This is localised dilation of an artery within the brain. They rarely occur in veins. They are a major risk factor for subarachnoid haemorrhage.

Epidemiology & Aetiology

Occur in 5% of the population
Risk factors include:



  • Most commonly occur at the circle of Willis, but do occur at other sites
  • They are not usually circular, but instead made up of many small sacs, hence the name berry aneurysms
  • Usually <2.5cm in diameter

Signs and Symptoms

  • Vast majority are asymptomatic
  • Occasionally might compress a local structures, typically the optic chiasm and the  III, IV, V and VI cranial nerves
  • Aneurysms themselves don’t cause headaches but they may cause subarachnoid haemorrhage, and in 6% of these cases, there may be a preceding headache, thought to be due to a small bleed of the aneurysm before rupture.


Is usually made with MR/CT angiography., and is often discovered incidentally.


  • <7mm risk of rupture is low, and these are not treated, unless causing a local compression problem
  • >7mm they should be treated, with surgical clipping, platinum coiling or less commonly, stenting and balloon treatments.
    • Clipping –  has a lower failure rate, but greater risk of intracranial bleed / mortality as well as post-operative epilepsy
    • Stenting – slightly higher failure rate but safer. A catheter is passed up through the femoral artery. A platinum coil (or several) is left in the aneurysm which causes thrombus of the aneurysm, eliminating it.
    • To coil or clip? – is a decision for the consultants and is usually considered on a case by case basis. Recurrences have better outcomes if clipped.

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