Essentials

• Recurrent fever and dyspnoea may be caused by exposure to environmental organic particles or biological dusts, such as mould spores or bird droppings, in patients with allergic alveolitis.

Aetiology

• Allergic alveolitis is caused by repeated or continuous exposure to organic particles or biological dusts. In some countries the condition is known as hypersensitivity pneumonitis. Several causative factors are known.
• A common form of the disease is farmer's lung, which is caused by handling mouldy hay, litter or straw. In Finland the incidence of farmer's lung has nowadays decreased.
• Allergic alveolitis may also be caused by exposure to mouldy sawdust, wood chippings or mushroom compost.
• Cage bird droppings may cause allergic alveolitis known as bird fancier’s lung (bird breeder's lung). It is in many countries one of the most common forms of allergic alveolitis.
• New causes of allergic alveolitis may be found even nowadays.

Symptoms

• The symptoms of acute allergic alveolitis develop about four to eight hours after exposure.
  • The most common symptoms are cough, dyspnoea and fever.
  • Muscle and joint pains, headache and nausea may also appear.
  • If the condition persists, loss of appetite and weight loss may occur.
• The symptoms associated with farmer’s lung usually occur during the cattle indoor feeding season. The symptoms generally appear after the working day in the evening or at night, and they abate within a few days. With repeated exposure, the attacks recur.
• The subacute form is the most common form of allergic alveolitis, and it may manifest as febrile episodes, bronchitic symptoms, malaise, loss of appetite, weight loss and exertional dyspnoea.
• In cases where the symptoms are of a long duration a careful history should direct towards the possible diagnosis.
• Chronic allergic alveolitis may resemble idiopathic pulmonary fibrosis (Idiopathic pulmonary fibrosis), and exposure to some factor cannot always be found.

Clinical findings

• Lung auscultation often reveals fine inspiratory crackles.

Imaging and laboratory tests

• A chest x-ray may be normal or show an increased micronodular pattern, most prominent in the basal lobes, or more diffuse infiltrates. In the chronic form, signs suggesting pulmonary fibrosis may be found.
• Typical findings of high resolution CT scanning (HRCT) include air trapping and, in particular, centrilobular ground glass opacities. Reticular abnormalities are rarely seen in the acute and subacute phase, but they are common in the chronic phase. Honeycombing and traction bronchiectasis can also be encountered in the chronic phase, whereby the finding may be classified as so-called usual interstitial pneumonia (UIP). Sometimes the HRCT finding may resemble that of so-called nonspecific interstitial pneumonia (NSIP). Abnormalities usually show diffuse distribution.
• ESR and CRP may be elevated at the early stages. Leucocytosis may also be present.
• The patient’s serum often contains allergen-specific IgG antibodies against mould dust or other antigens. The presence of antibodies indicates prior exposure but does not necessarily represent disease.
• A bronchoalveolar lavage will show lymphocytosis and the T cell CD4/CD8 ratio may be reduced.

Pulmonary function tests

• Oxygen saturation when measured with a pulse oximeter may be reduced.
• The partial pressure of oxygen in an arterial sample is often reduced.
• Diffusing capacity is usually reduced.
• Restrictive changes are often evident on spirometry.

Diagnosis

• Diagnosis is based on three major criteria:
  1. exposure to organic dust can be demonstrated
  2. symptoms compatible with allergic alveolitis
  3. radiological findings compatible with allergic alveolitis.
• In addition to the major criteria two out of the six minor criteria should be fulfilled. The minor criteria are:
  • fine crackles on lung auscultation
  • reduced diffusing capacity
  • restrictive defect on spirometry
  • reduced partial pressure of oxygen in an arterial sample, and/or oxygen saturation, at rest or during exercise
  • lung biopsy compatible with the disease
  • positive provocation test.
• If an acute case is suspected, expertise in pulmonary medicine should be sought immediately in order to arrange for diagnostic investigations as soon as possible. If these investigations are not done until, for example, after the sick leave period, the results may have returned back to normal.
• New international guidelines are being developed for the diagnosis and treatment of chronic allergic alveolitis.

Differential diagnoses

• Pneumonia (Pneumonia)
• Sarcoidosis (Sarcoidosis)
• Tuberculosis (Diagnosing tuberculosis)
• Interstitial and fibrotic lung diseases (Idiopathic pulmonary fibrosis (IPF))
• Pulmonary drug reactions
• Lymphangitis carcinomatosa
• Organic dust toxicity syndrome (ODTS).

Treatment

• Measures to eliminate allergen exposure and avoidance of further exposure are the cornerstones of treatment.
• Glucocorticoids are used in severe cases to accelerate recovery, for example prednisolone with a starting dose of 30–60 mg a day followed by gradual dose reduction. The duration of drug treatment is usually about 2–4 weeks, and it does not alter long-term prognosis.
• Exposure leading the recurrence of allergic alveolitis should be prevented, for example by using an air-supplied helmet respirator.
• Allergic alveolitis due to occupational exposure, for example in farmers, may be considered an occupational disease entitling the patient to compensation.
• In the chronic form, treatment methods of other types of pulmonary fibrosis are applied.
• In a recent study, nintedanib was found to slow down the rate of decline in the forced vital capacity (FVC) in some patients with a progressive fibrosing interstitial lung disease, such as chronic allergic alveolitis, for example. Official indications and reimbursement policies concerning nintedanib in allergic alveolitis may vary between countries.

Prognosis

• Long-term, untreated illness can become chronic and may cause, for example, pulmonary fibrosis and emphysema.
• Usually lung function will more or less normalise, provided that the disease is diagnosed and treated in time.

References