An 18 year old female with epilepsy presents with status epilepticus requiring anaesthesia and invasive ventilation. Despite sedation with midazolam infusion, there was still evidence of seizures on EEG. Her usual doses of levetiracetam and sodium valproate were increased. Lacosamide and subsequently phenobarbitone were added after which the seizures eventually terminated. She is allergic to phenytoin. Initial CT head and CSF analysis were normal. Sedation was ceased with no evidence of further fitting. 5 days after stopping phenobarbitone, her GCS remained 3. What is the next appropriate step to take?