Aortic aneurysms are classified along several overlapping axes rather than a single system. Doctors sort them by where they sit along the aorta, what their shape looks like, what caused them, and how the wall itself is affected. Each axis answers a different clinical question, from how urgently someone needs treatment to what kind of repair is most likely to succeed. The interplay of these categories is what makes an individual aneurysm’s risk profile more complex than its diameter alone might suggest.
True Aneurysm, False Aneurysm, and Dissection
The most fundamental distinction is whether all layers of the aortic wall are involved. In a true aneurysm, the vessel balloons outward but the wall still contains its normal layers. A false aneurysm, also called a pseudoaneurysm, forms when the aortic wall actually ruptures and blood is held in place only by a thin shell of scar-like tissue that lacks the normal structure of the aorta. A dissection is different from both: instead of ballooning, the layers of the wall split apart and blood tracks between them, creating a channel that can extend along the length of the vessel.1ARYA Atherosclerosis. Diagnostic dilemma: Saccular aneurysm or pseudoaneurysm of the ascending aorta with dissection above level of leaflets
This three-way distinction matters because the treatment urgency and approach differ sharply. A true aneurysm can be watched with regular imaging for years. A pseudoaneurysm is typically more fragile and carries a higher rupture risk, making earlier intervention the norm. A dissection is often a surgical emergency, especially when it involves the ascending aorta. Imaging alone can sometimes make the distinction tricky, which is why CT angiography and sometimes MRI are used in combination to evaluate wall integrity before deciding on a plan.
Shape Matters More Than You Might Expect
When you picture an aneurysm, you probably imagine a balloon-like bulge. That image actually corresponds to a saccular aneurysm, which pouches out on one side of the vessel. The other common shape is fusiform, where the aorta dilates symmetrically around its entire circumference, more like a sausage than a balloon. Most degenerative aneurysms are fusiform. Saccular ones are less common and tend to carry a different risk profile.
A large Japanese study using a national clinical database found that saccular abdominal aortic aneurysms ruptured at smaller diameters than fusiform ones. The median diameter at rupture was about 56 mm for saccular aneurysms compared with 68 mm for fusiform ones. In the 40- to 54-mm range, saccular shape was an independent risk factor for rupture, roughly two and a half times the odds compared with fusiform shape after adjusting for sex and diameter.2PubMed Central. Saccular and Fusiform Abdominal Aortic Aneurysms Treated With Endovascular Repair Differ in Presentation and Treatment Threshold: Analyses Using a National Clinical Database in Japan A similar pattern holds in the chest: patients with saccular thoracic aortic aneurysms underwent emergency or urgent repair at smaller diameters than those with fusiform ones.3PubMed Central. Clinical presentation, outcomes, and threshold for repair by sex in degenerative saccular vs fusiform aneurysms in the descending thoracic aorta
The practical takeaway is that saccular aneurysms are generally treated at smaller sizes. If your imaging report describes a saccular shape, the threshold for recommending repair is lower than the numbers commonly quoted for fusiform aneurysms. The asymmetric bulge concentrates wall stress in a smaller area, which is one reason it tends to be more rupture-prone at a given diameter.
Location Along the Aorta
The aorta runs from the heart down through the chest and abdomen, and where an aneurysm sits dictates almost everything about how it is managed. The broadest anatomical split is between thoracic aortic aneurysms, which involve the ascending aorta, aortic arch, or descending thoracic aorta, and abdominal aortic aneurysms, which most commonly affect the section of the aorta below the renal arteries.4PubMed. Pathogenesis of thoracic and abdominal aortic aneurysms Some aneurysms span both territories and are called thoracoabdominal, a category that brings its own surgical complexity because many vital branch arteries arise in the transition zone.
Even within a single region, sub-locations change the picture. An ascending aortic aneurysm near the aortic valve may require valve-sparing root replacement or a composite graft. An arch aneurysm sits where the vessels supplying the brain branch off, making repair one of the most technically demanding operations in cardiac surgery. A descending thoracic aneurysm is increasingly treated with stent grafts placed through the groin, while infrarenal abdominal aneurysms are the most common type and the one for which endovascular repair is most established.
