I think you will find that it is all to do with the lower cabin pressure, weakened blood vessel and probable hypertension. The effect of the hypertension will be increased in the lower cabin pressure effectively increasing the pressure within the blood vessel thus increasing the risk of rupture and further brain damage. Better to wait until the prvious weak spot has had time to heal fully - I think you will agree. There may well be other reasons too but it would be better to talk this over properly with a doctor.