Chapter 2
The clinical problem. Morfology, etiology, imaging
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Chapter 2
The clinical problem.
Morfology, ethiology, imaging
2.1 Morfology of abdominal aortic aneurysms
An aneurysm is an abnormal localized sac or an irreversible dilation caused by
weakness (decreased elastin) of the arterial wall. Aneurysms are classified as either
fusiform, round shape with a diameter that can reach even 20 cm (fig. 2.1), or
saccular, tube-like shape (fig. 2.2); the former is a much more common type than the
latter.
In saccular aneurysms, one side of the artery wall has a large strain they vary between
5 to 20 cm in diameter and are the result of processes which affect only part of the
circumference of the vessel wall. Their lumen is connected with the lumen of the
affected vessel through an opening of varying size. As a result, flow through the
aneurysm is turbulent and thrombosis is common. A subtype of saccular aneurysm is
the Berry aneurysm, these are localized dilatation of an intracranial artery.
In fusiform aneurysms, the entire circumference of the artery is strained; they are
shaped like a spindle ("fusus" means spindle in Latin) with widening all around the
circumference of the aorta.
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Fig 2.1 Fusiform aneurysm.
The drawing shows the typical spindle-like
shape, naturally every different fusiform
aneurysm is characterized by having his
own size and shape, however the blood
flow is always coaxial with the main part
of the aneurysm.
Fig. 2.2 Saccular aneurysm.
It is possible to note that saccular aneurysm
is a spherical dilatation which project from
one aspect of the affected vessel.
This work has been focused only on fusiform aneurysms and from now on I will refer
to fusiform abdominal aneurysms simply as abdominal aneurysms.
It is important to realize that an aneurysm is not just an extremely wide aortic
diameter but also reflects changes in the aorta wall, not present in normal aortas, that
make it susceptible to rupture.
The abdominal aorta is the most common site for an aneurysm to develop, therefore it
is important to localize exactly the piece at issue. The abdominal aorta begins at the
aortic hiatus of the diaphragm, in front of the lower border of the body of the last
thoracic vertebra, and, descending in front of the vertebral column, ends on the body
of the fourth lumbar vertebra, commonly a little to the left of the middle line, by
dividing into the two common iliac arteries. It diminishes rapidly in size, in
consequence of the many large branches which it gives off. As it lies upon the bodies
of the vertebræ, the curve which it describes, is convex forward, the summit of the
convexity corresponding to the third lumbar vertebra (fig. 2.3).
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Fig. 2.3
The diameter of the human abdominal aorta in non-aneurysmatic subjects varies
according to gender. In studies of subjects older than 50 years the mean ulrasound
diameter of the infrarenal abdominal aorta ranges from 12 to 19 mm in women and
from 14 to 21 mm in men [J.L. Cronenwett et al., 1985]. AAA is most common in
Caucasians, with men affected four times more than women, they are most prevalent
between the ages of sixty and seventy and first-degree relatives of persons with AAA
develop dilation at earlier ages (fig. 2.4).
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Fig. 2.4 Age and sex distribution for open reconstructed
aneurysms (n = 462) [H. J. C. M. Pleumeekers, 1995]
Some studies take this variance of the normal aorta diameter into account and several
cut-off points are used to evaluate the presence of an aneurysm. An aneurysm is
considered to be present for instance if:

the diameter of the abdominal aorta is larger than the diameter of the aorta at
the renal bifurcation or

there is an increase of 5 mm of the abdominal aorta compared with the
suprarenal segment of the aorta or

there is a localized dilatation with at least a 50% increase in diameter
compared to the aorta diameter at the bifurcation of the renal artery etc.
If an AAA is untreated its natural course is to expand and rupture and this leads to
hemorrhage in the space surrounding the aneurysm, considering that the vessel at
issue is the aorta (the main artery of the body), it is easy to understand the importance
of this disease.
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2.2 Ethiology
The exact cause of abdominal aortic aneurysms is unknown.
AAAs were first thought to be of arteriosclerotic origin. Martin [P. Martin, 1978] was
the first to study this question, suggesting that arteriosclerosis may not be the cause
rather the consequence of aortic degeneration.
