Chapter 49: Bronchiolitis

INTRODUCTION

Diseases of the bronchioles occur throughout the bronchiolar structures, from bronchiolar airways to alveolar ducts and alveoli (Table 49-1). Acute and chronic bronchiolitis are seen from near the bronchi, all the way to the respiratory bronchioles; constrictive bronchiolitis is seen in the mid-bronchioles, while diffuse panbronchiolitis occurs from the distal bronchioles to the respiratory bronchioles, and smoker’s bronchiolitis involves the respiratory bronchioles. Bronchiolitis obliterans organizing pneumonia (BOOP) includes both the terminal bronchioles and alveoli and is discussed more fully in Chapter 55.

This chapter includes a discussion of the pathologic, clinical, and radiographic findings, as well as treatment of the bronchiolar airway disorders.

BRONCHIOLAR ANATOMY

Bronchioles are noncartilagenous small airways that are usually 1 mm or less in diameter; they have been called the bridge between the bronchi and alveoli.¹ The bronchioles have cartilage and mucus glands that are commonly found in the bronchi, but bronchioles also contain ciliated epithelium, smooth muscle, and bronchiolar exocrine cells.² Neuroendocrine cells are common in the proximal bronchioles.

More distal in the airways are approximately 30,000 terminal bronchioles that have an average diameter of about 0.6 mm. These bronchioles have circular smooth muscles in their walls; the surface cilia gradually disappear distally. Terminal bronchioles branch into 224,000 respiratory bronchioles that differ from the bronchioles: respiratory bronchioles have two to three alveolar structures in the walls containing columnar cells with cuboidal type II cells and squamous type I cells. These structures terminate in 13.8 million alveolar ducts and 300 million alveoli.

THE CLINICAL SPECTRUM OF THE BRONCHIOLAR DISEASES

The wide variety of bronchiolar diseases that may be seen in clinical practice are discussed below.

Acute and Chronic Cellular Bronchiolitis

Acute and chronic cellular bronchiolitis is characterized pathologically as acute or chronic inflammation of the bronchioles without a fibrotic component.^3,4 The inflammation may be submucosal, mural, or peribronchiolar. Although common in infants and children, in the adult, bronchiolitis is rare and may be caused by respiratory syncytial virus, adenovirus, influenza, Mycoplasma pneumoniae, Streptococcus pneumoniae, and Haemophilus influenzae as well as acute toxic inhalational injury. Symptoms include a flu-like illness with fever and persistent nonproductive cough of several weeks’ duration. There is generally no wheezing and no airflow obstruction. The chest x-ray is often normal, although the chest CT scan may show tree-in-bud opacities, ill-defined nodules, and ground-glass opacities.⁴ The illness usually subsides over time. Cough suppressants may be utilized. Sometimes, a brief ...