Blindness describes the inability to process visual information. It is always a symptom of a serious disorder, but can have several underlying causes. It can be the result of:

  • Damage to the eye itself, such as damage to the cornea, clouding of the vitreous humor (fluid in the eyeball) or a detached retina;
  • Damage to the optic nerve that carries impulses to the brain, such as can be caused by several neurological disorders;
  • Damage to the brain, which can prevent the signals from being properly processed.

A patient need not be totally unable to perceive light to be classified as blind. Usually, the criteria is that their best corrected vision is worse than 20/200 (being able to perceive something from 20ft that a person with normal sight could perceive from 200ft). In addition, the whole field of vision may not be affected. In some conditions, the retina cannot perceive visual information in the center of the field, in others at the periphery of the field.

Blindness can be an unusual phenomena. In some cases, persons unable to perceive visual information are not able to realize they are blind (see Euphoria (Part 1)). Their brain continues to process visual information as if the retina were receiving it.

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