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.

•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

Differential Diagnosis