Today I am showing chest radiographs of a 39-year-old man with high fever and malaise.
What do you see?
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Findings: Chest radiographs show bilateral air-space infiltrates in both upper lobes (A and B, arrows). There is widening of the right superior mediastinum (A, asterisk), displacing the right wall of the trachea. Moderate scoliosis.
Three weeks after treatment the infiltrates have healed. Widening of the upper right mediastinum persists. The clue to the diagnosis lies in in the absence of the aortic knob on the left, placing the aortic arch on the right side (C, white arrow) and simulating a mediastinal mass. Due to the scoliosis, the pulmonary arch is more evident and simulates a low-lying aortic knob (C, red arrow). Unenhanced coronal CT (D) confirms the findings (A aorta, P pulmonary artery).
Final diagnosis: right aortic arch in a patient with scoliosis and healed pneumonia
I am presenting this case to discuss the importance of negative findings. In radiology teaching we stress the importance of positive findings, such as increased opacity of the lung, pulmonary nodules, etc. However, we may fail to note absence of a structure that should be present. Recognizing this absence may be vital to reach a correct diagnosis (Fig. 1).
The value of negative findings is exemplified by the following dialogue taken from the short story “Silver Blaze” by Arthur Conan Doyle:
Gregory (Scotland Yard detective): “Is there any other point to which you would wish to draw my attention?”
Holmes: “To the curious incident of the dog in the night-time.”
Gregory: “The dog did nothing in the night-time.”
Holmes: “That was the curious incident.”
Sherlock Holmes was clever enough to detect a negative finding. The fact that the dog did not bark indicated that it knew the murderer and, based in this clue, Holmes solved the case.
We radiologists should pay special attention to similar negative findings that may help solve our cases.
Negative findings can be classified into two groups:
1. Anatomic structures that are not visible
2. Findings that should be present but are not
Today I will concentrate on the first group, leaving the second for the next Diploma
Failure to visualize a normal anatomic structure can be due to three reasons:
1. It is absent
2. It is not in its normal position
3. It is hidden
There are two causes for absent anatomic structures: congenital malformation or previous surgery.
Congenital abnormalities are not rare in adults. In some cases, a structure is missing and this fact may be overlooked, especially in routine examinations. This occurred in the following patient (Fig. 2), who had yearly check-ups for five years. The chest features were reported as unchanged until 2016, when the radiologist discovered that the left hilum was absent.
Previous surgery is the most common cause of a missing anatomic structure. Detecting this finding may be important for the radiologic diagnosis, as demonstrated by the cases below (Figs. 3 and 4).
Sometimes, anatomic structures are not seen because they are not in their normal location. A common example is right-sided aortic arch, which can simulate a mediastinal mass unless we note that the aortic knob is absent on the left side (Fig. 5).
The same occurs with the gastric bubble, which is visible in 90% of patients. Occasionally, it is not seen because the abdominal organs are inverted and the stomach lies under the right hemidiaphragm (Fig. 6) (See Caceres’ Corner, cases 178 and case 194).
Anatomic structures may not be seen because they are hidden. The best example is provided by the pulmonary hila, which hide behind the cardiac shadow when displaced downwards by lower lobe collapse (Figs. 7 and 8).
Follow Dr. Pepe’s advice:
1. Detecting negative findings is important in the diagnostic process
2. Anatomic structures may not be seen for the following reasons:
a) They are absent
b) They are not in their normal location
c) They are hidden