diagnosis of HP is in imaging and BAL with high lymphocyte count


Diagnostic guidelines for hypersensitivity pneumonitis (HP) were updated by the American Thoracic Society (ATS) in 2020, with a new set to be published by the American College of Chest Physicians in the near future. The ATS HP guidelines use clinical presentation, radiographic imaging features, and histopathologic findings to make a multidisciplinary diagnosis of HP with varying levels of confidence. New to these guidelines is the revision of the definition to include nonfibrotic vs fibrotic HP rather than acute, subacute, and chronic HP and incorporation of a BAL fluid lymphocyte cellular analysis to the diagnostic algorithm, with a higher proportion of lymphocytes noted in the BAL fluid in HP vs idiopathic pulmonary fibrosis or sarcoidosis. The goal of the patient evaluation is to make a confident diagnosis of HP by using the least invasive tests and procedures.

In this patient, the physical examination findings of crackles and squeaks suggest both parenchymal lung disease and small airway disease, as is consistent with the pathophysiology of this inhalation interstitial lung disease (ILD). The high-resolution CT chest scan has imaging findings typical of HP, including upper lobe predominant ILD, a mosaic attenuation pattern, and air trapping that worsened on expiratory views. The most specific radiographic sign of fibrotic HP is the "three-density pattern," formerly known as the "headcheese sign," which demonstrates the small airway characterized by decreased attenuation and vascularity and the parenchymal lung disease characterized by ground-glass opacities along with adjacent normal lung lobules. In the setting of a high-resolution CT scan that has imaging findings typical for HP and an identified organic antigen (bird in the home), bronchoscopy with BAL fluid demonstrating an elevated lymphocyte count >30% supports a high-confidence diagnosis of HP (choice A is correct).

Transbronchial lung cryobiopsy is a newer bronchoscopic technique that has a higher yield than traditional transbronchial biopsy in diagnosing HP, but the quality of the evidence from studies supporting this determination is low, and biopsy is not deemed necessary when exposure to an organic antigen has been identified and the CT scan shows imaging findings that are associated with high confidence for HP (choice B is incorrect). While published data suggest complications are similar between the two procedures if cryobiopsy is performed at a center with experience with this procedure, difficult-to-control bleeding and pneumothorax remain concerns if performed at centers with limited experience with lung cryobiopsy.

After a comprehensive multidisciplinary evaluation, in patients in whom HP is still suspected but an antigen is not identified and the CT scan is not typical for HP, a lung biopsy should be considered, with local expertise determining whether transbronchial lung biopsy or cryobiopsy or surgical lung biopsy is performed. Caution should be exercised in performing surgical lung biopsy in a patient with advanced fibrosis and/or a usual interstitial pneumonia type–pattern because morbidity and mortality are increased. On histopathology, the expected findings of loosely formed noncaseating granulomas would support the diagnosis of HP (choice B is incorrect). Serum antibodies for mold and avian antigens may be sent and incorporated into the diagnostic decision-making, and are especially helpful if an identified antigen cannot be confirmed by historical exposure assessment, but they are neither sensitive nor specific enough to confirm the diagnosis of HP (choice C is incorrect). A methacholine bronchial provocation study would not be helpful in this setting (choice D is incorrect).

Once HP is diagnosed, outcome is critically dependent on the identification and remediation of the antigen or antigens causing the HP. Studies have shown an increased mortality for patients with HP in whom an antigen cannot be identified or if pulmonary fibrosis is present. Patients who have HP from avian antigen exposure must not only remove the bird or birds from the home but also professionally clean their dwelling place because avian antigens have been identified months later, even after removal of the bird from the home. Additional sources of exposure must be mitigated as well, so patients and/or their family members must not visit the bird if it has been taken to board locally because this can trigger an acute exacerbation or cause worsening fibrosis. Other sources of avian antigen exposure must also be remediated, including removal of down pillows and bedding from the bed and home. Initiation of additional therapies, including corticosteroids, the immunosuppressive agents azathioprine and mycophenolate mofetil, and/or the antifibrotic nintedanib, depends on the extent of the physiological impairment, radiographic pattern of inflammation and fibrosis, and response to antigen mitigation.1

Footnotes

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