Fig 2: Perimesencephalic haemorrhage: Pattern of blood confined to the midbrain cisterns with no evidence of intraventricular or intracerebral haemorrhage (click to enlarge)
This group accounts for the remaining 20% of cases of SAH.
Of these, over half are due to perimesencephalic haemorrhage. This pattern of SAH, in which extravasated blood is confined to the cisterns around the midbrain, was first described in 1985 .
The condition is defined by this characteristic distribution of blood on CT scan (Fig 2) and a normal angiographic study.
Unlike all other forms of SAH, the clinical outcome in perimesencephalic haemorrhage is invariably good and no specific treatment is needed .
10-15% of subarachnoid haemorrhages are due to perimesencephalic haemorrhage. In contrast to aneurismal bleeds the outcome is invariably good [19-20].
The remainder of SAH cases are due to vascular rarities such as arterio-venous malformations (AVMs), dissections, tumours and vasculitides .