What are the key points of Hypersensitivity Pneumonitis (HP), a type of interstitial lung disease?

From the Guidelines

Hypersensitivity Pneumonitis (HP) is a complex, immunologically mediated lung disease resulting from inhalational exposure to various environmental and/or occupational organic and nonorganic inciting antigens (IAs) 1.

Key Points of HP

• HP can occur at any age, with most patients presenting after the fourth decade of life, and has a prevalence estimated to range from one to two cases per 100,000 per year in North America and Europe 1.
• The disease is characterized by a complex interplay among host risk factors, the IA, and environmental factors, leading to chronic respiratory symptoms and resultant pulmonary fibrosis in more than half of the subjects 1.
• Traditional diagnostic criteria, which required the presence of an identifiable IA, are no longer considered sufficient, as the IA often goes unrecognized or has ceased prior to diagnosis 1.
• A multidisciplinary approach, including radiologists and pathologists, is necessary for diagnosis, as HP shares features with other acute and chronic pulmonary diseases, such as idiopathic pulmonary fibrosis (IPF) 1.
• Exposure assessment is crucial, and any exposure should be considered along a probability spectrum based on the strength of the identified antigen's association with HP, the intensity of the exposure, and the timing of exposure in relation to disease activity 1.
• Lymphocytosis in BAL fluid and inspiratory squeaks on chest auscultation can increase the likelihood of HP, but these findings may lack sensitivity in patients with more established fibrosis 1.
• A provisional HP diagnostic approach and criteria are provided, emphasizing a patient-centered and teamwork-oriented approach 1.

Diagnostic Challenges

• The absence of a comprehensive clinical practice guideline has led to practice variation and diagnostic disagreement among clinicians 1.
• The elusiveness of the IA, combined with the array of clinically heterogeneous HP phenotypes, frequently leads to misdiagnosis 1.
• Recent guidelines aim to provide rigorously developed contemporary guidance to clinicians on the HP diagnostic process to improve disease recognition, diagnostic accuracy, and individual care and outcomes of HP patients 1.

From the Research

Definition and Causes of Hypersensitivity Pneumonitis (HP)

• Hypersensitivity pneumonitis (HP) is a complex interstitial lung disease caused by chronic inhalation of a wide variety of antigens in susceptible and sensitized individuals, commonly associated with an occupational exposure 2.
• HP is an inflammatory interstitial lung disease caused by a wide variety of organic particles and certain small-molecular weight chemical compounds that provoke an exaggerated immune response in susceptible individuals 3.
• The antigens involved are widely diverse, and the list of causes of HP is frequently expanding 4.

Clinical Presentation and Diagnosis

• HP is multifaceted and may mimic almost any interstitial lung disease, some infectious diseases, and even bronchiolar disorders 5.
• The clinical manifestations are heterogeneous and have been classically described as acute, subacute, and chronic 3.
• Diagnosis of HP requires a combination of clinical, environmental, radiologic, physiologic, and pathologic findings that represent a diagnostic challenge for clinicians 5.
• The diagnosis could be considered with high confidence if the high-resolution computed tomography (HR-CT) shows a typical HP pattern associated with a lymphocytosis over 30% in the broncho-alveolar lavage (BAL) 4.

Treatment and Management

• The cornerstone of therapy is antigen avoidance 3.
• Corticosteroid treatment is the first-line medical treatment for severe forms and aims to prevent the development of fibrosis 4.
• Anti-fibrotic therapy is now an option for patients with progressive ILD and failure of immunomodulatory/immunosuppressive therapies 4.
• The differentiation of HP from other ILDs is important due to distinct causes, pathophysiology, prognosis, and management implications 6.

Challenges and Future Directions

• HP seems to be under-diagnosed owing to its highly heterogeneous presentation in both the non-fibrotic and fibrotic subtypes 4.
• The recognition of HP cases is essential to ensure appropriate therapy for the patient 4.
• Further research is required to address the gaps in the literature and to improve the diagnosis and management of HP 6.