Interstitial lung disease

Interstitial lung disease (ILD) is an umbrella term that encompasses a large number of disorders that are characterized by diffuse cellular infiltrates in a periacinar location. The spectrum of conditions included is broad, ranging from occasional self-limited inflammatory processes to severe debilitating fibrosis of the lungs. 

Terminology

ILD is considered as a misnomer by some, as many of the diseases also involve the alveolar spaces.  

Clinical presentation

Interstitial lung diseases classically produce the "3Cs": cough, clubbing of the nails, and coarse crackles on auscultation ⁶.

Functional respiratory tests commonly show an abnormal restrictive pattern and reduced diffusing capacity. 

Pathology

Etiology

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, Langerhans 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
  • antisynthetase syndrome
  • ankylosing spondylitis
  • Sjögren syndrome
  • mixed connective tissue disease
• miscellaneous, e.g. amyloidosis, alveolar proteinosis

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

• usual interstitial pneumonia (UIP): idiopathic pulmonary fibrosis
• cryptogenic organizing 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, but patterns vary among the different etiologies. Please refer to the articles in each specific etiology listed above for specific details on their imaging pattern.