A 67-year-old man experienced sudden loss of vision in the left eye 3 hours ago. You record visual acuity as OD 20/20 and OS no light perception. The right pupil responds to light directly but not consensually, the left responds to light consensually but not directly. Directed fundus of the right eye is normal. The left eye shows a white opacified retina, a cherry red spot in the macular and a sluggish retinal circulation. What do you think the patient has and how to treat?