Renovascular Disease: Aneurysms and Arteriovenous Fistulae

True Aneurysms

More than 90% of true renal artery aneurysms are extraparenchymal. ^, The peak incidence is in patients between the ages of 40 and 60 years. Stanley and Stanley et al. have suggested that true aneurysms are probably due to either atherosclerosis or a congenital defect. Although arteriosclerotic changes have been identified in most aneurysms in patients with multiple lesions, these aneurysms are more likely due to a congenital medial degenerative process with weakness of the elastic lamina. ^, Lesions typically occur at the primary or secondary renal artery bifurcations and are rarely confined only to the main trunk of the renal artery, often making treatment challenging.

Approximately 75% of true renal artery aneurysms are saccular. Saccular aneurysms occur almost invariably at the main renal artery bifurcation. Fusiform aneurysms are usually associated with atherosclerosis or are a result of a poststenotic dilatation distal to a hemodynamically significant renal artery stenosis, the latter of which results from atherosclerosis or fibromuscular disease. ^{, , ,} Fusiform aneurysms usually affect the main renal artery trunk, which may be associated with more straightforward endovascular or open surgical intervention.

Arterial fibrodysplasia is often a direct contributor to the development of renal artery aneurysms. ^, Medial fibroplasia is typically associated with multiple stenoses and poststenotic dilatation of the distal two-thirds of the renal artery. Renal artery aneurysms in association with fibromuscular dysplasia are generally only a few millimeters in diameter. The typical angiographic appearance of a renal artery involved with medial fibroplasia is a “string of beads.” In a series of 23 aneurysms that were examined histologically, 16 (70%) were found to be of dysplastic origin. Larger aneurysms can also occur, however, and in one study, renal artery microaneurysms were found in 9.2% of adults with fibromuscular dysplasia. A rare cause of renal artery aneurysms is Ehlers–Danlos syndrome. This disorder is associated with extreme arterial fragility and spontaneous rupture.

False Aneurysms (Pseudoaneurysms)

False aneurysms of the renal artery arise from blunt or penetrating trauma and occasionally from iatrogenic causes such as renal artery catheterization or after nephrectomy. They represent contained ruptures of the renal artery, with only inflammatory and fibrous tissue encasing the leak.

Dissections

Spontaneous dissections confined to the renal artery that do not arise from the adjacent aorta are rare. However, primary dissections causing pseudoaneurysms affect the renal arteries more than any other peripheral artery. ^{, ,} Poutasse and Stanley et al. reported that 14 of 57 cases of renal artery aneurysms were due to spontaneous dissection. An intimal defect of the renal artery due to atherosclerosis is probably the underlying cause of spontaneous renal artery dissection causing aneurysms, along with dysplastic renovascular disease and trauma. The incidence of dissection in patients with fibrodysplastic renal arteries ranges from 0.5% to 9.0%. ^, Dissection often extends into the branches of the renal artery and may pose particularly challenging reconstruction problems.

Traumatic renal artery dissection can occur secondary to blunt abdominal trauma or catheter-induced injury. Blunt trauma accounts for the higher prevalence of dissection in men and is more likely to result in right-sided injuries, possibly because of ptosis-related physical stresses affecting the renal pedicle. Blunt trauma can cause renal artery dissection by either severe stretching of the artery, with fracture of the intima, or compression of the artery against the vertebra. Renal artery dissection caused by guide wires or catheters can occur, but is rare, having been observed in only 4 of 2200 selective renal artery arteriograms.

Intrarenal Aneurysms

Less than 10% of renal artery aneurysms are intraparenchymal. ^, Intrarenal aneurysms are usually multiple and may be congenital, are associated with collagen vascular disease, or are posttraumatic. They may be associated with arteriovenous fistulae (AVFs), possibly as a result of spontaneous closure of a fistula. Intrarenal aneurysms can occur with polyarteritis nodosa and are usually in the renal cortex. ^,

Rupture and Prevention of Rupture

Rupture of a renal artery aneurysm is an indication for emergency intervention, as it is for virtually any arterial aneurysm. Probably less than 3% of renal artery aneurysms rupture. ^, This complication is associated with a mortality rate of approximately 10% in men and nonpregnant women. ^{, , ,} In a hemodynamically stable patient, an emergent CT scan may reveal the pathology and allow the surgeon to plan the operative repair.

Prevention of rupture is the most common indication for intervention in cases of asymptomatic renal artery aneurysms. Traditionally, repair has been recommended for renal artery aneurysms greater than 2 cm in diameter. ^, We believe this recommendation is overly aggressive, and more conservative guidelines for intervention based on more recent studies with longer follow-up have been suggested.

Older series suggested higher rates of rupture. Harrow and Sloane reported one of the highest rates of rupture of renal artery aneurysms, noting 14 ruptures in 100 cases. In another series of 126 renal artery aneurysms, six ruptured. However, most other series of asymptomatic renal artery aneurysms in men and nonpregnant women report a much lower incidence of rupture. Only 1 of 62 patients with aneurysms 4 cm in diameter or smaller ruptured after follow-up from 1 to 17 years. None of 19 small aneurysms in another series ruptured. A group of 21 patients was observed for an average of 3 years without rupture. In another series of 18 patients, no renal artery aneurysms ruptured that were less than 2.6 cm who were followed for 1 to 16 years. There were no ruptures in a series of 32 patients (who eventually underwent surgery) with renal artery aneurysms that ranged from 0.7 to 9 cm. Of 83 renal artery aneurysms found on arteriography and followed without surgery, none ruptured or became symptomatic after a mean of 4.3 years. In a pooled analysis, there were no ruptures in more than 200 renal artery aneurysms observed for up to 17 years. Because of the very low risk of rupture and the relatively high rate of significant postoperative complications, especially with open surgery, we agree with Coleman and Stanley that a more conservative approach be used for these cases and that a 2-cm guideline for intervention is too aggressive and unwarranted. We also agree with the recommendations of the Vascular Low-Frequency Disease Consortium that identified 865 renal artery aneurysms in 760 patients at 16 institutions. The authors recommended that repair should be considered for asymptomatic renal artery aneurysms >3 cm diameter in men and in women of non-childbearing age.

Besides size, other factors may play a role in the consideration of elective surgery for asymptomatic renal artery aneurysms. Calcification of the aneurysm has been thought to protect against rupture. Poutasse suggested that a heavily calcified renal artery aneurysm may be less likely to rupture than a noncalcified or minimally calcified one. In a review of cases through 1959, 14 of 100 noncalcified aneurysms ruptured. In a more recent series, 15 of 18 ruptured renal artery aneurysms were noncalcified. However, in a series of 62 solitary aneurysms less than 4 cm in diameter, one-third were not calcified, and only one aneurysm in the entire series ruptured after 1 to 17 years of follow-up. Because of these conflicting data, some authorities believe that the presence or absence of calcification is not relevant when predicting the risk of rupture. The Vascular Low-Frequency Disease Consortium found that calcification does not protect against enlargement or rupture of renal artery aneurysms.

Most authorities agree that pregnancy is associated with a significantly increased risk of rupture for renal artery aneurysms. ^{, , ,} Pregnancy may increase the risk of rupture because of a hyperdynamic state with increased blood volume and cardiac output, hormonal influences, and increased intraabdominal pressure due to the gravid uterus. ^, Cohen and Shamash reported 18 cases of rupture during pregnancy. In another series of 18 patients who underwent surgery for renal artery aneurysms, the only two ruptures were in women at childbirth; both of these aneurysms measured only 1 cm in diameter. In a review of 43 ruptured renal artery aneurysms, 81% occurred in women; 21 of the 35 women in this series were younger than 40 years old, and 18 were pregnant. Of the 18 aneurysms of known size, three ruptured when they were less than 2 cm. Rupture of renal artery aneurysms in pregnancy has been associated with a maternal mortality rate of 55% and a fetal death rate of 85%. ^, Vascular surgeons should maintain an aggressive surgical or endovascular approach for pregnant women with renal artery aneurysms of any size, namely more than 1.5 times the diameter of the normal adjacent artery.

It seems prudent to recommend repair of renal artery aneurysms in good-risk men and women older than childbearing age when the diameter is at least greater than 3 cm in good-risk patients when there is reasonable certainty that nephrectomy will not be required. ^, In a recent review by the Vascular Low-Frequency Consortium of 865 renal artery aneurysms from 16 institutions, the authors concluded that these aneurysms rarely rupture when asymptomatic (even if >2 cm), major complications occurred in 10% of patients postoperatively, and recommended repair for renal aneurysms greater than 3.0 cm in asymptomatic patients of non-childbearing age. An even more conservative approach reserving repair for aneurysms greater than 4 cm has been suggested; in our opinion, this is not unreasonable. As previously mentioned, any renal artery aneurysm in women of childbearing age should be repaired – namely, more than 1.5 times the diameter of the normal adjacent artery.