Acute hypersensitivity pneumonitis
Acute hypersensitivity pneumonitis, also known as acute extrinsic allergic alveolitis, refers to the episodic form of this condition usually happening in just a few hours after the antigen exposure and often recurring with the re-exposure. It represents the most inflammatory side of the spectrum of hypersensitivity pneumonitis and has the potential to resolve with treatment.
For a general discussion of the condition, refer to the parent article on hypersensitivity pneumonitis.
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Clinical presentation
Fever, chills, myalgia, headaches, coughing, chest tightness, dyspnea, and leukocytosis can occur in various combinations and typically occurs 4-12 hours after exposure 1-3.
The 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.
Pathology
Microscopic appearance
Acute hypersensitivity pneumonitis is histologically characterized 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.
Radiographic features
Plain radiograph
Chest radiographs are often normal in patients with mild symptoms and can remain normal despite severe symptoms 3.
When abnormal, it may present with:
- bilateral areas of increased opacities that may be either heterogeneous or homogeneous and can simulate pulmonary edema 2,3
- numerous poorly defined small (<5 mm) opacities throughout both lungs, sometimes with sparing of the apices and bases
- a pattern of fine reticulation may also occur
- zonal distribution is variable from patient to patient and may even show temporal variation within the same patient
CT
The acute phase is dominated by air space abnormality (alveolitis) with no features of fibrosis. Findings include:
- homogeneous groundglass opacities that are usually bilateral and symmetric, but sometimes patchy and concentrated in the middle part and base of the lungs or in a bronchovascular distribution
- numerous "soft" centrilobular opacities: usually <5 mm in diameter (occasionally these opacities have well-defined borders and soft-tissue attenuation)
- hypoattenuation and hypovascularity of scattered secondary lobules: hypoattenuating regions that persist on expiratory CT scans are indicative of air trapping, which is caused by bronchiolar inflammation and obstruction: this may give a mosaic attenuation pattern
- there may be a lower zonal predilection in the acute form
Differential diagnosis
Related Radiopaedia articles
Chest
- imaging techniques
-
chest x-ray
-
approach
- adult
- pediatric
- neonatal
-
airspace opacification
- differential diagnoses of airspace opacification
- lobar consolidation
-
atelectasis
- mechanism-based
- morphology-based
- lobar lung collapse
- chest x-ray in the exam setting
- cardiomediastinal contour
- chest radiograph zones
- tracheal air column
- fissures
- normal chest x-ray appearance of the diaphragm
- nipple shadow
-
lines and stripes
- anterior junction line
- posterior junction line
- right paratracheal stripe
- left paratracheal stripe
- posterior tracheal stripe/tracheo-esophageal stripe
- posterior wall of bronchus intermedius
- right paraspinal line
- left paraspinal line
- aortic-pulmonary stripe
- aortopulmonary window
- azygo-esophageal recess
- spaces
- signs
- air bronchogram
- big rib sign
- Chang sign
- Chen sign
- coin lesion
- continuous diaphragm sign
- dense hilum sign
- double contour sign
- egg-on-a-string sign
- extrapleural sign
- finger in glove sign
- flat waist sign
- Fleischner sign
- ginkgo leaf sign
- Golden S sign
- Hampton hump
- haystack sign
- hilum convergence sign
- hilum overlay sign
- Hoffman-Rigler sign
- holly leaf sign
- incomplete border sign
- juxtaphrenic peak sign
- Kirklin sign
- medial stripe sign
- melting ice cube sign
- more black sign
- Naclerio V sign
- Palla sign
- pericardial fat tag sign
- Shmoo sign
- silhouette sign
- snowman sign
- spinnaker sign
- steeple sign
- straight left heart border sign
- third mogul sign
- tram-track sign
- walking man sign
- water bottle sign
- wave sign
- Westermark sign
-
approach
- HRCT
-
chest x-ray
- airways
- bronchitis
- small airways disease
-
bronchiectasis
- broncho-arterial ratio
- related conditions
- differentials by distribution
- narrowing
-
tracheal stenosis
- diffuse tracheal narrowing (differential)
-
bronchial stenosis
- diffuse airway narrowing (differential)
-
tracheal stenosis
- diverticula
- pulmonary edema
-
interstitial lung disease (ILD)
- drug-induced interstitial lung disease
-
hypersensitivity pneumonitis
- acute hypersensitivity pneumonitis
- subacute hypersensitivity pneumonitis
- chronic hypersensitivity pneumonitis
- etiology
- bird fancier's lung: pigeon fancier's lung
- farmer's lung
- cheese workers' lung
- bagassosis
- mushroom worker’s lung
- malt worker’s lung
- maple bark disease
- hot tub lung
- wine maker’s lung
- woodsman’s disease
- thatched roof lung
- tobacco grower’s lung
- potato riddler’s lung
- summer-type pneumonitis
- dry rot lung
- machine operator’s lung
- humidifier lung
- shower curtain disease
- furrier’s lung
- miller’s lung
- lycoperdonosis
- saxophone lung
-
idiopathic interstitial pneumonia (mnemonic)
- acute interstitial pneumonia (AIP)
- cryptogenic organizing pneumonia (COP)
- desquamative interstitial pneumonia (DIP)
- non-specific interstitial pneumonia (NSIP)
- idiopathic pleuroparenchymal fibroelastosis
- lymphoid interstitial pneumonia (LIP)
- respiratory bronchiolitis–associated interstitial lung disease (RB-ILD)
- usual interstitial pneumonia / idiopathic pulmonary fibrosis (UIP/IPF)
-
pneumoconioses
- fibrotic
- non-fibrotic
-
lung cancer
-
non-small-cell lung cancer
-
adenocarcinoma
- pre-invasive tumors
- minimally invasive tumors
- invasive tumors
- variants of invasive carcinoma
- described imaging features
- adenosquamous carcinoma
- large cell carcinoma
- primary sarcomatoid carcinoma of the lung
- squamous cell carcinoma
- salivary gland-type tumors
-
adenocarcinoma
- pulmonary neuroendocrine tumors
- preinvasive lesions
-
lung cancer invasion patterns
- tumor spread through air spaces (STAS)
- presence of non-lepidic patterns such as acinar, papillary, solid, or micropapillary
- myofibroblastic stroma associated with invasive tumor cells
- pleural invasion
- vascular invasion
- tumors by location
- benign neoplasms
- pulmonary metastases
- lung cancer screening
- lung cancer staging
-
non-small-cell lung cancer