There are 4 main parts of the human eye that aid in vision.

Disease at any of these levels can cause a

The Cornea:

Clear part of the eye that vaults over the iris, the colored part of the eye
Contact lenses sit on this part of eye. Painful ulcers and scars can occur if contacts are not worn or cleaned properly.
Curvature of the cornea (known as astigmatism) can affect vision, but is correctable with glasses/contacts.

The Optic Nerve:

Sends signals to and from the brain from the eye
Glaucoma is the most common disease affecting the optic nerve. Glaucoma can cause loss of peripheral (side) vision and can sometimes lead to blindness.

The Lens:

Sits behind the iris, helps us focus on objects and images
Part of the eye that swells with increases in blood sugar; this can cause a shift in one's glasses prescription and can aid in the diagnosis of diabetes
Muscles surrounding the lens become less flexible with age and the lens thickens, causing a decrease in near vision around the age of 40 (known as presbyopia)
Cataracts affect this part of the eye. Most patients with age-related cataracts have the lens removed in their 60s-70s.

The Retina:

The tissue that lines the back of the eye that is composed of several layers, one of which contains millions of cells called rods and cones
Rods help us with peripheral and low-light vision, while cones are located in the macula and help us with color, detailed, and central vision
The retina sends signals to the optic nerve and this helps us see
Some conditions affecting the retina include:
- Macular Degeneration
- Diabetic Retinopathy
- Hypertensive Retinopathy
- Retinal Tear / Detachment