A 34-year-old man presented with one day of fever and a cough
productive of yellowish sputum. He complained of generalized myalgia
and had vomited twice.
He had a history of intravenous drug use and had last injected
drugs over one year earlier. He was tested and found to be PPD negative
and HIV negative two months earlier.
On examination, he appeared healthy but had persistent cough.
His vital sign were: blood pressure 118/78 mm Hg, pulse
88 beats/min, respiratory rate 24 breaths/min,
temperature 100.8°F (oral), 103.4°F (rectal). Pulse oximetry oxygen
saturation was 96% on room air.
Lung auscultation revealed bibasilar crackles. His neck was supple
and oropharynx was clear. He was anicteric and had no rash, oral
thrush, or lymphadenopathy. He had no heart murmur or hepatosplenomegaly.
A chest radiograph was obtained and interpreted as normal (Figure
- Do you agree with this interpretation?
In most patients, pneumonia is easy to detect. However, when
the infiltrate is small or has only slightly greater opacity than
adjacent normal lung, more subtle radiographic signs must be sought.
Silhouette signs occur when the
airspaces of the lung are filled with fluid resulting in either
the formation of a new abnormal air/fluid interface or
obliteration of a normal air/fluid interface (loss of a
silhouette). This silhouette effect, which occurs with pneumonia
as well as other conditions that cause airspace filling, is one
of the basic principles of chest radiography.
For an air/fluid interface to be visible on a radiograph
(i.e., form a silhouette), it has to have a relatively sharp margin
that is parallel to the direction of the x-ray beam. When the margin
of a fluid collection is either gradual or not parallel to the x-ray
beam, it appears indistinct (ill-defined) or may be completely invisible.
There are four types of silhouette signs (Table 1). The first
most well known is loss of a normal air/fluid
interface that occurs when an
abnormal fluid collection such as pneumonia lies adjacent to a soft
tissue structure such as the heart or diaphragm. For example, a
right middle lobe pneumonia obliterates the right heart border because
the right middle lobe lies against the right atrium. This is often
referred to as “the silhouette sign,” although
it is actually a misnomer because there is loss of a normal radiographic silhouette
(air/fluid interface). In addition, a silhouette sign does
not necessarily mean that the fluid collection is within the lung.
A large pleural effusion can also obliterate the margin of the diaphragm.
Table 1 Four Silhouette Signs