Visual damage may be the most prominant long term complication. When blood glucose level is changing rapidly, eyesight may suddenly becomes blur because of the diopter change of lenses resulted from the change of fluid concentration inside. Such changes may subsides with tranquilization of blood sugar level. However, when long term visual damage takes place, it is not reversible.

Uncontrolled diabetes over time can cause permanent damage to the small vessels that supply the retina, and thus cause damage to the visual system. This complication is called diabetic retinopathy.

In its early stages, retinopathy does not affect vision and patients are not alert. As the damage gets worse, however, blood vessels can leak, causing swelling in the back of the eye (macular edema), and if left untreated, can eventually cause blindness. Proliferative retinopathy occurs when new, weak vessels form after the previous ones broke down. Blood leakage from these weak vessels can likewise result in blindness.

Retinopathy can be treated with laser therapy and surgery to seal off leaking vessels and remove the blood blocking the retina. Only early treatment can prevent or delay vision loss.
So it is important to prevent diabetic retinopathy:

Check your blood glucose and keep it under control.Sometimes it might be difficult to keep it in a normal level because of the elder age or medication intolerance.In such cases, we would say, tranquilization of blood glucose level is more important, of course, not too high.
Do not smoke.
Control your blood pressure (less than 130/80).
Have an annual dilated eye exam.Have annual checkups with an ophthalmologist or an optometrist.
Notify your doctor, ophthalmologist, or optometrist of any changes in your vision.

If you have retinopathy, avoid activities that increase blood pressure and intense activities.