What is the diagnosis and treatment for Hypersensitivity Pneumonitis (HP)?

From the Guidelines

Hypersensitivity pneumonitis treatment should primarily focus on identifying and avoiding the offending antigen, and for acute or severe cases, corticosteroids are the mainstay of therapy, typically starting with prednisone 0.5-1 mg/kg/day (usually 40-60 mg daily) for 1-2 weeks, followed by a gradual taper over 2-3 months, as recommended by the most recent guidelines 1.

Diagnosis and Evaluation

In patients with suspected hypersensitivity pneumonitis (HP), gathering a thorough clinical history of exposures is crucial to establish the type, extent, and temporal relationship of exposure(s) to symptoms 1. This includes completing a comprehensive environmental and occupational questionnaire tailored to the geographic region. If the inciting antigen is thought to be related to an occupational exposure, considering the inclusion of an occupational medicine specialist and an environmental hygienist during the multidisciplinary diagnostic workup is suggested, especially when the source of exposure is obscure or unverified 1.

Classification and Treatment

Patients with HP should be classified based on the likelihood of an occupational or environmental inciting antigen exposure, and the disease should be classified as fibrotic or nonfibrotic based on the presence or absence of fibrosis on high-resolution computed tomography (HRCT) of the chest 1. The presence or absence of lung fibrosis provides important prognostic information, and correct identification of the inciting antigen and subsequent elimination of that exposure facilitates the management and helps determine the prognosis of HP.

• Key points in management include:
  • Identifying and avoiding the offending antigen
  • Use of corticosteroids for acute or severe cases
  • Potential addition of immunosuppressants like mycophenolate mofetil or azathioprine for chronic or progressive disease
  • Supportive care with supplemental oxygen and pulmonary rehabilitation as needed
  • Regular monitoring with pulmonary function testing and high-resolution CT scans to assess disease progression, as supported by earlier guidelines 1.

Prognosis and Monitoring

The prognosis of HP is significantly influenced by the identification and avoidance of the inciting antigen, as well as the presence or absence of lung fibrosis 1. Regular surveillance for exposure and patient education focused on antigen avoidance at every visit is crucial, particularly for those unwilling to remove the antigen source, patients with disease progression despite management, or those with a recurrence of symptoms after an initial appropriate response. Given the potential for disease progression to irreversible fibrotic lung disease, early diagnosis and intervention are critical to improve outcomes in patients with hypersensitivity pneumonitis.

From the Research

Definition and Causes of Hypersensitivity Pneumonitis

• Hypersensitivity pneumonitis (HP) is a group of lung disorders caused by the inhalation of a wide variety of organic particles by susceptible individuals 2.
• HP can be caused by various antigens, including bacteria, fungi, amoebae, plant and animal proteins, and inorganic low-molecular-weight haptens such as isocyanates and drugs 3.
• The disease occurs mainly in nonsmokers, but smoking may promote an insidious and chronic disease 2.

Clinical Presentations of Hypersensitivity Pneumonitis

• HP can present in three different clinical forms: acute, subacute, and chronic 2, 3.
• The acute form is characterized by flu-like symptoms, dyspnea, and dry cough, which subside a few hours or days later 2.
• The subacute and chronic forms result from recurrent low-level antigen exposure and are characterized by progressive dyspnea, dry cough, and other constitutional symptoms such as fatigue, anorexia, and weight loss 2, 3.

Diagnosis of Hypersensitivity Pneumonitis

• The diagnosis of HP can be difficult and requires a history of exposure and a constellation of clinical, imaging, laboratory, bronchoalveolar lavage, and pathologic findings 2, 4, 5.
• General laboratory tests show an increase of acute phase reactants, and specific precipitating antibodies can be evidence of antigen exposure 2.
• Chest radiograph and high-resolution CT scans can reveal ground-glass attenuation, nodular or reticulonodular shadowing, and air trapping 2, 5.
• Pulmonary function tests are characterized by a predominantly restrictive ventilatory defect with loss of lung volume and hypoxemia at rest that worsens with exercise 2, 4.

Treatment and Prognosis of Hypersensitivity Pneumonitis

• The treatment of choice is avoidance of the offending antigen, and systemic corticosteroids may be effective in suppressing the inflammatory response 2, 4, 5.
• The prognosis depends on early diagnosis and effective antigen avoidance, and recurrent acute, subacute, and chronic HP may progress to irreversible lung fibrosis or provoke emphysematous changes 2, 6.
• Treatment of chronic or residual disease is supportive, and the use of inhaled steroids is anecdotal 5.