extrinsic allergic alveolitis

Question 1

definition

Answer 1

Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis, is the result of non-IgE mediated immunological inflammation. HP is caused by repeated inhalation of non-human protein, which can be of natural plant or animal origin or can be the result of a chemical conjugated to a human airway protein, such as albumin. The inflammation of HP manifests itself in the alveoli and distal bronchioles. The clinical manifestations of HP depend on the concentration and frequency of exposure. The clinical syndromes - acute, sub-acute, and chronic HP - present differently. Some authors are now reporting that there are only two clinical syndromes; however, they vary as to whether they group sub-acute with the acute or the chronic form. Many cases of HP are caused by occupational exposure.

Question 2

aetiology

Answer 2

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.

Question 3

history/symptoms

Answer 3

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 increasing breathlessness and decreasing 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.

Question 4

signs/examination

Answer 4

Acute: Rapid shallow breathing, pyrexia, inspiratory crepitations.

Chronic: Fine inspiratory crepitations (see Cryptogenic fibrosing alveolitis). Finger clubbing
is rare.

Question 5

investigations

Answer 5

Blood: FBC (neutrophilia, lymphopenia), ABG (decreased PO2, decreased 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 (decreased/normal FEV1, decreased FVC with preserved or increased ratio), decreased TLCO.

Bronchoalveolar lavage: Increased cellularity with increased CD8 þ suppressor T cells. Lung biopsy
(transbronchial or thorascopic).