially CT) features. For a precise diagnosis, a surgical lung biopsy with histological examination may be needed; however, even this procedure does not always give a clear answer. The microscopic  appearance of the lung should be interpreted according to the recent American Thoracic Society (ATS)/European Respiratory Society (ERS) consensus classification of the idiopathic interstitial pneumonias (IIPs), of which one of the commonest is idiopathic pulmonary fibrosis (IPF).

Therapy includes anti-inflammatory and antifibrotic agents, but in advanced disease it may be limited to palliative care. In ILDs resulting from known exogenous causes, it is crucial to avoid further exposure. Despite treatment, some forms of ILD, such as IPF, have a downward course, and lung transplantation may need to be considered.

Major ILDs of known aetiology (~35% of all patients with ILDs)
             Pneumoconioses (e.g. asbestosis, silicosis)  
Extrinsic allergic alveolitis (hypersensitivity pneumonitis)  
Iatrogenic ILD caused by drugs and/or radiation  
Post-infectious ILD  
Major ILDs of unknown aetiology (~65% of all patients with ILDs)
Idiopathic interstitial pneumonias, of which the most important are:
      IPF with a histopathological pattern of usual interstitial pneumonia (~55%
      of IIPs)
      Nonspecific interstitial pneumonia (~25% of IIPs)
      Respiratory bronchiolitis ILD, occurring in smokers (~10% of IIPs)
      Desquamative interstitial pneumonia (~5% of IIPs)
      Cryptogenic organising pneumonia (~3% of IIPs)
      Lymphoid interstitial pneumonia (~1% of IIPs)
      Acute interstitial pneumonia (~1% of IIPs)
ILD in CTDs and in collagen-vascular diseases, of which the most important are:
      ILD in rheumatoid arthritis
      ILD in progressive systemic sclerosis
Table 1 – Classification of interstitial lung diseases (ILDs). IIP: idiopathic interstitial pneumonia; IPF: idiopathic
pulmonary fibrosis; CTDs: connective tissue diseases.

See the entire Interstitial lung diseases Chapter