To your eye doctor, your eyes are windows to your overall health.  Many illnesses show their earliest signs in the eyes, and many diseases can affect your vision.  While you're being examined, your eye doctor will look for signs of:

The appearance of irregular blood vessels on the retina could indicate high blood pressure.

Small, circular areas of bleeding on the retina may be an early sign of diabetes.  Increases and decreases in blood sugar level could cause blurred vision and retinal changes.

Headaches may arise from or be worsened by vision problems such as eye strain.

The eye is susceptible to its own diseases, many of which have minimal associated symptoms, that can cause a range of problems from blurry vision to blindness.  You need to do your part.  Check vision in both eyes every day.  Do your eyes feel good, see good, look good.  Your eye doctor routinely performs tests and looks for signs and symptoms of many eye diseases, including:

High levels of pressure inside the eye due to failure of the eye to regulate its own internal pressure.  Early detection is crucial to prevent loss of sight.

The vessels in the eye may point to a risk of stroke.

Hypertension often goes hand-in-hand with high cholesterol levels.  The condition of the veins and arteries in the eyes could warn of high cholesterol, which may be a risk factor for heart disease.

Double vision, visual field loss, and uncoordinated or involuntary eye movements might be early signs of MS.

Of course, and eye examination cannot take the place of a general examination by your medical doctor. However, your eyes may reveal more about your health than you know.

The internal lens of  the eye becomes cloudy and must be removed and replaced with a tiny synthetic lens.

Each eye forms its own image, and it is the brain's job to fuse the two images into one.  For the brain to do its job, the eyes must be aligned, move together, and produce images of similar size and shape.  When the eyes are not coordinated, they have to work hard to see clearly, often causing eyestrain or headaches.  Although you might not know it, your eye doctor performs a whole series of tests to asses the ability of the eye muscles to keep the two eyes working together as a team.