Asbestosis

Pathology and Pathogenesis

Asbestosis is defined as diffuse pulmonary fibrosis caused by the inhalation of excessive amounts of asbestos fibers. Histologically, asbestosis is characterized by discrete foci of fibrosis in the walls of respiratory bronchioles accompanied by asbestos bodies. As the fibrotic process progresses, it extends distally to the alveolar ducts and proximally to the membranous (terminal) bronchioles. The fibrosis also extends radially to involve alveolar septa distant from the respiratory bronchiole. In the most advanced cases, honeycomb fibrosis is present. The hallmark of asbestos exposure is the asbestos body, namely, a rodlike, beaded, or dumbbell-shaped structure with golden-brown coating and a thin, translucent core. Detection of asbestos bodies may be facilitated by the use of iron stains, which impart a deep blue color. Pleural plaques consist of layers of acellular hyalinized collagen, arranged in a Òbasket weaveÓ pattern, or appear as compact layers of collagen. A mild lymphocytic infiltrate sometimes accompanies the fibrosis [19].

Symptoms and Signs

Clinically, asbestosis manifests as dry cough, dyspnea, Þne inspiratory crackles on auscultation, and a restrictive defect in pulmonary function test with or without Þnger clubbing [15]. Sputum production and wheezing are less common. Patients may be asymptomatic in mild cases.

CT Findings

HRCT with the patient prone is the most sensitive imaging technique to detect asbestosis. Findings in early disease include subpleural dots or branching structures, intralobular lines, thickened interlobular septa, subpleural curvilinear lines, pleura-based irregular small nodules, patchy areas of GGO, and small, cystic spaces [17] (Fig. 17.4). These abnormalities tend to involve mainly the dorsal subpleural regions of lower lobes. Honeycombing is a common Þnding in advanced-stage disease. On comparative study of asbestosis and IPF, subpleural dot-like or branching opacities, curvilinear lines, band-like opacities, and air trapping are more common in asbestosis, whereas honeycombing, visible bronchioles, and bronchiectasis within consolidation are more common in IPF [6]. However, the presence of parietal pleural thickening in association with lung Þbrosis is the most important feature differentiating asbestosis from IPF.

CT–Pathology Comparisons

HRCT-pathology correlation studies have shown that subpleural dots and branching structures correspond to peribronchiolar Þbrosis [20]. Extension of Þbrous tissue into the parenchyma between affected bronchioles results in pleurabased nodular irregularities. Thickened interlobular septa on HRCT may correspond to the Þbrosis of the septa themselves or to Þbrosis in the periphery of lobule. GGOs are related to mild alveolar wall Þbrosis. In the most severe cases, diffuse interstitial Þbrosis leads to parenchymal remodeling and honeycombing.

Patient Prognosis

No speciÞc treatment is available other than prevention of further exposure to asbestos, smoking cessation, and treatment for coexisting chronic obstructive pulmonary disease and cor pulmonale if present. The prognosis of asbestosis is highly variable and depends on the extent of lung involvement. Development of lung cancer or pleural tumor should be closely monitored.

Honeycombing with Upper Lung Zone Predominance

Definition

Pathologically, honeycombing (HC) represents destroyed and Þbrotic lung tissue containing numerous cystic airspaces with thick Þbrous walls [1]. In addition, HC on pathology is characterized by cysts usually measuring <1 mm, often below the resolution of CT, and therefore not necessarily concordant with macroscopic HC seen on CT images. On thin-section CT (TSCT) scans, the appearance is of clustered cystic air spaces, typically of comparable diameters on the order of 3Ð10 mm but occasionally as large as 25 mm [2] (Please refer to section ÒHoneycombing with Subpleural or Basal PredominanceÓ) (Fig. 17.5).

Diseases Causing the Pattern and Distribution

In familial idiopathic pulmonary fibrosis (IPF), honeycombing (HC) is seen in approximately one-third of patients, and lung lesions demonstrate still lower lung zone predominance (67 %, 6 of 9 patients). However, upper lung zone predominance (33 %) is higher than nonfamilial usual interstitial pneumonia (UIP) [21] (Fig. 17.5).

Fig. 17.5 Biopsy-proven usual interstitial pneumonia showing upper lung zone-predominant honeycomb cysts in a 51-year-old man. Patient did not have family history for usual interstitial pneumonia.(a, b) Lung window images of thin-section (1.5-mm section thickness) CT scans obtained at

levels of great vessels (a) and aortic arch (b), respectively, show patchy areas of honeycomb cysts (arrows) in both lungs with upper lung zone predominance. (c) Coronal reformatted image (2.0-mm section thickness) demonstrates upper lung zone-predominant honeycomb cysts (arrow)

According to a study, HC is seen in approximately 60 % of patients with chronic hypersensitivity pneumonia (HP) with upper lung zone predominance [22]. In advanced fibrotic sarcoidosis, traction bronchiectasis, HC, other types of cystic destruction, bullae, and paracicatricial emphysema are encountered, mainly in middle or upper lung zone predominance [23].

Distribution

In all three categories of diseases, the various patterns of lung abnormalities including HC demonstrate upper lung zone predominance.

Clinical Considerations

The clinical presentation in familial IPF is similar to those in nonfamilial IPF and consists of dry cough and progressive dyspnea. Survival in this group of patients is considerably longer, with a 5-year survival rate of 67 % [21].

Although HC is seen in approximately 60 % patients with chronic HP, the HC is not a discriminating factor of the disease from UIP or Þbrotic nonspeciÞc interstitial pneumonia [22].

In end-stage Þbrotic sarcoidosis, there distinct patterns of lung lesion distribution are seen; the bronchial distortion pattern (47 % of patients) with or without coexisting masses, the HC pattern (29 %), and the linear pattern (24 %). The patterns are associated with different functional proÞles [24].