Renal Artery Aneurysm 

The first published report of a renal artery aneurysm (RAA) was in 1770 by Rouppe, who described the demise of a sailor who fell onto his right flank.^[1] Autopsy revealed a large false aneurysm with rupture. Since that time, many more case reports and case series have provided most of the data on this rare pathological entity.^{[2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]}

A renal artery aneurysm is defined as a dilated segment of renal artery that exceeds twice the diameter of a normal renal artery. Symptomatic RAAs can cause hypertension, pain, hematuria, and renal infarction. Asymptomatic RAAs may seem benign, but the potential for rupture and fistulization increases with size. Asymptomatic patients can be referred for elective repair, but if patients are symptomatic, further investigation with possible surgical intervention should be considered.

Frequency

Based on autopsy studies, the incidence rate of RAA is 0.01%.^[16] However, selected patients who undergo renal arteriography have an incidence rate of 0.3-1%.^[17] In one of the largest series over a 16-year period, 62 patients had RAA repair. The average age was 46 +/- 18, and 68% were women.^[18]

Many causes of RAA exist, each with different morphologies and locations along the renal artery, as depicted in the image above, as follows:

• True Aneurysms: These include all layers of the artery and are usually inherited disorders. They can be fusiform or saccular in appearance and are extraparenchymal in 90% of cases.^[11]

  • Fibromuscular dysplasia (FMD)
  • Ehlers-Danlos syndrome^[19]
• False/Pseudoaneurysm: These do not include all layers of the artery, are usually acquired, saccular in appearance, and can be extraparenchymal or intraparenchymal.

  • Blunt abdominal trauma
  • Anastomotic
  • Iatrogenic during endovascular procedures
  • Spontaneous^[7]
  • Dissection
  • Mycotic^[20]
  • Kawasaki Disease^[21]
• Intrarenal Aneurysms:^[22] These are either true or false aneurysms within the renal parenchyma. They deserve special classification because their management is usually with nephrectomy or coil embolization.

  • Polyarteritis nodosa
  • Tuberculosis
  • Neurofibromatosis

In true aneurysms, a weakening then dilatation of all layers of the arterial wall occurs. In FMD, the degenerative fibroplasia-type changes lead to this wall weakening. Often, renal artery stenosis is associated.^{[23, 15]} Most patients with FMD are healthy, young, hypertensive women, and on angiography the renal artery appears as a string of beads, with the aneurysm at the renal artery bifurcation.

Ehlers-Danlos is an autosomal dominant disorder characterized by fragility of medium and large arteries due to type III procollagen deficiency. This condition leads to dissections and aneurysms in any artery, including the renals.^[24]

In false or pseudoaneurysms, there is a focal disruption in one or every layer of the artery that causes a saccular outpouching at the weaken area. In blunt trauma, anterior displacement of the relatively mobile kidneys with rapid deceleration generated tension generated in the vascular pedicle causing a fracture of the intima, predisposing it to subintimal dissection then aneurysmal degeneration. Another mechanism involves direct arterial wall contusion against the vertebral bodies.^[7]

Anastomotic leaks from previous renal artery reconstructive procedures become walled off by the body, creating a pseudoaneurysm. In this situation, the wall of the aneurysm contains only fibrotic/inflammatory tissue. Iatrogenic endovascular–related aneurysms are caused by intimal trauma and focal dissection, leading to aneurysmal degeneration. Spontaneous renal artery dissections cause aneurysms by the same mechanism described above.^[25]

Intraparenchymal aneurysms are believed to arise primarily from inflammatory changes of the vessel wall. These commonly develop into microaneurysms.

Although pregnancy is not associated with an increased incidence of RAA formation, it is associated with a higher rate of rupture. The increased blood flow, intra-abdominal pressure, and vessel wall changes due to the hormonal and metabolic changes associated with gestation are believed to be contributory. Most ruptures occur late in the pregnancy, usually in the third trimester, and are left renal artery predominant.^[4]

In the pediatric age group, RAAs are due to trauma, infection, arteritides, Kawasaki disease, or vascular dysplasias. Multiple idiopathic arterial aneurysms that include renal artery involvement have been described but are extremely rare.^[26]

Asymptomatic

Most RAAs are asymptomatic and are found incidentally while investigating other intra-abdominal pathologies using diagnostic imaging studies such as computed tomography (CT), duplex ultrasonography, angiography, magnetic resonance imaging (MRI), or magnetic resonance angiography (MRA).

