Dr. Cura
is Chief Resident in the Department of Radiology, Mount Sinai
Medical Center, Miami Beach, FL.
The nontraumatic acute abdomen is one of the most common
presentations to the emergency room, with appendicitis being one of
the most common causes of the acute abdomen. Up to 30% of patients
suspected of having acute appendicitis will present with atypical
signs and symptoms.
1
There are many conditions that mimic acute appendicitis.
2
The percentage of unnecessary appendectomies that result from a
clinical false-positive diagnosis of appendicitis is reported to be
8% to 43%, with a mean of approximately 20%.
3
The use of computed tomography (CT) before planned surgery has
decreased the negative appendicectomy rate for patients with
suspected acute appendicitis. Recognition of the typical and
atypical CT signs of appendicitis is important to optimize the
diagnosis yield of the examination. Visualization of an appendix
with normal characteristics is the most important finding to
exclude appendicitis.
To determine the individual frequency of CT signs of acute
appendicitis and common features of the normal appendix, CT results
obtained in 113 patients examined at the author's hospital for
suspected appendicitis were evaluated retrospectively.
Subjects and methods
A total of 113 patients with suspected acute appendicitis were
referred for thin-section helical CT from the departments of
surgery and emergency medicine. Patients ranged in age from 8 to 80
years (5 patients were younger than 15 years of age) and included
71 males and 42 females. CT diagnoses were recorded
retrospectively. The studies were performed on an Advantage HI
speed helical CT scanner (GE Medical Systems, Milwaukee, WI) using
3-, 5-, or 7-mm slice thickness. Some patients received oral
enteric contrast. Intravenous contrast was administered to 29 of 57
(51%) of patients with acute appendicitis.
Cases were reviewed for the following CT findings: maximum
transverse diameter of the appendix, the "target" sign, calcified
appendicolith, periappendiceal inflammation, appendiceal tip
stranding, the "comet-tail" sign, the psoas sign, the "arrowhead
sign," the presence of appendiceal air (intraluminal, intramural,
and periappendiceal), the position of the appendix (retrocecal or
right hemipelvis and low abdomen), and the presence of free
intraperitoneal fluid.
Final diagnoses were established based on pathologic findings of
surgical specimens, clinical follow-up, or both. All cases of
appendicitis were confirmed by surgical and pathologic findings.
Negative diagnoses were confirmed surgically or through chart
review until patient discharge. The variation and frequency of
various CT signs of the inflamed appendix and characteristics of
the normal appendix were noted.
Results
Among the 113 patients studied by preoperative helical CT, 57
patients had surgically and pathologically proven appendicitis. The
other 56 patients did not have either surgical or clinical evidence
of appendicitis at discharge.
The appendiceal lumen contained air or contrast in only 8.7% (5
of 57) of patients with acute appendicitis and in 75% (42 of 56) of
patients without appendicitis (Figures 1 and 2). The maximum
cross-sectional diameter of an inflamed appendix ranged from 8 to
22.5 mm. All of the 57 patients with appendicitis had an
appendiceal diameter >0.6 cm, and 82% (47 of 57) had an
appendiceal diameter >1 cm. Overlap in size between inflamed and
normal appendices was noted although other signs of appendicitis
were present in acute appendicitis.
The "target sign" is a hyperattenuated or intense
contrast-enhancing thickened appendiceal wall. Intense contrast
enhancement was defined as attenuation equal or greater than that
of normal bowel wall. It was noted in 96% (27 of 29) of patients
with acute appendicitis who underwent contrast-enhanced CT (Figures
3 and 4).
An appendicolith is a focus of high attenuation that varies in
size, shape, and number; they are usually seen within the proximal
appendiceal lumen. One or more appendicolith was seen in 28% (16 of
57) of patients with appendicitis (Figure 5).
Periappendiceal fat stranding represents inflammation of the
periappendiceal fat, mesenteric fat that surrounds the appendix.
