Chapter 7

Pneumonia

Gebhard Mathis

Correspondence: Gebhard Mathis, Dr Summer-Strasse 3, 6830, Rankweil, Austria. E-mail: Gebhard.mathis@cable.vol.at

Parts of this chapter are reproduced and modified from the following, with the kind permission of Springer Nature. Mathis G, ed. Chest Sonography. 4th Edn. Berlin, Springer, 2017.

Pneumonia has been documented as a forgotten killer [1]. According to the data released by the World Health Organization (WHO), lower respiratory tract infection is the leading cause of infectious disease-related mortality worldwide and refers to the top-ranking cause of death in low-income countries. Pneumonia accounts for 16% of all deaths in children <5 years of age and is thought to be the highest cause of mortality in children around the globe [1]. Pneumonia is also a serious disease in adults, with high rates of mortality and hospitalisation [2]. One-year all-cause mortality rates range 1% (age 18–49 years, low risk) to 36% (age ≥65 years, high risk) [3].

The most common cause of dyspnoea in a study of 2683 patients in emergency departments (ED) was pneumonia (38% of diagnoses). Thoracic ultrasound is emerging as a significant potential diagnostic option in this common and morbid disease.

The aim of this chapter is to familiarise the reader with sonomorphology of pneumonia including LUS-differential diagnoses. The accuracy of LUS in diagnosing pneumonia will also be discussed, and a comparison with standard imaging modalities, such as CT and CXR, will be considered.

Sonomorphology of pneumonia

Pathophysiological prerequisites

In cases of lobular and segmental pneumonia, large amounts of air are displaced from the lung as a result of extensive fibrinous exudation. Affected lobes or segments are depleted of air and sink in water. The phase of engorgement and hepatisation, i.e. the first week of the disease, offers good conditions for pathological echo transmission. In this phase, pneumonia is well imaged on the sonogram. In the phase of lysis, the inflamed portion of the lung is ventilated to an increasing extent. Air reflexes superimpose deeper infiltrations. The image on sonography at this time may underestimate the actual extent of disease [4].

Focal pneumonias and interstitial pneumonias barely extend up to the pleura and are therefore poorly accessible to sonographic imaging. However, bronchial pneumonias are often accompanied by involvement of the pleura and are therefore partly visualised by sonography [4].

Phase of engorgement

In the initial phase of disease, i.e. in the phase of engorgement, the pneumonic lesion is hypoechoic, relatively homogeneous, and hepatoid or tissue-like in echotexture. Its configuration is bizarre. It is rarely explicitly segmental like the pulmonary infarction or rounded like carcinomas and metastases. Its margins are irregular, serrated and somewhat blurred (figure 1) [5].

Figure 1. A 68-year-old severely ill man with clinical signs of acute pneumonia. In the upper lobe of the lung on the left side, there is a liver-like consolidation with a bronchoaerogram. Reproduced and modified from [4] with permission.

Bronchoaerogram

A marked bronchoaerogram (bronchopneumogram, air bronchogram) with tree-like ramifications is seen in 70–88% of cases and is the most important sign of bacterial pneumonia. The intensive reflexes of the bronchial tree run between consolidated portions of the parenchyma. In all stages of the disease the bronchoaerogram is more pronounced than in cases of pulmonary embolism. Quite often one finds a small number of, and in most cases numerous, lenticular internal echoes just a few millimetres in size (figures 1 and 2). These echoes indicate the presence of air in the small bronchi [6–15]. The bronchoaerogram visualised by LUS cannot be equated with that seen on a radiograph [4].

Figure 2. A 28-year-old woman with H1N1 influenza. She had a high fever, a moderate cough and mild pleuritic pain. LUS shows a consolidation with air inlets. At the border of the consolidation are some B-lines, a sign of a focal interstitial syndrome. Reproduced and modified from [4] with permission.

In patients with lung consolidation in whom air bronchograms are displayed on US, the dynamic air bronchogram indicates pneumonia, differentiating it from resorptive atelectasis. Static air bronchograms have been found in most resorptive atelectases cases and in one-third of pneumonia cases [16].

Viral and fungal pneumonias are often less ventilated and/or reveal less pronounced bronchoaerograms. They are smaller and more compact than bacterial pneumonia and may resemble large pulmonary infarctions. In contrast to pulmonary infarctions, they are strongly perfused with blood (figure 3). The latter conclusions have been drawn from personal experience and have been confirmed in expert meetings [4].

Figure 3. A 52-year-old woman with pain on inspiration, fever and haemoptysis. a) Sonography revealed a consolidation measuring 5×3.5 cm in size, with less bronchoaerogram. b) Colour Doppler showed regular perfusion: viral pneumonia. c) The corresponding CT showed a deeper infiltration. Reproduced and modified from [4] with permission.

Fluid bronchogram

The fluid bronchogram is a further sonographic criterion of pneumonia. It is marked by anechoic tubular structures along the bronchial tree. The bronchial wall is echogenic and the fluid in the segmental bronchi is hypoechoic. Given good resolution, the bronchial walls are ribbed and the vessel walls are smooth; therefore, tubular structures can be easily classified on B-mode images. In case of doubt, colour-coded duplex sonography helps to distinguish between vessels and bronchi. The fluid bronchogram is seen in approximately 8% of patients with pneumonia and develops in the early phase of the disease as a result of bronchial secretion or owing to bronchial obstruction. A persistent fluid bronchogram always raises suspicion of obstructive pneumonia and is an indication for bronchoscopic investigation. A tumour may be found or ruled out; the obstructive secretory embolus is aspirated and the material obtained for bacteriological investigation [4, 5, 17].

Vascularisation

On colour-coded duplex sonography pneumonia has a typical appearance: vascularisation is uniformly increased, ramified and the vessels run a normal course. In fact, circulation is increased in the entire infiltrate up to beneath the pleura (figures 3 and 4). This is interesting when pneumonia needs to be differentiated from pulmonary infarctions that have poor or no blood flow, or even from tumours with an irregular circulation pattern. Owing to neovascularisation, vessels in the margins of carcinomas are characterised by a typical corkscrew pattern [4, 10, 18].

Figure 4. Regular vascularisation in pneumonia with a pleural effusion (PE).

CEUS of lung consolidations

In comparison with other contrast-enhanced imaging procedures, CEUS has the following inherent advantages: the possibility of assessing the extent of enhancement in real-time examination during the arterial and parenchymal phases; the ability to differentiate pulmonary arterial vascularity from bronchial arterial vascularity by measuring the time to enhancement; and the ability to perform repeated examinations.

With CEUS, pneumonias are rapidly enriched with contrast medium and achieve intensive saturation after just 8–10 s. In patients with pleurisy and subpleural lesions of unknown cause that were found in LUS, CEUS allows the diagnosis or exclusion of pleuropneumonia and differentiation from pulmonary embolism [19–21].

The European Federation of Societies for Ultrasound in Medicine and Biology has recommended the following uses and applications for CEUS. CEUS is useful in differentiating inflammatory from embolic lung consolidation, especially in patients with equivocal CT findings. CEUS may be used in equivocal cases to diagnose lung abscesses within pneumonia, as CEUS appears better than conventional B-mode US or CXR [22].

Abscess formation

Bacterial pneumonias tend to develop colliquations and form abscesses. This is seen in approximately 6% of lobar pneumonia patients (figure refers to radiographic investigations). Sonography more commonly reveals microabscess [23, 24].

Pulmonary abscesses have a highly characteristic sonomorphology: round or oval and largely anechoic lesions (figure 5). Depending on whether a capsule is formed, the margin is smooth, echodense and white. Blurred internal echoes are indicative of high cell content or viscous pus rich in protein. Microabscesses can be easily distinguished from vessels on colour Doppler imaging.

Figure 5. Colliquated abscess with persistent fever after pleuropneumonia. Sonography-guided needle aspiration was successful. The patient went into complete remission.

If antibiotic therapy does not yield the desired result, LUS examination and US-guided transthoracic aspiration are useful and safe diagnostic procedures for the collection of specimens that enable the accurate diagnosis of lung abscesses [23–28].

Small abscesses measuring ≤2 cm in size can be punctured and emptied with ordinary aspiration. Lung drainage and lavage of large abscesses is rarely necessary. Drainage may be performed under sonographic guidance. The catheter is seen as a two-layered reflex. The investigator should use the shortest access and puncture through consolidated lung tissue to minimise the risk of pneumothorax and to avoid bronchopleural fistula. The results are equally as good as in CT-guided drainage. However, the position of the catheter can be supervised using LUS at bedside (table 1) [4].

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