Appendicitis can be acute or chronic. Acute appendicitis is the most
common surgical abdominal emergency in the western world occurring
in 7%–12% of the general population [
1]. Prompt and
accurate diagnosis reduces the morbidity and mortality of acute appendicitis.
Due to its high sensitivity and specificity, computed tomography
(CT) is becoming the preferred imaging modality for suspected acute
appendicitis, particularly in adults [
2]. CT is more accurate in the diagnosis
of acute appendicitis since it is less operator dependent than ultrasonography
(US) [
3]. Therefore, the use of CT has been advocated so
far in the minority of patients with acute appendicitis that present with
atypical clinical features [
4].
Although in most cases the diagnosis of acute appendicitis is usually
clear on the basis of clinical features, there is a significant negative
laparotomy rate. Therefore, some authorities now recommend CT for all
patients with suspected acute appendicitis or for those with equivocal
acute appendicitis. CT may also be helpful in the preoperative evaluation
of patients undergoing laparoscopic appendectomy [4].
CT seems to be more sensitive (96% vs. 76%) and accurate (94% vs.
91%) than US in diagnosing acute appendicitis, whereas they are almost
equal when it comes to specificity (89% vs. 91%). CT imaging tailored
to evaluate acute appendicitis has proven to be particularly successful
with a sensitivity of 100%, specificity of 95%, positive predictive value
of 97%, negative predictive value of 100%, and accuracy of 98% [5,6].
Multidetector-row CT (MDCT) currently has an important role in the diagnosis
of acute appendicitis and its severity. Some authors suggest that
they can diagnose acute appendicitis with an accuracy of 99%. It is also possible
to reconstruct the entire form and position of appendices from successive
CT findings because of high-resolution thin-slice MDCT images [7].
CT examination protocol
The patient is prepared with 800–1000 ml of oral contrast medium
for bowel opacification 60–90 min prior to scanning. The scan is performed
with the patient in the supine position, following an intravenous
injection of 100–120 ml of iodinated contrast medium at a rate
of 3 ml/s and a scan delay of approximately 60 s. The combination of
oral and intravenous contrast medium provides the most information
about the inflamed appendix and the surrounding tissues [5]. It was
reported that oral administration of up to 800 ml of contrast medium
at least 1 h before CT scanning enables opacification of both the small
bowel and the right colon in most patients [8].
CT appearances of the normal appendix
Although in the initial period after the development of CT the visualization
of the normal appendix was poor, this changed with refineintraments
in CT technology, standardization
of the technique, and increasing
experience [4]. Compared with plain
radiography, barium enema, and US,
CT achieved both higher accuracy and
greater consistency for appendiceal imaging
[9]. The identification of a normal
appendix excludes appendicitis
effectively and with a greater degree
of confidence than does the lack of
CT findings indicative of appendicitis
[10]. A normal appendix is visualized
in 43%–58% of CT examinations of the
abdomen. With 5-mm sections a normal
appendix was identified in 75%
of cases. Ingestion of contrast medium
improves the detection rate and is recommended
to enhance the appendiceal
and cecal walls [4]. Some authors
reported a normal appendix on CT
scans in 94% of patients with negative
findings using rectal contrast material
and additional decubitus series.
The normal appendix is usually not
conspicuous and needs to be looked
for diligently. It sometimes may be
seen throughout its entirety as a tubular,
linear, or curvilinear structure in
a single CT section. More often, however,
because of its convoluted course,
the appendix appears in multiple contiguous
sections and needs to be followed
to its origin from the posteromedial
wall of the cecum 2.5–3.0 cm
below the ileocecal valve for confirmation
[4]. The position of the tip of the
appendix varies and may be retrocecal
(65%), pelvic (31%), paracolic (in the
sulcus of the other side of the cecum),
pre-ileal, post-ileal, promontory (pointing
toward the sacral promontory), or
subcecal. The appendix can even be located
in the left lower quadrant if there
is a visceral transposition. Congenital
absence and duplication of the appendix
have been reported, but are rare
[2]. The 3 most common positions are
descending, pelvic, and retroperitoneal
(in a fixed retroperitoneal position in
60% and mobile in 40% of cases) [4].
