Extrinsic allergic alveolitis
D E FI N I T ION
Interstitial inflammatory disease of the distal gas-exchanging parts of the
lung caused by inhalation of organic dusts. Also known as hypersensitivity pneumonitis.
AE T IOLOGY
Inhalation of antigenic organic dusts containing microbes (bacteria, fungi or
amoebae) or animal proteins induce a hypersensitivity response (a combination of type III
antigen–antibody complex hypersensitivity reaction and a type IV granulomatous lymphocytic
inflammation) in susceptible individuals. Examples:
Farmer’s lung: Mouldy hay containing thermophilic actinomycetes.
Pigeon/budgerigar fancier’s lung: Bloom on bird feathers and excreta.
Mushroom worker’s lung: Compost containing thermophilic actinomycetes.
Humidifier lung: Water-containing bacteria and Naegleria (amoeba).
Maltworker’s lung: Barley or maltings containing Aspergillus clavatus.
E P IDEMIOLOGY
Uncommon, 2% of occupational lung diseases, 50% of reported cases
affect farm workers (incidence is about 4–10 in 100 000/year), marked geographical variation
reflecting dependence on occupational causes.
H ISTORY
Acute: Presents 4–12 h post-exposure. Reversible episodes of dry cough, dyspnoea, malaise,
fever, myalgia. Wheeze and productive cough may develop on repeat high-level
exposures.
Chronic: Poorly reversible manifestation in some, slowly " breathlessness and # exercise
tolerance, weight loss. Exposure is usually chronic, low level and there may be no history
of previous acute episodes.
Full occupational history and enquiry into hobbies and pets important.
EXAMINA T I ON
Acute: Rapid shallow breathing, pyrexia, inspiratory crepitations.
Chronic: Fine inspiratory crepitations (see Cryptogenic fibrosing alveolitis). Finger clubbing
is rare.
INVE S T I G A T IONS
Blood: FBC (neutrophilia, lymphopenia), ABG (# PO2, # PCO2).
Serology: Precipitating IgG to fungal or avian antigens in serum; however, these are not
diagnostic as are often found in asymptomatic individuals.
CXR: Often normal in acute episodes, may show ‘ground glass’ appearance with alveolar
shadowing or nodular opacities in the middle and lower zones. In chronic cases, fibrosis is
prominent in the upper zones.
High-resolution CT-thorax: Detects early changes before CXR. Patchy ‘ground glass’
shadowing and nodules.
Pulmonary function tests: Restrictive ventilatory defect (# FEV1, # FVC with preserved or
increased ratio), # TLCO.
Bronchoalveolar lavage: Increased cellularity with " CD8þ suppressor T cells. Lung biopsy
(transbronchial or thorascopic).
MANAGEMENT
Advice: Complete avoidance of exposure to the antigen (e.g. change of work practice or
hobby), if this is problematic, then minimize exposure and encourage use of respiratory
protection masks.
Medical:
Acute flare: Spontaneous recovery usually within 1–2 days, high-dose corticosteroids for 2–4
weeks may accelerate recovery but do not appear to affect long-term outcome.
Chronic disease: Trial of high-dose oral prednisolone for 1 month may be carried out, this is
gradually reduced, or stopped if no objective response demonstrated.
Extrinsic allergic alveolitis (continued)
General: Regular follow-up to monitor lung function. Environmental assessment is necessary
for risk posed to others. In UK, farmer’s lung patients are entitled to compensation,
depending on the degree of disability.
COMPLICATIONS
Progressive lung fibrosis, pulmonary hypertension, right heart failure.
P ROGNOSIS
The acute form generally resolves if further exposure is prevented, with
chronic disease some patients will improve while a minority progress to lung fibrosis.
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