Arteriosclerotic changes in the intimal layers of the abdominal aorta are thought to
affect the diffusion of oxygen and nutrients to the outer layers of the abdominal aorta.
This may lead to changes in the aortic wall structure, making the aorta susceptible to
dilatation [D. Reed et al., 1992].
Sterpetti [A. V. Sterpetti et al., 1988] proposed the existence of two types of
abdominal aneurysms. The first type is associated with arteriosclerotic occlusive
disease and the other is not. In their study of 526 patients undergoing aneurysmal
resection, 25% of aneurysms were believed to be non-arteriosclerotic. There were
significantly more ruptures in this group, compared to the arteriosclerotic group,
moreover there were a generalized weakness of the aorta wall in non-arteriosclerotic
group.
In the Rotterdam study they founded that a history of inguinal hernia surgery at a
relative young age was related to the presence of AAAs indicates that congenital
weakening of the collagen structure may predispose to aneurysm formation. In this
study the majority (approximately 90%) of abdominal aortic aneurysms, however, can
be attributed to risk factors associated to arteriosclerosis [H.J.C.M. Pleumeekers,
1995]. The finding that men with a first degree relative with an AAA experience a 10
fold increased risk of developing an abdominal aneurysm, provides a strong argument
for a genetic component [H.J.C.M. Pleumeekers, 1995].
Another interesting theory is the trace metal theory.
It is based on the observation that in blotchy mouse aneurysm formation is related to
an X-linked chromosome defect leading to an abnormality of the cupper metabolism
[D.H. Rowe et al., 1974]. Copper is thought to play a role in the cross linkages of
collagen and elastin, which forms the extracellular matrix of the aortic wall. A
deficiency of the copper metalloenzyme lysyl oxidase could result in a deficiency of
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collagen and elastin and weaken the aortic wall, thus making it prone to aneurysm
formation.
However, risks associated with AAA include atherosclerosis, hypertension and
smoking. Other risks include trauma to the arterial wall and infection of the artery
wall. Alterations in the delivery of oxygen and nutrients to the artery wall may also
play a role in the pathophysiology, because the infrarenal aorta lacks medial vasa
vasorum (literally vessels of the vessels, they carry oxygen and nutrients to the cells
of the blood vessel wall, fig.2.5) even though the aortic wall is nearly as thick as the
proximal segments that have medial vasa vasorum.
Fig. 2.5a High power view
(300x) shows the vasa
vasorum, which supply blood
for theFig.
wall4c
of this large vein.
Fig. 2.5b Vasa vasorum
openings show continuity of
endothelial lining (SEM
2000x).
Most AAAs occur below the level of the renal artery and involve the bifurcation of
the aorta as well as the proximal ends of the iliac arteries. Stasis of blood can lead to
thrombus formation along arterial wall. Peripheral emboli can develop causing
arterial insufficiency.
A person with an abdominal aortic aneurysm often becomes aware of a pulsing
sensation in the abdomen. The aneurysm may cause pain, typically a deep,
penetrating pain mainly in the back. The pain can be severe and is usually steady,
although changing position may relieve it. The anatomic structure of abdominal
aneurysms seems to be of importance for the risk of rupture. Longer, fusiform
aneurysms have a poorer prognosis than saccular ones [K. Ouriel et al., 1989]. Aortic
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blebs, consisting of protrusions in the aortic wall and filled with thrombus en debris,
are an indication of impending rupture [G.C. Hunter et al., 1989].
Also the risk of rupture seems to be higher when there is no evidence of peripheral
vascular disease.
The first sign of a rupture is usually excruciating pain in the lower abdomen and back
and tenderness over the aneurysm. With severe internal bleeding, a person may
rapidly go into shock. A ruptured abdominal aneurysm is often fatal.
Once an aneurysm forms it often increase in size and consequently the chance of
rupture also increase. Aneurysm rupture can lead to haemorrhage and death.
Aneurysms in the segment of the aorta that passes through the abdomen (AAA) tend
to run in families. Many times, these aneurysms occur in people with high blood
pressure.
The mechanic role of high blood pressure in the formation of aneurysms may seem
obvious. Studies in this issue, however, have produced conflicting results.
