Discussion
The frequent use of CTA scanning has raised concerns related to the
added cost, as well as cumulative radiation exposure and the use of
nephrotoxic agents (6-10,27,28). The Society for Vascular Surgery
practice guidelines scaled back and currently recommend
contrast-enhanced CT scanning at 1 and 12 months during the first year
after EVAR and adds if neither endoleak nor aneurysm expansion is
detected subsequent duplex follow up may be a reasonable alternative
(4). All the guidelines suggest that type I and III endoleaks should be
treated immediately with a strong recommendation (4,5). For type II
endoleaks aneurysm expansion should be surveilled. Expansion of sac
diameter ≥ 1 cm detected during the follow up after EVAR using the same
modality and measurement method may be considered as a reasonable
threshold for significant growth (4,5).
Surveillance after EVAR is universally accepted eventhough there are
currently no ideal frequency or standart regimen. Compliance of patients
to surveillance after EVAR is around 60% in the literature (29). As a
vascular surgeon we must improve patient compliance for EVAR
surveillance without any adverse consequences in the long term. Besides
distant homeland of the patient, the awareness of the risks of CT
surveillance is a critical issue on this topic. A noninvasive, easy to
apply and repeatable tool will be pleased by the patients also.
CDUS does not carry associated radiation exposure and nephrotoxicity
risk as well as the obvious advantage of being readily available and
non-invasive manner. These specifications make it a more desirable
imaging modality for long term surveillance. However, it is definitely
operator dependent and the quality of the images can be adversely
affected by obesity or excess bowel gas. Concerns have been raised in
the past regarding its variable sensitivity in detecting endoleaks
almost always because of type II endoleaks (10, 13, 16, 30, 31). Based
on recent reports, some investigators have proposed that follow up with
duplex ultrasound as the sole imaging modality is proper (9, 10, 16).
CDU sensitivity for type I and III is excellent. CDUS has the advantage
of identifying the flow direction of endoleaks over CT.
Sensitivity refers to the ability of a test to detect a positive
finding; a highly sensitive test will detect most real problems but may
return some false positives. In contrast, a highly specific test will be
very trustworthy when it is positive, though it will not necessarily
detect all true positives. Accuracy refers to the overall rate of true
test results. In literature, there are many diverse reports about
sensitivity and specificity concerning the endoleak detection of CDUS.
These conflicts may be due to heterogeneity of study designs, lack of
standardized protocols, equipment, techniques, or training. Notably some
rigorous CDUS reporting 90-minute search for endoleaks or some CT
protocols did not include delayed phase images (19, 25, 32). In our
study, the CDUS examination took approximately 10 to 20 minutes
depending on the patient’s status, bowel gas or obesity. Raman et al,
one of the largest studies, reports 43% sensitivity may have been a
consequence of older equipment and shorter scan times (23). Elkouri
declaring CDUS sensitivity of only 25% may be because of the lack of a
standardized technique and protocol (24). Nagre et al reported a very
low overall sensitivity of 35% however in each case requiring
reintervention, CDUS detected the endoleak on subsequent visit or
detected a significant increase in aneurysm sac (33). In our study, the
sensitivity and specificity of CDUS was 63,6% and 100% respectively.
The accuracy was 95,2%. Positive and negative predictive values were
100% and 94,8%. According to our findings, CDUS is highly sensitive
and capable of diagnosing Type I and III endoleaks which require
reintervention. There was no endoleak which were detected on CT and
missed on CDUS requiring reintervention. Lack of detecting type II
endoleaks may be negligible as sac enlargement was the key for
reintervention in this situation and CDUS has a remarkably high
correlation with CTA in sac diameter measurement.
Our results showed strong correlation among CDUS and CTA in measuring
the sac diameter, as it is in the literature (23,24,34,35). The
correlation of CTA and CDUS for diameter measurement and endoleaks were
remarkably strong with statistical significance (r = 0,884 and r = 0,777
respectively, p < 0,001). These results are comparable with
Gray (r=0,96) (10) and Arko (r=0,93) (15) to the degree of correlation.