Some aneurysms extend into the iliac arteries. An early morphometric classification system categorized abdominal aortic aneurysms partly by whether they had an adequate “neck” of normal aorta above the bulge and whether the disease spread into the iliacs. In one such study, over 70 percent of aneurysms extended into the iliac arteries, and just over half of those were eligible for a bifurcated stent graft.5Journal of Endovascular Therapy. Morphometry and Classification in Abdominal Aortic Aneurysms: Patient Selection for Endovascular and Open Surgery Isolated iliac artery aneurysms have also been given their own classification to guide whether and how endovascular repair can be performed.6PubMed. Isolated common iliac artery aneurysms: a revised classification to assist endovascular repair
What Caused It
Aneurysms are also grouped by their underlying cause, which influences both natural history and treatment decisions. Most abdominal aortic aneurysms are degenerative, driven by a combination of aging, smoking, high blood pressure, and atherosclerosis. But several other etiologies have distinct clinical pictures.
Inflammatory abdominal aortic aneurysms make up roughly five to ten percent of all abdominal aortic aneurysms. They stand apart because the aneurysm wall is markedly thickened, surrounded by a cuff of inflamed, fibrotic tissue that can glue adjacent structures to the aorta. Patients tend to be younger and are usually symptomatic, often presenting with back or abdominal pain. Blood tests typically show elevated inflammatory markers, and CT scans reveal the characteristic soft-tissue rind.7PubMed. Inflammatory abdominal aortic aneurysm This matters surgically because the dense adhesions make open repair considerably more difficult, and the inflammation can obstruct the ureters.
Infected, or mycotic, aneurysms result from a bacterial or fungal infection in the aortic wall. Despite the name, fungi are actually the less common culprit; the term is historical. On imaging, mycotic aneurysms tend to look lobulated and irregular, sometimes with gas in the wall or surrounding tissues.8PubMed Central. Imaging features of mycotic aortic aneurysms Their shape is usually saccular, in contrast to the fusiform shape typical of inflammatory aortic aneurysms.9Journal of Cardiology. Infected aortic aneurysm and inflammatory aortic aneurysm—In search of an optimal differential diagnosis Distinguishing infected from inflammatory aneurysms is critical because infected aneurysms typically require antibiotics plus surgery, and the infected tissue often needs to be excised rather than simply stented.
Genetic connective-tissue disorders represent another major etiological category. Marfan syndrome, Loeys-Dietz syndrome, and vascular Ehlers-Danlos syndrome all predispose to aortic aneurysms, dissections, and ruptures, often at younger ages and smaller diameters than degenerative disease. These syndromes affect the structural proteins that give the aortic wall its strength, and the aortic problems are just one part of a complex clinical picture that can involve the eyes, skeleton, and skin.10PubMed Central. Genetic Basis, New Diagnostic Approaches, and Updated Therapeutic Strategies of the Syndromic Aortic Diseases: Marfan, Loeys-Dietz, and Vascular Ehlers-Danlos Syndrome The surgical thresholds for these patients are set lower, and endovascular stent grafts are often avoided in vascular Ehlers-Danlos because the tissue is too fragile to anchor a device reliably.
Acute Aortic Syndromes
Not every aortic emergency involves a classic aneurysm. Acute aortic syndromes encompass three related conditions defined by a sudden breakdown of wall integrity: classic aortic dissection, intramural hematoma, and penetrating aortic ulcer.11PubMed. Diagnosis and management of acute aortic syndromes: dissection, intramural hematoma, and penetrating aortic ulcer All three can occur in an aorta that was previously aneurysmal or in one that was normal-sized, so they overlap with but are not synonymous with aneurysm classification.
