• Clinical science

Hypersensitivity pneumonitis (Extrinsic allergic alveolitis)

Abstract

Hypersensitivity pneumonitis (or extrinsic allergic alveolitis) is a hypersensitivity reaction following exposure to environmental allergens. It is associated with inflammatory interstitial lung disease. Occupational groups affected by hypersensitivity pneumonitis are most commonly exposed to birds, hay, or certain reactive chemical species. Hypersensitivity pneumonitis can be classified into acute, subacute, or chronic forms depending on the frequency, length, and intensity of exposure, severity of symptoms, and duration of subsequent illness. The acute form is characterized by fever, flu-like symptoms, and leukocytosis. The subacute form is characterized by the insidious onset of productive cough, dyspnea, and fatigue over weeks to months. Chronic hypersensitivity pneumonitis is characterized by long-term progressive dyspnea, weight loss, cough, and fatigue. Diagnosis is usually based on occupational history and evidence of fibrosis on imaging, bronchoalveolar lavage with lymphocytosis, positive inhalation challenge testing, or histopathology showing poorly formed noncaseating granulomas or mononuclear cell infiltrate. The preferred treatment for the acute form includes allergen avoidance (e.g., change of occupation) and administration of glucocorticoids for symptomatic relief . The prognosis for the acute form is good. Once severe damage to lung structures has occurred, however, few treatment options remain. Chronic hypersensitivity pneumonitis may lead to pulmonary fibrosis, which may ultimately require transplantation.

Etiology

  • Combined type III and type IV hypersensitivity reaction with genetic predisposition
  • Inhalation of organic particles; (< 5 microns), primarily through occupational exposure (notifiable occupational disease)
Antigen Source Disease
Animal proteins
Microorganism (s)
  • Actinomycete spores from air conditioners, humidifiers, and water reservoirs
  • Actinomycete spores from moldy hay
  • Actinomycete spores from sugar cane
  • Penicillin casei or P. roqueforti spores from cheese casings
  • Actinomycete spores from moldy compost
  • Grain weevil dust
  • Various bacteria in saw dust (through logging)
Chemical substances
  • Isocyanates (used in the adhesive and foam industry)



Smokers are less likely to be symptomatic because they have a decreased immune response to new antigens!

References:[1][2][3][4]

Clinical features

  • Acute (commencing 4–8 hours after exposure)
    • Flu-like symptoms: Fever, chills, malaise, cough, headache
    • Dyspnea without wheezing
    • Diffuse fine crackles upon auscultation
    • Symptoms subside after 12 hours to several days (in the absence of additional exposure)
  • Subacute (weeks to months after continuous exposure)
    • Insidious onset of fatigue; Possible progression to persistent productive cough and dyspnea
    • May begin noticing weight loss
  • Chronic (months after continuous exposure)
    • Insidious onset of fatigue, productive cough, progressive dyspnea, cyanosis
    • Bilateral rales
    • Weight loss

A recurrent 'common cold' with an irritating cough and fever may indicate hypersensitivity pneumonitis!

References:[3][4][5]

Diagnostics

Primarily a clinical diagnosis based on history of exposure and typical clinical presentation, which is supported by the presence of any one of the following:

  • Positive serology: IgG, IgA, or IgM antibodies
  • Chest x-ray or CT
    • Acute: Patchy reticulonodular or diffuse infiltrates may be found in the mid to upper zone.
    • Chronic : ground-glass attenuation with honeycombing (fibrotic changes) +/- emphysema
  • Pulmonary function test: : restrictive or mixed restrictive / obstructive pattern
  • Bronchoalveolar lavage (BAL): : lymphocytic predominance
  • Inhalation challenge
    • Procedure: inhalation of known antigens to observe symptoms within 8-12 hours after exposure in a controlled setting
    • Not routinely done
  • Lung biopsy : histopathological confirmation

In hypersensitivity pneumonitis, IgG antibodies are found during an allergic reaction!

References:[4][6]

Pathology

  • Granulomatous inflammation with lymphocytes and polynuclear giant cells

References:[3][4]

Differential diagnoses

References:[4]

The differential diagnoses listed here are not exhaustive.

Treatment

  • Antigen avoidance
  • Glucocorticoid therapy
  • Last resort: lung transplant for persistent cases that are not responsive to medical therapy

References:[4]

Complications

  • Respiratory failure
  • Pulmonary heart disease

References:[3]

We list the most important complications. The selection is not exhaustive.

Prognosis

  • Favorable in the acute stage, but the disease recurs and worsens upon re-exposure
  • Worsens with severity of fibrosis

References:[4]