pulmonary disorders
Hypersensitivity Pneumonitis
Diagnosis of Hypersensitivity Pneumonitis
High-resolution CT (HRCT) Pulmonary function tests Bronchoalveolar lavage Histologic examination and serologic tests Diagnosis of hypersensitivity pneumonitis requires a high index of suspicion in patients with compatible symptoms and a compatible occupational, avocational, or domestic exposure history. Hypersensitivity pneumonitis must be considered in patients with newly identified interstitial lung disease ( Diagnosis reference ). HRCT and Clues in the historyinclude Recurring atypical pneumonias Symptom onset after moving to a new job or home A hot tub, a sauna, a swimming pool, or other sources of standing water or water damage in the home or regular exposure to them elsewhere Having birds as pets Exacerbation and relief of symptoms in and away from specific settings Examination often is not useful in making the diagnosis, although abnormal lung sounds and clubbing may be present. Imaging tests are typically done for patients with appropriate history, symptoms, and signs. Chest x-rayis neither sensitive nor specific for detecting disease and is frequently normal in patients with acute and subacute forms. It may show reticular or nodular opacities, usually when symptoms are present. Chest x-rays of patients with chronic disease are more likely to show reticular or nodular opacities in the upper lobes with reduced lung volumes and honeycombing, similar to that of idiopathic pulmonary fibrosis. High-resolution CTis far more likely to show abnormalities and is considered standard for evaluating parenchymal changes in hypersensitivity pneumonitis. The most typical HRCT finding in acute and subacute disease is the presence of profuse, poorly defined centrilobular micronodules. Occasionally, ground-glass opacification (attenuation) is the predominant or only finding. It is usually diffuse but sometimes spares the periphery of the secondary lobule. Focal areas of hyperlucency, similar to those present in bronchiolitis obliterans, may be a prominent feature in some patients (eg, mosaic attenuation with air trapping on expiratory HRCT). In chronic hypersensitivity pneumonitis, there are findings of lung fibrosis (eg, lobar volume loss, linear or reticular opacities, or honeycombing), and centrilobular nodules may be absent. Some nonsmoking patients with chronic hypersensitivity pneumonitis have findings of upper lobe emphysema. Mediastinal lymphadenopathy is uncommon, thereby distinguishing hypersensitivity pneumonitis from Sarcoidosis . Overview of Tests of Pulmonary Function should be done as part of the standard evaluation of suspected cases of hypersensitivity pneumonitis. The syndrome can cause obstructive, restrictive, or a mixed pattern of airway changes. Advanced disease most commonly causes a restrictive defect (decreased lung volumes), a decreased diffusing capacity for carbon monoxide (DLCO), and hypoxemia. Airway obstruction is unusual in acute disease but may develop in chronic disease. 1.0) is more characteristic of Sarcoidosis . Other findings may include mast cells Lymphocyte transformation testing is an in vitro test of sensitization and is particularly useful in detecting sensitization to metals. The test can be done on peripheral blood but is better done on bronchial lavage fluid. In this test, the patient's lymphocytes are exposed to potential antigens. If the lymphocytes transform into blasts and proliferate, they (and hence the patient) were previously sensitized to that antigen. Transbronchial or surgical lung biopsy may be indicated when noninvasive testing is inconclusive. Findings vary but typically include peribronchiolar metaplasia, poorly formed non-necrotizing granulomas, and organizing pneumonia. Interstitial fibrosis may be present in chronic cases. Further testing is indicated when additional support for the diagnosis is required or to detect other causes of interstitial lung disease. Circulating precipitins (specific precipitating antibodies to the suspected antigen) are suggestive of an exposure that may be the cause of the illness. However, the presence of circulating precipitins is neither sensitive nor specific. Identification of a specific precipitating antigen may require that industrial hygiene specialists do detailed aerobiologic and/or microbiologic assessment of the workplace, but workplace assessments usually are guided by known sources of inciting antigens (eg,Bacillus subtilisin detergent factories). Skin tests are not helpful, and eosinophilia is absent. Tests helpful in detecting other disorders include serologic tests and cultures (for psittacosis and other pneumonias) and autoantibodies (for systemic rheumatic disease). Elevated eosinophil levels may suggest Chronic Eosinophilic Pneumonia . Hilar and paratracheal lymph node enlargement is more characteristic of 1.Raghu G, Remy-Jardin M, Ryerson CJ, et al: Diagnosis of hypersensitivity pneumonitis in adults. An Official ATS/JRS/ALAT Clinical Practice Guideline.Am J Respir Crit Care Med202(3):e36–e69, 2020. doi: 10.1164/rccm.202005-2032ST 