Hi, Thanks for writing in. Fainting all of a sudden in the elderly can be due to low blood glucose (hypoglycaemia) if the brain MRI is normal. Rarely it can be due to vasovagal syncope, a condition where there is sudden blackout due to stimulation of the vagal nerve. Such an incident can also occur due to vertebral basilar insufficiency, where there is reduced blood supply to the posterior brain through the vertebral arteries leading to fainting spells. It is important that she gets tested in detail for any low blood glucose levels and also a carotid and vertebral artery Doppler to study blood supply to brain.