Hypersensitivity Pneumonitis

Chapter 75


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Hypersensitivity Pneumonitis


Katharine Kevill, MD, MHCDS, FAAP


Introduction/Etiology/Epidemiology


Hypersensitivity pneumonitis (HP), also called extrinsic allergic alveolitis, has no consensus definition. Most definitions include several common features.


It is a pulmonary disease with or without systemic manifestations (such as fever and weight loss).


It is caused by the inhalation of an antigen to which the subject is sensitized and hyperresponsive.


Sensitization and exposure alone in the absence of symptoms do not define the disease.


HP was first reported in children in 1967 as pigeon breeder’s lung, with clinical features that included


Severe interstitial pneumonitis and prolonged exposure to pigeons.


Chronic cough, progressive dyspnea, weight loss, and acute symptoms of fever and chest pain.


Before this, HP was thought to be a disease in adults that results from occupational exposure to environments such as moldy hay (farmer’s lung) and sugar cane residue (bagassosis).


Epidemiologically, HP is rare, but probably underreported.


Between 1960 and 2005, 95 cases of HP in children were reported in the literature. Inciting antigens included various birds and molds.


The incidence of HP is unknown.


Twenty-three cases over 3 years in all of Germany were identified by the German Surveillance Unit for Rare Pediatric Disorders.


Pathophysiology


HP is one of many heterogeneous disorders found in the broader category of diffuse and interstitial lung disease.


It is not mediated by immunoglobulin E (IgE).


Features of both immune complex–mediated (type III) and T cell– mediated (type IV) reactions have been described.


The patient with HP develops immunoglobulin G (IgG) antibodies to the inciting environmental agent. However, the presence of serum IgG antibodies released in response to the antigen also occurs in individuals who have been exposed but have no symptoms.


Murine model studies indicate that toll-like receptors linked to the MYD88 intracellular pathway play a role in the response to the inciting antigens.


Clinical Features


Clinical features are variable and depend on the severity of the disease at presentation.


Symptoms


Usually include cough, dyspnea, and fatigue


Sometimes include fever and weight loss


Signs


Usually include crackles and hypoxemia


Often include tachypnea and clubbing


Occasionally include wheezing


Radiographic Findings


Plain chest radiographs usually show a reticulonodular pattern of interstitial markings.


Thin-section chest computed tomography usually demonstrates nodules in a centrilobular distribution; it sometimes shows ground-glass opacities; in late stages, it shows lobar volume loss and honeycombing.


Pulmonary Function Test Findings


Usually include a restrictive pattern (decreased lung volumes)


Usually include decreased diffusing capacity for carbon monoxide (DLCO)


Can include an obstructive pattern or mixed restrictive and obstructive disease


Findings at Bronchosopy


Usually lymphocytosis


Findings at Lung Biopsy


See Figure 75-1 for lung biopsy findings of HP.


Bronchiolocentric, chronic inflammation is always found.


Alveoli are often filled with lymphocytes—usually T cells.


Often, noncaseating histiocytic granulomas with multinucleated giant cells are found.


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Figure 75-1. Photomicrograph (hematoxylin-eosin stain; original magnification, ×20) of a lung biopsy sample from an 8-year-old boy with a recent history of cough, shortness of breath, and fever shows a cellular interstitial lymphocytic infiltrate admixed with multinucleated giant cells (arrow). Courtesy of Gail H. Deutsch, MD, Seattle, WA.


Differential Diagnosis


The signs and symptoms of HP overlap with a range of infectious, respiratory, and systemic diseases. See Box 75-1.


Consider HP in the differential for


Any child in whom diffuse and interstitial lung disease is suspected


Restrictive respiratory disease without a clear cause


Fulminant respiratory symptoms that resolve in the hospital and then recur


Any child with persistent respiratory symptoms of unclear etiologic origin


Diagnostic Considerations


There is no definitive test for HP.


Start with a detailed history and physical examination. Include environmental history–ask specifically about exposures to birds and mold.


If HP is included in the differential, order DLCO and lung volume studies, in addition to spirometry.


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Aug 22, 2019 | Posted by in PEDIATRICS | Comments Off on Hypersensitivity Pneumonitis

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