A 44 year old male reported some disturbance in the vision of his left eye. It was of sudden onset. The vision is just not the same in each eye. He owns his own business and works long hours.

Rx from previous eye exam two years before: R +0,50  6/6.  L +0,50  6/6. No prescription was filled at the time.

Today:

Ophthalmoscopy – no apparent defects

Slit lamp – anterior segment all clear

R +0,50. 6/6   L +1,25.  6/9-

There are no other significant findings.
What is your preliminary diagnosis?
What further tests or plan of action would you employ.