Extrinsic allergic alveolitis is a lung disorder resulting from repeated inhalation of organic dust, usually in a specific occupational setting. In the acute form, respiratory symptoms and fever begin several hours after exposure to the dust. The chronic form is characterized by gradual changes in the lung tissue associated with several years of exposure to the irritant.
Hypersensitivity pneumonitis (HP) is categorized as acute, subacute, and chronic based on the duration of the illness.
In the acute form of HP, symptoms may develop 4–6 hours following heavy exposure to the provoking antigen. Symptoms include fever, chills, malaise, cough, chest tightness, dyspnea, rash, swelling and headache. Symptoms resolve within 12 hours to several days upon cessation of exposure.
Acute HP is characterized by poorly formed noncaseating interstitial granulomas and mononuclear cell infiltration in a peribronchial distribution with prominent giant cells.
On chest radiographs, a diffuse micronodular interstitial pattern (at times with ground-glass density in the lower and middle lung zones) may be observed. Findings are normal in approximately 10% of patients." In high-resolution CT scans, ground-glass opacities or diffusely increased radiodensities are present. Pulmonary function tests show reduced diffusion capacity of lungs for carbon monoxide (DLCO). Many patients have hypoxemia at rest, and all patients desaturate with exercise.
Patients with subacute HP gradually develop a productive cough, dyspnea, fatigue, anorexia, weight loss, and pleurisy. Symptoms are similar to the acute form of the disease, but are less severe and last longer. On chest radiographs, micronodular or reticular opacities are most prominent in mid-to-lower lung zones. Findings may be present in patients who have experienced repeated acute attacks.
The subacute, or intermittent, form produces more well-formed noncaseating granulomas, bronchiolitis with or without organizing pneumonia, and interstitial fibrosis.
In chronic HP, patients often lack a history of acute episodes. They have an insidious onset of cough, progressive dyspnea, fatigue, and weight loss. This is associated with partial to complete but gradual reversibility. Avoiding any further exposure is recommended. Clubbing is observed in 50% of patients. Tachypnea, respiratory distress, and inspiratory crackles over lower lung fields often are present.
On chest radiographs, progressive fibrotic changes with loss of lung volume particularly affect the upper lobes. Nodular or ground-glass opacities are not present. Features of emphysema are found on significant chest films and CT scans.
Chronic forms reveal additional findings of chronic interstitial inflammation and alveolar destruction (honeycombing) associated with dense fibrosis. Cholesterol clefts or asteroid bodies are present within or outside granulomas.
In addition, many patients have hypoxemia at rest, and all patients desaturate with exercise.
In General following symptoms are common
- Breathing difficulty
- Dry cough
- Fever with prolonged exposure
- Rales with prolonged exposure
- Cyanosis with prolonged exposure
- Coughing up blood with prolonged exposure
Health and Safety measures at work, avoiding pastimes which provoke the illness, adequate maintenance of hot tubs/indoor swimming pools and humidifiers.
The following criteria are good diagnostic predictors:
- Exposure to a known offending antigen
- Positive precipitating antibodies to the offending antigen
- Recurrent episodes of symptoms
- Inspiratory crackles on physical examination
- Symptoms occurring 4-8 hours after exposure
- Weight loss
The diagnosis is based upon a history of symptoms after exposure to the allergen and clinical tests. A physician may take blood tests, seeking signs of inflammation, a chest X-ray and lung function tests. The sufferer shows a restrictive loss of lung function.
Precipitating IgG antibodies against fungal or avian antigens can be detected in the laboratory using the traditional Ouchterlony immunodiffusion method wherein 'precipitin' lines form on agar plate. The ImmunoCAP technology has replaced this time consuming, labor-intensive method with their automated CAP assays and FEIA (Fluorescence enzyme immunoassay) that can detect IgG antibodies against Aspergillus fumigatus (Farmer's lung or for ABPA) or avian antigens (Bird Fancier's Lung).
Although overlapping in many cases, hypersensitivity pneumonitis may be distinguished from occupational asthma in that it is not restricted to only occupational exposure, and that asthma generally is classified as a type I hypersensitivity. Unlike asthma, hypersensitivity pneumonitis targets lung alveoli rather than bronchi.
- Most patients recover lung function completely, but this may take several years.
- Most patients with farmer's lung recover with only minor functional abnormalities and very few progress to disability. A significant number of farmers develop mild chronic lung impairment, which is predominantly obstructive airflow disease associated with mild emphysematous changes on high-resolution CT scan images.
- Bird fancier's lung is less well studied but it appears to have a much worse prognosis than farmer's lung. The poorer outcome may be due to higher antigenic exposure and persisting avian antigens in the home environment, even after birds are removed. These factors may account for the substantial 5-year mortality rate of 30%.
- Pigeon fanciers tend to have a chronic and fairly benign course despite the fact that many choose to continue to keep pigeons.
The best treatment is to avoid the provoking allergen, as chronic exposure can cause permanent damage. Corticosteroids such as prednisolone may help to control symptoms but may produce side-effects.