In a classic dissection, a tear in the inner layer lets blood split the wall apart. An intramural hematoma involves bleeding within the wall, usually from small vessels that rupture inside it, without a clear entry tear. A penetrating aortic ulcer is an atherosclerotic plaque that erodes through the inner lining into the muscular layer. These three conditions are grouped together because they share the feature of compromised wall integrity and can evolve into one another.12PubMed Central. Diagnosis and Management of Acute Aortic Syndromes: Dissection, Penetrating Aortic Ulcer, and Intramural Hematoma A penetrating ulcer can progress to an intramural hematoma, which in turn can give way to a full dissection. Recognizing which entity you are dealing with helps determine whether the patient needs immediate surgery, an urgent stent graft, or aggressive blood-pressure control with close monitoring.
Size Thresholds and When to Intervene
For decades, the conventional wisdom for abdominal aortic aneurysms has been to repair at 5.5 cm in men and 5.0 cm in women. These numbers come from older randomized trials, but more recent modeling suggests the picture is more nuanced than a single cutoff. A 2024 simulation study concluded that for a 60-year-old woman in average health, the optimal repair size to minimize aneurysm-related death was about 6.1 cm, and life expectancy barely changed across a range from 5.7 to 7.1 cm. For a 60-year-old man in average health, the optimal threshold was around 6.9 cm, with a similarly flat curve from 6.0 to 7.4 cm. People in poor health had even higher optimal thresholds.13PubMed Central. Size thresholds for repair of abdominal aortic aneurysms warrant reconsideration
These figures are provocative because they suggest current guidelines may push some patients toward repair too early, especially men and patients with significant comorbidities. Still, these results come from a modeling study, not a new randomized trial, so they complement rather than replace existing guidelines. What they do reinforce is that diameter alone is a blunt tool, and that age, sex, overall health, and aneurysm morphology all feed into the decision.
Why Sex Changes the Risk Calculus
Women get abdominal aortic aneurysms far less often than men, but when they do, the aneurysms tend to rupture at smaller sizes. One study found that for women, aneurysm diameter indexed to body size was the strongest predictor of rupture, whereas for men, raw diameter alone was most predictive. Women with the largest-diameter aneurysms and the smallest body sizes faced the highest rupture risk.14PubMed Central. Relative importance of aneurysm diameter and body size for predicting abdominal aortic aneurysm rupture in men and women A more recent analysis supported using the aortic size index, which divides the aneurysm diameter by body surface area, as a way to account for these differences and reduce sex-based disparities in rupture prediction.15PubMed Central. Aortic Size Index Versus Aortic Diameter in the Prediction of Rupture in Women With Abdominal Aortic Aneurysm
Data from thoracic aneurysms echo this theme. One study of descending thoracic aneurysms found that sex-specific treatment thresholds made sense for fusiform aneurysms but not for saccular ones, where shape-related risk overrode the sex difference.16PubMed Central. Clinical presentation, outcomes, and threshold for repair by sex in degenerative saccular vs fusiform aneurysms in the descending thoracic aorta The clinical implication is that a one-size-fits-all diameter threshold misses real variation in who is actually at risk.
Endovascular Zoning and Anatomical Planning
When repair is done through a stent graft rather than an open operation, the exact anatomy of the aneurysm and the surrounding normal aorta determines what kind of device can be used and where it needs to land. For arch aneurysms, zone-based planning maps like the Ishimaru classification divide the arch into numbered zones based on proximity to the great vessels. A newer refinement called the Modified Arch Landing Areas Nomenclature provides finer gradations, and entry-focused schemas help categorize dissections by where the tear starts and whether blood flow to branch arteries is compromised.17PubMed Central. Contemporary Management of the Aortic Arch: A Narrative Review
For infrarenal aneurysms, device suitability hinges on the anatomy of the neck, which is the stretch of normal aorta between the renal arteries and the top of the aneurysm. Standard device instructions specify a minimum neck length (typically 15 mm), maximum angulation, and acceptable diameter range. Extended criteria push those limits, allowing necks as short as 10 mm and somewhat sharper angles.18PubMed Central. Suitability of the Aortic Neck Anatomy for Endovascular Aneurysm Repair in Korean Patients with Abdominal Aortic Aneurysm Patients whose anatomy falls outside even the extended criteria are generally steered toward open repair or more complex endovascular options like fenestrated or branched grafts.
