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 Introduction
Severe Acute Respiratory Syndrome (SARS)
is a term used to describe a serious
respiratory illness which appears to be
less infectious than influenza. The
incubation period is believed to be short,
around three to six days (maximum 10
days). Its main symptoms are high fever
(>380 C), dry cough, shortness of breath
or difficulty in breathing. Changes in
chest X-rays indicative of pneumonia also
occur. Most patients identified to date
have been previously healthy adults, aged
25-70 years. Approximately 10% of SARS
patients will develop severe disease that
will require intubation. The global death
rate for “probable SARS”
cases is 4%. The cause of SARS has now
been documented by WHO (World Health
Organization) to be “SARS coronavirus” (SARS
CoV,) a new member of the coronavirus
family. Based on currently available
evidence, close contact with an infected
person poses the highest risk of the
infective agent spreading from one person
to another. SARS is a new disease, which
has its origins in Guangdong Province,
China. The earliest known cases were
identified in mid-November 2002. Since
then, probable cases of SARS have been
reported in at least 28 countries(1-8).
Background
Health care workers
need to be alert to the possibility of
encountering patients with SARS and able
to diagnosis the disease. The current WHO
case definition(9) of the disease is a
person presenting after 1 November 2002
with history of: high fever > 38 C, along
with one or more of the respiratory
symptoms (cough, dyspnea) and with
radiographic evidence of infiltrates
consistent with pneumonia or respiratory
distress syndrome (RDS) on chest X-ray (CXR),
plus one or more of the following:
exposure during the 10 days prior to onset
of the symptoms (close contact with a
person who is a suspect or probable case
of SARS); history of travel to, or
residing in, an affected area.
A sudden outbreak of Severe Acute
Respiratory Syndrome cases would create an
unprecedented and dire situation for
caregivers. At present, the symptoms and
signs of the disease are nonspecific,
therefore history of travel and/or close
contact is essential. Diagnostic imaging
will play a vital role in the management
of this unprecedented healthcare crisis,
because radiologists will be able to
triage cases from suspected cases of SARS
to probable cases of the disease.
Chest
radiographic findings
In the early stage of
the disease, a focal unilateral or
bilateral homogeneous
peripheral/pleural-based opacity, without
obscuration of the underlying vessels, may
be the only abnormality. This may range
from “ground glass” to consolidation in
appearance. A particular area to view is
the paraspinal region behind the heart.
In
author experience, this is frequently
where lung lesions are detected on high
resolution computed tomography (HRCT) in
suspected SARS patients with normal
radiographs(10-13).
In the more advanced
cases, there is widespread unilateral or
bilateral homogeneous opacification, with
obscuration of the underlying vessels,
which may be ground glass-like or
consolidative, affecting large areas.
This
tends to affect the lower zones first and
is not a feature of this disease(10-13).
In recent study, the
initial chest radiographs were abnormal in
108 of 138 (78.3%) patients and showed
air-space opacity. Lower lung zone (70 of
108, 64.8%) and right lung (82 of 108,
75.9%) were more commonly involved. In
most patients, peripheral lung involvement
was more common (81 of 108, 75.0%). Unifocal involvement (59 of 108, 54.6%)
was more common than multifocal or
bilateral involvement(12).
Radiological finding
of cavitation, lymphadenopathy, or
pleural effusion are not seen in SARS
patients and other causes of pneumonia
should be investigated(13).
Figure 1. Chest radiography of
SARS Patients. Panel A.
Frontal chest radiograph in a 25-year-old
woman showing ill-defined air-space
shadowing (arrows).
Panel B. Frontal chest
radiograph in a 38-year-old woman showing
patchy consolidation in the right lower
zone (arrows). Panel C.
Frontal chest radiograph in a 46-year-old
man showing obvious area of air space
shadowing (arrows). Panel D.
Frontal chest radiograph in a 46-year-old
man showing multiple, bilateral areas of
air space disease. In all of the
above no cavitation, adenopathy, or
pleural effusion noticed.
High
Resolution Computed Tomography (HRCT)
In a recent study of
73 patients, thin-section CT scans were
abnormal in those patients with symptoms
of SARS, in keeping with criteria from the
Centers for Disease Control and
Prevention, and a positive chest
radiograph but also in those patients
with high clinical suspicion of SARS and
a normal radiograph. However, the scans
were normal for those patients with minor
symptoms and a normal chest radiograph(13).
Common findings
included solitary, or multiple, patchy
area (s) of ground glass opacification of
interstitium or interlobular interstitium,
and/or consolidation, and/or interlobular
septal and intralobular interstitial
thickening. A combination of the above is
not uncommon(10, 13).
These changes tend
to occupy a sub-pleural position rather
than axial. Again, calcification,
cavitation, pleural effusion or
lymphadenopathy are not features of this
disease(10, 12).
The size of each
lesion and the total number of segments
involved were smaller in those patients
with high clinical suspicion of SARS, but
a normal radiograph. The affected segments
were predominantly in the lower lobes (91
of 149 affected segments). A majority of
patients with symptoms of SARS and a
positive chest radiograph (14 of 23) had
mixed central and peripheral lesions.
In
symptomatic SARS patients with normal
chest xray, the peripheral lesions were
more common (10 of 17). A purely central
lesion was uncommon in SARS patients(13).
Figure 2. Selection of HRCT
from SARS patients. Panel A.
High-Resolution CT Scan of a 72-year-old
man showing subpleural areas of
consolidation and ground-glass
opacification with air bronchogram (arrow)
affecting the posterior aspects of the
lungs, particularly the lower lobes.
Panel B. High-Resolution CT Scan showing the
characteristic ground-glass abnormality in
a subpleural location. Panel C.
High-Resolution CT Scan showing the
characteristic wide spread ground-glass
abnormality. There is no cavitation,
lymphadenopathy or pleural effusion.
Progression
of disease
The majority of
patients (78.3%) with severe SARS had
abnormal chest radiography on presentation
in one study, consisting of air space
opacification. Approximately 74.6% of
those patients showed deterioration in the
form of unilateral or bilateral multifocal
opacification. HRCT in those patients
showed a progression from unilateral, or
bilateral, peripheral focal lesions to
multiple unilateral, or bilateral, ground
glass and/or consolidation that
incorporated the central or perihilar
regions.(14)
In one study of 10
patients with severe SARS, the
progression of radiography is fast, within
24 hours of presentation, regardless of
the initial radiographic pattern(10).
Four patterns of
radiographic progression were recognized:
1.
Type 1 (initial
radiographic deterioration to peak level
followed by radiographic improvement) in
97 of 138 patients (70.3%),
2.
Type 2
(fluctuating radiographic changes) in 24
patients (17.4%),
3.
Type 3 (static
radiographic appearance) in 10 patients
(7.3%), and
4.
Type 4
(progressive radiographic deterioration)
in seven patients (5.1%). Initial focal
air-space opacity in 44 of 59 patients
(74.6%) progressed to unilateral
multifocal or bilateral involvement
during treatment (11,
12).
Imaging
protocol
A recently published imaging protocol to
be followed if SARS is suspected is as
follows: If SARS is clinically suspected,
a chest radiography should be performed.
If the chest radiography is abnormal, then
no further imaging investigation is
required other than serial radiographs for
follow up. If the chest radiograph is
normal, an HRCT is performed. This may show
changes one or two before they become
radiographically apparent (12).
Currently,
all patients admitted with severe SARS
have abnormal chest imaging findings.
Summary
SARS is contagious
disease affecting health care workers,
therefore appropriate personal precautions
need to be taken, as well as appropriate
cleansing of radiographic or CT equipment (15).
High Resolution CT
Scan should be used with caution as it may
result in overdiagnosis. It should be
used only if there is a history of
contact or the clinical signs, such as a
continuing fever, Leucopenia etc., are
strongly suggestive of SARS, and the
initial chest radiograph is normal.
Radiological finding of cavitation,
lymphadenopathy, or pleural effusion are
not seen in SARS patients and other causes
of pneumonia should be investigated.
References
Other
Topics:
Review Article # 1
- Clearing the
Cervical Spine in the Acutely
Injured
Review Article # 3
- Optimal Strategies of Coronary
Reperfusion in Acute ST-segment Elevation
Myocardial Infarction
: Thrombolysis vs. Percutaneous
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