Italy: Ultrasound First with Pediatric Pneumonia

Published on February 7, 2013
Ultrasound now allows physicians to forego harmful radiation when scanning for pneumonia in children.

The diagnosis of pneumonia in children relies on physical examination, blood tests and chest X-rays. These methods have a low accuracy in the adult population that is even lower in the critically ill (1). A systematic review of the utility of the clinical examination in diagnosing community-acquired pneumonia concluded that there were no combinations of the history and physical examination that could reliably confirm or refute the presence of pneumonia. The chest radiograph has been used as the reference tandard for the diagnosis of pneumonia in most studies on adult patients. Nevertheless, technical limitations in the interpretation of chest radiograph of patients with possible pneumonia are well documented (2). The reliability of this test is limited by significant interobserver and intraobserver variability in interpretation (3). The chest X-ray is the most commonly used imaging tool in pneumonia because of availability and an excellent cost-benefit ratio. Computed tomography is mandatory in unresolved cases or when complications of pneumonia are suspected (4).

Current estimates indicate that a single CT scan will result in a lethal malignant transformation at least once in every 3,000 adult studies and that this rate may exceed 1 in 500 for children. Thoughtless and indiscriminate imaging of extremely low-risk patients becomes an ethical problem, as does imaging that is primarily obtained to protect the clinicians (5).

One in 1,250 adult patients, after being exposed to 15 mSv of radiation from a pan-scan (computed tomography of the head, neck, chest, abdomen and pelvis), will die from cancer. Children have 10 times the sensitivity to radiation, and the mortality approaches and possibly exceeds that of a laparotomy for a negative appendectomy (6).

These limitations along with the risk of ionizing radiations, mandate the search for a safe diagnostic tool for patients with suspected pneumonia. Enter portable bedside ultrasound. Lung scan using ultrasound can be adopted as a simple and non-invasive method for evaluating children with pneumonia. It is easy to perform at the bedside, allows close follow-up and avoids the use of ionizing radiation (7).

Scanning Technique

Neonates and infants are best imaged with a high resolution 5-10 MHz linear- array transducer; children and adolescents may require a 2-4 or 4-7 MHz sector or linear-array transducer. Transternal, para-sternal and intercostal approaches are good for imaging of the lung, pleura and anterior mediastinum. In the subxyphoid and trans-diaphragmatic approaches the liver is used as an acoustic window for evaluating juxtaphrenic paravertebral lesions. Suprasternal and supraclavicular approaches facilitate evaluation of the upper mediastinum and lung apices. Ultrasound is performed in the supine, prone or decubitus position. Images are obtained in the transverse, longitudinal, and inclined transverse or inclined longitu- dinal planes to maximize demonstration of the lesion (8).

Normal Findings

Ultrasonsonography has been underused or often ignored as a diagnostic tool in the chest, especially in the lung , because air and the bony thorax were traditionally considered an obstacle to transmission of the ultrasound beam. However, since ultrasound has been gaining recognition as a highly useful tool in the evaluation of pleural lesions, its role in imaging of the lung and extracardiac mediastinum has expanded and its usefulness has been recognized (1,8).

The normal paediatric lung picture does not differ from that of the adult lung. The superficial layers of the thorax consist of subcutaneous tissues and muscles. The ribs, on longitudinal scan, appear as curvilinear structures associated with posterior acoustic shadowing. The pleura appears as a regular echogenic line (pleural line) moving continuously during respirations. Pleural movement has been described as the “lung sliding” sign. The amplitude of the lung sliding is minimal at the apices and maximal at the bases. Lung sliding can be objectified and documented with M-mode. The ribs and the pleural line, in the longitudinal view, outline a characteristic pattern, the “bat sign” (Fig. 1).

