Arterial Aneurysms: Etiology, Epidemiology, and Natural History

Introduction

The term aneurysm describes dilatation of any blood vessel. Arterial aneurysms occur throughout the body but are most prevalent in the infrarenal aorta (AAA). These aneurysms represent the primary cause of the death and disability attributed to arterial aneurysms. In the United States, AAAs were directly responsible for 9928 deaths in 2017 and roughly 120,000 procedures are performed annually to prevent ruptured AAAs. ^, The incidence of aneurysms increases with age, but aneurysmal disease can occur in any decade of life. These lesions may be the result of several processes, including degenerative, inflammatory, infectious, genetic, and traumatic conditions. Although aneurysms can cause a variety of clinical conditions (embolism, compression on adjacent organs, and fistulas) the primary danger from intracavitary aneurysms, such as aortic, is rupture with uncontrolled hemorrhage, and death. Nonaortic aneurysms may also rupture, but they more often embolize, thrombose, or compress surrounding structures, and they rarely rupture into an open space where blood loss is catastrophic.

The risk of rupture of any aneurysm is related to both the absolute size of the aneurysm and its size relative to the normal diameter of the inflow and outflow artery from which it arises. The normal sizes for arteries in men and women have been reported, with normal ranges, based on location. A variety of other factors, including etiology, growth rate, and aneurysm morphology (i.e., fusiform vs. saccular) are also critical in assessing aneurysm risk.

Historical Perspective

The presence of arterial calcification and atherosclerotic change has been documented in Egyptian mummies from 3500 years ago. The Ebers Papyrus ( c. 2000 bce ) clearly identifies an arterial aneurysm, recommending, “Treat it with a knife and burn it with a fire so that it bleeds not too much.” Antyllus repaired aneurysms of the extremities with proximal and distal ligation, followed by packing of the aneurysm sac. Few advances occurred during the next 1000 years, until John Hunter ushered in the era of modern vascular surgery based on a scientific understanding of anatomy and physiology. He studied the collateral circulation resulting from the occlusion of arteries, and in 1785 successfully ligated the superficial femoral artery to treat a popliteal aneurysm. The patient did well and maintained function of his lower extremity.

Rudolph Matas (1860–1957), in 1888, was the first to perform endo-aneurysmorrhaphy in the treatment of a brachial artery aneurysm. This procedure augmented the principle of proximal and distal ligation by Hunter, using evacuation of the aneurysm sac and ligation of tributaries from within, while preserving important collaterals, and remains an important principle in modern surgery. ^, In 1923, Matas performed the first successful aortic ligation. Little progress was made during the next two decades. Bigger’s summary of therapy for aortic aneurysms at the 1940 American Surgical Association meeting probably represented the consensus of the era: “Judging from the literature, only a small number of surgeons have felt that direct surgical attack upon aneurysms of the abdominal aorta was justifiable, and it must be admitted that the results obtained by surgical intervention have been discouraging.”

The modern era of aneurysm therapy was ushered in by Charles Dubost in 1951 with the first successful graft replacement of the aorta with an aortic homograft. The introduction of Vinyon N cloth aortic prostheses, by Voorhees, Jaretski, and Blakemore, addressed the poor availability of aortic homograft and initiated the era of prosthetic vascular reconstruction. ∗

∗ Large samples of Vinyon N were made available for research following World War II. The material was manufactured to make parachutes for an invasion of Japan, which was cancelled following the atomic bombings of Hiroshima and Nagasaki and Japan’s subsequent surrender. Interestingly, polytetrafluoroethylene (PTFE) was used to help contain uranium hexafluoride, a toxic byproduct of atomic bomb production during the Manhattan Project. (Foundation CH, Plunkett RJ, 2013. Available from: < http://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/petrochemistry-and-synthetic-polymers/synthetic-polymers/plunkett.aspx >.)

The eventual introduction of polyester, championed by DeBakey, and the introduction of new knitting machine technologies to make seamless grafts of multiple sizes and shapes led to wider availability of materials and ultimately diffusion of surgical techniques for repairing aneurysms. In the 1990s, many surgical series documented mortality for open aneurysm repair below 5%, with some at 1% or lower.

