Diagnosing Pneumonia: the ultrasound is mightier than the x-ray
The following is adapted from The POCUS Manifesto: Expanding the limits of our physical exam with point-of-care ultrasound (POCUS). You can get a copy here.
"Atelectasis versus pneumonia, correlate clinically"
In 1895, German physicist Wilhelm Rontgen was experimenting with electrical discharges passing through vacuum tubes when he came across a remarkable discovery: what appeared to be an image of his own skeleton, plastered on a piece of lead near the tube. He spent 6 weeks reproducing this miraculous phenomenon, culminated by a radiograph of his wife's hand. "I have seen my death!" she famously exclaimed upon seeing her skeleton bearing her ring in this iconic image:
This was one of the most remarkable discoveries in medical history, giving the medical community collective "x-ray vision" and ushering in a new era of more precise diagnostic medicine. 127 years later we still rely on x-rays for diagnosing everything from broken bones to pulmonary edema, pleural effusions to pneumonia.
Chest x-rays and pneumonia
Whether in an outpatient clinic, urgent care, or hospital setting, a chest X-ray is routinely ordered to evaluate a patient presenting with shortness of breath or cough. If this chest X-ray does not detect a lung consolidation (also known as an opacity), then it is often assumed that the patient does not have pneumonia.
However, this is not necessarily the case.
A chest X-ray of the lungs is analogous to a photograph of a large pine tree. It is a static two-dimensional snapshot in time that portrays a triangular-shaped tree with spiky-green-pine-covered branches emerging from the trunk. The trunk of the tree may be visualized through the more sparse portions while hidden through the dense ones. But it would not portray a light breeze displacing the branches every few seconds, or an animal nesting in the back. And it is these 2-D properties that result in low resolution and poor ability to detect important lung abnormalities.
In a study looking at 97 patients presenting to the hospital with respiratory symptoms, chest X-rays missed 27% of cases of pneumonia that were found on gold-standard CT scans, while another study of 188 febrile, immunosuppressed patients found that 60% of them had CT findings of pneumonia with a normal corresponding chest X-ray. Researchers at Vanderbilt University studied over 3,400 patients and found that chest X-rays missed over half of pneumonias seen on CT scans, while only one-quarter of the patients with chest X-rays interpreted by the radiologist as having pneumonia actually did.
They concluded that chest X-rays “cannot independently rule-in or rule-out pneumonia,” and that given the millions of annual emergency department visits for respiratory symptoms, “an ‘opacity equals antibiotics’ reflexive management strategy is likely to lead to frequent overuse of antibiotics and unnecessary pressure for the development of antibiotic resistance.”
This strongly-worded conclusion is not fully appreciated by most, yet becomes abundantly clear once one has ultrasounded hundreds of lungs. In contrast to a chest X-ray's one-dimensional nature, lung ultrasound is a series of semi-immediate dynamic snippets of lung tissue separated by ribs. It is more like a drone-mounted video camera, traveling around the pine tree in horizontal circles as it makes its way down the trunk. Unlike the photograph, this drone can appreciate the real-time subtle movement of the tree branches, as well as any animals that may be dwelling in the back. Like a pointillist painting of your respiratory organ, these snippets of ultrasound can then be mentally pieced together to form a three-dimensional image of the lung.
Ruling in or out pneumonia with ultrasound
When attempting to diagnose pneumonia with lung ultrasound there are two main abnormalities to look for: focal B-lines with absent lung sliding, and consolidations. Dr. Daniel Lichtenstein, the godfather of POCUS, convincingly demonstrated that cardiogenic pulmonary edema causes the interlobular septae to swell and emit lung rockets across multiple rib spaces. He characterized this type of non-infectious, non-inflammatory fluid present with the B-profile as “a lubricant which does not impair lung sliding.” Yet when edema from inflammation or infection fills the alveoli and the interlobular septae swell, lung rockets appear as well; however, they are often focused in discrete regions of the lung and lung sliding commonly becomes impaired (what he called the B-prime profile). This exudative fluid as it is called, “is a biologic glue. We assume that each exudative B-line acts as a nail. Since B-lines are numerous, these multiple nails should appear sufficient for sticking the lung to the wall.” Such conditions arise in diseases like acute respiratory distress syndrome, pulmonary fibrosis, or pneumonia, where lung rockets are present but lung sliding is often impaired.