Beyond the pleura-lung interface, the lung is air-filled and does not allow further visualization of normal lung parenchyma. However the large change in acoustic impedance at the pleura-lung interface results in horizontal parallel artifacts below the pleural line. Those artifacts have been termed “A lines” (Fig. 2). Vertically-oriented “comet-tail” artifacts arising from the pleural line, also called “B lines”, are absent in the normal lung. They arise from the pleural line, are well defined, reach the lower edge of the screen, erase A lines and move with lung sliding (1,7). The presence of B lines, or comet-tails, is related to pathological findings and results from the fluid-rich subpleural interlobular septae, which are surrounded by air and identify an alveolar-interstitial syndrome (Fig. 3).

Pathological Findings

The airless lung is similar in echogenicity and echotexture to the liver and spleen. Within the solid-appearing area of echogenicity, multiple bright dot-like and branching linear structures are found. These findings represent air in the bronchi and scattered residual air in alveoli within the consolidated or atelectatic lung. This appearance is termed a sonographic air bronchograms (Fig.4).

In consolidation the lung volume is increased by fluid or tissue, but the bronchi are spared and retain their normal branching pattern. In atelectasis, overall lung volume is decreased; supplying bronchi of the involved lung can be crowded together in very close apposition in one plane, appearing as parallel-running bright lines (Fig. 5).

Occasionally when the bronchial tree is filled with fluid rather than air, as in mucoid impaction, US may demonstrate a branching pattern of anechoic or hypoechoic tubular structures within consolidated lung. Demonstration of fluid-filled bronchi, an appearance termed a sonographic or mucous bronchogram, is a specific indicator of pulmonary parenchymal consolidation, equivalent to the air bronchogram (1, 10).

Sonographic or air fluid bronchograms may not be visible, particularly in the peripheral lung. In this case color flow US demonstrates the normally branching pattern of pulmonary vessels in consolidated lung (5).

Pathological processes that involve the pleura and manifest as fluid collections, are ideal for imaging with US because of their acoustic properties. The different types of pleural effusion depend on the nature of the fluid collection: serous, purulent, hemorrhagic or chylous. Serous fluid is usually a transudate, and purulent fluid is an exudate or empyema. At ultrasound, pleural fluid may be characterized as a simple effusion, a complicated effusion or fibrothorax (pleural thickening or fibrosis). A simple effusion (Fig. 6) appears as a clear anechoic or cloudy hypoechoic fluid with or without swirling particles. A complicated effusion appears as a septated or multiloculated, hypoechoic fluid, partitioned by fibrin strands, with no clear demarcation between the lung and the pleural components. Fibrothorax appears as a thickened, echogenic rind of pleural plaque (8).

Reference

  1. Lichtenstein, et al: Ultrasound in the management of thoracic disease. Crit Care Med 2007; 35 (5 suppl.): S250-61
  2. Basi, et al: Patients admitted to hospital with suspected pneu- monia and normal chest radiographs: epidemiology, microbiology, and outcomes. Am J Med 2004; Sept 1, 117: 305-311
  3. Katz, et al: Radiol- ogy of pneumonia. Clin Chest Med 1999; Sept 20 (3): 549-563
  4. Sharmar, et al: Radiological imaging in pneumonia: recent innovations. Curr Opin Pulm Med 2007; 13: 159-169
  5. Mower, et al: Ra- diation doses among blunt trauma patients: assessing risks and benefits of computed tomographic imaging. Ann Emerg Med 2008; Aug 52 (2.): 99-100
  6. Snyder, et al: Whole-body imaging in blunt multisystem trauma patients who were never exam- ined. Ann Emerg Med 2008; Aug 52 (2.): 101-103
  7. Copetti R, Cat- tarossi L. Ultrasound diagnosis of pneumonia in children. Radiol Med 2008, 113: 190-198
  8. OkH,WooSK, Min J K, Jun Y J. US in the diagnosis of pediatric chest diseases. Radio- Graphics 2000, 20: 653-671

From the Fall 2012 issue of Emergency Physicians International

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