This article is part of the operation script for grafting an abdominal aortic aneurysm. The article can be used on its own, or, to go to the next part of the operation, click on NEXT PART OF THE OPERATION at the bottom of this page.
AORTIC ANEURYSM 050 - ANATOMY REVIEW
Subrenal Aorta
Aneurysms most commonly occur in the aorta below the renal arteries.
In health, this section of the aorta is only 5 - 6cm. long and 16-20 mm. in diameter.
Surface markings of the subrenal aorta
The renal arteries run laterally from the aorta at the level of the xiphisternum. The right renal artery is usually 5mm. or more lower than the left.
The aorta bifurcates at the level of the umbilicus. This makes good access to the upper abdomen essential for grafting the aorta.
Relations of the subrenal aorta
A clear knowledge of the anatomy of the subrenal aorta between the renal arteries and the upper end of an aortic aneurysm is the key to successful aneurysm surgery.
This section of aorta is surrounded behind, on both sides, above, behind and in front by large, interconnecting and fragile veins.
Posterior relations of the subrenal aorta
The subrenal aorta lies on the front of the bodies of the 2nd to 4th lumbar vertebral bodies, sharing this position with the vena cava on its right.
• The subrenal aorta lies behind the peritoneum of the posterior abdominal wall,
- As a result of the lordosis of the lumbar vertebrae, it may be only 5cm. posteriorto the anterior abdominal wall.
• The aorta lies on the anterior longitudinal ligament of the vertebral column.
• The aorta gives off pairs of lumbar arteries which contribute to the blood supply of the spinal cord.
- There are usually adequate anastomotic channels in the spine to sustain a blood supply to the spinal cord if the lumbar arteries are stitched off during aortic surgery.
• Fragile lumbar veins run behind the aorta to drain into the vena cave.
• Very rarely the left renal vein runs behind the aorta to the vena cava and may be double.
Anterior relations of the subrenal aorta
• The upper half of the subrenal aorta is crossed from anterior to posterior by :
- The transverse colon.
- The fourth part of the duodenum.
- The left renal vein.
- These structures are met in this sequence when exposing the aorta.
• The lower half of the subrenal aorta lies immediately behind the peritoneum.
- The mesentery of the small bowel lies on the right and the vessels of the descending colon on the left.
- This is a convenient route for access to the aorta.
Right lateral relations of the aorta
• The right side of the aorta is closely attached to the vena cava.
- Dissection of the aorta above an aneurysm may tear the vena cava.
- An aneurysm may erode into the vena cave, producing a lethal arteriovenous fistula.
Left lateral relations of the aorta
• The inferior mesenteric artery arises from the left antero-lateral side of the subrenal aorta.
- It continues downwards in the pelvis as the superior rectal artery. It supplies blood to the large bowel.
- It anastomoses with branches of the superior mesenteric artery above and the middle and inferior rectal arteries below.
- These anastomoses can usually sustain the blood supply to the large bowel if the inferior
- mesenteric artery is tied off during aortic surgery.
• The inferior mesenteric vein accompanies the artery in the lower subrenal area.
- It then parts company and runs up to the left of the duodenum.
- It finally turns right to run into the portal vein.
- It can cause bleeding if damaged during the dissection of the aorta, but can be tied
- off without harming the large bowel.
• Paired gonadal arteries arise from the front of the aorta between the origins of the renal and inferior mesenteric arteries, but rapidly run laterally.
- The left gonadal artery runs down the left side of the aorta, behind the mesenteric ::vessels, but in front of the left ureter.
• If the gonadal arteries are occluded by a subrenal aneurysm, as is generally the case, the gonads obtain an adequate blood supply from local anastomoses.
• The left gonadal vein drains into the left renal vein.
- Damage at the junction of these vessels during dissection of the upper end of an
aneurysm can cause serious bleeding.
• The left side of the subrenal aorta is related to the inferior mesenteric artey and vein, the left gonadal vessels behind and the left ureter behind them.
Surgical approach to the upper subrenal aorta.