Spread of the appendiceal inflammation to surrounding mesenteric
fat results in stranding of the right lower quadrant fat. Stranding
of the mesenteric fat in the right lower quadrant was observed in
73% (42 of 57) of patients with acute appendicitis. When the
appendix is borderline in size, this finding helps support the
diagnosis of appendicitis (Figure 6). Appendiceal tip stranding was
demonstrated in 68% (39 of 57) of patients. Inflammation may begin
in the distal end of the appendix.
The psoas sign is defined as asymmetric obliteration of the fat
immediately anterior to the right psoas muscle as compared with the
left side. It was noted in 43% (25 of 57) of patients with acute
appendicitis. Among patients with acute appendicitis, the psoas
sign was noted in 43% (17 of 39) of lower abdominal and pelvic
appendices and in 44% (8 of 18) of retrocecal appendices (Figures 7
through 9).
The "comet-tail" sign, defined as thickening of the right
lateroconal fascia, was present in 26% (15 of 57) of patients with
acute appendicitis. Right lateroconal fascia thickening was noted
in 66% (12 of 18) of retrocecal and 7% (3 of 39) of low abdominal
and pelvic appendices among patients with acute appendicitis
(Figure 10).
Cecal signs of appendicitis, such as focal cecal apical
thickening or the "arrowhead" sign, were under-evaluated because
rectal administration of contrast was not used routinely in this
study. The "arrowhead" sign occurs when cecal contrast material
funnels symmetrically at the cecal apex to the point of appendiceal
occlusion.
4
In 2 patients, CT of the abdomen and pelvis with oral contrast was
performed and the appendix was not visualized initially. Delayed
images with additional oral contrast produced more distention of
the cecum and subsequently the appendix was identified.
Free intraperitoneal fluid was present in 15% (9 of 57) of the
patients with acute appendicitis. Inflamed appendices were seen in
the lower abdomen and pelvis in 68% (39 of 57) of patients and were
retrocecal in 31% (18 of 57) of patients (Figures 8 and 11 through
15). Intraluminal appendiceal air was seen in 8.7% (5 of 57) of
patients with acute appendicitis. On the other hand, normal
appendices demonstrated intraluminal appendiceal air in 75% (42 of
56) of patients without evidence of appendicitis (Figure 16).
Of the 56 patients without evidence of appendicitis, normal
appendices were seen in the low abdomen and pelvis in 45 (80%) and
were retrocecal in 11 (19%) (Figures 1, 16, and 17). The mean
thickness of the normal appendix ranged from 6 and 10 mm. Of these
56 patients without acute appendicitis, an appendix with a diameter
<0.6 cm was seen in 46% (26); a diameter >0.6 cm and <1 cm
was noted in 51% (29); and a diameter >1 cm was seen in 1%
(1).
Discussion
Presentation of many acute abdominal conditions overlap, and CT
has been demonstrated to be a rapid and accurate tool in the
diagnosis of many of these conditions.
5
In order to confirm or exclude appendicitis on CT in a patient with
clinical suspicion of appendicitis, finding the appendix is the
initial step. Identification of anatomic landmarks (such as the
cecum and terminal ileum), dynamic cine review, and reconstruction
of CT images on the monitor have been shown to be useful tools
6
(Figure 1). Normal appendices are identified in 67% to 100% of
adults without appendicitis on helical CT
3,7,8
(Figure 16).
A normal appendix on CT is seen as a tubular structure, linear
or curved with a blinded end arising from the posteromedial aspect
of the cecum 1 to 2 cm below the ileocecal valve and measuring up
to 10 mm in maximum diameter. Recognizing a normal appendix is the
most important CT finding to exclude appendicitis
9
(Figures 1 and 16).