In addition to focal lung rockets, if a consolidation is present it can often be visualized deep to the pleural line in the lung tissue itself. By nature of the erratic accumulation of fluid, pus, or irregular cells that occurs during the normal course of an infection or a cancer, consolidations that occupy a portion of the lung will have irregular, shredded borders with underlying normal lung deep to it. This is called the shred sign, and it can be used to identify a consolidation when it is seen (Figure below). The vast majority of these consolidations reside towards the base of the lung on the posterior-lateral side, a point Lichtenstein dubbed the PLAPS point, an acronym for the posterolateral alveolar and/or pleural syndrome point mouthful.
Ultrasound protocols to evaluate for pneumonia vary but generally involve looking at multiple rib spaces in both lungs, usually two anterior, one or two lateral, and the PLAPS point looking for impaired lung sliding, focal lung rockets, or lung consolidations. A 2020 study in a Minneapolis, Minnesota ICU found that when compared with gold standard CT scans, using a similar protocol detected up to 92.5% of consolidations as compared to only 65.7% with chest X-ray.
Another study of Chinese patients with community-acquired pneumonia showed that lung ultrasound and chest X-rays had sensitivities of 94% and 77% respectively, with similar specificities.
This superiority of lung ultrasound for detecting pneumonia has been confirmed in multiple populations and locations around the world. In a study completed in Dutch kids under 16 years old, for example, they compared the results of chest X-ray to lung ultrasound for those presenting to the ED with clinical signs and symptoms of pneumonia. During the course of the study, they observed that "several small areas of consolidation identified on [ultrasound] were not visible on the corresponding chest X-rays," which they realized is because ultrasound can detect small consolidations that X-rays could not. In 8 patients with negative X-rays, ultrasound identified 17 areas of consolidation. On average these consolidations were 9mm, significantly smaller than the average 26mm size consolidations seen with both.
Lung ultrasound's ability to detect smaller pneumonias that chest x-rays could not resulted in impressive diagnostic accuracy, with a sensitivity of 98% and a specificity of 92%.
Similar results were seen in a meta-analysis of 18 pediatric studies, in Chinese neonates, in Greek ICU patients, in adults with COVID-19 pneumonia, and even in patients presenting to a remote emergency department in Nepal.
With sensitivities and specificities in the 90% range, lung ultrasound can rule in or rule out pneumonia with near certainty. Validated by multiple meta-analyses, this research begs the question why lung ultrasound, with its zero ionizing radiation icing-on-the-cake, is not the first line pulmonary imaging option for all patients.
This is especially true in the outpatient setting. In Europe for example, as much as 90% of antibiotics are prescribed by general practitioners, the majority of which are for suspected respiratory tract infections. In a clinic environment with no access to immediate imaging and no good screening tests, the decision to prescribe antibiotics is largely subjective, ultimately leading to excess antibiotic use and resistance.
While there are many contributors to the antibiotic overprescription problem, including patient expectations or medical-legal repercussions, it is fundamentally a function of poor screening tests. If there was a nasal swab or blood test that could say with over 90% sensitivity and specificity that the patient did or did not have pneumonia, then this would not be such a problem. While we don’t have such a laboratory test, we do have lung ultrasound with similarly impressive diagnostic capabilities, yet it is rarely or ever evoked as a method for narrowing our antibiotic use for respiratory infections.
One can only speculate how many fewer antibiotics we would use, how much antibiotic resistance we could spare, or how much radiation exposure could be averted if we follow the protocols developed by Dr. Lichtenstein and used our ultrasound probe in place of our stethoscope when examining a patient with respiratory symptoms.
This was adapted from The POCUS Manifesto: Expanding the limits of our physical exam with point-of-care ultrasound (POCUS). You can get a copy here.