Tunnel Vision is the gradual loss of peripheral vision. The outer edges of ones vision becomes dark gradually moving inward until one can only see what is directly in front of them. It resembles looking down a tunnel. Glaucoma and Migraines can cause this to happen to one's vision. My dad had migraines without the headache, instead he had visual disturbances that involved wavy lines floating across his eye/ and or tunnel vision. In my dad's case his eyes would return to normal vision when the migraine was gone. Those that have tunnel vision from glaucoma could lose their vision if not treated.
I layered this image with different tones of darkness going inward to resemble how tunnel vision affects the eyes.