Aneurysms

Aneurysm: dilatation of any blood vessel.

Classification:

• Size:
  • Defined as 'a permanent localised dilatation of an artery having at least a 50% increase in diameter compared with the expected normal diameter of the artery in question.'
  • 'Physical forces of wall tension are dictated by the absolute radius rather than by any ratio to a standardised normal size.'

• True vs false
  • True: dilatation of the entire vascular wall
  • False: locally contained haematoma resulting from disruption of the vessel (i.e. pseudoaneurysm). The wall is effectively connective tissue formed in reaction to the contained haematoma. Aetiology - vascular trauma or peri-anastomosis.
  • Note that in many cases it doesn't really matter whether it's true or false in determining treatment, except in the case of pseudoaneurysms formed as a result of vascular interventions, which tend to have a narrow neck, amenable to injectable thrombotics.
• Location and extent
  • Ectasia - Enlarged, but <50% greater than normal.
  • Arteriomegaly - diffuse, continuous enlargement of multiple arterial segments, dilated to >50% of normal. This is a descriptive term, rather than a specific diagnosis.
  • Aneurysmosis - multiple aneurysms at several different locations, or the combination of aneurysmal degeneration in the setting of arteriomegaly.
• Morphology
  • Fusiform - generalised increase in the entire diameter of the affected vessel. More often seen with true degenerative aneurysms.
  • Saccular - localised, often eccentric in shape. Can arise from a focal ulcer or weakness in the arterial wall due to trauma or infection. Can be eccentric (pseudoaneurysms, renal, intracranial/cerebral aneurysms) or concentric. Saccular morphology aneurysms have less well-characterised risks, and therefore may be seen as an indication for intervention at a lower threshold than fusiform aneurysms. Saccular are often easier to treat endovascularly because of their focal nature.

Aetiology

• Degenerative (i.e. atherosclerotic)
  • There is an unproved causal relationship between aneurysms and typical atherosclerosis, and it is probably a complex relationship. However, they are seen in the same patient groups.
  • Factors a/w degenerative:
    • Metalloproteinases in the media of specimens
  • Intensity and duration of cigarette smoking is the most significant risk factor
• Inflammatory
  • Aneurysms with an exaggerated inflammatory component, inciting a fibrotic reaction around the aneurysm
  • Infra-renal aorta most often affected by this process
  • Seen on imaging as a thick inflammatory 'rind'
  • Tough to dissect from surrounding structures
  • Associated inflammatory conditions: Takayasu arteritis, GCA, polyarteritis nodosa, Behcet disease, Cogan syndrome, cystic medial necrosis
• Aneurysms associated with arterial dissection
  • Spontaneous tear of the intima of an artery and subsequent propagation of that tear along the anatomic plane within the media
  • Driven by acute arterial distension and arterial pressure
  • Occurs most often in the aorta, but can also occur in peripheral locations
• Traumatic
  • Pseudoaneurysms
• Developmental and congenital abnormalities
  • Highly variable depending on the underlying defect
• Infectious
  • Primary infection of the arterial wall (resulting from haematogenous seeding or extension of an adjacent infectious process) resulting in an area of weakness and then aneurysm
  • Most primary infective aneurysms have an eccentric saccular shape
  • Wide variety of organisms including bacteria, fungi, TB and syphilis have been implicated
  • Can occur in essentially any vessel
  • Risk factors: drug injection, immunosuppressed
  • Repair is difficult because prosthetic material is contraindicated and there are limited durable autogenous agents available, and the open approach is also more difficult due to the friable tissue.

Risk factors

• Family history - 30% increased risk of having an aneurysm
• Connective tissue disorders - Ehlers-Danlos, Marfan, Loeys-Dietz
• Other aneurysms
• Men
• Smokers
• Atherosclerotic disease
• History of MI
• PVD
• HTN

Cerebrovascular

• Aetiology
  • Dissection can lead to late aneurysmal degeneration in ICA and VA
• Distribution
  • Extracranial cerebral arteries are rarely affect by aneurysmal degeneration
  • Can occur in ICA, CCA and ECA (in that order of frequency)

Upper extremity

• Aetiology
  • Repetitive trauma a/w thoracic outlet syndrome (subclavian artery)
  • Iatrogenic brachial artery pseudoaneurysms

