Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with a doctor.
Without the ability to examine and obtain a history from your grandmother, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
Orthostatic hypotension is common in the elderly partly from impaired ability to change heart rate in response to changes in position to compensate for drops in blood pressure. There are many causes for this. These can include diabetes, prolonged immobility, parkinsonism plus diseases such as multiple system atrophy or progressive supranuclear palsy, amylodiosis, medications, and endocrine dysfunction. (This is a small list). Many times the diagnosis is aided by an MRI of the brain (make sure it includes a sagittal image and GRE), and autonomic testing such as tilt table and QSART. It is good that your grandmother has had blood testing already. However, I would make sure it includes a thorough neuropathy workup (such as rheumatological panels, thyroid studies, cancer screening in serum and urine, etc), which can be discussed with her neurologist.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.
She is currenly taking meds that are supposed to help with the orthostatic hypotension but so far we had not seen much in the way of good results. She is also on medication for the Alzheimer's/dementia. The doctors have adjusted her meds to make sure that they are not causing her symptoms. She has had blood work, chest x-rays, CT Scans, MRI's, and so forth and so on...and still no answers. One doctor apparently mentioned Parkinson's Disease but the neurologist said he didn't think that was it. Home health has been visiting her at the house and so has Physical Therapy, in which the PT says that there should be some kind of answer to her health issues as when it happens they come on suddenly and tend to worsen with each "spell" in severe orthostatic hypotension that she has. Just weeks ago she was up and on her own and was still pretty cognitive and then she had this severe issue with the drop in BP and she can not do anything with out assistance.