Definitions

An aneurysm is a localised dilatation of an artery by greater than 50% of the original diameter.

An increase of arterial diameter by less than 50% is termed ‘ectasia’.

Aneurysms can be either true or false:

• A true aneurysm is a localised dilatation of an artery including all three layers (intima, media and adventitia).
  
• A false aneurysm (pseudoaneurysm) is a contained sac containing flowing arterial blood communicating with the lumen of the artery through a defect in its wall. It is usually surrounded by haematoma and contained by surrounding tissues. It doesn’t have any arterial wall layers.

True Aneurysms

True aneurysms can also be divided into type:

• Fusiform:  Wide in the middle and tapers at both ends, classically an abdominal aortic aneurysm.
  
• Saccular:  May be almost spherical and projects from one point on the arterial wall; classically an intracerebral aneurysm, but still contains all three wall layers.