Pneumonitis is a general term for inflammation of lung tissue. Chronic inflammation of lung tissue can lead to irreversible scarring (pulmonary fibrosis). Pneumonitis is not a specific disease but a sign of an underlying problem.

Acute chemical pneumonitis causes swelling of the lung tissue, movement of fluid into the air spaces in the lung and reduced ability to absorb oxygen and remove carbon dioxide. In severe cases, death may result from hypoxia.

Chronic pneumonitis may follow low levels of exposure to the irritant over long periods of time, causing inflammation which may lead to fibrosis, resulting in decreased gas exchange and stiffening of the lung, and ultimately leading to respiratory failure and death.

Causes of pneumonitis

Causes of pneumonitis include:

• Pneumonia.
• Inhalation of foreign matter, usually of stomach contents when vomiting (aspiration pneumonitis).
• Pertussis.
• Exposure to an inhaled allergen (hypersensitivity pneumonitis), e.g. humidifier lung, farmer's lung, bird fancier's lung.
• Systemic lupus erythematosus.
• Adverse reaction to a drug or toxic chemical; many household and industrial chemicals can cause acute and chronic pneumonitis:
  • Exposure to dangerous levels of chlorine gas may occur at home when using cleaning materials, in industrial accidents or when near to swimming pools.
  • Inhalation of dangerous substances can occur during smelting, welding or other metalwork, in the production or use of solvents or pesticides, fires and when handling grain.
  • Medication: a variety of medications can cause interstitial pneumonitis, e.g. interferon therapy, amiodarone, nitrofurantoin.
• Radiation therapy.¹
• Sepsis: the body's inflammatory response to infection.

Epidemiology

• It is common, if all causes of pneumonitis are considered.
• The annual incidence of interstitial lung diseases has been estimated as 30:100,000 with hypersensitivity pneumonitis accounting for less than 2% of these cases.²

Presentation

• History of exposure to a precipitating cause, e.g. birds, radiotherapy, dusts, drugs, chemicals.
• Clinical features will depend on the severity and underlying cause and may include:
  • Shortness of breath.
  • Cough.
  • A burning sensation in the chest.
  • Symptoms which chronic pneumonitis may lead to: fatigue, weight loss, exercise intolerance, cyanosis and finger clubbing.

Differential diagnosis

Any other cause of acute, subacute or chronic respiratory distress or cough - for example:

• Infection, including legionella, Q fever, tuberculosis.
• Sarcoidosis.
• Lung cancer.

Investigations

• Blood tests: FBC (may be raised neutrophilia, lymphocytosis, eosinophilia), raised ESR and CRP.
• Blood gases: hypoxaemia.
• Sputum or culture of lung secretions with bronchoscopy.
• CXR: may be normal or show micronodular or reticular opacities.
• CT scan: may also be normal but far more sensitive. May show diffuse, patchy ground-glass attenuation and small, poorly defined centrilobular nodules; patchy areas of air-trapping; evidence of pulmonary fibrosis and honeycombing may be seen in chronic and advanced disease.
• Pulmonary function tests: spirometry usually shows restrictive changes but may be a mixed obstructive/restrictive picture.
• Lung biopsy: this is occasionally required if other tests fail to establish the diagnosis.

Management

• Avoidance of any established precipitating cause.
• Treatment of pneumonitis depends on the underlying cause and may include medications such as:
  • Systemic corticosteroid therapy, which can speed resolution of hypersensitivity pneumonitis.
  • Antibiotics for infection.

Complications

• Severe acute or subacute flares may cause severe respiratory distress and can be life-threatening.
• Recurrent pneumonia.
• Pneumothorax.
• Pulmonary fibrosis.
• Cor pulmonale.
• Chronic obstructive pulmonary disease.

Prognosis

• Prognosis is good with early diagnosis and management.
• Late-diagnosed chronic pneumonitis may lead to progressive, irreversible lung disease.

Prevention

Avoid exposure to the cause, e.g. control of occupational hazards, routine maintenance of heating, ventilation and air-conditioning equipment.

Document references

1. Ta V, Aronowitz P; Radiation Pneumonitis. J Gen Intern Med. 2011 May 3.
2. Lacasse Y, Cormier Y; Hypersensitivity pneumonitis. Orphanet J Rare Dis. 2006 Jul 3;1:25. [abstract]

Internet and further reading

• Farber HJ et al; Pediatric Hypersensitivity Pneumonitis, Medscape, Mar 2010