In asymptomatic patients complications from RAA are relatively low. In one study, 62 asymptomatic patients with solitary saccular aneurysms of mean size 1.5 cm (0.3 to 4.0 cm) were followed over a mean period of 5.7 years (median of 8 years). No ruptures, need for operations, or new symptoms developed. Eight patients (12%) did expire, but they were all unrelated to the aneurysm.^[9]

In another series, 34 RAAs were managed nonoperatively and followed with serial arteriograms. Over a mean interval of 35 months, no changes were found in 28 (82.4%) RAAs, and slight changes were found in the other 6 (17.6%). Again no ruptures were found during follow up.^[27]

Symptomatic

• Hypertension

  • This is the most common symptom found in RAA, with reported incidence as high as 90%. Often, renal artery stenosis is associated with a poststenotic fusiform aneurysm. In this case, the hypertension can be attributed to the renal artery stenosis and activation of the renin-angiotensin system, with increased angiotensin II levels resulting in fluid retention and vasoconstriction.^[25]
  • Hypertension in RAA without a renal artery stenosis is not as well understood. Possible causes of hypertension in these cases may be from renal ischemia secondary to thromboembolization distal to the aneurysm; in cases of large aneurysms, anatomical kinking of the renal artery has been reported.^[28]
• Flank or abdominal pain: In case series, 8-25% of patients presented with abdominal pain.^{[6, 9, 11]} Patients with RAAs caused by dissection may present with flank pain, although most of those with spontaneous dissections are asymptomatic. New or worsening pain may also be indicative of a rapidly expanding aneurysm or impending rupture.
• Hematuria: Hematuria may be another manifestation of dissecting RAA. Intraparenchymal aneurysms, which rupture into the collecting system, may also manifest as hematuria.
• Collecting system obstruction: Collecting system obstruction is a rare presentation but has been documented in patients with larger aneurysms.
• Renal infarction: Renal infarction may be visualized on CT scan images and is the result of embolization from the aneurysm sac.
• Rupture: Patients do not usually present with rupture. Patients with RAA rupture typically have signs and symptoms of an abdominal catastrophe and may be in frank shock.^[8]

Indications for intervention ^{[29, 6, 11, 12]}

• Rupture
• Symptomatic RAA

  • Hypertension (from associated renal artery stenosis, refractory to medical management)
  • Pain
  • Renal ischemia or infarction secondary to embolization from the aneurysm sac
• Renal artery aneurysms (RAAs) in females who are pregnant or in those contemplating pregnancy
• Diameter greater than 2 cm

  • Currently, no consensus exists for the size at which an RAA should be repaired in an asymptomatic patient.
  • Experts have recommended RAA repair at diameters ranging from 1.5-3 cm, although most suggest 2 cm.
  • Note that aneurysm rupture at 1.5 cm diameter has been reported.
  • Also, complete calcification of the wall of the aneurysm sac, as depicted in the image below, manifest in approximately 40% of patients. This was previously believed to confer protection against rupture;^[23] however, more recently, this theory has been questioned.^[16] (A) Aortogram with calcified left renal artery aneurysm (RAA). (B-C) Same RAA in magnified view, demonstrating the RAA is saccular, arising from the main renal artery.
• Enlarging RAA
• RAA associated with acute dissection

The renal arteries arise from the aorta at the level of the intervertebral disc between the L1 and L2 vertebrae. Cadaveric studies have shown that more than one renal artery is present in 15% and 20% of cases on the right and left sides, respectively. A great deal of variety can be found in the anatomy of the renal artery and its branches, but most often the main renal artery splits into an anterior and posterior division. Within the hilum, the anterior division gives rise to apical, anterior, and inferior segmental branches, as depicted in the image below. Segmental vessels then penetrating the renal parenchyma becoming lobar, interlobar, arcuate, interlobular arteries, afferent arterioles, then finally reach the capillaries and glomeruli.^[30]

Measurements of renal artery diameter can differ depending on imaging modality used. In one study, ultrasound found mean renal artery measurements of 5.04 ± 0.74 mm; with angiography, they were found to be 5.68 ± 1.19 mm. In this study, the authors also found that when accessory renal arteries were found, the main renal artery measurements were significantly smaller in diameter than when one renal artery was present.^[31]

Aneurysm of the renal artery can be classified in relationship to the parenchyma of the kidney, as depicted in the image above. Extraparenchymal aneurysms predominate, comprising approximately 85% of all RAAs.^{[11, 15]} The other 15% are intraparenchymal. Of the extraparenchymal type, roughly 70% are saccular, 20% are fusiform, and 10% are dissecting.^[29]

Of patients with RAAs, 20% present with bilateral pathology and 30% have multiple aneurysms.^[15] RAAs occur equally in men and women, although ruptures are more common in reproductive-aged women.

• Asymptomatic, small (< 2 cm diameter) renal artery aneurysms (RAAs) do not usually require treatment.
• One notable exception is a female who is pregnant or contemplating pregnancy. Even small asymptomatic aneurysms should be repaired in this population given the increase risk of rupture.
• Regular follow-up examination with ultrasound or CT scan is recommended in patients who are treated expectantly. Spontaneous cure by thrombosis of small aneurysms has been described.