Allen [P. I. M. Allen et al., 1987] found 5.3% prevalence of abdominal aortic
aneurysms among 168 hypertensive men and women, however this report did not
include a control group and were of limited size. In two other studies comprising a
control group, the prevalence of AAA in non-hypertensive was similar to that in
hypertensive patients [J. Collin et al., 1988; R.A.P. Scott, 1991].
O’ Kelly
[J. O’Kelly et al., 1989] demonstrated a significantly higher risk of
abdominal aneurysms in patients with a systolic blood pressure of 180 mmHg or over
compared with subjects with a lower systolic blood pressure (a 4.1% prevalence
versus 1.2%). No increased risk in patients with diastolic hypertension was found.
Such aneurysms often become larger than 76mm and may rupture.
There is a 20% chance of rupture within a year, for an aneurysm that is larger than 6
cm. Ruptured aneurysms occur in approximately 5 out of 10,000 people that reveals
an aneurysm.
The study of Pleumeekers [1995] shows a pronounced increase from and hospital
discharge rates for aneurysms of the abdominal aorta in The Netherlands during the
last two decades (’72 and ’92). This increase remained after adjustment for age and
was more prominent in men than in women.
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Several factors have to be considered when interpreting an upward trend in data
obtained from routine statistics.
1. A first explanation could be an increase in detection rate. The widespread use
of ultrasound in hospitals since the mid seventies will have lead to the
detection of many aneurysms previously unknown (In 1989 the board of
radiologist in The Netherlands recommended that during all sonographic
examinations of the abdomen an attempt should be made to visualize the
abdominal aorta).
2. Changes in coding practice could be a second explanation for the observed
increase. The introduction of a new, promising diagnostic method and the
improvement in surgical techniques will have increased the medical
awareness for AAAs. This might result in improved case finding and more
deaths attributed to aneurysms and can explain part of the observed increase in
mortality from AAAs. However, results from autopsy studies still suggest that
many aneurysms remain undetected during life [M.J. Mc Farlane, 1991].
Apart from the, undisputed, improvement in diagnostic capabilities and perhaps a
change in coding practice, the rise in mortality and morbidity from AAAs could
reflect a true increase in incidence. First of all, the increase was not the same in men
and women. Both for mortality and for the number of hospital admissions men
showed a stronger increase than women. If the increase was solely caused by an
improved detection rate one would expect similar effects in both sexes. Secondly,
there is a marked increase in the number of ruptured aneurysms where
ultrasonographic detection does not play a major role.
2.3 Diagnostic and imaging techniques
Given the high rate of morbidity and mortality associated with abdominal aortic
aneurysms, accurate diagnosis and preoperative evaluation are essential for improved
patient outcomes. In general physical examination is considered to be inadequate to
detect asymptomatic aneurysm because only very large aneurysm can be palpated, it
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is estimated that only 30% to 50% of all abdominal aneurysms can be detected
through abdominal palpation [F.A. Lederle et al., 1988], few is know about the value
of auscultation. A plain X-ray of the abdomen may give an adequate estimate of the
diameter of the aorta but calcification of the aorta wall is needed to make
measurement of the aortic diameter possible and these calcification are present in
about 75% of the patients with an AAA.
The aim of imaging is to accurately delineate the proximal and the distal landing sites
in the infarenal aorta and common iliac arteries. Visualization of the attachment sites
is vitally important to guarantee the success of the exclusion procedure and to prevent
endoleaks. Negative or adverse anatomical or morphological features must be
detected in order to size or select the stent correctly.
Imaging obtained before, during and after stent placement by pressure injector is of
cardinal importance to ensure procedural success of endovascular repair of AAA,
especially to detect endoleak formation. It is an integral part of the technique of
endoluminal repair of AAA and high-quality imaging is essential.
