There are many different causes of tunnel vision, and some are more serious than others.
Damage to the optic nerve
The optic nerve transmits signals from the eyes to the brain. Any damage to the optic nerve, either through trauma or disease, can cause tunnel vision to occur. Glaucoma is a fairly common eye disease that can damage the optic nerve by increasing the pressure inside the eye. Glaucoma is associated with a slow but steady loss of peripheral vision, and can lead to blindness if left untreated.
Damage to the retina
Damage to the peripheral areas of your retina can affect your peripheral vision. A condition called retinitis pigmentosa is known to cause tunnel vision by damaging the retina, but it is quite rare.
Damage to the brain
A stroke or loss of blood to the brain can also result in a loss of peripheral vision.
Temporary tunnel vision
Sometimes, tunnel vision can occur temporarily when the body produces high levels of adrenalin. Extreme panic, stress or anger can all cause temporary tunnel vision. Alcohol and drugs have also been shown to have an effect on peripheral vision. In these cases, normal vision will usually return without treatment.