Evaluating Growth Patterns of Abdominal Aortic Aneurysm Diameter With Serial Computed Tomography Surveillance

Abstract

Importance: Small abdominal aortic aneurysms (AAAs) are common in the elderly population. Their growth rates and patterns, which drive clinical surveillance, are widely disputed.

Objective: To assess the growth patterns and rates of AAAs as documented on serial computed tomography (CT) scans.

Design, setting, and participants: Cohort study and secondary analysis of the Non-Invasive Treatment of Abdominal Aortic Aneurysm Clinical Trial (N-TA3CT), a randomized, double-blind placebo-controlled clinical trial conducted from 2013 to 2018, with CT imaging every 6 months for 2 years. The trial was a multicenter, observational secondary analysis, not related to treatment hypotheses of data collected in the N-TA3CT. Participants included 254 patients with baseline AAA diameter between 3.5 and 5.0 cm.

Exposures: Patients received serial CT scan measurements, analyzed for maximum transverse diameter, at 6-month intervals.

Main outcomes and measures: The primary study outcome was AAA annual growth rate. Secondary analyses included characterizing AAA growth patterns, assessing likelihood of AAA diameter to exceed sex-specific intervention thresholds over 2 years.

Results: A total of 254 patients, 35 women with baseline AAA diameter 3.5 to 4.5 cm and 219 men with baseline diameter 3.5 to 5.0 cm, were included. Yearly growth rates of AAA diameters were a median of 0.17 cm/y (interquartile range [IQR], 0.16) and a mean (SD), 0.19 (0.14) cm/y. Ten percent of AAAs displayed minimal to no growth (<0.05 cm/y), 62% displayed low growth (0.05-0.25 cm/y), and 28% displayed high growth (>0.25 cm/y). Baseline AAA diameter accounted for 5.4% of variance of growth rate (P < .001; R2, 0.054). Most AAAs displayed linear growth (70%); large variations in interval growth rates occurred infrequently (3% staccato growth and 4% exponential growth); and some patients' growth patterns were not clearly classifiable (23% indeterminate). No patients with a maximum transverse diameter less than 4.25 cm exceeded sex-specific repair thresholds at 2 years (men, 0 of 92; 95% CI, 0.00-0.055; women, 0 of 25 ; 95% CI, 0.00-0.247). Twenty-six percent of patients with a maximum transverse diameter of at least 4.25 cm exceeded sex-specific repair thresholds at 2 years (n = 12 of 83 men with diameter ranging from 4.25 to <4.75 cm; 95% CI, 0.091-0.264; n = 21 of 44 men with diameter ranging from 4.75-5.0 cm; 95% CI, 0.362-0.669; n = 3 of 10 women with diameter ≥4.25 cm; 95% CI, 0.093-0.726).

Conclusions and relevance: Most small AAAs showed linear growth; large intrapatient variations in interval growth rates were infrequently observed over 2 years. Linear growth modeling of AAAs in individual patients suggests smaller AAAs (<4.25 cm) can be followed up with a CT scan in at least 2 years with little chance of exceeding interventional thresholds.

Trial registration: ClinicalTrials.gov Identifier: NCT01756833.

Figure 1.. Distribution of Yearly Abdominal Aortic Aneurysm (AAA) Maximum Transverse Diameter (MTD) Growth Rates

Histogram and box plots showing distribution of yearly MTD growth rates. Patients were included if they had measurement data from at least 2 computed tomography scans (n = 250). Growth rates were calculated in centimeters per year based on the linear regression lines calculated for each patient. Shapiro-Wilk test for normal distribution showed data to be not normally distributed (P < .001).

Figure 2.. Distribution of Maximum Segment Slope Difference Supports Most Abdominal Aortic Aneurysms (AAAs) Grow Linearly

Histogram and box plot distribution of maximum segment slope difference values for all patients with at least 3 computed tomography scans (n = 241). The maximum segment slope difference values for each patient measure the difference between the highest growth rate (centimeters per year) between 2 consecutive scans and the lowest growth rate between 2 consecutive scans. Shapiro-Wilk test for normal distribution showed data to be not normally distributed (P < .001). Low differences between slope segments in the majority of patients emphasize that linear modeling of maximum transverse diameter growth rates per patient is appropriate.

Figure 3.. Association of Yearly Maximum Transverse Diameter (MTD) Growth Rates With Baseline Abdominal Aortic Aneurysm (AAA) Diameter

Linear regressions were calculated for each patient based on measurement and study day. Patients were included if they had measurement data from at least 2 computed tomography scans (n = 250). Linear regression growth rates in centimeters per year are plotted vs baseline MTD in centimeters. The shaded blue area represents 95% confidence limits. Linear regression trendlines show a small but significant association between baseline aneurysm size and yearly growth rate.