Ultrasonography is the standard method of screening and monitoring AAAs that have
not ruptured. In the past, aortography was commonly used for preoperative planning
in the repair of AAAs. More recently, computer tomography (CT) has largely
replaced older, more invasive methods. Recent advances in CT imaging technology,
such as helical CT and CT angiography, offer significant advantages over traditional
CT. These methods allow for more rapid scans and can produce three-dimensional
images of the AAA and important adjacent vascular structures. Use of endovascular
stent grafts has increased recently and is less invasive for the repair of AAAs in
selected cases. Aortography and CT angiography can precisely determine the size and
surrounding anatomy of the AAA to identify appropriate candidates for the use of
endovascular stent grafts. Helical CT and CT angiography represent an exciting
future in the preoperative evaluation of AAAs. However, this technology is not the
standard of care because of the lack of widespread availability, the cost associated
with obtaining new equipment, and the lack of universal protocols necessary for
acquisition and reconstruction of these images [Am. Fam. Physician, 2002].
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The choice of imaging modality depends on the clinical presentation of the patient
(Table 1). Screening and serial monitoring are performed most efficiently with
ultrasonography. Aortography is invasive and it underestimates the aneurysm
diameter when a thrombus is present.
Ultrasonography and computer tomography (CT) of the abdomen have an accuracy
varying from 97% to 100%. Of these two, ultrasonography is relatively less expensive
and easier to perform, which makes it the method of choice for screening
symptomatic patients with stable vital signs are usually evaluated with CT in an
emergency situation, whereas those who are hemodynamically unstable typically go
directly to surgery. Two drawbacks are the problem of contrast-induced: renal
insufficiency in patients with renal impairment, and parallax errors which may
contribute to sizing and interpolation errors.
In table 1 a general overview of the imaging techniques is shown.
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Imaging modality
Ultrasonography
Aortography
Advantages
Disadvantages
Lower cost
Sub optimal in obese patients
Widely available
Sub optimal in patients with increased
bowel gas
Non-invasive
Increased interobserver variation
Visualize reno-vascular
disease
Invasive
Identifies anomalous
vessels
Higher cost
Aids placement of
endovascular stent grafts
Increased patient morbidity
Underestimates aneurysm size
Exposure to iodinated contrast
MRI
Non-invasive
Higher cost
Lack of ionizing radiation
Motion artefact
Contraindications with metal clips and
pacemakers
Patient claustrophobia
Availability of scanner and software
CT
Non-invasive
Use of ionizing radiation
Highly predictive of
aneurysm size
Higher cost compared with
ultrasonography
Localize proximal extent
of aneurysm
Limited information regarding arterial
anatomy
Identify other abdominal
pathology
Procedure of choice for
suspected rupture
Helical CT and
CTA
Non-invasive
Higher cost
Faster scanning time
Lack of availability of scanner and
software
Use in conjunction with
endovascular stent grafts
Use of ionizing radiation
Table 1. Comparison of diagnostics methods. MRI = magnetic
resonance imaging; CT = computer tomography; CTA = computer
tomographic angiography.
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Ultrasonography
Ultrasonography is the examination of choice for screening and monitoring the rate of
AAA enlargement. Figures 2.6 and 2.7 are longitudinal and transverse sonograms,
respectively, of an AAA.
Fig. 2.6 Longitudinal view of the
abdominal aorta demonstrating a focal
area of enlargement (arrows)
consistent with an abdominal aortic
Fig 2.7 Transverse sonogram of the
abdominal aorta demonstrating an
AAA (arrows) with a small amount of
mural thrombus (arrowhead).
aneurysm.
Ultrasonography has a sensitivity of nearly 100% in the diagnosis of AAAs [P.
Vowden et al., 1989]. This procedure is largely preferred for screening and
monitoring because of its relatively low cost, availability, and noninvasive nature.
Ultrasonography is accurate to within 0.3 cm in estimating aneurysm diameter on
serial scans and has little interobserver variation [K. Myers and T. Devine, 1997].
However, the procedure does have limitations. Images tend to be suboptimal in
patients who are obese or who have excessive bowel gas. Ultrasonography cannot
reliably identify the presence of periaortic disease, or the proximal and distal extent of
the aneurysm. It also cannot determine the patency of visceral vessels, the relationship
of the aneurysm with respect to renal vessels, or the presence of iliac aneurysms. As a
result, ultrasonography alone cannot fully provide the necessary information required
in the preoperative evaluation of a patient undergoing elective aneurysm repair.
