Journal of Vascular Surgery
Volume 49, Issue 4 , Pages 881-885, April 2009

Natural history of the common iliac artery in the presence of an abdominal aortic aneurysm

  • Toby Richards, BSc, MBBS, FRCS, MD

      Affiliations

    • Department of Vascular Surgery, John Radcliffe Hospital, Oxford, United Kingdom
    • Corresponding Author InformationReprint requests: Toby Richards, RCS, MD, St. Vincents Hospital, Victoria Parade, Fitzroy, Melbourne 3068, Australia
    • ,
  • Asela Dharmadasa, MBBS

      Affiliations

    • Department of Vascular Surgery, John Radcliffe Hospital, Oxford, United Kingdom
    • ,
  • Rachael Davies, BSc, MSc, AVS

      Affiliations

    • Oxford Centre for Evidence Based Medicine, Churchill Hospital Campus, Oxford, United Kingdom
    • ,
  • Michael Murphy, MBBS, MD, FRCS

      Affiliations

    • Department of Vascular Surgery, John Radcliffe Hospital, Oxford, United Kingdom
    • ,
  • Rafael Perera, BA, MSc, Dphil

      Affiliations

    • Oxford Centre for Evidence Based Medicine, Churchill Hospital Campus, Oxford, United Kingdom
    • ,
  • Jackie Walton, BSc, AVS

      Affiliations

    • Department of Vascular Imaging, John Radcliffe Hospital, Oxford, United Kingdom

Received 12 July 2008; accepted 7 November 2008. published online 23 February 2009.

Article Outline

Objective

Patients with an abdominal aortic aneurysm (AAA) often develop common iliac artery (CIA) aneurysms. We wished to assess the natural history of the CIA in the presence of an AAA and develop a model to predict CIA growth.

Methods

Data were gathered at a single center from 1996 to 2006 in patients undergoing AAA surveillance. Maximum size of AAA and both CIAs at yearly intervals were collected. CIA > 16 mm was defined as being an aneurysm. A mixed effects regression model was generated to predict CIA growth rates.

Results

One hundred ninety-one patients with AAA underwent duplex ultrasound on at least two occasions (median, 4; range, 2-11). Average baseline CIA was 12 mm (standard deviation, 5.0); 41% of patients had one CIA over 16 mm. A CIA > 16 mm was more likely to expand (81% vs 53%, P = .0001) particularly in patients with an AAA that expanded (73% vs 43%, P = .0005). A larger AAA was associated with a larger CIA (P = .0341). CIA growth rate was proportional to baseline size. A CIA of 16 mm was predicted to take 10 years to reach 25 mm (156% or 5.6% per annum) or if 23 mm at baseline 10 years to reach 35 mm (152% or 5.2% per annum). Overall, a CIA was predicted to increase in diameter by 5.7% (± 0.5%) per annum.

Conclusion

The CIA in the presence of an AAA expands over time. CIA > 16 mm are more likely to increase. Routine duplex examination of a CIA less than 16 mm may not be necessary when following up AAA. These data may be used to aid planning and intervention during AAA repair.

 

Population screening for abdominal aortic aneurysms (AAA) is recommended.1 Data from the Multicentre Aneurysm Screening Study (MASS) trial suggest this to be feasible and cost effective on a national scale.2 The natural history of small AAA is well recognized and most patients are followed up by annual duplex scanning if the diameter is greater than 30 mm.3 At the time of AAA surveillance, it is routine practice at our center to assess the common iliac arteries (CIA). However, the natural history of the CIA in the presence of an AAA is less well understood. Knowledge of growth rates of the CIA in the presence of an AAA would guide the need for CIA surveillance.

Patients with AAA often have a co-existing aneurysm of the CIA. Normal CIA diameter ranges from 9.7 mm to 12.3 mm.4, 5 Definitions of size for a CIA aneurysm are variable; suggestions range from 15 mm to 20 mm to 24 mm.6, 7, 8 Similarly, the size recommended for CIA aneurysm repair ranges between 30 mm and 40 mm.6, 7, 9 The objective of repair for either an AAA or a CIA aneurysm is to replace the diseased artery in order to prevent rupture in the future. In the United Kingdom, 31% of open operations performed for AAA involved replacement of the CIA with a bifurcated graft.10 While not all these cases were for CIA aneurysm disease, the majority it can be assumed, were; others have reported 20% of CIA to be over 24 mm in patients undergoing AAA repair.8 EndoVascular Aneurysm Repair (EVAR) for AAA is increasing. Data from the European collaborators on stent/graft techniques for aortic aneurysm repair (EUROSTAR) registry showed that a concomitant CIA aneurysm was associated with increased risk of endoleak.11 During EVAR, there are two objectives for adequate placement of the limb extension in the CIA; to ensure distal sealing to prevent endoleak, and to provide fixation and stability for the whole endograft. Data on the natural history of the CIA in the presence of an AAA will aid graft planning for EVAR.