• The anatomy of the section of aorta just below the renal arteries and above the aneurysm is of particular importance.
• This is the site of the upper anastomosis with the graft.
• If this section of aorta is aneurysmal itself, it will contraindicate an operation.
• The section is surrounded on three sides, (ie on the left, cephalically and on the right ) by bowel superficially and by veins more deeply.
- Damage to any of these surrounding structures may lead to a fatal outcome to the operation.
• The bowel on the left is the descending colon, cephalically the transverse colon overlying the 4th part of the duodenum, and on the right, the small bowel.
• The vein on the left is the left gonadal vein, running into the left renal vein cephalically, which in turn runs into the vena cava on the right side.
• There are also lumbar veins posteriorly and the inferior mesenteric vein
anteriorly.
Autonomic nerves and plexuses
• The autonomic nerve supply to the pelvic organs (genito-urinary derivatives of the hindgut) forms a series of interconnecting plexuses related to the aorta and major arteries.
- The plexuses contain both parasympathetic and sympathetic fibres.
- The fibres also supply the lower bowel up as far as the middle of the transverse colon.
• Damaging this autonomic nerve supply can cause debilitating urinary
retention, loss of control of micturition, penile erection (parasympathetic) and ejaculation (sympathetic).
- Damage to the autonomic nerve supply to the lower bowel does not appear to be
- harmful.
• Most of the sympathetic nerve fibres come down to these plexuses directly from the T12, L1 , L2 sympathetic ganglia on the sides of the relevant vertebral bodies, behind and lateral to the aorta and vena cava.
• Parasympathetic nerve fibres come from spinal levels S1,2,3,4 via pelvic splanchnic nerves (sacral nerves, or nervi erigentes - arousal nerves).
• The aortic plexus lies on the front of the lower aorta, connected to the coeliac plexus higher up the aorta.
- The aortic plexus is connected to the midline hypogastric plexus (presacral nerve or lumbar splanchnics).
- The hypogastric plexus lies on the front of the 5th lumbar vertebra and upper sacrum.
• The hyogastric plexus divides into left and right hypogastric nerves,
which run down to the pelvic plexuses on the inner wall of the pelvis.
• The hypogastric nerves and plexuses are joined on their medial aspects by the parasympathetic pelvic splanchnic nerves.
• In aortic surgery, the aortic plexus (on the front of aorta)`and the hypogastric plexus
(on the front of the sacrum) and hypogastric nerves (on the lateral pelvic wall) are mainly at risk.
- The enlargement of the wall of the aorta may damage the aortic plexus.
- Incision and particularly transection may be damaging.
- The patient should be warned of this happening before the operation starts.
- At an elective operation, identification and preservation of the hypogastric plexus and
- nerves are usually possible.
- Infiltration of blood into the tissues from a leaking aneurysm usually prevents such identification.
Iliac and femoral arteries
• The vessels are prone to anatomical variation as well as to the changes of vascular disease.
• The aorta bifurcates into the left and right common iliac arteries over the 4th lumbar vertebra (behind the umbilicus).
• The common iliac arteries bifurcate into the internal and external iliac arteries over the sacro-iliac joints.
• The internal iliac arteries run into the depths of the pelvis to supply the pelvic organs, the lower bowel, the pelvic wall, soft tissues and skin of the pelvis.
• The external iliac arteries run round the pelvic brim to pass under the inguinal ligaments.
- There, they continue as the common femoral arteries, which, in health, are 5-6mm. diameter.
• The combined length of a common and an external iliac artery is about 15cm.
- The two index fingers are long enough to tunnel a path alongside the arteries for the limbs of a trouser graft.
- The curved course of the vessels means that the fingers meet at right angles.
- The fingers should keep to the lateral side of the arteries to avoid damage to the iliac and femoral veins running medially to the arteries.
• The common femoral artery continues as the superficial femoral artery, after the deep (profunda femoris) artery has branched off.
- This is usually 3-5 cm. below the inguinal ligament.
• The profunda artery arise from the lateral side of the common femoral artery.
