From the Guidelines
Hypersensitivity pneumonitis is an immunologically mediated form of lung disease resulting from inhalational exposure to a large variety of environmental and/or occupational organic and nonorganic inciting antigens, occurring in genetically susceptible individuals previously sensitized to the inhaled antigen. This condition can occur at any age, with most patients presenting after the fourth decade of life, and its prevalence is estimated to range from one to two cases per 100,000 per year in North America and Europe 1. The symptoms of hypersensitivity pneumonitis can vary, ranging from flu-like illness with fever, chills, and cough in acute cases to progressive shortness of breath and fatigue in chronic cases.
Key Characteristics
- The condition is caused by repeated inhalation of environmental antigens to which a person has developed sensitivity.
- It can present in acute, subacute, or chronic forms.
- Diagnosis typically involves a combination of exposure history, physical examination, pulmonary function tests, chest imaging, and sometimes lung biopsy.
- Treatment primarily focuses on identifying and avoiding the triggering antigen, which can lead to symptom resolution in early disease.
Diagnostic Approach
According to recent guidelines, a thorough clinical history of exposures is essential in diagnosing hypersensitivity pneumonitis, focusing on establishing the type, extent, and temporal relationship of exposure(s) to symptoms 1. The classification of patients based on the likelihood of an occupational or environmental inciting antigen exposure is also suggested, although the quality of evidence for this recommendation is very low 1.
Management and Prognosis
Identifying and avoiding the triggering antigen is crucial for managing hypersensitivity pneumonitis, as it can lead to symptom resolution in early disease 1. Corticosteroids such as prednisone may be prescribed to reduce inflammation, particularly in severe or progressive cases. The presence or absence of lung fibrosis on high-resolution computed tomography (HRCT) of the chest provides important prognostic information, with fibrotic disease generally having a worse prognosis than nonfibrotic disease 1.
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From the Research
Definition of Hypersensitivity Pneumonitis
Hypersensitivity pneumonitis (HP) is a respiratory syndrome involving the lung parenchyma and specifically the alveoli, terminal bronchioli, and alveolar interstitium, due to a delayed allergic reaction 2. It is also known as extrinsic allergic alveolitis and is caused by a hypersensitivity reaction after repeated inhalation of finely dispersed antigens, mainly organic particles or low molecular weight chemicals 3.
Causes and Risk Factors
The causes of HP include a wide range of organic dusts of animal or vegetable origin, as well as chemicals 2. The list of aetiological agents is long and new sources of antigens are constantly being identified 3. Host risk factors are poorly characterized, with the exception of those linked to exposure factors 3. Environmental factors and cofactors may be critical for the pathogenesis of the disease 3.
Clinical Features and Diagnosis
HP can present as an acute, subacute, or chronic disease and may follow various clinical courses 4. The diagnosis is made from a combination of clinical features, radiographic abnormalities, lung function tests, and immunological tests 3. High-resolution computed tomography (HRCT) of the lung, evaluation of possible antigen exposure, and bronchoscopy with bronchoalveolar lavage and, if necessary, forceps biopsy are used to make the diagnosis 5.
Treatment and Management
The mainstay in managing HP is the avoidance of the causative antigen, though the complete removal is not always possible 2. Antigen avoidance is the key element in the treatment 3. Steroid therapy is helpful for symptomatic relief, but probably does not affect the long-term prognosis 4. Immunotherapy and other treatments may be necessary in some cases, especially for patients suffering from chronic hypersensitivity pneumonitis 6.
Key Points
- HP is a complex syndrome that needs urgently more stringent and selective diagnostic criteria and validation 2.
- The diagnosis of HP is often considered on the basis of clinical history of exposure with resulting respiratory symptoms, but the definitive diagnosis requires a constellation of clinical, radiologic, laboratory, and pathologic findings 4.
- A multidisciplinary board is critical in making the diagnosis 5.
- HP is not a uniform disease entity, but a complex dynamic clinical syndrome such that different patterns of disease emerge over time 3.