The aims of this study are to assess the natural history of the CIA in the presence of an AAA, to develop a model to predict growth rates of the CIA for a given size, and to assess the size when a CIA is likely to continue to increase in diameter. We wished to use these predictions to make recommendations on surveillance and planning for EVAR.

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Methods 

Data were gathered from one center, where all men aged 65 were offered AAA screening by duplex examination and all patients with AAA > 30 mm were entered into an annual surveillance program until operation. Also included in surveillance were those patients who, on examination for other reasons, were found to have incidental AAA. Details on 191 patients were assessed; all patients had at least two separate examinations. Data on the maximum size of AAA and both CIAs at yearly intervals were prospectively collected. For the purpose of analysis, a CIA > 16 mm, one and half times the average size of a normal CIA, was defined as being an aneurysm.4, 5

Duplex Scanning 

Vessels were scanned using a Philips ATL HDI 5000 (Royal Philips Electronics, Amsterdam, The Netherlands) with a C5-2 curvilinear array probe (frequency, 2 MHz to 5 MHz). Vessel identification was by color Doppler and b-mode (gray scale) to undertake measurements. Diameters of AAA and CIA were recorded in the anterior/posterior (AP) plane by freezing the image at maximum systole. Limitations included tortuous vessels, depth (overly large patients and bowel gas), and heavy vessel wall calcification. To overcome these, vessels were followed from the groin, in a longitudinal view to the bifurcation, to ensure the correct iliac artery. If a longitudinal view was not feasible due to calcification, an oblique view was obtained and documented for subsequent scans. When gas obscured views, various adjunctive procedures were employed; the probe was used to massage the abdomen, the patient rolled, images taken during expiration, or, alternatively, a repeat scan undertaken on the fasted patient. In addition for large patients, a more lateral view and power Doppler may be used. In cases of limited view, a second technologist confirmed the measurements; a recognized margin of error was ± 2 mm.

Statistical Analysis 

Median number of scans per patient was four (range, 2-11), with an average length of follow up of 3.4 years (range, 1-10 years). In total, 1589 individual recordings were made on CIA diameters. An increase in vessel size, either AAA or CIA, was defined as growth of 2 mm or more over the surveillance period. Data were compared between two patient groups, those with a CIA greater than or less than 16 mm. In order to assess five- and ten-year growth rates for AAA and CIA of a given size, regression analysis was used to establish a model to predict AAA and CIA growth rates. The relationships between AAA and CIA (baseline size, growth and time, and/or the interactions between time and baseline level) were explored. For these data, as there were two CIAs per individual (left and right), we used a mixed-effects regression model, which takes into account that although there are two measurements, they come from the same patient. The patient was the identifier. The dependent variable was the total increase in CIA diameter from baseline at separate times. The independent variables were baseline size, time from baseline measure to new measure, and the interaction between baseline and time. These estimates were based on 378 observations on 189 patients. To obtain unbiased and consistent estimates, we used the small-sample Swamy-Arora estimator individual-level variance component and the Huber/White/sandwich estimator of variance (robust estimates of variance). Data were analyzed using SPSS (SPSS Inc, Chicago, Ill) and STAT 10 software (StataCorp LP, College Station, Tex) (www.spss.com, www.stata.com).

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Results 

Data were collected from 191 patients; the average age in the follow up group was 76.5 years (standard deviation (sd), ± 8; range, 49-100 years). The male to female ratio was 164:27.

The average size of AAA at entry into surveillance (baseline) was 39 mm (sd, 8.3 mm). Most patients at with an AAA of 55 mm underwent operation; a few patients (n = 10) deemed high risk for operation were retained in surveillance at the discretion of the supervising consultant. The average size of AAA increased steadily, as expected over time. During the course of surveillance, most AAA (74%) increased more than 2 mm (Table).