- It then runs deeply behind the superficial femoral artery into the adductor muscles on the medial side of the thigh.
- It is closely related to one or more deep femoral veins, which may need to be ligated for a satisfactory exposure of the deep femoral artery.
• NB Do not confuse these veins, which accompany the profunda artery,
with the femoral vein which accompanies the superficial femoral artery.
• The medial and lateral circumflex arteries, running into the thigh muscles, arise from the profunda artery or sometimes directly from the common femoral artery.
• In addition to controlling the 3 main femoral arteries (common, superficial and deep) when preparing for a femoral arteriotomy, minor arterial branches may need to be controlled nearby.
- They are the superficial epigastric, the superficial and deep external pudendal and the ::deep pudendal arteries.
• The superficial femoral artery runs down the medial aspect of the thigh.
- It passes through the adductor muscles to the back of the knee (popliteal fossa) via the adductor canal.
• The superficial femoral artery then continues as the popliteal artery.
- In the popliteal fossa, it divides into 3 terminal branches (trifurcation):
- The anterior and posterior tibial and peroneal arteries.
Iliac and femoral veins.
• The iliac veins are closely related to the corresponding arteries. The veins are easily damaged during dissection of the arteries.
- Mass temporary clamping of the common iliac vessels is commonly performed as an alternative to dissection when controlling the iliac arteries.
• All the femoral and iliac veins are to be found medial to their corresponding arteries with one exception.
• The exception is the right common iliac vein, which passes laterally and behind the right common iliac artery.
• The right common iliac vein is joined by the left common iliac vein behind the right common iliac artery to form the inferior vena cava.
• The long saphenous vein and its tributaries (which correspond to the minor arterial branches), may need ligation for a satisfactory exposure of the femoral arteries.
• The femoral vein may be confused with the superficial femoral artery during dissection of the femoral area.
- The femoral vein is bluish-white and does not have vasa vasorum running across its surface.
Ureters
• The ureters run from the hila of the kidneys, behind the renal veins and arteries.
• The left ureter runs down the posterior abdominal wall 1-2cm. lateral to the aorta, behind the inferior mesenteric vessels and the gonadal vessels.
- It can be at risk here with an aneurysm larger than 5cm. or in a retroperitoneal haematoma.
• The left ureter runs over the left iliac vessels at the pelvic brim, covered by the sigmoid colon.
- It then runs down the lateral wall of the pelvis to the bladder.
• The right ureter is separated from the aorta by the vena cava.
• The ureters usually cross the common iliac arteries where they are bifurcating into the internal and external iliac arteries.
- They are particularly at risk at this point during aortic surgery.
- They then run round the inner wall of the pelvis to the bladder where they are related
- to the autonomic pelvic plexuses.
• The ureters are pale pink with longitudinal vessels.
- They vermiculate as they propel urine to the bladder or if gently squeezed with forceps.
Horseshoe kidney
• Horseshoe or pelvic kidney is a congenital abnormality occuring in less than 1 in 200 cases of aortic aneurysm.
- The diagnosis should be made on scanning during the work up.
• The left and right kidneys are fused together by their lower poles.
- This will usually block access to the aorta from the abdomen, necessitating a retroperitoneal approach.
• The ureters run anteriorly from horseshoe kidneys to the bladder.
• The blood supply to the kidneys dictates the operative procedure.
- There may be multiple renal arteries arising from abnormal sites such as the subrenal
- aorta or iliac arteries.
• Reimplantation of renal vessels into the aortic graft may be necessary.
• Successful stenting has been recorded.
• Sometimes the 2 halves of a horseshoe kidney just consists of a fibrous band in the midline.
•Simple division of the fibrous band may be all that is required to obtain access to an aortic aneurysm.
Further reading
http://www.veithsymposium.org/pdf2004/122.pdf
Current optimal management of abdominal aortic aneurysms with a horseshoe kidney. Michael Horrocks MS FRCS Bath, UK Hoballah JJ, et al Aortic aneurysm rupture into a retroaortic left renal vein. Ann Vasc Surg. 1993 Jul ;7(4) :363-7.
