An aneurysm represents a localized weakness in a blood vessel wall that causes a bulge or ballooning, similar to a bubble forming on a tire. The two types most commonly discussed are cerebral aneurysms, which form in the arteries of the brain, and aortic aneurysms, which occur in the body’s main artery, the aorta. The primary concern with an aneurysm is its potential to rupture, which is directly linked to its size and its rate of growth. However, the speed at which an aneurysm enlarges is highly variable and often unpredictable.
Understanding Aneurysm Growth Rates
Research indicates that most aneurysms remain relatively stable or grow at a very slow pace over time. For unruptured cerebral aneurysms, the annual increase in size typically falls within a range of 0.34 to 1.63 millimeters per year, demonstrating a slow progression for many individuals. Many small aneurysms are considered stable, meaning they show minimal or no measurable change over extended monitoring periods.
Growth is often measured in two ways: as an absolute change in diameter (millimeters per year) or as a percentage of aneurysms that show any growth, which is reported to be around 1.5% to 3.0% annually. Conversely, an aneurysm is considered to be growing rapidly if it increases by 1 millimeter or more between surveillance scans.
The growth rates for abdominal aortic aneurysms (AAAs) are often reported in centimeters per year. Small AAAs, those between 3.5 and 5.0 centimeters, typically exhibit a mean growth rate of approximately 0.19 centimeters (1.9 millimeters) per year. This rate accelerates significantly with size; aneurysms measuring 5.0 centimeters or larger can grow over 4 millimeters annually.
Key Factors Influencing Progression
The progression of an aneurysm is driven by a combination of physical forces and biological risk factors. The aneurysm’s initial size and location are significant determinants. Larger cerebral aneurysms, particularly those measuring over 7 millimeters, are inherently more susceptible to growth and rupture than smaller ones.
Aneurysm location also plays a role; those situated in the posterior circulation of the brain have statistically higher growth rates compared to those in the anterior circulation. The physical stress exerted on the vessel wall, known as hemodynamics, is a major factor. High blood pressure (hypertension) subjects the weakened wall to constant, excessive pressure, which encourages expansion.
Patient health and lifestyle choices accelerate the weakening of the vessel wall. Smoking is a substantial and independent risk factor for both the formation and growth of aneurysms. Nicotine and other toxins damage the lining of the blood vessels, promoting inflammation and the breakdown of structural components. Other biological factors associated with a higher likelihood of aneurysm growth include:
- Family history of aneurysms.
- Female sex.
- Increasing age.
How Aneurysm Size is Monitored
Physicians rely on serial imaging to track the size and morphology of an aneurysm over time. The primary tools used for monitoring include non-invasive imaging techniques such as Computed Tomography (CT) angiography and Magnetic Resonance Angiography (MRA). These scans produce detailed, three-dimensional images of the blood vessels, allowing for precise measurement of the aneurysm’s maximum diameter.
For abdominal aortic aneurysms (AAAs), ultrasound is frequently used for initial screening and routine follow-up due to its accessibility and lack of radiation exposure. The frequency of these follow-up scans, known as surveillance, is determined by the aneurysm’s current size and its observed growth pattern. A small, stable aneurysm might be scanned every year or two, while a larger or rapidly growing one may require monitoring every three to six months.
Monitoring determines if the aneurysm has crossed the threshold for intervention. This threshold is the size at which the risk of rupture outweighs the risk of treatment. For AAAs, this is generally set at a maximum diameter of 5.5 centimeters. Evidence of significant growth, such as an increase of 5 millimeters within a six-month period, is often a separate indication for treatment, even if the maximum size threshold has not been reached.