My sister, age 85, in supposedly good health is having weak and lightheaded spells...a funny feeling in her head that comes with weakness. She has had an echocardiogram, an EKG, an EEG, scan of her carotid arteries, been checked by the ENT, worn monitors for her heart, etc. The spells come on quickly and may last for several hours. Blood pressure fluctuates sometimes and sometimes it is normal. She had a fainting spell about two months ago due to dehydration. The spells started a few days after this incident. We are at a loss as to what is causing this. She is scheduled for a stresss test on Wednesday. She was diagnosed recently with slightly low thyroid and will be taking synthroid.