The eye is a unique organ to have transparent structures such as cornea, lens and vitreous. The transparency of these parts of the eye is vital to its function as an organ for visual perception. The light reflected from the objects around us passes through these parts to reach the retina and the visual information is then conveyed to the brain via the optic nerve. The occipital cortex of the brain actually interprets the visual information (sees the images).
It is because of this transparency of cornea, lens and vitreous that we are able to see the internal structures of the eye themselves. Apart from important eye tissues like optic disc, macula and peripheral retina we can also see the vessels – arterioles and venules. They can be seen with ophthalmic instruments like ophthalmoscope, some special lenses and fundus camera.
These vessels look and function the same way they do elsewhere in the body. Circulatory system is a vital part of body in all animals. All nutrients, oxygen, fluids and even infective agents and drugs reach all parts of body through the circulatory system.
Many diseases like diabetes and hypertension (high blood pressure) affect the vasculature especially small vessels. In the whole body, the eye is the only organ where these vessels can be directly visualized to study the severity of the systemic disease. It is generally assumed that the amount of damage caused by affected vessels in the eye gives a fair idea of tissue damage in other organs of the body, for example heart, brain, liver, kidneys, intestines etc.
The degree of hemorrhage, exudation, tissue destruction and consequent scarring gives a lot of information not only about the severity but also possible duration of the disease. We also get the information as to how the disease is affecting other parts of the body.
Almost all the cases of diabetes are advised a dilated eye examination. This helps in determining the severity of diabetes and if the eye itself is affected (diabetic retinopathy), it can be treated with appropriate intervention like laser photocoagulation.
Interestingly in many cases, diabetes is first detected by an optometrist or ophthalmologist during a routine eye examination.