Endoleak Classification After Stent Graft Repair
Once a stent graft is in place, the aneurysm sac is supposed to be sealed off from blood flow and gradually shrink. When blood continues to reach the sac, the leak is categorized by its source:
- Type 1: Blood leaks around the attachment site where the graft meets normal aorta, either at the top end or the bottom end. This usually requires prompt reintervention.
- Type 2: Small branch arteries (like lumbar arteries or the inferior mesenteric artery) feed blood back into the sac. This is the most common type and is often watched rather than immediately treated.
- Type 3: The graft components separate or the fabric tears, allowing direct flow into the sac. This needs repair.
- Type 4: Blood seeps through the pores of the graft fabric, typically seen only in the immediate postoperative period and usually resolves on its own.
- Type 5: The sac keeps expanding without a visible leak on imaging, a phenomenon sometimes called endotension. The cause is debated.
This five-type system is standard in post-repair surveillance and determines how aggressively a leak is pursued.19SpringerOpen. Secondary Endoleak Management Following TEVAR and EVAR
Beyond Diameter Alone
Diameter has been the workhorse of aneurysm risk assessment for decades, but researchers have long recognized its limitations. Two aneurysms of identical diameter can have very different rupture risks depending on wall thickness, the amount of clot lining the sac, vessel tortuosity, and overall wall surface area. Computational modeling that estimates peak wall stress has shown better accuracy and sensitivity for predicting rupture than maximum diameter alone.20PubMed Central. The role of geometric and biomechanical factors in abdominal aortic aneurysm rupture risk assessment These approaches use patient-specific CT data to build a virtual model of the aneurysm and calculate where the wall is under the most strain.
CT angiography itself has become the dominant imaging tool for classifying aneurysms. One study comparing it with catheter-based angiography found no significant difference in diagnostic accuracy: CTA correctly identified the type, site, and shape of the aneurysm in about 95 percent of cases, compared with roughly 98 percent for the invasive method.21PubMed Central. Research Study on the clinical value of computed tomography angiography in the diagnosis of aortic aneurysm Because CTA is noninvasive and widely available, it has become the standard first-line imaging study for both initial classification and long-term surveillance.
Artificial Intelligence and Circulating Biomarkers
Two frontiers in aneurysm classification aim to go beyond what a human eye can see on a scan. Machine-learning models are being developed that take in imaging data, patient demographics, and biomechanical features and output a rupture-risk prediction. One approach combining fluid-structure interaction modeling with a neural network reported 98.6 percent accuracy in predicting rupture risk from numerical simulation data, suggesting that deep-learning tools could eventually make patient-specific biomechanical assessment fast enough for routine clinical use.22Arabian Journal for Science and Engineering. Investigation of Rupture Risk of Thoracic Aortic Aneurysms via Fluid–Structure Interaction and Artificial Intelligence Method A broader review of AI-based predictive modeling for aortic aneurysms has highlighted the growing range of supervised, unsupervised, and deep-learning strategies being explored for screening, risk assessment, and prognosis.23PubMed Central. Artificial Intelligence-Based Predictive Modeling for Aortic Aneurysms
On the blood-test side, researchers are investigating whether circulating molecules could eventually classify aneurysms by biological aggressiveness rather than just size. A recent narrative review cataloged over 40 individual circulating biomarkers plus several multi-protein and microRNA panels that showed correlations with aneurysm disease risk.24PubMed Central. A narrative review of recent literature of circulating biomarkers of abdominal aortic aneurysm Among the more promising candidates, one study found that a specific microRNA, miR-15a-5p, was elevated in aneurysm patients and higher still in those with aneurysms exceeding 50 mm in diameter, suggesting it could serve as both a diagnostic and a progression marker.25Communications Medicine. MicroRNA-15a-5p mediates abdominal aortic aneurysm progression and serves as a potential diagnostic and prognostic circulating biomarker None of these biomarkers has entered routine clinical practice yet, but the direction of the research points toward a future where a blood draw might help decide whether a borderline aneurysm warrants earlier intervention.