Thunderclap headache can be symptomatic of a brain bleed. If this happens, rush to the hospital.

SIR Alex Ferguson is believed to have suffered a subarachnoid haemorrhage, which can be confused for a stroke and causes similar speech loss and paralysis.

Often caused by smoking and uncontrolled high blood pressure, it kills about one in five people immediately.

Another third are left so disabled that they need permanent 24-hour care.

However, between a third and a half of people make a full recovery, returning to a normal quality of life in just a few months.

Ranjeev Bhangoo, a consultant neurosurgeon at King’s College Hospital in London, said: ‘The prognosis all depends on what Sir Alex was like before the bleed happened.’

He added: ‘If he had only a headache and a reasonably good level of consciousness, with no weakness in his arms or legs, the prognosis is reasonably good.

‘If he had greatly diminished consciousness and severe weakness, then the chances of making a good recovery are a lot less.’

The haemorrhage is a bleed between the skull and surface of the brain, which usually starts with a sudden ‘thunderclap’ headache so painful that many people lose consciousness.

© Daily Mail