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.