Aortic

• Aetiology
  • Ascending aorta - typically degenerative or the sequela of prior dissection
  • Infra-renal:
• Distribution:
  • Majority in infra-renal segment - 30% of all aortic aneurysms
  • 1% of men 55-64 have a AAA > 4cm, which increases with advancing age by 2-4% per decade
• Natural history
  • Natural history is of progressive enlargement (2-3mm per year) leading to increased risk of rupture
• Rupture risk:
  • Maximal AAA diameter is the standard basis
  • RFs for rupture: female, larger initial AAA diameter, smoking, lower FEV1, higher mean BP. Saccular morphology.
  • Imaging characteristics for increased risk of rupture: dissection, mural thrombus, dissection of the peripheral calcification of the aneurysm sac.
  • Risk of rupture declines when smoking is ceased and HTN is controlled.
• Diagnosis
  • History
    • Typically asymptomatic
    • Can get NSAP and/or lower back pain
    • Sometimes feel pulsations
    • Can sometimes see a pulsatile mass
  • Examination
    • Pulsatile mass, most commonly supra-umbilical and in the midline, but the location is variable
    • Remember to check for POPA and femoral artery too
  • Imaging
    • USS has excellent sensitivity and specificity - good for screening and surveillance. Not an ideal method for detecting rupture (sensitivity might be lower than 50%).
    • CT - better reproducibility of diameter measurements than USS. Best pre-op preparation.
    • MRI - good, but does not detect aortic wall calcification, which can be important. Only really useful in inflammatory aneurysms where it helps show the extent.
    • Angiography - only used where detailed characterisation of an aneurysm is required or for intervention, or in the case or pre-operative embolisation of an accessory renal artery prior to EVAR.
• Screening:
  • See bottom of this page for full table
  • Rough consensus:
    • One-off USS screening for all men at age 65
    • Screen first-degree relatives of those with AAA once they reach 50 years old
    • Selective screening for those at high risk (women >65 who smoke, have CVA or FHx
    • Generally, those who screen negative initially are at very low risk of developing a AAA later
• Surveillance
  • Those with infra-renal AAA 4-5.4cm should have an USS every 6-12 months
• Medical therapy
  • Lots of things (ACE inhibitors, beta blockers, statins, anti-platelets) have slowed AAA growth in mice, but none have worked in humans to date
• Indications for repair
  • Consider: operative risk (use VSGNE risk index), rupture risk, morbidity due to rupture
  • Symptomatic
    • Thrombosis
    • Embolization of mural debris - lower extremity ischaemia/trash foot syndrome
    • Compression of adjacent organs
    • Aortic dissection
    • Rapid expansion/impending rupture
    • Frank rupture - haemodynamic instability
      • Consider direct to theatre. Alternative is rapid resus with permissive hypotension and rapid CTA
      • CT: heterogeneous mural thrombus, loss of fat planes around aorta, periaortic inflammation, retroperitoneal haematoma
  • Asymptomatic
    • Aneurysm diameter >5.5cm in men and 5.0cm in women (UK Small Aneurysm Trial showed very low risk of rupture for <5cm: 2.5% annually)
      • Use risk factors for rupture (above) to personalise decision
      • The real issue is timing rather than eventual necessity of repair - over 80% of patients in the ADAM trial with aneurysms 5-5.4cm EVENTUALLY underwent repair
      • However earlier repair (<5.5cm) provides no survival benefit compared with surveillance and subsequent repair once it gets >5.5cm
    • 12-month growth rate of 10mm or greater in either gender
    • Saccular aneurysms
    • Dissection of mural thrombus
    • Fracture of saccular calcification
• Surgery
  • See 'Open AAA repair' page for discussion on choice between open vs EVAR

Visceral

• Aetiology
• Distribution
  • Splenic artery most common (60% of all visceral). Occur more frequently in women and portal HTN. Pregnancy is a risk factor for rupture.
• Splenic artery aneurysm
  • Focal dilation >150% of normal vessel diameter
  • Can be true or false (different pathophysiology)
  • Same RFs as for AAA
  • Usually saccular, usually seen in middle or distal third of splenic artery
  • Intervene for size >2cm, symptomatic, women of childbearing years (due to high pregnancy rupture rate), cirrhosis planning to undergo transplant or shunt
  • Surgery:
    • Resection and primary anastomosis/bypass for mid-artery aneurysms
    • Resection and splenectomy for distal aneurysms
  • Endovascular:
    • Covered stent
    • Coiling
    • Liquid embolic agents

Renal

• Aetiology
  • Renal circulatory bed has low resistance and high flow, contributing for the frequency of aneurysms here
  • Often saccular
  • A/w fibromuscular dysplasia, and can also result from the full spectrum of other aetiologies
• Natural history
  • Rate of rupture is low, but is elevated in third trimester, with resulting maternal and fetal mortality rates of 70% and 100% respectively.
• Treatment
  • Indications for repair:
    • Size greater than 2-3cm
    • Lesions occurring in women of child-bearing age
    • Refractory HTN
    • Flank pain
    • Haematuria
  • Treatment approach
    • Both endovascular and open approaches play a role

Iliac

• Aetiology:
  • Most are degenerative
  • Traumatic also occurs
  • A/w vascultides
• Distribution:
  • Most frequently CIA (70%), often in the presence of concomitant infra-renal AAA (89% !)
  • Next IIA - more commonly occur in isolation. Mortality 30%, present with rupture in 40%. Hard to control surgically when ruptured.
  • EIA - rare - usually traumatic or non-degenerative.
• Natural history:
  • One study showed no ruptures at less than 3.8cm and median expansion rate of 0.29cm in CIA aneurysms

Femoral

• Aetiology:
  • Pseudoaneurysms are common
  • Infected pseudoaneurysms are high-risk from IVDU
• Distribution:
  • 26% are bilateral
  • 88% have a synchronous aneurysm (85% have a AAA)

Popliteal

• Aetiology
• Distribution
  • Most common peripheral arterial aneurysm
  • Frequently bilateral
  • Frequently a/w aortic aneurysms - 62% of those with a popliteal aneurysm also have a AAA
• Natural history:
  • Primary morbidity is acute thrombosis or embolisation with resulting ischaemia and potential limb loss