Average length of the appendix is 9
cm (range, 4–25 cm). The diameter of
the appendix outer wall does not exceed
6 mm and the lumen may contain
fluid, fecal material, air, or contrast medium.
It is very important to determine
the maximum thickness of the normal
appendix with CT in order to diagnose
acute appendicitis and to rule out other
etiologies of acute abdominal pain
[3]. An appendicolith may be present
within the lumen of the appendix in asymptomatic patients. The wall of the
appendix is well depicted by the surrounding
fat and is thin, measuring
less than 1 mm in thickness [2]. The
normal appendix may be collapsed.
Ileocolic vessels may be mistaken for
the appendix if their branching is not
readily visualized. Gerota's fascia may
have a similar appearance, but is readily
differentiated by its continuity with
the lateral conal fascia [4].
Surgeons and clinicians often ask
whether the normal appendix was
identified, as this increases their confidence
in the negative predictive value
of a negative CT report [10]. Non-enhanced
limited CT is not operator dependent
and does not require the same
level of extensive experience that is
needed for accurate US results [11].
CT appearances of appendicitis
The appearance of appendicitis on
CT depends on the extent and severity
of inflammation, and the presence or
absence of complications [4]. Inflammation
of the appendix results from
obstruction of its lumen from fecaliths,
foreign bodies, lymphoid hyperplasia,
parasites, or tumors (primary or metastatic)
[2].
A prompt and accurate diagnosis
of acute appendicitis significantly decreases
morbidity and mortality. Although
in most cases clinical symptoms
and signs may strongly suggest
a diagnosis of acute appendicitis, the
clinical presentation is atypical in 20%
of cases, while in another 20% the condition
is misdiagnosed. The clinical
features in children are often atypical,
with generalized rather than localized
abdominal pain, whereas in the elderly
there is a wider range of differential diagnosis than in the younger population
because of the frequency of age-related
diseases such as diverticulitis. The
diagnosis may also be delayed in the
elderly as they complain less of pain
than younger patients do and clinical
signs are less pronounced. There is also
an increased risk of misdiagnosis in
young females because gynecological
diseases can mimic acute appendicitis
[2]. Women suspected of having appendicitis
benefit mostly from preoperative
CT or US, and they have significantly
lower negative appendectomy
rate than women who do not undergo
preoperative imaging [12]. For some
female patients clinicians order pelvic
US to be performed within 24 h of a
CT study.
The diagnosis of acute appendicitis
is usually based on clinical symptoms
and laboratory tests; however, one
third of patients with acute appendicitis
show atypical clinical symptoms
and physical findings. In this group of
patients radiological imaging can play
an important clinical role.
The inflamed appendix shows a
variable degree of distension, has a
diameter measuring 6–40 mm, and
wall thickness of 1–3 mm. The wall is
usually asymmetrically thickened and
enhances with intravenous contrast
medium [13]. As the disease progresses
a periappendiceal inflammatory mass
called phlegmon may develop (Fig. 1).
Thickening and enhancement with intravenous
contrast medium may also
be observed in the adjacent wall of the
cecum or ileum if they are involved in
the inflammatory process (Fig. 2). Progression
of the inflammatory process
may lead to the findings ranging from
a sealed abscess to widespread intra-abdominal inflammatory seeding with
multiple abscesses. An abscess with a
well-defined border usually indicates
chronicity and the presence of air bubbles
or air fluid levels inside indicates
the presence of gas-forming organisms
or communication of the abscess with
the bowel (Fig. 3). If the periappendiceal
fat is involved in the inflammatory
process then it shows an increased haziness,
streaky densities, and/or fluid collection
(Fig. 4). In 30% of appendicitis
cases the arrowhead sign is present and
it has 100% specificity. It describes focal
thickening of the cecal wall around
the root of the appendix, which funnels
toward the point of obstruction of
the appendiceal lumen [14].