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Aortography
Conventional aortography now has a limited role in the preoperative evaluation of
AAAs. Aortography is often reserved for special situations, such as when patients
have suspected renovascular stenosis, chronic mesenteric ischemia, occlusive iliac
disease, juxtarenal or suprarenal AAAs, horseshoe kidneys (fig. 2.8), previous
colectomy, and thoracoabdominal extension, and is also used in endovascular graft
placement. Advantages of aortography include consistent visualization of the renal
artery origins, renal and visceral artery stenoses, accessory renal arteries, and
extension of an aneurysm into the iliac arteries [M. L. Errington et al., 1997]. It takes
ca. 5 minutes to obtain images like the ones shown in fig. 2.9 and 2.10, but they are
bi-dimensional.
Fig. 2.8 Horseshoe kidneys have an
abnormal axis. Draw a line between the
uppermost calyx and the lowermost
calyx. In normal kidneys this line will be
closer to the vertebral column at the
upper pole. In horseshoe kidneys, the line
will be closer to the vertebral column at
the lower pole. Horseshoe kidneys
usually have multiple renal arteries.
Fig 2.9 Early arterial phase of
a posteroanterior abdominal
aortogram showing a bilobed
aneurysm (arrows) originating
just below the renal arteries.
Fig 2.10 Late arterial phase of
the same posteroanterior
abdominal aortogram, further
defining the bilobed abdominal
aortic aneurysm.
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The use of aortography in the imaging of AAAs also has several drawbacks that
prevent its more routine use. For instance, aortography tends to underestimate the size
of the aneurysm because it only demonstrates the patent lumen, and typically there is
a circumferential organized thrombus within most aneurysms. In some cases, an
aneurysm is missed altogether. Other disadvantages of aortography include its
invasive nature, cost, and risk of exposure to large amounts of iodinated contrast.
MRI
MRI is minimally invasive and, when combined with magnetic resonance
angiography (MRA), can provide excellent details for the preoperative evaluation of
AAAs. MRI with MRA has 100% sensitivity in detecting aneurysms, and successfully
identifies the proximal and distal extent of the aneurysms, the number and origins of
renal arteries, and the presence of inflammation [M. L. Errington et al., 1997]
(fig.2.11). Renal artery stenoses greater than 50% can be detected with a sensitivity of
84 to 100% [J. R. Durham et al., 1993]. The advantages of MRI with MRA include
lack of ionizing radiation and iodinated contrast medium exposure. Disadvantages
include increased cost, patient claustrophobia during longer scanning times, and
difficulties with patient compliance in lying still during the procedure to avoid motion
artefact. The entire procedure time (since the patient is ready on the table) to obtain
MRI images is ca. 20 minutes long, the longest part of the analysis is the data
elaboration with the calculator. A limitation of this techniques is that contrast
injection is needed, and so the trombotic tissue is not visible.
During magnetic resonance (MRI) scan, a narrow table moves the patient through a
tunnel-like structure which creates a magnetic field through which radio waves are
sent, creating a 3-D image of the internal structure (fig. 2.12).
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Fig. 2.11 An MRI image of an AAA, with
this kind of image it is possible to
accomplish some geometrical considerations.
Fig. 2.12 MRI set-up.
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The clinical problem. Morfology, etiology, imaging
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CT
CT has a broad array of uses in the imaging of AAAs. It is used as a screening test
when ultrasound images are sub optimal; as a diagnostic test when a
haemodynamically stable, ruptured AAA is suspected; and in the preoperative workup for the repair of AAAs. CT is a superior diagnostic modality compared with
ultrasonography because it offers the clinician valuable information about not only
the aneurysm but also the surrounding anatomy (fig. 2.13). Unlike ultrasonography,
there is no concern regarding reproducibility of results, interobserver technical error,
or limitations caused by body size or the presence of bowel gas. CT technology is
widely available, non-invasive, and can be performed quickly when needed to rule
out a suspected AAA rupture (it takes no more than 20 minutes to obtain 2-D images,
a bit more for 3-D images). While CT has numerous advantages, there are limitations
to traditional CT; CT images contain limited information on arterial anatomy.