Table. Number of patients with enlarged common iliac arteries (CIA) over 16 mm and grew 2 mm or more during surveillance of a small abdominal aortic aneurysm (AAA) (3-5.5 cm)
NumberBilateral(%)Left(%)Right(%)Either(%)P =
CIA191
CIA > 16 mm at start 33(17.3)54(28.3)57(29.8)78(40.8)
CIA < 16 mm at start 158(82.7)137(71.7)134(70.2)113(59.2)
CIA > 16 mm at end 61(31.9)78(40.8)94(49.2)111(58.1).015a
CIA < 16 mm at end 130(68.1)113(59.2)97(50.8)80(41.9)
CIA that grew > 2 mm12352(27.2)90(47.1)85(44.5)123(64.4)
CIA > 16 mm7815(19.2)39(50.0)39(50.0)63(80.8).0001b
CIA < 16 mm11337(32.7)51(45.1)46(40.7)60(53.1)
AAA that grew > 2 mm142
CIA that grew > 2 mm 46(32.4)75(52.8)73(51.4)102(71.8).0005c
CIA > 16 mm5112(23.5)29(56.9)30(58.8)47(92.2).00001d
CIA < 16 mm9134(37.4)46(50.5)43(47.3)55(60.4)
AAA that did not grow49
CIA that grew > 2 mm 6(12.2)15(30.6)12(24.5)21(42.9)
CIA > 16 mm273(50.0)10(66.7)9(75.0)16(76.2).019e
CIA < 16 mm223(50.0)5(33.3)3(25.0)5(23.8)

Patients were analyzed according to size of CIA (more or less than 16 mm) at the start and end of surveillance, if they grew 2 mm or more and if they were in a patient with an AAA that grew 2 mm or more.

AAA, abdominal aortic aneurysm; CIA, common iliac artery.

aAt the end of surveillance were more likely to have an isolated CIA > 16 mm on the right than the left.

bOverall, a CIA > 16 mm was more likely to increase in size.

cCIA was more likely to increase in size in patients who had an AAA that also increased in size.

dIn patients who had an AAA that increased, most CIA > 16 mm also increased.

eIn patients with an AAA that did not increase, those with a CIA >16 mm were more likely to increase in size.

Average size of CIA entry into surveillance (baseline) was 12 mm (sd, 5.0); 41% of patients had one CIA over 16 mm. During the course of surveillance, two-thirds of CIA increased in size. A CIA > 16 mm was more likely to increase in size than a CIA < 16 mm (81% vs 53%, P = .0001). A CIA was more likely to increase in size in a patient who had an AAA that increased (73% vs 43%, P = .0005); similarly, this was more likely if the baseline CIA was greater than 16 mm (Table). Patients were more likely to have an isolated CIA > 16 mm on the right than the left at the end of surveillance (33 vs 17, P = .015). Comparing average size of AAA and average size of CIA, overall, a larger AAA was associated with a larger CIA (Fig 1) (Kruskal-Wallis Test [non-parametric analysis of variance (ANOVA)] P = .0341). However, as both AAA and CIA size are time-dependent, time may act as a confounding factor and these results may reflect the effect of time rather than the effect of AAA size on CIA size. No correlation was found between AAA growth rate and CIA growth rate or AAA size and CIA growth rate.

  • View full-size image.
  • Fig 1. 

    Average common iliac artery (CIA) size for a given abdominal aortic aneurysm (AAA) size, median ± interquartile range (IQR). Larger AAA were associated with larger CIA (Kruskal-Wallis Test [non-parametric analysis of variance (ANOVA)] P = .0341).

As the average length of follow up was 3.4 years, we wished to combine patients' individual data on size and growth to make a statistical model to predict vessel growth rates over a five- and ten-year period. Using the data from the AAA and CIA measurements, a mixed-effects regression model was used to predict AAA and CIA growth rates. For aortic scans, the majority were performed on small AAA (3-5.5 cm); few patients remained in screening beyond 5.5 cm. Consequently, the model provided good fit and accuracy of predictions for growth rates of small AAA; a 3 cm AAA was predicted to grow 1.55 mm and a 4 cm AAA 1.68 mm per annum.

For the CIA, there were between 90 and 160 measurements taken at each mm between 12 mm and 20 mm, whereas there were less over 26 mm. The model, as assessed by comparison of predicted values to original data, showed accuracy for the predictions up to a baseline CIA of 26 mm with good correlation between observed and predicted growth rates. A CIA of 16 mm was predicted to take 10 years to reach 25 mm (156% or 5.6% per annum) or if 23 mm at baseline 10 years to reach 35 mm (152% or 5.2% per annum) (Fig 2). When comparing percentage increase in arterial diameter, overall, the CIA was seen to increase at a similar rate as an AAA. This was independent of size for a given CIA. Overall, a CIA was predicted to increase in diameter by 5.7% (± 0.5%) per annum.

  • View full-size image.
  • Fig 2. 

    Results from a mixed-effects regression model used to predict CIA growth rates. Predicted growth for different sizes of CIA at baseline are shown. The larger the CIA at baseline, the greater the predicted growth over a ten year period.

