A 58-year-old man with uncontrolled hypertension presents with sudden-onset severe headache, vomiting, and right-sided hemiparesis. On examination, he has a Glasgow Coma Scale of 12 and neck stiffness. Non-contrast CT head shows a hyperdense lesion in the left basal ganglia with intraventricular extension. What is the most appropriate next investigation to assess prognosis and guide management?