Hypersensitivity pneumonitis

Hypersensitivity pneumonitis (also called extrinsic allergic alveolitis, EAA) is an inflammation of the alveoli within the lung caused by hypersensitivity to inhaled organic dusts. Sufferers are commonly exposed to the dust by their occupation or hobbies.

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Types

Hypersensitivity Pneumonitis (HP) may also be called many different names, based on the provoking antigen. These include:

Bird-Breeder's Lung 

Also called Bird-Fancier's Lung, Pigeon-Breeder's Lung, and Poultry-Worker's Lung. Caused by avian proteins. Exposure is from feathers and bird droppings.

Farmer's Lung 

Caused by the molds Thermophilic actinomycetes, Aspergillus species, Saccharopolyspora rectivirgula, and Micropolyspora faeni. Exposure is generally from moldy hay but may be found elsewhere.

Bagassosis 

Caused by Thermophilic actinomycetes. Exposure is from moldy bagasse (pressed sugarcane).

Malt Worker's Lung 

Caused by Aspergillus clavatus. Exposure is from moldy barley.

Humidifier Lung 

Caused by the bacterias T. candidus, Bacillus subtilis, B. cereus, and Klebsiella oxytoca; the fungus Aureobasidium pullulans; and the amoebae Naegleria gruberi, Acanthamoeba polyhaga, and Acanthamoeba castellani. Exposure is from mist from standing water.

Mushroom Worker's Lung 

Caused by Thermophilic actinomycetes. Exposure is from mushroom compost.

Compost Lung 

Caused by Aspergillus. Exposure is from compost.

Peat Moss Worker's Lung 

Caused by Monocillium sp. and Penicillium citreonigrum. Exposure if from peat moss.

Suberosis 

Caused by Penicillum frequentans. Exposure is from moldy cork dust.

Japanese Summer-Type HP 

Caused by Trichosporon cutaneum. Exposure is from damp wood and mats.

Cheese-Washer's Lung 

Caused by Pencillum casei or P.roqueforti. Exposure is from cheese casings.

Metalworking Fluids HP 

Caused by Nontuberculous Mycobacteria. Exposure is from mist from metalworking fluids.

Hot Tub Lung 

Caused by Mycobacterium avium complex. Exposure is from mist from hot tubs.

Mollusc Shell HP 

Caused by aquatic animal proteins. Exposure is from mollusc shell dust.

Isocyanate HP 

Caused by TDI, HDI, and MDI. Exposure is from paints, resins, and polyurethane foams.

TMA HP 

Caused by Trimellitic anhydride. Exposure is from plastics, resins, and paints.

Beryllium

Caused by Beryllium. Exposure in the electronics industry.

Of these types, Farmer's Lung and Bird-Breeder's Lung are the most common. "Studies document 8-540 cases per 100,000 persons per year for farmers and 6000-21,000 cases per 100,000 persons per year for pigeon breeders. High attack rates are documented in sporadic outbreaks. Prevalence varies by region, climate, and farming practices. HP affects 0.4-7% of the farming population. Reported prevalence among bird fanciers is estimated to be 20-20,000 cases per 100,000 persons at risk." ^[1]

Symptoms

Hypersensitivity Pneumonitis (HP) is categorized as acute, subacute, and chronic based on the duration of the illness.

Acute

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, and headache. Symptoms resolve within 12 hours to several days upon cessation of exposure.^[1]

Acute HP is characterized by poorly formed noncaseating interstitial granulomas and mononuclear cell infiltration in a peribronchial distribution with prominent giant cells.^[1]

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 "diffusing capacity of lungs for carbon monoxide. ... Many patients have hypoxemia at rest, and all patients desaturate with exercise.^[1]

Subacute

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-upper lung zones.^[1] 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.^[1]

Chronic

In chronic HP patients often lack a history of acute episodes. They have an insidious onset of cough, progressive dyspnea, fatigue, and weight loss. Removing exposure results in only partial improvement. ... Clubbing is observed in 50% of patients. Tachypnea, respiratory distress, and inspiratory crackles over lower lung fields often are present.^[1]

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.^[1]

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.^[1]

In addition, many patients have hypoxemia at rest, and all patients desaturate with exercise.

Diagnosis

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.

Management

The best treatment is to avoid the provoking allergen, as chronic exposure can cause permanent damage. Sufferers might try a face mask if they cannot avoid the allergen. Corticosteroids such as Prednisolone may help to control symptoms but may produce side-effects.

Notes

1. ^ ^{a b c d e f g h i} Sharma, Sat. Hypersensitivity Pneumonitis. eMedicine, June 1, 2006.

Oxford Handbook of Clinical Medicine