An aneurysm is a localized, abnormal bulging or ballooning in the wall of a blood vessel, caused by a weak spot that yields to the pressure of blood flow. These weakened areas can occur in any artery, but they are most commonly found in the aorta, the body’s largest artery, and in the brain’s blood vessels, known as intracranial aneurysms (IAs). The primary concern with any aneurysm is its potential to grow, as growth directly increases the mechanical stress on the vessel wall, raising the risk of rupture. The rate at which an aneurysm grows is highly variable and often unpredictable, which is why tracking its progression is a central part of clinical management.
Measuring and Tracking Growth Rates
Aneurysm growth is typically measured in millimeters per year and tracked using serial medical imaging. For unruptured intracranial aneurysms (UIAs), the majority—often over 80%—remain stable and show no significant growth over long periods of time. When enlargement does occur, the average growth rate is generally slow. Small UIAs (less than 7 mm) may grow at a rate of less than 3% per aneurysm-year, while abdominal aortic aneurysms (AAAs) often show a median growth rate around 2.1 millimeters annually.
Growth is clinically defined as an increase of at least 1 millimeter in one or more dimensions or a significant change in shape. For AAAs, the growth rate varies with initial size; larger aneurysms (50 millimeters or more) show a more rapid rate of expansion. The unpredictability of this progression necessitates continuous monitoring to distinguish stable lesions from those that are actively expanding.
Factors That Accelerate Aneurysm Growth
Several biological, physiological, and lifestyle factors accelerate aneurysm growth. Uncontrolled arterial hypertension (high blood pressure) is considered a major mechanical accelerator. This condition subjects the already weakened arterial wall to increased pressure, directly contributing to further ballooning.
Smoking is another powerful factor, introducing inflammatory components that actively weaken the structure of the vessel wall. Smoking history has been associated with a higher growth rate in UIAs, increasing the risk of growth from about 3.5% per year to 5.5% per year compared to non-smokers. The initial size also influences growth; UIAs larger than 10 millimeters have a significantly higher growth rate (9.7% per year) compared to those smaller than 10 millimeters (2.9% per year).
The specific location and shape of the aneurysm also influence its tendency to grow. Aneurysms located at arterial bifurcation points, where blood flow creates higher local stress, are more prone to growth. Posterior circulation aneurysms in the brain, for example, have been observed to have a higher growth rate (3.8% per year) than those in the anterior circulation (2.7% per year). Furthermore, aneurysms with a nonsaccular or irregular shape demonstrate significantly higher growth rates than the more common saccular type.
Growth Progression and Rupture Risk
The progression of aneurysm growth is directly and exponentially linked to the probability of rupture. As the vessel diameter increases, the tension on the wall rises according to physical laws, making it more susceptible to tearing. This relationship means that growth progression can quickly shift an aneurysm from a low-risk category to a high-risk one.
For abdominal aortic aneurysms (AAAs), the annual rupture risk is less than 1% for those smaller than 4 centimeters but increases sharply with size. For example, an AAA between 5 and 6 centimeters carries a 3–5% annual rupture risk, escalating to over 20% per year for those larger than 7 centimeters. Similarly, for intracranial aneurysms (IAs), the annual rupture rate rises substantially with increasing size, especially when coupled with other risk factors.
Rapid growth over a short period is a particularly concerning indicator of imminent risk, often surpassing the significance of absolute size. For AAAs, rapid growth is defined as expansion greater than 0.5 centimeters in six months and is an independent predictor of future rupture. For UIAs, the detection of any growth is associated with a dramatic increase in the annual rupture rate, jumping from 0.1% for stable aneurysms to 3.1% for those that have grown. Clinical data suggests that after growth is detected in a UIA, the absolute risk of rupture is approximately 4.3% within the following year.
Clinical Monitoring and Surveillance Schedules
Given the variable nature and rupture risk associated with growth, healthcare providers rely on structured monitoring and surveillance schedules. The primary tools for tracking aneurysm size are advanced imaging modalities like Magnetic Resonance Angiography (MRA) and Computed Tomography Angiography (CTA). For abdominal aneurysms, Duplex ultrasound (DUS) is often used as the first-line surveillance tool, although CT or MRI may be used if the ultrasound cannot provide adequate measurements.
The frequency of surveillance imaging is determined by the aneurysm’s size and location. Smaller, stable aneurysms have extended monitoring schedules. AAAs measuring 3.0 to 3.9 centimeters are typically monitored every three years, while those between 4.0 and 4.9 centimeters require annual follow-up. Aneurysms approaching the intervention threshold (5.0 to 5.4 centimeters) are generally imaged every six months.
The decision to transition from monitoring to intervention (surgical repair or endovascular treatment) occurs when the risk of rupture outweighs the risks associated with the treatment procedure. For AAAs, elective repair is generally recommended when the aneurysm reaches 5.5 centimeters in men and 5.0 centimeters in women, or if the growth rate is rapid. For UIAs, the detection of growth is a strong factor that often accelerates the timeline for considering active treatment.