Brief Answer:
Orthostatic hypotension should be considered

Detailed Answer:
I read your question carefully and I understand your concern.

Since MRI hasn't shown anything (at least you do not mention it to then a brain or spinal cord lesion has fortunately been excluded. I suppose also causes like anemia, vitamin B12 deficiency, metabolic alterations etc have been excluded by blood tests.

In that case given the lack of symptoms when laying down and appearance when standing erect the possibility of orthostatic hypotension should be considered. It happens due to an excessive drop in blood pressure when standing. Some drop due to gravity effect on blood flow happens in all of us but in orthostatic hypotension it is an excessive drop. To diagnose it pressure measurements supine and standing are the first step. If inconclusive tilt table test is commonly used.

If that is not found to be the case another common cause would be an inner ear issue. In that case medication with meclizine would be first choice.

I remain at your disposal for other questions.

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