HYPERSENSITIVITY PNEUMONITIS

Hypersensitivity pneumonitis (HP) represents an immune reaction to inhaled organic antigens. The possible sources of these antigens are diverse and include microbes, animals, plant material, and various chemicals.

There are three possible presentations of HP: acute, subacute, and chronic. Acute HP is rare and involves a large antigen exposure leading to the rapid onset of cough, dyspnea, and fever. Exposure to moldy hay in farmer’s lung is the most typical example of acute HP. Subacute HP is common and demonstrates symptoms that are similar to, but less severe than acute HP. Exposures leading to the development of symptoms are more prolonged, extending over weeks to months. Chronic HP is also common and involves long-term exposure to low levels of antigen over a period of years.

There are striking differences in the prevalence of HP based upon antigen exposure. For instance, up to 15% of pigeon breeders will develop HP. Of note, however, only 50% of patients with HP will have an identifiable exposure. This is one of the major challenges in the diagnosis of HP as most other inhaled diseases, such as pneumoconioses, have an easily identifiable exposure. Smoking is thought to be relatively protective against the development of HP, but it does not preclude this diagnosis.

HRCT Findings

The HRCT findings of HP vary depending upon the clinical presentation.

Acute HP

The findings of acute HP have not been well studied, as most patients are not imaged in this stage. The HRCT findings are likely similar to subacute HP (discussed below), but may be more extensive (Fig. 13.1) and demonstrate an increased incidence of consolidation.

Subacute HP

The diagnosis of subacute HP is usually based on the presence of a combination or constellation of abnormalities, including ground glass opacity (GGO), centrilobular nodules of GGO, mosaic perfusion or air trapping, and the headcheese sign (Table 13.1).

Figure 13.1

Ground Glass Opacity

GGO in HP is often patchy and bilateral in distribution. It may be seen in isolation, in which case it is a nonspecific finding (Fig. 13.2). Isolated GGO may be seen with a variety of acute diseases (infections, edema, diffuse alveolar damage, and hemorrhage) and chronic diseases (invasive mucinous adenocarcinoma, interstitial pneumonias, organizing pneumonia, eosinophilic pneumonia, lipoid pneumonia, and alveolar proteinosis). When associated with other findings of subacute HP, such as air trapping, the specificity of GGO for a diagnosis of HP increases.

Centrilobular Nodules of GGO

This is a common finding in subacute HP and reflects peribronchiolar inflammation, cellular infiltration, and poorly marginated granulomas. HP is the most common cause of centrilobular GGO nodules in association with chronic symptoms. The nodules are often diffuse or symmetric (Fig. 13.3). In the setting of an identifiable exposure to an organic antigen, centrilobular GGO nodules are considered diagnostic of subacute HP. The differential diagnosis of centrilobular GGO nodules includes respiratory bronchiolitis, follicular bronchiolitis, atypical infections, and vascular diseases, including pulmonary hemorrhage.

Mosaic Perfusion, Air Trapping, and the Headcheese Sign

Mosaic perfusion and air trapping may be seen in association with other abnormalities (Figs. 13.3B and 13.4) or may be seen as an isolated abnormality (Fig. 13.5). Mosaic perfusion appears as one or more focal areas of decreased lung attenuation associated with reduced size of vessels within the lucent region (see Chapter 5). It reflects the presence of bronchiolitis and bronchiolar obstruction occurring as a result of bronchiolar inflammation. It may persist after treatment of HP and following resolution of GGO or nodules. When seen as an isolated abnormality, the differential diagnosis includes asthma and constrictive bronchiolitis.

The combination of patchy mosaic perfusion and GGO has been termed the headcheese sign; it is highly suggestive of HP (Fig. 13.6). Both GGO and mosaic perfusion must be present. GGO reflects the cellular infiltration typical of HP, while the mosaic perfusion reflects bronchiolar obstruction. It is called the headcheese sign because of its resemblance to a sausage of the same name.

Figure 13.2

Figure 13.3

Figure 13.4

Figure 13.5

Figure 13.6

The differential diagnosis of the headcheese sign includes desquamative interstitial pneumonia (DIP)/respiratory bronchiolitis, follicular bronchiolitis/lymphoid interstitial pneumonia, and atypical infections. Of note, mosaic perfusion and air trapping may persist after treatment even when other findings have resolved.

Consolidation

The presence of consolidation is rare in HP. However, it may be the predominant feature in cases in which secondary organizing pneumonia is also present (Fig. 13.7). On HRCT, patchy consolidation in HP may be indistinguishable from cryptogenic organizing pneumonia, eosinophilic pneumonia, invasive mucinous adenocarcinoma, and other causes of chronic consolidation.

Distribution of Abnormalities

In patients with HP, abnormalities typically predominate in the mid-lungs, with sparing of the inferior costophrenic angles (Fig. 13.8). However, an upper lobe distribution is not uncommon. Distribution may help distinguish HP from the interstitial pneumonias (usual interstitial pneumonia [UIP], nonspecific interstitial pneumonia [NSIP], and DIP) that typically predominate in the lung bases, with involvement of the costophrenic angles. Also, in the axial plane, HP is usually diffuse or central in distribution, whereas the interstitial pneumonias are typically peripheral and subpleural.

Figure 13.7

Figure 13.8

Related posts:

HRCT Indications, Technique, Radiation Dose, and Normal Anatomy Pulmonary Edema, Diffuse Alveolar Damage, the Acute Respiratory Distress Syndrome, and Pulmonary Hemorrhage Pneumoconioses Complications of Medical Treatment:Drug-Induced Lung Disease and Radiation Reticular Opacities Nodular Lung Disease

Stay updated, free articles. Join our Telegram channel

Join