Reversed Halo Sign

Definition

Reversed halo sign, which was named originally from the disease cryptogenic organizing pneumonia, refers to a focal, rounded area of ground-glass opacity surrounded by a more or less complete ring of consolidation [1, 2] (Fig. 8.1).

Diseases Causing the Sign

The reversed halo sign was Þrst described as being relatively speciÞc for cryptogenic organizing pneumonia (COP) (Fig. 8.2) or COP-like reaction but was later observed in several other infectious and noninfectious diseases. Pulmonary mucormycosis (former pulmonary zygomycosis) (Fig. 8.3) and invasive pulmonary aspergillosis may manifest the reversed halo sign [3]. Other causes of the reversed halo sign include noninvasive fungal infections such as paracoccidioidomycosis, histoplasmosis, and Pneumocystis jirovecii pneumonia [4]. Furthermore, pulmonary infarction [5], antineutrophil cytoplasmic antibodyassociated granulomatous vasculitis (former WegenerÕs granulomatosis), radiofrequency ablation, and lymphomatoid granulomatosis (Fig. 8.4) may also lead to this Þnding [6, 7] (Table 8.1).

Distribution

The reversed halo sign in COP may have its distribution along the subpleural lungs or the bronchovascular bundles [1]. In invasive fungal infection in immunocompromised patients, the sign appears as a large single lesion. In noninvasive fungal infection, the reversed halo sign(s) is usually associated with ground-glass opacity, small centrilobular nodules, or areas of consolidation, and the lesions are usually

bilateral and asymmetric [4]. The reversed halo sign in chronic eosinophilic pneumonia may show upper and mid lung zone predominance, whereas pulmonary infarction may show subpleural location in pulmonary infarction.

Clinical Considerations

In neutropenic patients, the presence of reversed halo sign may suggest invasive fungal infection. Approximately 40 % of patients with chronic eosinophilic pneumonia have asthma; thus, reversed halo sign in asthmatics may imply chronic eosinophilic pneumonia [8]. In the situation of clinically pulmonary embolic disease, the presence of reversed halo sign hints the diagnosis of pulmonary infarction.

Key Points for Differential Diagnosis

1.On CT scans, the reversed halo sign is observed in approximately 20 % of COP patients [1].

2.The reversed halo sign, along with the halo sign, is

highly suggestive of early infection by an angioinvasive fungus; the halo sign and reversed halo sign are most commonly associated with invasive pulmonary aspergillosis and pulmonary mucormycosis, respectively [3].

3. In paracoccidioidomycosis, the reversed halo sign is seen in 10 % of patients with active

Paracoccidioides brasiliensis infection [4].

4.The reversed halo sign Þndings may be speciÞcally useful for making a diagnosis of pulmonary infarction, when direct Þndings of thromboembolism are absent, especially when CT examination is delayed relative to symptom onset [5].

Fig. 8.1 Reversed halo sign in a 55-year-old asthmatic woman and chronic eosinophilic pneumonia. (aÐc) Lung window images of thinsection (1.5-mm section thickness) scans obtained at levels of the great vessels (a), aortic arch (b), and azygos arch (c), respectively, show multifocal areas of ground-glass opacity in both lungs. Please note reversed halo sign (ground-glass opacity surrounded by a more or less complete ring of consolidation) (arrows) in both upper lobes

Cryptogenic Organizing Pneumonia and

Reversed Halo Sign

Pathology and Pathogenesis

COP is a predominantly airspace-Þlling, active Þbrosing process that involves distal bronchioles, alveolar ducts, and peribronchiolar alveoli in varying proportions (Fig. 8.2). The Þbrosis is easily recognized at low magniÞcation because it

The areas of consolidation correspond histologically to the regions of lung parenchyma that show airspace Þbrosis [13]. The ground-glass opacities correlate with areas of alveolar septal inßammation and minimal airspace Þbrosis. The small nodules are related to foci of organizing pneumonia limited to the peribronchiolar region and/or to Þbroblastic tissue plugs within the bronchiolar lumen. The central groundglass opacity of the reversed halo sign corresponds histopathologically to the area of alveolar septal inßammation and cellular debris, and the ring-shaped peripheral airspace consolidation of the reversed halo sign, to the area of organizing pneumonia within the alveolar ducts [1].

Patient Prognosis

In general, COP improves rapidly with the administration of corticosteroid therapy. Complete recovery with physiologic