Neurologymiddle twist
A 41-year-old woman presents nine hours after the onset of a severe headache that struck like a blow to the back of the head while she was lifting weights and reached its worst within a minute. She has vomited once, and the pain has only partially eased with paracetamol. She has never had a similar headache before. Neurological examination, including fundoscopy, is normal. Non-contrast CT of the brain, reported by a radiologist, is normal. Which one of the following is the most appropriate next step in investigation?