Extrinsic Allergic Alveolitis

Question 1

Define

Answer 1

Interstitial inflammatory disease of the distal gas-exchanging parts of the lung caused by inhalation of organic dusts.

Also known as hypersensitivity pneumonitis.

Question 2

Causes

Answer 2

Inhalation on antigenic organic dust containing microbes (bacteria/fungi/microbes) or animal proteins

→ Induce a hypersensitivity response in susceptible individuals

→ Combination of type III antigen-antibody complex hypersensitivity reaction and type IV granulomatous lymphocytic inflammation

Question 3

Risk factors

Answer 3

Farmers lung - mould hay, thermophilic actinocycetes

Pigeon fancier’s lung - bloom on feathers and excreta

Mushroom worker’s lung - compost containing thermophilic actinocycetes

Humidifier lung - water-containing bacteria and Naegleria (amoeba)

Maltworker’s lung - barley/maltings with Asp. Clavatus

Chemical worker’s lung - trimellitic anhydride, diisocyanate and methylene diisocyanate act as the antigens during the manufacture of plastics, polyurethane foam and rubber

Question 4

Epidemiology

Answer 4

• Uncommon, 2% occupational lung disease
• 50% cases affect farm workers
• Marked geographical variation reflecting dependence on occupational causes

Question 5

Symptoms

Answer 5

ACUTE

• Present 4-12 hrs after exposure
• REVERSIBLE episodes of:
  
  • Dry cough
  • Dyspnoea
  • Malaise
  • Fever
  • Myalgia
• Wheeze and productive cough may develop if repeat high-level exposure

CHRONIC

• Slowly increasing breathlessness
• Decreased exercise tolerance
• Weight loss
• Exposure is usually chronic, low-level and there may be no history of previous acute episodes

IMPORTANT: make sure you get a full occupational history and enquire about hobbies and pets

Question 6

Signs

Answer 6

ACUTE

• Rapid shallow breathing
• Pyrexia
• Inspiratory crepitations

CHRONIC

• Fine inspiratory crepitations
• Clubbing (rare)

Question 7

Investigations

Answer 7

Bloods

• FBC - neutrophilia, lymphopenia
• ABG - reduced PO2 + PCO2

Serology

• Test for IgG to fungal or avian antigens
• NOTE: these are not diagnostic because you may find these in normal individuals

CXR

• Often NORMAL in acute episodes
• Fibrosis may be seen in chronic cases

High Resolution CT-Thorax

• Detects early changes
• May show patchy ‘ground glass’ shadowing and nodules

Pulmonary Function Tests

• Restrictive defect (low FEV1, low FVC)
• Preserved or increased FEV1/FVC ratio
• Reduced total lung capacity

Bronchoalveolar Lavage

• Increased cellularity
• Lung biopsy can also be performed