HP hypersensitivity pneumonitis is divided into fibrotic and nonfibrotic types

Hypersensitivity pneumonitis (HP) is a syndrome with a wide array of clinical manifestations. It occurs as the result of an immune-mediated response (often to an organic antigen) within the lung parenchyma. The immune-mediated lung inflammation may progress to fibrosis in some patients, but not all patients. It is not fully understood why some people develop HP after exposure to the inhaled antigen while others do not. Furthermore, why some with HP progress to pulmonary fibrosis while others do not is poorly understood. Guidelines categorize HP into nonfibrotic (purely inflammatory) and fibrotic (mixed inflammatory plus fibrotic or purely fibrotic) types on the basis of lung imaging and/or histopathologic evaluation (choice C is correct).

The key tools to evaluate patients suspected of having HP are a detailed history, pulmonary function tests (PFTs), and chest imaging. BAL may be useful for detecting alveolar inflammation in patients for whom HP is being considered. The most common finding is a high percentage of lymphocytes (>20%). However, for many patients with a compelling history and PFT and high-resolution CT (HRCT) findings, it may not be necessary. Likewise, with the advent of HRCT scanning, the need for histopathological confirmation for the diagnosis of HP has fallen substantially.

Although the vast majority of lung diseases are exacerbated by cigarette smoke exposure, results from several studies suggest that smoking may confer a benefit that reduces the risk of developing HP, as well as sarcoidosis (pulmonary diseases that are less common among smokers than nonsmokers). The mechanisms behind this benefit are not well understood, but investigators in many studies have reported a decreased prevalence of precipitating antibodies in smokers exposed to avian or mold antigens.

This patient has the typical PFT findings seen in HP. These include restrictive lung disease with reduced lung volumes, concomitant reduced flows with increased elastic recoil, and reduced DLCO. Occasionally, patients may also have obstructive physiology, although it is not seen in this case. Importantly, the restrictive lung physiology and reduced DLCO are seen in both nonfibrotic HP and fibrotic HP. Therefore, PFTs are not helpful in identifying which type of HP an individual patient has. With appropriate anti-inflammatory treatment and avoidance of antigen exposure, a patient's PFTs may show substantial improvement over time. This is much more likely to occur in those with nonfibrotic HP (choice B is incorrect).

Patients with nonfibrotic HP have a substantially improved prognosis compared with those with fibrotic HP. The mainstays of therapy are avoidance of the antigen and corticosteroid treatment. Although there are no results from randomized trials to guide the specifics of such treatment, expert guidelines suggest 0.5 to 1.0 mg/kg of prednisone as a starting dose. This should be accompanied by frequent reassessment with PFTs (every 2-3 months) and HRCT scanning (every 6 months), with an eye toward tapering the corticosteroids as tolerated. The use of steroid-sparing agents, such as azathioprine or mycophenolate, is typically focused on those with fibrotic HP. There is no role for steroid-sparing agents in the management of nonfibrotic HP (choice D is incorrect).1

Footnotes

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