Click to Enlarge |
Figure 1: On axial CT image of an acute appendicitis case, periappendiceal soft tissue mass,
with a variable degree of enhancement representing a phlegmon is evident. |
Click to Enlarge |
Figure 2: Axial CT image shows the inflammatory changes thickening the cecal wall and
involving the peri-cecal fat due to appendicitis. |
Click to Enlarge |
Figure 3: On axial CT image, an area of ill-defined and variable enhancement with pockets of
extraluminal gas is present due to an appendiceal abscess. Note that there is thickening of the
peri-cecal fascia. |
Click to Enlarge |
Figure 4: Axial CT image shows dilated appendix lumen filled with fluid, with thickened
and enhanced walls, following the administration of intravenous contrast medium. Note the
presence of an adjacent fluid collection. |
There are 5 specific CT findings
for perforated appendicitis: abscess,
phlegmon, extraluminal air, extraluminal
appendicolith, and focal defect
in the enhanced wall of the appendix.
Among these findings with 100% specificity,
a focal defect in the enhanced
appendiceal wall has the highest sensitivity.
Many surgeons now avoid a
surgical approach once perforation
has occurred because of perioperative
complications. Instead, they choose
conservative medical treatment or percutaneous
CT drainage (Fig. 5), with
or without interval appendectomy
[15–18].
Click to Enlarge |
Figure 5: Appendiceal abscess in the process of being drained under CT guidance. |
Perforation without abscess formation,
peri-appendicitis (serosal inflammation
of the appendix due to disease
outside the appendix), or inflammation
limited to the mucosa or submucosa
of the appendix makes the diagnosis
of appendicitis on CT difficult. Microscopically
some of the small blood vessels
in the submucosa show fibrinoid
necrosis with neutrophilic infiltrations
in acute appendicitis in Henoch-Shonlein
purpura cases [19].
The advantage of using intravenous
contrast material is that it allows assessment
of appendiceal wall enhancement,
differentiation of pelvic
blood vessels from a retrocecal appendix,
and identification of other pathologic
conditions [8]. The ability of CT
to depict periappendiceal pathologies,
which may alter the management
plan, is of primary importance to clinicians
[20].
Although most cases of acute appendicitis
can be diagnosed correctly with
a meticulously obtained history and
physical examination, 22%–33% of patients
do not present with typical signs, symptoms, and accurate diagnosis
[20]. CT has been shown to be accurate
in the diagnosis of acute appendicitis.
Negative appendectomy rates were as
high as 30% in the pre-CT era. Its use
has decreased the negative laparotomy
rate to 4%–7% [11,21]. With the progressive
increase in the use of CT has
also come a decrease in negative appendectomy
rate to 2% [22].
CT classification of appendicitis
Based on CT findings, acute appendicitis
may be classified into 4 categories
of increasing severity:
Category 1: Simple appendicitis in
which findings are limited to the appendix,
and the lumen of the appendix
may be distended with a thick and enhancing
wall (Figs. 6, 7).
Click to Enlarge |
Figure 6: a, b. Axial CT images of two different cases showing a thickened appendix wall with homogenous enhancement after intravenous
contrast medium administration. |
Click to Enlarge |
Figure 7: On axial CT image, the dilated lumen of the appendix
is filled with fluid and shows thickened and enhanced walls after
intravenous contrast medium administration. |
Category 2: Appendicitis with periappendiceal
inflammatory changes. In this case, the fat surrounding the
appendix and/or cecum appears enhanced
(Figs. 4, 8, 9).
Click to Enlarge |
Figure 8: Axial CT image without oral or intravenous contrast medium
administration in acute appendicitis. Note the dirty fat sign. |
Click to Enlarge |
Figure 9: Unenhanced CT image demonstrating an appendicolith and an inflammatory
process surrounding the appendiceal lumen. |
Category 3: Appendicitis with appendiceal
phlegmon or abscess (Figs. 1,
3, 5, 10).
Click to Enlarge |
Figure 10: Axial CT image shows air-fluid level due to an abscess in an appendicitis case. |
Category 4: Appendicitis with distal
inflammatory changes. This is the
most severe type, indicating perforation
of the appendix with dissemination
of the inflammatory process.
Chronic appendicitis
Chronic appendicitis is rare and
may be seen in cystic fibrosis where
mucoid material occupies the lumen
of the appendix. It is also seen with
recurrent episodes of acute appendicitis
or when the appendix has been
incompletely removed [8]. Continuous
symptom duration of more than 3
weeks (chronic appendicitis) or previous
episodes of similar symptoms (recurrent
appendicitis) occur in 6% and
13% of patients with appendicitis referred
for CT, respectively. Recurrent
and chronic appendicitis are indistinguishable
from acute appendicitis on CT. Stump appendicitis is a rare variation
of recurrent appendiceal disease,
which occurs after appendectomy
with simple ligation and without appendiceal
stump investigation. The
residual appendiceal stump acts as a
small appendix or diverticulum that
can become obstructed and inflamed.
