Review of Respiratory Medicine - Volumen 24, Número 3 - September 24

Review Article

CT Findings in Organizing Pneumonia

Hallazgos tomográficos en neumonía en organización

ABSTRACT

Organizing pneumonia consists of the presence of granulomatous inflammation in the small airways, alveolar ducts, and alveoli. It is a nonspecific pathological response that can occur when there is a pulmonary injury. Cryptogenic organizing pneumonia is a specific clinical and pathological entity for which no etiology has been found. It can pres­ent with different patterns in tomographic studies, some of which are highly suggestive of this condition, while others are nonspecific. The objective of this review is to describe the tomographic characteristics of organizing pneumonia.

Key words: Cryptogenic organizing pneumonia, Reversed halo sign, Interstitial lung diseases

RESUMEN

La neumonía en organización consiste en la presencia de inflamación granulomatosa localizada en la pequeña vía aérea, conductos alveolares y alvéolos. Es una respuesta patológica no específica que puede generarse ante alguna lesión pulmonar. Existe la neumonía en organización criptogénica, entidad específica, clínica y patológica, en la cual no se encuentra una etiología. Puede presentarse con diferentes patrones en los estudios tomográficos, de los cuales algunos de ellos son muy sugestivos de esta entidad, mientras otros son inespecíficos. El objetivo de esta revisión es describir las características tomográficas de la neumonía en organización.

Palabras clave: Neumonía en organización criptogénica, Signo del halo invertido, Enfermedades pulmonares intersticiales

Received: 10/06/2023

Accepted: 02/09/2024

INTRODUCTION

Organizing pneumonia (OP) consists of the pres­ence of granulomatous inflammation in the small airways, alveolar ducts, and alveoli.¹ It is a nonspe­cific pathological response that can occur following any pulmonary injury, such as infections, exposure to smoke or toxic gases, radiotherapy, connective tissue diseases, neoplasms, and drug toxicity.² When potential causes of this condition are ruled out, it is called cryptogenic organizing pneumonia (COP). This is a specific clinical and pathological entity of unknown etiology, first described in 1983,³ and is considered an idiopathic interstitial lung disease.⁴ Histopathologically, granulation tissue is generated by part of the alveolar epithelium in response to injury, accumulating within the alveoli and spreading through the alveolar ducts and terminal bronchioles. These intraluminal ac­cumulations of loose granulation tissue are known as Masson bodies.^5,6 The condition typically ap­pears between the 5th and 6th decades of life, has no gender preference, and non-smokers may be at higher risk.⁷ The most commonly found symp­toms are a mostly non-productive cough, fever, dyspnea, and general malaise, with less frequent occurrences of chest pain, arthralgia, and weight loss. The most characteristic findings on physi­cal examination are crackles, with less frequent occurrences of cyanosis and wheezing.⁸ There is currently no laboratory finding that confirms this disease, though half of the patients will have leukocytosis, and it is common to see elevated acute-phase reactants such as erythrocyte sedi­mentation rate and C-reactive protein.9 Some of the tomographic findings are often suggestive of this disease, allowing for a presumptive diagnosis prior to histological confirmation.

CT FINDINGS

Ground glass opacities and consolidations

The most characteristic findings consist of bi­lateral and asymmetric consolidations. The dis­tribution is predominantly subpleural, and less frequently, peribronchovascular, or a combination of both, and may be associated with ground-glass opacities⁹ (Figure 1). When there is bronchocen­tric or peribronchovascular involvement, it often affects the lower lobes, and the air bronchogram sign can be observed¹⁰ (Figure 2). These opacities may migrate within a few weeks, although this has not been evidenced in the majority of patients.⁷

Patients often respond to steroid treatment, but the disease may relapse in different lobes or in different segments within the same lobe when the dose is reduced or if the treatment is interrupted¹¹ (Figure 3).

Consolidation bands

In OP, consolidation bands that extend from the hilum to the pleura can be observed, leaving the pleura intact. These bands are over 8 mm thick and may sometimes present with an air broncho­gram.^12,13 Curved linear consolidations parallel to the pleura can also be observed. These signs are characteristic of OP and have been suggested to correspond to linear atelectasis secondary to proximal airway involvement⁹ (Figures 4 and 5). This finding, from a tomographic evaluation, does not indicate interstitial involvement, as it does not correspond to the contour of the secondary pulmonary lobule.¹⁴

Perilobular consolidations

This refers to the presence of arched and curvilin­ear consolidations, distributed around the second­ary lobules, that often come into contact with the pleura. They can be located anywhere, but occur predominantly in the middle and lower lobes, with both peribronchovascular and subpleural distribu­tion.¹² They are often accompanied by ground-glass opacities or consolidations.⁹ The presence of these changes is not indicative of fibrosis. This is a spe­cific sign of OP and has been reported to represent 57% of tomographic findings¹⁵ (Figures 6 and 7).

