Brilliant white 'after-images' after exercise

Hi, I ride my bicycle in to work daily - about 3 miles. Over the past year I have had a number of occasions where at the end of the ride, I suddenly see a brilliant white kidney-shaped image which is reminiscent of a very strong after-image such as you would get after looking at the sun. This kidney-shape usually morphs into a full donut-shaped image and swells until the central hole is consumed by the whiteness. The whole image then fades out over 2 or 3 minutes until my sight is back to normal. Sometimes this happens on consecutive days, but usually several weeks elapse between these episodes. Sometimes only one eye is affected (it is hard to tell which one as the image is visible whether the eyes are shut or open) but sometimes there are two of these images superimposed on each other and they evolve at slightly different rates. The strange thing is that I have only ever had this happen at the end of my cycle rides into work (and never on the way home). There is a hill at the end of the ride in, so this is the most strenuous part of the ride, but it is only a short rise so this may be coincidence. I have never had this image occur at any other time besides arriving at work. But it is quite worrying as my eyesight is badly affected until the image dissolves. Sometimes the image also appears colored a bright purple and may morph into yellow. Does anyone have any idea what might be causing this?

[Ray T Oyakawa, MD]

Check your blood pressure and make sure you are not getting dehydrated.

Dr. O.