AAA Rupture Rate in England Supports Current Thresholds for Surveillance, Intervention—With Some Caveats

New data from the United Kingdom’s nationwide abdominal aortic aneurysm (AAA) screening program support the current referral threshold for intervening on this condition and point to a very low rate of rupture among men under surveillance.

The UK National Health Service (NHS) instituted a national AAA program in 2009 wherein all men over age 65 are eligible for screening. Those found to have aneurysms of 5.5 cm or greater are sent for a vascular consult, while those with smaller aneurysms are invited for regular ultrasound follow-up.

The findings from the current survey should be reassuring to physicians concerned that the advent of screening, with strict cut points to regulate who heads to surgery and who doesn’t, have not been associated with an uptick in rupture risk.

“The thing about a national program is that it standardizes the treatment,” Jonothan J. Earnshaw, DM, FRCS (Gloucestershire Hospitals NHS Foundation Trust, Cheltenham, England), told TCTMD. “Before there was an aneurysm screening program, if I saw somebody reasonably fit and healthy with a 5.2 cm aneurysm and I happened to have a slot on my operating list next week, [then] I might have talked to him and offered him the surgery below the threshold because he was young and fit and likely going to get the aneurysm done sometime anyway.”

Indeed, he continued, there’s a lot of evidence to suggest that many people with aneurysms below the accepted threshold undergo surgery or interventions, despite the risks of those procedures. “What the screening and surveillance program does is it doesn't involve the clinicians at any stage until the aneurysm has reached 5.5 cm in diameter, so it stops anybody operating below that threshold because they don't get to see the patient. We think that's important because a group of people who were hitherto having surgery are not having surgery, and we've shown in our study today that it's safe to wait for 5.5 cm.”

The paper was published online ahead of print January 14, 2019, in Circulation.

Screening, Surveillance, and Rupture

Guidelines for the treatment of abdominal aortic aneurysms dating back to 2003 estimate that the annual risk of AAA rupture in aneurysms measuring 5.0 to 5.9 cm ranges from 3% to 15%. More recent practice guidelines document an estimated rupture rate of 1% per year for aneurysms under 5.5 cm being tracked in surveillance programs.

The current study aimed to determine the annual risk among men in England invited to participate in the NHS screening program.

The study, led by Clare Oliver-Williams, PhD (University of Cambridge, England), followed more than 18,000 men between 2009 and 2017 who were invited to participate in the surveillance program and were found to have an initial AAA greater than 3.0 cm but less than 5.5 cm. The program included only the data for England (not the UK more broadly) and did not include women, who have a far lower incidence of AAA.

Over the study period, 1,763 men had a scan showing an AAA measuring 5.5 cm or greater—1,742 of these were referred to a vascular specialist. An additional 83 men with smaller aneurysms were also referred on the basis of symptoms, rapid growth, or other concerning signs. Thirty-one men had ruptured AAA in the surveillance program, and 29 died. This yielded an estimated rupture rate of 1 per 100 person-years for all diameters under the 5.5 cm cutoff, or 0.5% per annum. The rupture rate tended to increase with aneurysm size, ranging from 0.03% per annum in men with small aneurysms (3.0 to 4.4 cm) to 0.4% in men with aneurysms just below the threshold (5.0 to 5.4 cm).

Since the risk of rupture in surveillance is so low, I can conclude that the program that we've got at the moment is safe. Jonothan J. Earnshaw

These rates, say the authors, show that men with small and medium AAAs below the 5.5 cm threshold are not at “unacceptable” risk of AAA rupture.

“Since the risk of rupture in surveillance is so low, I can conclude that the program that we've got at the moment is safe,” Earnshaw told TCTMD. “Whether that is the optimal program, I don't know.”