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Discussion 

The data presented here show the natural history of the CIA in the presence of an AAA. Patients with small AAA often have common iliac arteries that are likely to increase in diameter over time, particularly if the aortic size increases. As with an AAA, the rate of CIA expansion was dependent on baseline size. A CIA over 16 mm was more likely to increase in size, and, if 16 mm is used to define a CIA aneurysm, then half of patients with an AAA in surveillance had a concomitant CIA aneurysm.

The data here reflects the natural history of the CIA in the presence of small AAA (3-5.5 cm). Predictions of growth for small AAA sizes were comparable to those seen by the Gloucester Vascular Group.3 They looked at data from population screening of 65 year-old men and found AAA growth rates were related to initial aortic diameter, which was 0.16 cm per year for a 3-3.4 cm AAA. The results for CIA were similar to those found by Santilli et al.6 They found small CIA aneurysms (15-30mm), most in the presence of a small AAA, had an overall growth rate of 1 mm per annum. One-third did not change over the study period as seen here. Those in the presence of an AAA appeared to increase faster. Our data looked at all CIA in the presence of a small AAA and suggested that CIA of all sizes, including those in the normal range, may increase in size over time. This was more likely in patients who had a baseline CIA > 16 mm and if the AAA also expanded. In patients who had a CIA > 16 mm, this was often bilateral, but if on one side, was more common on the right, although numbers are small. Overall, we found no difference in growth or predicted growth rates between left and right CIA, although others have reported no differences6, 7 some have found the CIA to be larger on the right side.8

Using the model presented here, a CIA of 16 mm on initial scanning would take approximately 10 years to reach 25 mm. Thus, if a CIA measured less than 16 mm for an AAA of 4 cm, it could be reasonable only to rescan the CIA when intervention for the AAA was planned. For those patients with a CIA of > 16 mm, the CIA could be reassessed at three to five years. However, the model was limited by being accurate only for small aneurysms and could not predict growth for larger CIA > 26 mm so does not provide data on rupture risk, which is rare below 40 mm.7, 8 Further, these data do not reflect the impact of smoking and hypertension.7

Although operative management for AAA is changing with increased use of EVAR,12 open repair remains common practice. In patients with an AAA, a CIA of 23 mm would be predicted to increase to 35 mm in ten years. Hassan-Khodja found the CIA continued to expand following open tube graft repair for an AAA, recommending concurrent repair for CIA > 25 mm or in fit patients with a CIA > 18 mm.13

EVAR is associated with a lower hospital mortality and complication rate than open repair.14, 15 Concerns exist over the long-term outcome of EVAR and need for secondary intervention.16, 17 About half of all AAA, in line with manufacturers guidelines, are suitable for EVAR,18, 19 although with experience, development of fenestrated and branched grafts this is increasing.7 Good graft placement in EVAR has focused on suitability of the proximal landing zone in the infrarenal aortic neck. Less attention has been paid to the landing zone in the CIA. Initial reports suggested a ‘bell bottom’ graft was able to achieve a good seal in a dilated CIA (> 14 mm).20 Follow-up data from the Zenith US multicenter trial showed no adverse outcome from EVAR in AAA with a concomitant CIA aneurysm (CIA 14-20 mm).21 Contrary to this, two recent reports found a higher distal type 1 endoleak (7.1% and 9.1%) in cases with a CIA aneurysm.11, 22 Heikkinen et al showed reduced stent migration with good iliac fixation defined as greater than 25 mm and within 10 mm of the iliac bifurcation, even in cases with poor aortic neck fixation.23 Benharash et al confirmed this for EVAR devices either with infrarenal or suprarenal fixation.24

These reports suggest the need to ensure adequate fixation of the CIA during EVAR. For a CIA of 16 mm, an 18 mm diameter graft would be recommended during EVAR. However, continued expansion of an uncovered CIA is seen following EVAR.25 The data presented here would predict a CIA >16 mm may exceed 18 mm within 2-3 years, which may explain the increased in type 1b endoleak seen in some series.5, 17 Alternatives for larger CIA aneurysms include extension into the external iliac artery or an iliac bifurcation device.26, 27

The natural history of a CIA > 16 mm is to continue expanding, Care should be taken to reline the CIA to the iliac bifurcation when undertaking EVAR in the presence of a CIA aneurysm, defined as >16 mm.

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Author contributions 


Conception and design: TR

Analysis and interpretation: RP, TR, AD

Data collection: RD, JW

Writing the article: AD, TR

Critical revision of the article: TR, RP

Final approval of the article: TR, MM

Statistical analysis: RP

Obtained funding: Not applicable

Overall responsibility: TR

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References 

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 Competition of interest: none.

PII: S0741-5214(08)01954-X

doi:10.1016/j.jvs.2008.11.025

Journal of Vascular Surgery
Volume 49, Issue 4 , Pages 881-885, April 2009

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