Nodular images

OP can present with nodular images, described as less than 5 mm (micronodules) and over 10 mm, with random and bilateral distribution.^13,15 The nodules may have well-defined or spiculated margins, so a differential diagnosis with metastatic cancer should be made.¹² These nodules can also cavitate or present with a halo sign. They may coexist with other tomographic patterns (Figure 8). OP can also manifest as a solitary pulmonary nodule, which could be due to a localized injury, such as an infectious process¹⁴ (Figure 9).

Reversed halo sign (atoll sign)

The reversed halo sign, also known as the atoll sign (a ring-shaped island with an inner lagoon)¹⁶ refers to the presence of an area of ground-glass opacity surrounded by a complete or partially complete ring of consolidation.¹⁷ It was first de­scribed in 2 patients with cryptogenic organizing pneumonia (COP), and it was initially considered a pathognomonic finding of OP. However, over time, other etiologies have been described, primarily infectious, that present with the same image.^18,19 The findings in the consolidative portion are significant, since the granular appearance is sug­gestive of active granulomatous disease, such as tuberculosis and sarcoidosis.²⁰ The reversed halo sign is one of the most specific findings of OP and has been reported to be present in 19% of patients with OP²¹ (Figure 10).

Fibrosing pattern

During the progression of the disease, reticula­tions and even honeycombing may appear, pre­dominantly in the basal segments of the lower lobes, along with traction bronchiectasis and loss of lung volume. These findings can coexist with other tomographic patterns.¹² Some series describe a high percentage of residual lesions in the tomography, whose characteristics sug­gest a pattern of fibrotic nonspecific interstitial pneumonia (NSIP). This progression to fibrotic lesions can be seen in rheumatic diseases such as antisynthetase syndrome.²² OP can also present as an added component of another interstitial lung disease, such as NSIP or hypersensitivity pneumonitis, or it may be part of the exacerbation of an interstitial lung disease.¹⁴

DISCUSSION

OP represents a process of lung repair second­ary to certain known injuries (secondary OP) or idiopathic injuries (COP). There are ongoing controversies about whether these are different entities or if they belong to the same pathology. So far, no substantial differences have been found between these groups in terms of tomographic findings, clinical characteristics, or evolution.^7,23 When evaluating these patients to determine the etiology of OP, it is important to create detailed medical records, including signs and symptoms of rheumatological diseases with the corresponding evaluation of autoantibodies, use of medications and illicit drugs, history of thoracic radiotherapy, ruling out pulmonary infectious diseases, including respiratory viruses, and considering the exposure history.⁸

The most specific tomographic signs of OP are perilobular consolidations, consolidation bands, and the reversed halo sign. Recognizing these images could lead to a diagnosis in many cases, in the context of a multidisciplinary team discussion, thereby avoiding the need for a biopsy. However, the most frequent signs are consolidations as­sociated with ground-glass opacities, which are not very specific,¹³ making the recognition of OP more difficult. Nodular patterns would imply a differential diagnosis with neoplastic or infec­tious conditions, possibly requiring some invasive method for the diagnosis.

A definitive diagnosis requires a lung biopsy, with a good yield from transbronchial biopsy, showing a sensitivity of 64% and a specificity of 86%,²⁴ and even higher yield from surgical lung biopsy.¹ This not only allows for a definitive di­agnosis but also helps detect underlying causes of OP. CT-guided transthoracic needle biopsy has demonstrated a diagnostic accuracy of 87.9% in a retrospective study.²⁵ Currently, transbronchial cryobiopsy seems to be a safe and efficient method in specialized centers.²⁶

OP has good response to treatment with sys­temic steroids, which can lead to complete remis­sion. However, relapses are common, occurring in approximately 50% of the cases in some series, appearing either in the same affected lobes or in different lobes with same or different topographic patterns. Among the risk factors associated with relapses are the presence of fever, elevated C-reactive protein, reduced diffusion capacity of carbon monoxide, extensive lung involvement, presence of traction bronchiectasis, and delay in treatment initiation, as well as partial remission with treatment.^27,28,29 Although most patients achieve complete resolution, some may be left with residual changes, such as airspace occupation or altered lung architecture.¹⁴

In conclusion, we emphasize the importance of recognizing tomographic patterns such as the reversed halo sign, perilobular consolidations, and consolidation bands, which are indicative of this condition.

Conflict of interest

The authors have no conflict of interest to declare.

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