Here is an important question clinicians need to ask. When can you have a pneumonia and not see it on a chest x-ray? You can have a patient who is clinically sick because of a pneumonia, but when you do an x-ray you just don’t see it there. There are three answers.
One is dehydration. A patient who is dry will not waste water on hydration of a lung that is infected. So, as we talked about under working up a patient who is dehydrated for hyponatremia, when you size up a patient with pneumonia who is dehydrated, it is valid for you to make the diagnosis of pneumonia without seeing it on the x-ray. You would put in your notes that the patient is clinically dehydrated, and I am confident that when hydrated the next day, and we again x-ray, a pneumonia will become apparent.
Chronic obstructive pulmonary disease patients can have pneumonia and not have it readily apparent on the chest radiograph. Remember that the pathology of COPD is that they have air trapping. So, they have disproportionate amount of air in their lungs and because there is more air in the lungs, a pneumonia can be more subtle. COPD patients are treated with antibiotics. If someone comes in with chronic obstructive pulmonary disease exacerbation, we are going to assume that there is indeed an infectious component and automatically treat with antibiotics. Of contrast, in asthma patients we understand the pathophysiology does not include infectious disease. So, when someone comes in with an asthma attack, very rarely would we put them on an antibiotic as opposed to someone with chronic obstructive pulmonary disease who we always place on antibiotics.
Lastly is a retrocardiac (or lingula) pneumonia. This is where behind the heart there is a pneumonia that would not be seen on an AP film. You would need to use a lateral film. It’s extremely difficult to evaluate a patient that has an under penetrated CXR. If they have a well penetrated film and you can see the vertebral bodies, you may be able to make out a lingula pneumonia on the chest x-ray. If not, you really need to get a lateral radiograph. Of concern here is patients with pneumonias that are low in the lungs and very close to the diaphragm, it is classic that their fever could present with a degree of abdominal pain.
So, someone comes in with abdominal pain and a fever, you have to be concerned about whether this could be a pneumonia in the lower fields of the lung. Let’s say you have a child who comes in with a fever and abdominal pain. You think appendicitis. The surgeon takes him into the operating room, removes the appendix and admits him to the hospital. In the postoperative phase, these patients are not placed on antibiotics. This could lead to a life threatening infection/sepsis. So, once again, when can you have a pneumonia and not see it on the chest x-ray? Dehydration, chronic obstructive pulmonary disease, and retrocardiac or lingular pneumonia.