Some causes of Sudden loss or reduction of vision are caused by circulation problems.
Amaurosis Fugax is due to temporary interruption of blood supply to retina causing painless temporary disturbance of vision usually in elderly people caused by Platelet Emboli.
Permanent but similar loss can be caused by Cholesterol emboli or Temporal Arteritis
Ensure there are no signs of Temporal Arteritis which is an ophthalmic emergency
Symptoms include headache, scalp tenderness, jaw cramps and PMR symptoms.
There are many other causes of reduced vision that exist.