Acute hypersensitivity pneumonitis (AHP) refers to the acute form of hypersensitivity pneumonitis although this classification system has recently been challenged 4. For a general discussion of the condition, refer to the parent article.
Fever, chills, myalgia, headaches, coughing, chest tightness, dyspnea, and leukocytosis can occur in various combinations. This typically occurs 4-12 hours after exposure 1-3. Symptoms may begin after patients return to an environment from which they have been absent for a while (e.g. resuming work following weekends or holidays) but in at times do not develop with uninterrupted, routine contact with the same antigen.
Acute hypersensitivity pneumonitis is histologically characterised by the presence of neutrophilic infiltration of the respiratory bronchioles and alveoli. A pattern of diffuse alveolar damage and temporally uniform, non-specific, chronic interstitial pneumonitis may also be seen.
Chest radiographs are often normal in patients with mild symptoms and can remain normal despite severe symptoms 3. Plain radiography may show bilateral areas of increased opacities that may be either heterogeneous or homogeneous and can simulate pulmonary oedema 2-3.
The acute phase is dominated by air space abnormality with no features of fibrosis. There are usually homogeneous ground-glass and alveolar opacities which are usually bilateral and symmetric but sometimes patchy and concentrated in the middle part and base of the lungs or in a bronchovascular distribution. There may be a lower zonal predilection in the acute form.
The combination of segmental or lobular air trapping, normal lung, and ground glass changes can give a head cheese appearance.
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