43-year-old woman Mary presents to hospital after becoming suddenly numb down the R half of her body. She has been otherwise well recently. She has no history of hypertension, hyperlipidaemia, diabetes, smoking, stroke, or heart disease. She has depression for which she takes venlafaxine and sees a psychologist; she reports a couple of episodes of numbness on her R side over the last months, but this lasted minutes and resolved on its own so she did not seek medical assistance. She also suffers from headaches, mostly on the left side of her head, associated with nausea and made worse by bright light; she recalls some terrible episodes preceded by flashing lights in her 20s. These have eased off in recent years. Her father is in a nursing home due to dementia and suffered from severe headaches when Mary was a child. On exam, Mary has loss of all sensory modalities on the R side of her body, with Romberg’s positive; otherwise, neurological exam is normal. A CT brain shows no evidence of acute haemorrhage or infarct, though does show extensive bilateral hypodensities in the subcortical white matter. Bloods are unremarkable. What is the likely diagnosis?