Aneurysms and AV fistulas

Aneurysms

Definition:
An aneurysm is a localized, permanent dilatation of an artery greater than 1.5 times its normal diameter. Aneurysms occur all over the body and in any vessels, including the aorta, and the iliac, femoral, popliteal, subclavian, axillary and carotid arteries.

Classification of aneurysms:

Aneurysms can be grouped according to their:
I- Wall:
True aneurysm: containing all 3 layers of an arterial wall (intima, media, and adventitia)
False aneurysms: having a single layer of fibrous tissue as the wall of the sac
II- Morphology (shape):
Fusiform: when involving the total circumference of the artery.
Saccular: when arising from a distinct portion of the wall
Dissecting:
III- Etiology:
Atherosclerotic
Mycotic (infectious due to bacteria rather than fungi)
Collagen vascular disease
Traumatic
Post-stenotic
The majority are true fusiform atherosclerotic aneurysms


Clinical features:
Aneurysms may be asymptomatic and discovered accidentally on routine examination as in most cases of abdominal aortic aneurysms.
Symptoms tend to arise from:
Aneurysm expansion and pressure on nearby structures like nerves, veins, arteries, bone, skin,…etc. e.g.; chronic backache in patients with AAA (abdominal Aortic Aneurysm).
Aneurysm thrombosis leading to ischemia of the organ or tissue supplied by that artery e.g. thrombosis of a splenic aneurysm may lead to splenic infarction. Thrombosis of a popletial aneurysm may lead to gangrene in the foot.
Aneurysm embolization: the sac of the aneurysm usually contains thrombi, which if embolize will lodge in distal arteries and lead to ischemia, e.g. blue toe syndrome due to abdominal or thoracic aortic aneurysm emboliztion.
Note: blue toe syndrome is gangrene of one or both big toes due to multiple embolizations from an aortic aneurysm (thoracic or abdominal)
Aneurysm rupture: leading to internal or external bleeding depending on the artery involved and the site of rupture. E.g. ruptured abdominal aortic aneurysm leads to a usually fatal retroperitoneal or intraperitoneal hemorrhage. Rupture of middle cerebral artery aneurysm leading to subarachanoid heamorrhage.
Most aneurysms of clinical significance can be palpated and typically an expansile pulsation is felt. Transmitted pulsation through a mass lesion, cyst or abscess lying adjacent to a large artery may be mistaken for aneurysmal pulsation. Similarly before incising a swelling believed to be an abscess it is essential to make sure that it does not pulsate. Finally a tortous artery usually the innominate or carotid may seem like an aneurysm to the inexperienced eye.

Investigations:

Duplex ultrasound
Angiography
CT and CT angiography
MRI and MRA

Treatment:

Depending on the involved artery, presentation and patients general condition, aneurysms can be treated by one of the following methods:
Aneurysm excision with graft interposition (treatment of choice) as in most cases of true major artery aneurysms. E.g. popliteal artery aneurysm, subclavian artery aneurysm,…etc. Here the aneurysm with the involved segment of artery is excised
Aneurysm repair: excision of the sac with repair of the artery as in most cases of false aneurysm repair. E.g. traumatic false aneurysm of descending thoracic aorta, femoral artery, popliteal artery,…etc
Aneurysm excision with resection of supplying tissue. E.g. excision of a splenic artery aneurysm with splenectomy, excision of a renal artery aneurysm with partial or total nephrectomy.
Aneurysm excision without arterial reconstruction proving that the blood supply to the distal tissue is unaffected. E.g. excision of a radial artery aneurysm in the presence of a normal and functioning ulnar artery.
Endovascular aneurysmal repair


Abdominal Aortic Aneurysm (AAA):
Abdominal aortic aneurysm is by far the commonest type of large vessel aneurysm and is found in 2% of the population at autopsy; 95% due to atheromatous degeneration and 95% occur below the renal arteries.

Etiology:

Atherosclerosis.
Syphilis.
Marfan syndrome.
Trauma.
Mycotic.
In most cases the etiology of aneurysm formation is not well understood.

Clinical features:

Asymptomatic: acounts for 75% of cases discovered accidently on clinical examination, radiology or ultrasonography done for some other reason.
Symptomatic: symptoms are usually caused by pressure on adjacent structures, distal embolization, dissection, thrombosis, or rupture. Symptoms include
Chronic vague abdominal or back pain in one third of patients
Severe back pain with abscence of rupture may occur due to errosion of the aneurysm into the spinal column or due to sudden expansion or dissection.
Gastrointestinal, urinary and venous symptoms may arise due pressure effect of the aneurysm on these structures
Symptoms of lower limb ischemia due to aneurysm thrombosis and/or embolization including blue toe syndrome
Aneurysm rupture: mid abdominal, flank or back pain which may radiate to the groin and thighs which is usually severe, constant and unaffected by position. The severity of hypotension varies from mild to profound as the duration of symptoms from few minutes to more than 24 hours

Natural history of AAA:

The natural history of AAA is to grow and eventually rupture. The average rate of aneurysm growth is 0.4 Cm per year with a more rapid growth seen as the aneurysm enlarges. And the risk of aneurysm rupture is directly related to aneurysm size.
The risk of rupture is significantly increased with
Larger aneurysm diameter
Elevated arterial blood pressure
Low FEV1
Chronic cough
COPD
Smoking
It is said that without surgery, 80% of those with a symptomatic aneurysm will be dead in a year; with surgery, 80% will be alive.
AAA ruptures posteriorly into the retroperitoneal space (80%) or anteriorly into the peritoneal space (20%). Less commonly the rupture may occur into the gut or vena cava. Less than 50% of patients with rupture survive to reach hospital. Anterior rupture result in free bleeding into the peritoneal cavity and very few patients reach hospital alive. Posterior rupture on the other hand produces a retroperitoneal hematoma, and the resulting hypotension and resistance of retroperitoneal tissue temporary arrest further hemorrhage maintaining the life of the patient for a brief period and if surgery is not undertaken death will be inevitable.


