Lifelong Ultrasound or CT Surveillance Deemed Necessary After EVAR

BY JEFF EVANS

LONDON -- Patients who undergo endovascular repair of abdominal aortic aneurysms should be kept under surveillance, but there is no consensus yet on how many tests or which modalities are best to use, according to speakers at the Charing Cross 28th International Symposium.

A schedule of surveillance visits at 1, 3, 6, 12, and 18 months after endovascular repair, followed thereafter by yearly visits, "is one of the most common schedules found around the world," said Dr. Vicente Riambau of the University of Barcelona.

He suggested that surveillance should include, in the following order, clinical status, biologic markers such as renal function and hemoglobin levels, physical examination, measurement of the ankle-brachial index, a contrast-enhanced CT scan, four projections of a plain x-ray, an angiogram, and possibly MR angiography or duplex ultrasound scans.

These exams would look for risk factors for a late rupture, such as a graft-related endoleak, abdominal aortic aneurysm (AAA) expansion, or stent migration, which can cause proximal or distal failure of the anastomotic seal, dislocation of one of the stent components, or mechanical stress resulting in stent fracture, said Dr. Geoffrey L. Gilling-Smith, a consultant vascular surgeon at Royal Liverpool (England) University Hospital.

Dr. Riambau noted that a CT scan without contrast should be obtained to identify false endoleaks, whereas a contrast-enhanced CT scan can locate true endoleaks and measure aneurysm diameter and volume.

The integrity of a stent can be examined with multidetector CT scans or plain x-rays in four projections.

Surveillance every 3 months may be necessary when the size of the aneurysm has increased, when there is a type 2 endoleak without an increase in aneurysm size, or if the stent migrates proximally or distally more than 5 mm, Dr. Riambau said.

However, when the aneurysm shrinks completely in the presence of a fully intact stent, surveillance could be curtailed to once per year with plain x-rays and duplex ultrasound, CT scanning, or MR angiography.

Outside of the initial CT scan, duplex ultrasound may be the best modality to conduct routine surveillance of repaired AAAs, Dr. Gilling-Smith suggested, based on the results of a review of 99 patients who received a stent graft for an AAA at the University of Liverpool.

Each patient received CT and duplex ultrasound evaluations during at least 1 year of follow-up. The two modalities showed agreement on whether an AAA was expanding or stable/ shrinking in 78 patients.

Duplex ultrasound, but not CT scanning, showed evidence of expansion in 18 patients. In three patients, CT scanning showed AAA expansion 6 months before it was seen with duplex ultrasound. But no problems were identified in any of those three patients during the 6 months, Dr. Gilling-Smith said.

'CT SCANNING IS ONLY NECESSARY IF [ULTRASOUND] IS EQUIVOCAL, TECHNICALLY UNSATISFACTORY, OR REVEALS A PROBLEM.'

The cumulative dose of radiation absorbed during a CT surveillance scan is about 6-9 millisieverts, which is equivalent to 300-450 chest x-rays. That is "not insignificant," he said.

"CT scanning is only necessary if duplex ultrasound is equivocal, technically unsatisfactory, or reveals a problem, in which case you need further imaging to investigate it."

Although duplex ultrasound can detect graft-related endoleaks and AAA expansion, it cannot identify stent fracture or migration, Dr. Gilling-Smith said. Plain radiography can determine if migration is occurring, but not how far a stent has migrated.

Based on the results of the study, Royal Liverpool University Hospital is now using a surveillance protocol in which patients receive a CT scan 1 month after the operation and routine follow-up exams are performed annually with duplex ultrasound and plain abdominal x-rays.

Duplex ultrasound is operator-dependent and needs to be validated at institutions where it is implemented, Dr. Gilling-Smith stressed.

However, duplex ultrasound and abdominal x-rays have a number of limitations in their utility for surveillance, said Dr. Richard G. McWilliams, who is a consultant interventional radiologist at Royal Liverpool University Hospital.

Unlike CT scanning, duplex ultrasound is limited to certain anatomic sites, by the patient's size, as well as by the amount of air that is in the abdomen, he said.

In addition, CT also can make use of aortic and iliac reference points to detect and track stent migration and is not subject to the projectional, or parallax, errors that can occur with lateral plain radiographs, Dr. McWilliams added.