In this patient what is the likely cause?
An underlying vascular malformation should be suspected.
How would you further assess this patient?
Vascular imaging is required. CT angiography is usually the most easily obtained modality in the acute setting.
Extensive intraventricular blood is demonstrated, filling the fourth ventricle, third ventricle refluxing into the frontal horns of the lateral ventricles. Only a tiny amount is seen pooling dependently within the occipital horns. No convincing subarachnoid blood is identified. Hydrocephalus is present. The brain is otherwise unremarkable in appearance.