Brief Answer:
Read below.

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

The fact that the doctor hasn't found anything in physical exam is a good thing. Means he hasn't found any signs of issues such as stroke, brain tumor etc, making them less likely.

A common cause of balancs issues are inner ear issues as it is an organ involved in balance. May be related to a viral infection but often no cause is found, may be due to movement of free chrystals in the ear labyrinths. Antihistamine medication like meclizine may help. I wouldn't say it is the most likely cause though as usually it is a sensation of vertigo, spinning movement.

Another cause may be neck arthritis as it may compress upper cervical nerve roots which by sending the brain information on head and neck position are also involved in balance. Physical therapy may be of help.

An issue of the brain and supplying vessels such as stroke is another more worrying cause. Apart from an established stroke it may be due to narrowing of neck vessels supplying the posterior part of the brain with blood. This narrowing is most commonly due to atherosclerotic plaques and may lead to transitory episodes of insufficient blood flow to these areas which are responsible for balance, hence the symptoms. It would be a more serious issue as it means a higher stroke risk and need for prevention.

For that reason it is good that your doctor is being prudent and asking for MRI. I would only add (if he hasn't thought about it already)that it would be better to include in the test Angio MRI of neck vessels (MRI techniques which visualize neck vessels).

I remain at your disposal for other questions.

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