CT shows peri-cecal fat stranding and
focal thickening of the cecal apex
[23].
Distal appendicitis
Most cases of appendicitis begin with
luminal obstruction of the appendiceal
orifice; however, as many as 8% of patients
with appendicitis that undergo
appendiceal CT have at least 3 cm of
a normal proximal appendix with the
appendicitis confined to the distal
appendix. CT features to distinguish
distal from full-length appendicitis include
an absence of change in the cecal apex and a transition point between a
proximal normal and a distal inflamed
appendix [23].
Appendicitis in children
Acute appendicitis may be missed at
initial clinical examination in 28%–
57% of children aged 12 years and
younger, and in nearly 100% of children
under the age of 2 years. Diagnostic
imaging has an ever-increasing role
in the prompt and accurate diagnosis
of acute appendicitis in the pediatric
population [24]. Current reports in the
pediatric medical emergency and surgery
literature advocate imaging, particularly
CT, as the gold standard for
diagnosing appendicitis [25]. Researchers
found that 60.5% of children had
equivocal clinical findings and 14.7%
had negative appendectomies, and
when an imaging protocol was used
4.1% of cases had negative appendectomies.
After implementation of an
imaging protocol using US and CT, the
perforation and negative appendectomy
rates decrease [26]. In a recent article
[27], it was suggested that patients
with appendicolith should have an interval
appendectomy.
Differential diagnosis
A number of pathological conditions
may mimic appendicitis on CT imaging.
These include right-sided diverticulitis,
complicated cecal carcinoma
(Fig. 11), Crohn's colitis (Fig. 12), mesenteric
inflammation, complicated
ovarian cysts, endometriosis, ectopic
pregnancy, local lymphadenopathy,
and fibro-fatty proliferation [28]. Most
of them may be difficult to differentiate
from acute appendicitis. Alternative
diagnoses identified on CT scans
with a normal appendix may also include
right tubo-ovarian abscess, epiploic
appendagitis (Fig. 13), biliary
colic, or urinary tract infection [9].
Perforated duodenal ulcer, superior
mesenteric venous thrombosis, small
bowel ischemia, and abdominal wall
hernia are some abdominal pathologic
entities that require surgery and which
present with right lower abdominal
pain [29].
Click to Enlarge |
Figure 11: Axial CT image shows asymmetrical mural thickening of the cecum in a case of
cecal cancer, resembling an inflammatory mass. |
Click to Enlarge |
Figure 12: Axial CT image shows distended ileum loops in a case of Crohn’s disease and pericecal
inflammatory changes. |
Click to Enlarge |
Figure 13: Axial CT image shows increased attenuation of mesocolic fat in a case of epiploic
appendagitis. The location of acute epiploic appendagitis is most commonly adjacent to
the sigmoid colon, whereas acute omental infarction is typically located in the right lower
quadrant and often is mistaken for acute appendicitis. |
Carcinoid tumors of the appendix,
in most cases, are found incidentally
during appendectomies, especially
in young females, and are usually
smaller than 1 cm, which is the reason
for that there are no metastases.
These tumors are thought to be the most common type of appendiceal
neoplasms [30]. Some authors suggest
that a standardized CT protocol
for patients who present with right
lower quadrant pain that includes imaging
of the abdomen and pelvis with
routine oral and intravenous contrast
material provides an accurate diagnosis,
even without the preselection of
patients who are more likely to have
appendicitis. If CT can be postponed
until bowel preparation is feasible, it
can then be performed with a higher
diagnostic yield [31].
Limitations of CT
CT has been shown to be an excellent
diagnostic technique for patients
with acute abdominal pain and may
have a major effect on the treatment
of these patients. Exposure to radiation
limits the use of CT imaging during
pregnancy and for young women and
children [32]. Preliminary findings indicate
that it is feasible to optimize the
CT dose used to evaluate appendicitis
in children by using phantom-based
computer simulations [33].
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