There is also a consistent observation that CDUS underestimates size
compared to CTA (23, 24, 34). This diameter follow-up issue is very
important for Type II endoleak surveillance and endotension. Follow up
of sac diameter with the same modality will be of foremost importance
for successful CDUS surveillance.
In literature there is inconclusive CDUS results of 6-25% possibly due
to techniques, experience or how you define the technical adequacy and
factors concerning the patient (10, 19, 21, 24). In our experience, we
do not have an inconclusive CDUS result. We always inform patients about
fasting for 6-8 hours and all examinations were performed in the morning
session. To our knowledge, only examinations soon after implantation,
may be inadequate because of inflammation, or air captured inside the
deployment devices. Our ultrasound scan almost always took 10-20 minutes
of search of the aneurysm sac.
There are studies concerning the contrast enhanced doppler ultrasound
(CEUS) concluding superior sensitivity and accuracy over CDUS and also
CTA (12,31,36-39). Contrast enhancement will increase the sensitivity
also with the cost, iv administration, risk of allergy and longer time
duration. We do not used and/or needed any contrast in our study,
undoubtedly cost of surveillance will increase by CEUS and studies
confirm the only cost-effective modality is CDUS (12) .
The frequency, length of follow-up, imaging protocols are very
heterogenous, almost every clinic has its own protocol. Our clinical
approach for surveillance after EVAR begins with baseline CTA for all
patients as a reference point. It should be performed with in the first
three months after the procedure, according to the patient’s renal
status, aneurysm anatomy and risk of graft related complications.
Component overlap, sealing zones, positioning and endoleaks are
evaluated at this reference CTA, and the images are kept for every
patient in order to compare and comment for possible future
complications. After that, our follow up modality is CDUS for every 6
months in the absence of an endoleak or aneurysm sac enlargement. If
there is a type II endoleak in the CTA, we again follow the patient with
CDUS with concern to the sac enlargement every 6 months. We use CDUS as
a primary surveillance tool for EVAR, we reserve CTA scan, in case of
aneurysm sac enlargement, detection of an endoleak, inadequate CDUS or
in case of unexplained abdominal symptomatology. By this way we not only
avoid ionizing radiation or nephrotoxic agents, but achieve cost saving
issue also.
Limitation of CDUS is the detection of structural deformities. However,
if there is no endoleak or aneurysm sac enlargement we follow those
patients so it is not important issue. In complicated cases Type Ib and
Type III endoleaks may resemble each other however as this situation
should be confirmed with CTA, there will be no missed patients requiring
reintervention. The limitations of CDUS must be weighted with its
benefits. Limitation of our study is its single center, cross-sectional
structure. Patients are from various stages of follow-up and different
commercial endografts.
What must be the goal of the post-EVAR surveillance test? Not to miss a
diagnosis or not to miss an essential reintervention. For a vascular
surgeon, proper physical examination, symptomatology of the patient is
also of significant importance. Most reinterventions occur in
symptomatic patients and only 1.4 – 9 % of patients require
reintervention as a result of detected abnormalities (27,28,40-42).
Safety concerns, availability and inexpensive non-invasive manner make
it a more desirable imaging modality for long term surveillance.
CDUS is a convenient modality for monitoring the evolution of the
aneurysm sac and sufficient to identify complications requiring
reintervention. Also, CDUS better provides the characterization of the
endoleak by flow direction. Therefore, we perform CDUS for post-EVAR
patients as the primary diagnostic test, reserving CT for consulting the
reintervention strategy. It may easily be repeated without any safety
concerns. Modern equipment and highly experienced Doppler operators
remain crucial requirements for CDUS surveillance. Randomized clinical
trials should target if CDUS surveillance is satisfactory for all
patient group.