Complex cases involving AAAs may require additional imaging modalities to obtain
the necessary road map of nearby vessels for planning surgical repair.
Fig 2.13 Computed
tomographic scan of the
abdominal aorta at the level of
the kidneys (outline arrows)
showing an aneurysm (arrows)
with a small amount of mural
thrombus along the posterior
and lateral walls. The entire
abdomen is also clearly
visualized, providing useful
information about the
surrounding anatomy.
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Helical CT
The development of helical CT and CT angiography will likely lead to wide use of
these modalities in the preoperative evaluation of AAAs. Also, the use of
endovascular stent grafts in the repair of AAAs requires a technologically advanced
modality, such as helical CT and CT angiography, to provide the necessary detailed
preoperative anatomic information. Helical CT is accomplished by rotation of the
scanning device coupled with continuous table feed of the patient [G. D. Rubin et al.,
1993]. When compared with conventional CT, helical CT has faster scanning time (30
to 60 seconds) and the ability to obtain all images in one breath hold, thereby
eliminating the risk of respiratory artifacts. Dual-slice helical CT is a newer form of
helical CT technology that allows the volume of data scanned within a given time to
double, thus achieving even faster scanning times [S. D. Quanadli et al., 2000].
CT angiography is accomplished by combining all of the axial slices to produce a
three-dimensional reconstructed image of the AAA. This image can be rotated into
any plane that best demonstrates the relevant anatomy (Fig. 2.14). Dual-slice helical
CT correlates well with surgical findings in measuring the proximal and distal extent
of the aneurysm. CT and CT angiography are not only less invasive than conventional
aortography but also allow for more rapid scanning times and evaluation of the rest of
the abdomen.
A complete evaluation of the abdomen is important in identifying relevant associated
abnormalities, such as a horseshoe kidney, venous or arterial anomalies that would
alter the surgical approach, or inflammatory/fibrotic changes within the aneurysm.
Other details, including assessment for patency of the inferior mesenteric artery and
possible involvement of the iliac arteries, are also reliably achieved with CT and CT
angiography. Notably, even patients being considered for endovascular repair with a
stent graft sometimes have preoperative evaluation with CT and CT angiography
instead of aortography.
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Fig. 2.14 Reconstructed computed tomographic angiogram in the frontal
projection demonstrating the bilobed infrarenal aortic aneurysm (arrow
heads). The renal arteries are well visualized (small arrows), revealing a
severe stenosis of the proximal right renal artery (outline arrow) and the
proximity of the aneurysm neck to the renal arteries. The relationship of the
aneurysm to the iliac arteries and the aneurysmal dilatation of the right
common iliac artery (outline arrow head) are also well displayed.
2.4 Pulsatile Wall Motion (PWM): a way to identify
aneurysms
Although rupture of the aneurysm can be accelerated by the metabolic disorder in the
aneurysmal wall, aneurysm enlargement and rupture are not to be expected without
pressurization of the aneurysm sac.
Reduction of the aneurysmal diameter is probably the best evidence of successful
exclusion. Unfortunately, this diameter decrease takes 6 to 12 months to become
manifest.
Therefore it is important to develop a monitoring technique that ca demonstrate as
simply as possible both successful and unsuccessful exclusion of the aneurysm from
the circulation as soon as possible after the operation.
Imaging techniques are no sensitive enough to detect endoleaks.
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The PWM (pulsatile wall motion) is the movement of the aneurysmal wall during the
heart cycle; the reduction of PWM is smaller in presence of an endoleak or with
continued expansion of the aneurysm.
This means that the PWM may be predictive of the fate of the endovasculary treated
aneurysm. However, no relation appears to exist between the level of PWM and the
level of the mean pressure within the sac. A systemic level of pressure without
pulsatility in the aneurysmal sac will not generate PWM.
The presence of PWM is correlated with aneurysmal pulse pressure but not with the
mean level of pressure inside the aneurysm.
Therefore, PWM is related to a patent endoleak, but combination of a patent endoleak
with the absence of PWM and thrombosed endoleak (as visualized by means of
ultrasound scanning and Doppler scanning) with the presence of PWM also exist [G.
W. H. Schurink et al., 2000].
26