The era of endovascular, or transcatheter, repair of aortic and other peripheral aneurysms was introduced in 1991 by Juan Parodi, who used available balloon-expandable stents in combination with standard polyester grafts to create a device that could be delivered from the femoral artery into the infrarenal aorta, where it effectively excluded the aneurysm. The first devices approved for use in the United States by the US Food and Drug Administration were in June 1999. Today there are numerous endografts available for use and current estimates are that more than 70% of all aortic aneurysm therapy is performed with endograft technology (see Ch. 75 , Aortoiliac Aneurysms: Endovascular Treatment). Both approved devices and off-label combinations of various endovascular devices have been increasingly used where critical branches are present, leading to the technical ability to repair virtually any aneurysm, from the aortic valve to the popliteal and brachial artery. Formal clinical trials of protocol-driven endograft designs for treating the mesenteric and arch segments of the aorta are numerous and represent the most active area of current endograft innovation (see Ch. 81 , Aortic Stent Graft and Endovascular Treatment of Thoracoabdominal and Aortic Arch Aneurysms: Strategies for Operative Repair).

Modern approaches to peripheral aneurysm and nonaortic cavitary aneurysmal (e.g., splenic, renal) repair typically use techniques to bypass the vessel, after proximal and distal ligation of the aneurysm; their development has paralleled techniques used for arterial bypass. As with aortic aneurysms, a variety of endografting techniques for peripheral aneurysms have been introduced during the last 20 years, with both self-expanding and balloon-expandable stent-grafts. Embolization techniques have also been selectively used for the treatment of saccular visceral artery aneurysms and aneurysm providing blood supply to noncritical organs, or where good collateral circulation exists (see Ch. 85 , Lower Extremity Aneurysms and Ch. 87 , Visceral Artery Aneurysms).

Aneurysm Classification

True vs. False Aneurysms

The distinction of true or false aneurysm (also termed “pseudoaneurysm”) is dependent on the vascular wall, since a “true aneurysm” has all layers of the wall incorporating or outside the dilated area, while a pseudoaneurysm has part of the wall containing connective tissue or contained hematoma (see Ch. 50 , Anastomotic Aneurysms). Many peripheral pseudoaneurysms are the result of iatrogenic peripheral arterial access required for imaging or therapy ( Fig. 71.1 ). These pseudoaneurysms often have a “neck” or narrow conduit between the affected artery and the main pseudoaneurysm cavity. When this morphology exists, therapy directed at inducing aneurysm thrombosis is often effective in treating the aneurysm.

Another frequent cause of pseudoaneurysms is the loss of anastomotic integrity at the site of a prior surgical anastomosis. These peri-anastomotic pseudoaneurysms represent dehiscence of the original suture line from the graft to the vessel ( Fig. 71.2 ). Endovascular techniques have greatly simplified the repair of these lesions in many cases.

Morphology

The shape of aneurysms is typically described as fusiform or saccular. Fusiform aneurysms represent a generalized increase in the entire diameter of the affected vessel while saccular aneurysms are localized, eccentric defects arising from a focal ulcer or weakness in the arterial wall, often as a result of trauma or infection ( Fig. 71.3 ).

Most peripheral arterial aneurysms are associated with atherosclerotic degeneration of the entire wall, and a resultant fusiform or concentric saccular morphology. Some aneurysms routinely have eccentric saccular morphology, such as renal artery and cerebral aneurysms. Whereas fusiform aneurysms result from a generalized weakening of the entire circumference of the affected vessel, saccular aneurysms result from a focal weakness. Intrinsic causes associated with saccular aneurysms generally involve a focal “tear” or partial disruption of the arterial wall. Penetrating atherosclerotic ulcers and intramural hematomas are two such examples (see Ch. 84 , Penetrating Aortic Ulcers). These entities have become distinguished from true aneurysms, and relatively straightforward endovascular techniques are often effective in covering these focal lesions. ^,

The risk of rupture is well characterized for fusiform aneurysms, but is less well understood for saccular lesions. Saccular morphology is often used as a factor in advising intervention at a diameter less than that for fusiform aneurysms. However, data suggesting that saccular aneurysms are more prone to rupture are lacking. Symptoms, size, growth rate, and amenability to repair are still the most important factors used in the decision to repair these unusual lesions.