Optimal Risk/Benefit Thresholds Unknown

The uncertainty, he explained, is that while the current study supports the notion that the threshold should not be lowered, it doesn’t speak to whether it could potentially be raised. The cut point of 5.5 cm is based on decades-old trial data, Earnshaw noted. “If your annual risk of aneurysm rupture is 0.4% at 5.4 cm, I can't believe that when it gets to 5.6 cm there is a sudden increase in risk. [And yet] we counsel people all the time that in aneurysms greater than 5.5 cm over time there is a significant risk of rupture, and we encourage them to consider an elective aneurysm repair.”

According to the paper, elective aneurism repair in the United Kingdom currently carries an in-hospital mortality rate of 1.4%, although Earnshaw quoted numbers ranging as high as 3%—substantially higher than the rupture rate documented among surveilled patients in the current study. “I think you could make a strong case now that we really don't know the risk of rupture between 5.5 cm and 6.5 cm,” Earnshaw said. “I think it is time to start thinking of doing another randomized trial.”

Earnshaw is by no means the first person to question whether the benefits of screening outweigh the risks, particularly since the risk of aneurysm rupture has declined in tandem with population-level risk factor reductions, smoking in particular. In the last 15 years, the annual mortality from aneurysm rupture in England has declined from 5,000 to 3,000 deaths, said Earnshaw.

“So aneurysm screening has come along probably at the wrong time because aneurysms are getting fewer.”

He’s also the first to admit that it will be very difficult to persuade vascular surgeons, many of whom have made a reputation (and a living) from AAA repairs, that new randomized trials are needed to study repair versus watchful waiting in men with aneurysms above the current threshold.

“I've been a vascular surgeon for 26 years and I've sat in countless consultations of men with 5.5 cm aneurysms and counselled them that they should consider surgery,” he admitted. “And persuading somebody like me not to operate on someone, for example, with a 6.4 cm aneurysm—I mean, I'm fairly open-minded, but I would feel moderately uncomfortable about it. So it's going to be very tricky finding more information about these patients.”

Commenting on the study for TCTMD, Minna Johansson, MD, PhD (University of Gothenburg, Sweden), whose own work has questioned the validity of screening programs, reiterated a point made by the authors, namely that there is high-quality evidence from randomized controlled trials that early intervention for small/medium AAAs is not beneficial.

And yet, she said in an email, “a large proportion of men who undergo elective surgery do in fact have a diameter smaller than 55 mm. For example, in one US study, 59% of patients undergoing EVAR had an aortic diameter below 55 mm. This number is lower in countries where there are no personal financial incentives for the surgeons to perform surgery, but even in such countries the proportion of surgeries done on men with small/medium sized AAAs seems to be unacceptably high.

Perhaps it’s not just greed, but also human nature? If you discover what you perceive as a ticking bomb in your stomach, it might be difficult not to do anything about it. Minna Johansson

“Perhaps,” she continued, “it’s not just greed, but also human nature? If you discover what you perceive as a ticking bomb in your stomach, it might be difficult not to do anything about it.”

A second controversy broached in the paper—one that could perhaps be more easily addressed—is the issue of how aneurysms are measured. Currently, as the authors note, both ultrasound and CT are used to size an aneurysm, but while ultrasound measurements exclude the thickness of the aortic wall, CT does not.

“Our 5.5 cm aneurysm on ultrasound is a 6 cm aneurysm on CT, and yet vascular surgeons confuse the different modalities [and] use the same thresholds for intervention on ultrasound and CT,” Earnshaw elaborated. “That makes no sense at all. One should be careful before deciding when to treat someone with an aneurysm exactly what method you're using to do the scan.”

This is particularly important given the risks of the interventions themselves, Johansson stressed.

“Even though operative mortality has improved, this is a major surgery with a considerable risk of serious complications (with potentially life-changing consequences for the individual), as well as death,” she said. “This applies also to endovascular repair. Even if short-term mortality is lower, long-term is not, due to the high risk of long-term complications. Considering this, current trends with increasing rates of surgeries of men with AAAs below 55 mm is indeed alarming and, in my opinion, should be dealt with. Unfortunately, screening inevitably adds to this problem, since screening detects many small AAAs that would never have caused any symptoms even if they had remained undetected.”