What is the most likely cause?
Aneurysmal rupture is most likely, although other causes of subarachnoid haemorrhage include trauma, arteriovenous malformation (AVM), dural arteriovenous fistula (DAVF), and intradural arterial dissection.
Where are most aneurysms located?
Roughly 90% are located in the anterior circulation (anterior communicating artery (ACOM), middle cerebral artery bifurcation and supraclinoid internal carotid artery (including PCOM) roughly evenly represented). Roughly 10% are located along the vertebrobasilar circulation (posterior inferior cerebellar artery (PICA) and basilar tip being most common).
What would the next investigation (not shown in this case)?
Arterial imaging of some sort. Usually CTA is easiest to organise and can usually identify the source of haemorrhage and allow for management planning. MRI / MRA can be performed, but access and time constraints render it less useful in this setting. Catheter angiography remains the gold standard.
In addition to blood seen in all the basal cisterns and over the cerebral convexities, it has also refluxed into the ventricular system, visible filling the fourth ventricle (blue arrow) and pooling in the occipital horns (red arrows). The ventricles are distended, most easily appreciated in the temporal horns (yellow arrow).