Interstitial lung disease

Diffuse interstitial lung disease encompasses a large number of disorders that are characterised by cellular and extracellular infiltrates in a peri-acinar location. While some disease processes may cause signficant injury to the underlying lung parenchyma - such as UIP - some do not - pulmonary eosinophilia.

Pathology

Causes

The radiological appearances are not specific for the underlying cause of diffuse lung disease in many cases. It is therefore key to determine whether there is an underlying cause for the changes. A number of precipitants can cause diffuse interstitial disease such as

• smoking ¹
• organic dusts (causing extrinsic allergic alveolitis)
• inorganic dusts (causing pneumoconioses)
• gases or fumes
• drugs
• radiation
• Infection

Eliciting a history of underlying systemic disease is also helpful since they may involve the lungs in a diffuse and infiltrative manner. Examples include

• granulomatous diseases : e.g. sarcoidosis, Langerhan's cell histiocytosis
• neoplasia
  • primary : e.g. lymphoma, other lymphoproliferative diseases
  • secondary : e.g. pulmonary metastases, lymphangitis carcinomatosis
• vasculitis
• inherited diseases : e.g. neurofibromatosis
• autoimmune and collagen vascular diseases ³ :
  • systemic lupus erythematosus
  • rheumatoid arthritis
  • progressive systemic sclerosis
  • dermatomyositis and polymyositis
  • anti synthetase syndrome
  • ankylosing spondylitis
  • Sjögren syndrome
  • mixed connective tissue disease
• miscellaneous : e.g. amyloidosis, alveolar proteinosis

Where a cause is not determined, the idiopathic interstitial pneumonias (IIP) should be considered

• usual interstitial pneumonia (UIP) : idiopathic pulmonary fibrosis
• cryptogenic organising pneumonia (COP) : previously termed BOOP
• non-UIP IIP
  • non-specific interstitial pneumonia (NSIP) : non-smokers
  • respiratory bronchiolitis-interstitial lung disease (RB-ILD) : smokers
  • desquamative interstitial pneumonia (DIP) : end state of RB-ILD
  • lymphoid interstitial pneumonia (LIP) : women
  • acute interstitial pneumonitis (AIP)
• other entities :
  • combined pulmonary fibrosis and emphysema (CPFE)

Mnemonic : All Idiopathic Chronic Lung Diseases aRe Nonspecific

Radiographic features

The diffuse lung diseases tend to cause infiltrative opacification in the periphery of the lung. As the name of the group of diseases suggests, they are diffuse. While the consolidation or ground-glass change is usually bilateral, it may be localised, e.g. radiation pneumonitis.