Investigations
Confirmative investigations include:
Ultrasonography: its accuracy, safety and low cost has made it the diagnostic test of choice for diagnosis, screening and follow up of patients with AAA
Plain chest X-ray: calcification in the wall of the aneurysm will make it visible of X-ray. However the size cannot be determined and a normal radiograph will not exclude the presence of aneurysm
CT scan: computed tomography is the optimal imaging modality prior to planned AAA repair. It can identify its actual size, extent, involvement of renal, iliac and superior mesenteric arteries, presence of thrombus, aneurysm inflammation, leaking or rupture, and helps surgical planning.
MRA: no need for nephrotoxic contrast but still less informative than angiography and CT.
Angiography: not reliable for determining the diameter or extent of aneurysm but helpful in providing preoperative information about associated arterial disease involving renal, visceral and distal vessels. CT and MRI are increasingly taking over the use of angio.

Treatment:

The only curative treatment of AAA is surgery however it is not always indicated
Indication of surgery in AAA
Ruptured AAA, regardless of its size or cause is a surgical emergency
Symptomatic AAA regardless of its size
AAA 55 mm in diameter or more even if asymptomatic because of high risk of rupture.
Rapidly enlarging aneurysm (more than 5 mm per year)
Complicated AAA e.g., thrombosis, embolism, fistula formation, pressure on nearby structures, ...etc.
Atypical aneurysm, e.g., dissecting, false, mycotic ...etc

Surgical options:

Open surgical repair:
Under general anesthesia, through a midline or supraumbilical transverse incision. Upper and lower extent of the aneurysm identified, then the aneurysm opened, excised, and synthetic graft sutured between the two ends.
Complications of open or classical repair:
bleeding and/or anastomatic pseudoaneurysm formation
renal failure
lower limb ischemia due to thrombosis, dissection or embolization
injury to the ureter
ischemia to the spinal cord leading to paraplegia
ischemic colitis
graft infection


Endovascular aneurysmal repair (EVAR):
The principle of endovascular repair of AAA involves the transluminal implantation of an aortic stent graft that is fixed proximally and distally to the non-aneurysmal aortoiliac segment and thereby endoluminally exclude the aneurysm from the circulation.
Advantages of EVAR:
decreased operative time
less blood loss
shorter hospital stay
done under regional or local anesthesia
better for patients who are at increased risk for surgery because of comorbidity.
Disadvantages:
does not remove the sac of the aneurysm
risk of stent thrombosis
risk of renal artery occlusion due to stent malpositioning or migration
endoleak which is defined as persistence of blood flow outside the graft and within the aneurysmal sac.

Arteriovenous fistula

Definition:
Abnormal direct communication between the arterial and venous system that bypasses the capillary bed.

Etiology:

Congenital: also called arteriovenous malformations or hemangiomas, here there may be no direct communication between the arterial and venous system but abnormality of developement of certain arteries or veins.
Acquired which may be
Traumatic post penetrating injuries
Iatragenic post diagnostic or therapeutic procedures e.g., catheterization, percutaneous biopsy, embolectomy, mass ligation of artery and vein after splenectomy, nephrectomy or amputation.
Neoplastic e.g., hypernephromas and metastasis from thyroid carcinomas
Spontaneous as a complication of atherosclerotic or mycotic aneurysms e.g., aorto-caval fistula
Surgically created for hemodialysis, venous thrombosis, or to improve patency of arterial or venous bypass procedures.


Clinical presentation:
Congenital AV fistulas:
Many congenital AV malformations are disturbing solely because of their cosmetic appearance which may range from innocent looking varicose veins or simple skin discoloration to ulcerated, bleeding or pulsatile masses.
Others may be distributed into important organs and cause various symptoms from pressure effects on nearby structures like nerves, bones, …etc.,
Bleeding from the AV malformation which may be either internal and sometimes after trivial trauma,
Thrombosis or thrombophlebitis
Acquired AV fistula
These usually present as a painless pulsatile mass. A thrill is detected on palpation and auscultation reveals a loud continuous bruit (machinery murmur). An acquired AV fistula may cause:
Distal limb ischemia due blood stealing phenomenon as the blood is being redirected to the vein making less blood available for the tissue distal to the fistula. Leading to ischemic ulcers and gangrene.
Dilated tortuous veins in the area (varicose veins)
Limb edema with pain and discomfort due to the chronic venous hypertension
Increased venous return to the heart leading to increase work load on the heart and eventually heart failure
Aneurismal dilatation leading to what is called an "aneurismal fistula"
A chronic fistula in a limb of a growing child may affect the growth of that limb.

Investigations:

Duplex ultrasound, angiography, CT, MRA.

Treatment:

Congenital AV malformations may be difficult to treat needing staged operation, percutaneous embolization, multidisciplinary teams. And even so results may not always be satisfactory.
Acquired AV fistulas are usually easier to treat as a site of communication can be identified and treatment includes:
percutaneous embolization of the fistula,
excision with or without vascular reconstruction, or
4 limb ligation.