A bihemispheric anterior cerebral artery (ACA) is an anatomical variant with a hypoplastic A2 segment and dominant contralateral A2 segment supplying both ACA territories. The incidence is ~4.5% (range 2-7%).

Distribution of subarachnoid hemorrhage (SAH) according to location ¹:

• ACoA aneurysms: superiorly into the intrahemispheric fissure
• MCA bifurcation: Sylvian fissure
• internal carotid-posterior communicating artery (ICA-PCoA): suprasellar cistern
• vertebrobasilar: fourth ventricle, prepontine cistern, foramen of magnum

Modified Fisher scale (SAH grading) ¹:

1. equates to no visible SAH or intraventricular hemorrhage (IVH)
2. focal or diffuse thin (<1mm) layer of subarachnoid blood without IVH
3. IVH
4. focal or diffuse thick (>1mm) SAH without IVH
5. IVH with thick SAH