A 67-year-old man comes to the clinic with slowly worsening vision in both eyes.  He is not able to drive at night, as the symptoms are worse at night. His pupils are normal in diameter both in the light and darkness. Other medical history is unremarkable.  Ocular examination shows loss of the red reflex and blurry vision.  Acuity testing shows 50/100 vision in both eyes with normal visual field testing. His blood pressure is 130/70 and pulse is 68/min. the other physical examinations are normal.