Visualization of the appendix depends primarily on the amount of
intraperitoneal fat and appendiceal position. Other factors that
may help to recognize the appendix are the amount of ileocecal
bowel opacification, intravenous contrast, and type and quality of
the CT examination
10,11
(Figure 1). Large amounts of intra-abdominal fat have been shown to
provide excellent contrast to delineate the appendix and to
facilitate identification of periappendiceal fat stranding. A
low-lying appendix in the pelvis may be difficult to identify and
differentiate from the adjacent intrapelvic organs (Figures 8 and
15). False-negative examinations in patients with appendices that
lie low in the pelvis have been described.
12
A lack of inflammatory changes in the periappendiceal region may
further contribute to misinterpretation.
12
Distention of the bowel lumen by enteric contrast highlights the
anatomic landmarks, the cecum, and terminal ileum and permits
easier recognition of the appendix. Utilization of rectal contrast
distends the cecum more effectively than does oral contrast, and
cecal signs of appendicitis, such as focal cecal apical thickening
or the arrowhead sign, can be seen. An unfilled small bowel loop
may be confused with an enlarged appendix, but this confusion is
less likely with adequate oral contrast opacification of the small
bowel.
The CT characteristics of appendicitis may vary according to the
position of the appendix, the amount of intra-abdominal fat, and
evolution of the disease over time. An enlarged appendix with
periappendiceal stranding is a strong indicator of appendicitis. In
this study, CT demonstrated periappendiceal inflammation in 73% of
patients with acute appendicitis but up to a 98% incidence has been
reported in other series
8,13
(Figures 6 and 18).
When an enlarged appendix is noted, evaluation for
periappendiceal fat stranding is needed to ensure the proper
diagnosis (Figures 6 and 18).
Patterns of inflammation may vary depending on the appendix
position. Right lateroconal fascia thickening is seen more commonly
with an inflamed retrocecal appendix (Figure 10) than with a right
pelvic appendix. A positive psoas sign is seen as often in
retrocecal as in right hemipelvis and low abdominal appendicitis
(Figures 7, 9, and 19). The target sign (the enhancing,
high-attenuation, thickened wall of an inflamed appendix) is best
demonstrated after intravenous contrast (IV) administration.
Intravenous contrast may have a role in distinguishing between
early appendicitis and a normal appendix (Figures 3, 4, and
20).
Intraperitoneal fat is the intrinsic contrast medium for
nonenhanced CT examinations. Identifying inflammatory changes in
slender patients with little periappendiceal and pericecal fat may
be difficult; therefore, false-negative interpretations have been
reported in pediatric patients and young women.
14
An appendicolith has a high specificity for the diagnosis of
acute appendicitis (Figures 5, 11, 21, and 22). The initial event
in acute appendicitis is likely the appendicolith obstruction of
the appendiceal lumen.
15,16
The obstructed lumen leads to increased intraluminal pressures,
causing venous congestion, arterial compromise, and tissue
ischemia. Subsequently, there is epithelial mucosal barrier
interruption, bacterial invasion, and transmural appendiceal wall
inflammation.
17
Finally, appendiceal wall infarction and perforation occur, which
allows the inflammatory process to extend to the periappendiceal
mesenteric fat, parietal peritoneum, and adjacent structures.
17
Intraluminal appendiceal air is a finding on CT in both normal
and inflamed appendices. However, intraluminal air is seen more
commonly in normal appendices than in inflamed appendices. The
appendix is connected to the cecum, a gas-filled viscus, and air
probably regurgitates from the cecum into the nonobstructed
appendiceal lumen.
15
Intraluminal air can be seen in early appendicitis but is an
uncommon finding in the late stages of the disease; appendiceal
luminal air is probably resorbed after obstruction.
15
Intramural, extraluminal or periappendiceal air is more
characteristic of advanced inflammation and indicates a perforated
appendix
15
(Figure 7).
Summary
Intraluminal gas and an appendiceal diameter measuring up to 10
mm on CT are common findings of a normal appendix. Recognition of a
normal appendix is required to exclude appendicitis. An enlarged
appendix with periappendiceal inflammation is the most common
finding of acute appendicitis. Fat stranding may present in
different patterns depending on location of the appendix.
AR