What Causes Pulsatile Tinnitus?
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Detailed Answer:
I read your question carefully and I understand your concern.
The causes of pulsatile tinnitus (how hearing heartbeat is called) can be various. It can be due to brain related causes like some tumors, increased pressure in the brain, some brain blood vessel malformations. However since you have had a normal MRI these issues can be eliminated.
May be due to conditions which increase blood flow like anemia or high thyroid function. If you haven't be tested for those you should be.
At times frustrating as it is no cause can be identified. For these cases there can be two explanations. One may be abnormal tortuous blood vessels, but involving vessels of a small size which can't be visualized on imaging tests like MRI. The other possibility would be that of a psychological component. There is one factor which does support this last possibility, the fact it stops when you have to concentrate on something such as driving (other structural causes shouldn't be influenced by driving). Since the problem has persisted for so long I would consider a try of medication with an SSRI antidepressant like citalopram, most common medication used for anxiety. Actually amitriptyline is the one often used successfully for tinnitus, but it may cause drowsiness so since you mentioned driving I mentioned SSRI which has less of such a side effect.
I remain at your disposal for other questions.