Extrinsic allergic alveolitis, or hypersensitivity pneumonitis, was first described in Iceland in 1874. Radiographic and computed tomography (CT) scan images of the condition appear below.
The term heykatarr has been defined as "a group of related inflammatory interstitial lung diseases that result from hypersensitivity immune reactions to the repeated inhalation or ingestion of various antigens derived from fungal, bacterial, animal protein, or reactive chemical sources."  These antigens are often related to the patient's occupation. The most common antigens are thermophilic actinomycetes and avian proteins, and the most common diseases are farmer's lung and bird fancier's lung. The disease complex is characterized by diffuse inflammation of lung parenchyma and airways in previously sensitized patients.
Hypersensitivity pneumonitis has been traditionally classified into acute, subacute, and chronic phases. However, there are only 2 clinical phases or syndromes: acute and subacute/chronic. Most affected patients present acutely with flulike illness with cough. Patients can also present subacutely with recurrent pneumonia or chronically with exertional dyspnea, productive cough, and weight loss. Most patients recover completely after exposure to the inciting antigen ceases. [2, 3]
Patients with hypersensitivity pneumonitis may present acutely with a flulike illness with cough. They can also present subacutely with recurrent pneumonia or chronically with exertional dyspnea, productive cough, and weight loss.
Crepitant rales can be elicited in some patients. Pulmonary function tests generally reveal a restrictive defect in early disease and a restrictive, obstructive, or mixed defect in late disease. Specific precipitating antibodies are detectable in some cases.
The latent period between exposure to antigen and presentation varies from a few weeks to years. The onset of symptoms after acute exposure is usually between 4 and 12 hours. Some antigens provoke symptoms after repeated exposure; these include bioaerosols of microbial or animal antigens and a few reactive chemicals.
Conventional chest radiography is the examination of choice. Chest radiography is readily and universally available and has the added advantage of portability. The chest radiograph is abnormal in most patients with hypersensitivity pneumonitis. It is also useful for differential diagnosis in patients presenting with respiratory symptoms.In conjunction with the patient's clinical presentation, radiographic findings are generally sufficient to diagnose hypersensitivity pneumonitis, although high-resolution CT (HRCT) scanning is commonly performed to confirm the diagnosis and to rule out other possibilities. HRCT is often performed in the setting of chronic parenchymal lung disease. [11, 12, 13, 14, 15]
In many cases, lung biopsy is required for histologic confirmation of the diagnosis (see the images below).
Limitations of techniques
Conventional radiographs are nonspecific; without clinical input, a firm diagnosis of hypersensitivity pneumonitis cannot be made. The chest radiograph may be normal in established acute disease as well as chronic hypersensitivity pneumonitis. Chest radiography should be performed with caution in young or pregnant patients.
HRCT is comparatively expensive and exposes the patient to a radiation dose higher than that of other studies; it is indicated in patients in whom disease or poorly controlled airway disease is clinically suspected when a firm diagnosis has not been determined.
Ill-defined nonbranching centrilobular nodules with an upper lobe predominance or a diffuse distribution with or without ground-glass opacities are characteristic for acute or subacute hypersensitivity pneumonitis. These findings are diagnostic in an appropriate clinical setting, obviating biopsy. However, a normal HRCT scan does not exclude hypersensitivity pneumonitis, as HRCT scans can be normal in up to 50% of patients.
The most common abnormality in acute or subacute disease is small, bilateral pulmonary nodules, which are usually 1-5 mm in size (see the images below). The nodules may be well defined or indistinct. In 1 published series, only 2% of the nodules were unilateral. 
The nodules have a predilection for the midzones or lower zones, with comparative sparing of the upper zones. The nodules may appear within a few hours after exposure and take weeks or months to resolve. The nodules/opacities may be so small that they give a ground-glass appearance.
Focal consolidation is found in 10-25% of patients.  Interstitial type change is occasionally seen with accentuated bronchovascular markings and Kerley B lines. Generally, there are no pleural changes, but occasional thickening of minor fissures is noted.
Hilar lymphadenopathy is rare but has been reported in mushroom worker's lung,  farmer's lung, [19, 20, 21] and bird fancier's lung. [16, 22] In farmer's lung, plain chest radiographs are normal in up to 70% of cases. When the images are abnormal, acute disease is characterized by a pattern of diffuse airspace disease or a ground-glass pattern mimicking that of pulmonary edema. These changes are reversible and improve over several days. Over time, the disease progresses to a granulomatous reaction that may eventually lead to a small nodular pattern, followed by interstitial fibrosis and end-stage honeycomb lung. Pleural effusion is rare, and the disorder is not associated with hilar adenopathy.
