In general, an aneurysm is a localised or diffuse dilatation which is at least 50% greater than the normal size of an artery.
An abdominal aortic aneurysm is a permanent dilation of the abdominal aorta >3cm in diameter.
A true aneurysm is one where the 2 or more layers of the vessel wall forms the wall of the aneurysm (i.e. vessel wall is intact).
A false aneurysm is one where a collection of blood is held close to the vessel by a wall of connective tissue (i.e. contained leak from a vessel).
A fusiform aneurysm is one where the aneurysm is tapered at both ends. A sac-like aneurysm is more rounded.
Background
- The majority (~75-95%) of aortic aneurysms are abdominal and below the level of the renal arteries (~30% also involve the iliac arteries)
- Of those that are thoracic, the majority are either in the ascending or descending sections (rarely found in the arch)
- Occasionally, you can also get thoracoabdominal aneurysms. These can be classed using the Crawford classification
- The prevalence of AAAs is between 1.3-12.7%. AAA frequency increases with age (25/100,000 in 50 year old men vs 78/100,000 in >70 year old men)- around 1 in 20 men >65 increasing to almost 1 in 10 >75
- Male : Female ratio 6:1
Aetiology
- Family history/genetics are becoming increasingly apparent as a causal factor
- Smoking
- Male
- Age
- Hypertension
- COPD
- Heart disease/atherosclerotic disease
- Hyperlipidaemia
- Rarely, other causes e.g. trauma, infection (e.g. TB, HIV); inflammatory diseases (Behcet’s, Takayasu’s); connective tissue diseases (Marfan’s; Ehlers-Danlos type IV)
- Diabetes seems to be protective
- Can be associated with aneurysms elsewhere e.g. popliteal aneurysms
Pathophysiology
- In patients >50, the normal aortic diameter is 1.5cm in women and 1.7cm in men.
- Aneurysms are thought to arise due to failure of major structural proteins of the aorta (elastin and collagen)
- This can be genetic
- Inflammatory processes are also thought to contribute
- Proteases and metalloproteinases may be important, as are interleukins and various cytokines
- Atherosclerosis (degenerative disease) is a common finding in patients with AAA. However, not all patients with severe atherosclerotic disease will develop AAA. The association is not thought to be causative, but there may be a common underlying process in predisposed individuals. Some patients exhibit an inflammatory mechanism whereby vessel wall inflammation causes adhesions to adjacent structures e.g. duodenum, small bowel etc.
- Other causes include
- Mycotic/infective
- Traumatic (more commonly cause false aneurysms)
- Connective tissue disorders e.g. Marfan’s syndrome; Ehler Danlos; Tuberous sclerosis
- Other causes include
Progression
- Size is the best indicator of likelihood of rupture. Note that rate of growth is also a powerful indicator.
- <5cm have a <4% chance per annum
- 5-6cm have a 7% chance
- >6cm have a >20% chance (i.e. exponential increase with size)
- a growth of >10% per year significantly increases risk
Presentation
- Incidental- The majority of patients with an unruptured AAA will be asymptomatic and the diagnosis will be incidental either from
- Physical examination
- Expansile mass (i.e. expands – not just pulsatile as with normal pulses and in false aneurysms)
- Imaging (commonly abdominal USS, but x-ray/CT/MRI also possible)
- Physical examination
- Patients may present with central abdominal pain, back pain, loin pain or pain in the iliac fossa/groin (rarer)
- NB this is more common in inflammatory type aneurysms
- Rarely, AAAs can allow thrombi to form. These can then embolise to the lower limb causing acute ischaemia
- Occasionally patients may have features of compression/obstruction e.g. vomiting (duodenal obstruction); oedema/DVT (IVC obstruction)
- Rupture
- A ruptured TAA can cause sudden onset, severe chest pain (may be similar to MI). This can radiate to the back. This may also deteriorate rapidly.
- A ruptured AAA causes a ‘classical triad’ (few patients have all 3) of
- Back/flank pain
- Hypotension
- Pulsatile abdominal mass
- The patient may also feel nauseous (+/- vomit); light-headed (+/- syncope); pale/sweaty/cold; may also have bruising in flanks
- The pulse may be weak, rapid and ‘thready’.
- Mild cases may present with features similar to renal colic
Investigations
- Abdominal USS is the first line imaging investigation in patients with a stable AAA.
- CT is usually the gold-standard investigation for evaluation of the aneurysm
- Other investigations to consider would include an ECG, CXR, bloods
Management
- Uncomplicated aneurysm
- <5.5cm are generally monitored
- 3-4.4cm annually (USS)
- 4.5-5.4cm 3-monthly (USS)
- >5.5 (if unsuitable for surgery) 3 monthly (USS)
- >5.5cm – consider surgery (particularly for patients with high risk of rupture e.g. large diameter, smokers, female, hypertension, family history, rapid expansion, onset of sinister symptoms/signs e.g. pain/tenderness)
- Endovascular stent-graft repair preferred due to
- avoids open surgery and aortic clamping
- reduced mortality within 4 years
- Endovascular stent-graft repair preferred due to
- <5.5cm are generally monitored
- Ruptured aneurysm
- Surgical emergency
- ABCDE until theatre ready
- 95% overall mortality (50% in theatre)
- Most patients won’t survive (for those who even reach theatre, surgery mortality is 60-80%, more if there are impacting factors e.g. >80yo; shock non-responsive to resuscitation); cardiac arrest prior to/during surgery)
- Open repair
- Indication
- Elective if >5.5cm or if rapidly expanding or if patient presents with pain; all emergency cases (rupture)
- Pre-operative assessment
- BP control
- Smoking cessation
- Co-morbidity/mortality assessment
- Consent
- 5% mortality (significant)
- DVT; Chest infection; wound infection
- Graft infection (~1 in 500)
- Procedure
- Supine position
- Midline incision (usually)
- Retract bowel and duodenum to right
- Divide the posterior peritoneum and dissect the AAA
- Define the proximal and distal neck/extent of AAA
- Administer IV heparin
- Apply distal then proximal aortic clamps (warn anaesthetist) before incising sac
- Oversew arteries e.g. lumbars/IMA if patent
- Indication
AAA Screening
- Offered to all males >65 years old
- Those with small aneurysms are monitored and those with larger ones (>5.5cm) are offered surgery