Thinning blood vessels in the brain usually denotes cerebral aneurysm.
This occurs due to the weakness in the blood vessel wall.
This can be caused by congenital defects from diseases like Fibromuscular dysplasia, Marfan's syndrome, sickle cell anemia, lupus;
High blood pressure;
Atherosclerosis ( fatty deposits in the arteries), or
The symptoms associated can be headache, mental clouding, seizures, stroke symptoms, blurry vision, neurological weakness, rarely hormonal imbalances ( if pituitary gland is compromised by aneurysm).
For details on the treatment, you can go to this link: