Chapter 51: Bronchiolitis

Diseases of the Bronchioles

Diseases of the bronchioles occur throughout the bronchiolar structures, from bronchiolar airways to alveolar ducts and alveoli (Table 51-1). Acute and chronic bronchiolitis are seen from near the bronchi, all the way to the respiratory bronchioles; constrictive bronchiolitis is seen in the midbronchioles, 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. The bronchiolar component is the proliferative type of bronchiolitis obliterans and not the fibrosing type of bronchiolitis obliterans. The idiopathic form of BOOP is referred to as cryptogenic organizing pneumonia (COP) and is discussed in Chapter 57.

New bronchiolar disorders continue to be described, and new causes of bronchiolitis obliterans have been described. This chapter includes a discussion of the pathological, clinical, radiographic findings, and treatment of the bronchiolar airway disorders.

Bronchiolar Anatomy

Bronchioles are noncartilagenous small airways which are usually one millimeter 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 Clara cells.² Clara cells are columnar cells with apical surfaces capable of secreting proteins and surfactant. 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 millimeter. These bronchioles have circular smooth muscles in the 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 The ...