Chronic changes reflect healing by fibrosis and may occur after 1 or more attacks. Alternatively, they may develop insidiously in relation to chronic low-grade antigen exposure, eg, with bird fancier's lung related to budgerigars. The characteristic radiologic changes in the chronic stage are scarring process with loss of lung volume, which has a marked (85%) upper lobe preponderance. The principal opacities are reticular, sometimes with a definite honeycomb pattern. Larger ring shadows 1-4 mm in diameter are due to bullae, blebs, cysts, or bronchiectasis. Line shadows secondary to scarring are common, particularly in the upper zones. Parallel line shadows are caused by bronchiectasis or simple bronchial wall thickening; in the latter case, they are often transient.
Other changes sometimes seen in chronic stage include persisting small nodules, massive opacities, and evidence of pulmonary heart disease. Pneumothorax is recorded in the fibrotic stage, but this is uncommon.
Degree of confidence
Radiographic findings are often nonspecific but could be suggestive in appropriate clinical setting. Therefore, it is important to ask questions about etiologic, environmental, or occupational exposure when images show changes suggestive of extrinsic allergic alveolitis.The chest radiograph is often normal in patients with hypersensitivity pneumonitis. For this reason, HRCT is recommended in the proper clinical setting if the radiograph is normal.
There is some evidence that the radiographic changes are the same whatever the extrinsic provocative agent, but the evidence is incomplete. In the acute phase of the disease, the radiographic changes may be subtle, or the radiograph may appear normal. The radiograph may also appear normal in the chronic stage of the disease despite abnormal diffusing capacity [19, 23] and biopsy-proved allergic granulomatous disease. [20, 21]
In hypersensitivity pneumonitis, HRCT findings are related to the stage of disease. HRCT images depicting hypersensitivity pneumonitis are presented below.
With heavy acute antigen exposure, diffuse airspace shadowing may mimic that of pulmonary edema; if present, this finding generally resolves over a few days.
The subacute phase occurs from several days to months after exposure. In this phase, diffuse ground-glass opacities and small (1-5 mm) centrilobular and poorly defined nodules often affect all zones, though a middle and upper lobe predominance is often seen.
The ill-defined nodules can be found at all 3 stages of the disorder.
Associated areas of ground-glass opacification are common (86%) and often produce a mosaic pattern of attenuation. Less commonly, there is widespread ground-glass attenuation, which is indistinguishable from that of desquamative interstitial pneumonitis. The areas of ground-glass attenuation are usually diffuse, but they may spare the periphery. A mixed appearance with both regions of airtrapping (best seen on expiratory images) and ground-glass attenuation can be seen in patients with hypersensitivity pneumonitis.
About 20% of patients have perivascular interstitial thickening, which is often irregular.
Chronic inflammatory infiltrates along small airways produces bronchiolar narrowing that results in airtrapping.
Chronic hypersensitivity pneumonitis develops as a result of continuous or intermittent antigenic exposure over several months or years and commonly demonstrates intralobular interstitial thickening (evidence of fibrosis) predominantly involving the mid and upper lung zones. Usually present is relative sparing of the lung apices and the costophrenic sulci, which aids in distinguishing this disorder from idiopathic pulmonary fibrosis (IPF). 
Honeycombing is found in about 75% of patients with end-stage lung disease resulting from hypersensitivity pneumonitis.
Degree of confidence
The finding of poorly defined centrilobular nodules on HRCT scans in an appropriate clinical setting should prompt consideration of this disease. The sensitivity of HRCT for the detection of hypersensitivity pneumonitis reported in a population-based study is greater than that of chest radiography.
There are several causes of nodular and ground-glass attenuation on HRCT. Causes of a nodular pattern include sarcoidosis, silicosis, coal worker's pneumoconiosis, and pulmonary histiocytosis X. Causes of a ground-glass pattern include IPF, desquamative interstitial pneumonia, and alveolar proteinosis.
Micronodules may also be identified in respiratory bronchiolitis, but many patients with micronodules have a smoking history, which is generally not associated with hypersensitivity pneumonitis.
Nodules associated with sarcoidosis are usually larger, better defined, and denser than those seen in hypersensitivity pneumonitis. Ill-defined centrilobular micronodules can help distinguish hypersensitivity pneumonitis from IPF, whereas honeycombing and a lower lung zone or peripheral involvement suggest IPF.
Although patients with bronchiolitis obliterans with organizing pneumonia (BOOP) may have a clinical presentation similar to that of hypersensitivity pneumonitis, the areas of parenchymal opacification are typically denser and more patchy than those associated with hypersensitivity pneumonitis.
A normal HRCT scan does not exclude hypersensitivity pneumonitis, as HRCT scans can be normal in up to 50% of patients.