Sometimes the eye doctor is the only doctor people see with any regularity — especially some of us guys. Therefore it is comforting to know that there is a lot your eye doctor can tell you about your general health by examining your eyes. When you think about it, the back of your eye is the only place on your body that you can actually look at the blood vessels themselves. And the optic nerve is really a kind of cable extension from your brain. Hypertension and high cholesterol levels cause observable changes to your blood vessels.
Diabetes can cause small leaks in the walls of those retinal blood vessels, as well as premature cataracts and glaucoma. Melanoma cancer will alter the pigment and show swelling in the retinal tissue. Optic neuritis (an inflammation of that optic nerve cable) can be the first symptom of 75% of the patients who suffer from Multiple Sclerosis, Arthritic and rheumatoid conditions can often cause a dry eye condition and iritis. Many of these conditions will cause a change in color vision perception and compromise some area of you visual field as well.
An eye examination can tell you more than just how well you see! A vision check is "More than Meets the Eye".