Cortical visual impairment (CVI) is a form of visual impairment that is caused by a brain problem rather than an eye problem. (The latter is sometimes termed "ocular visual impairment" when discussed in contrast to cortical visual impairment.) Some people have both CVI and a form of ocular visual impairment. CVI is also sometimes known as cortical blindness, although most people with CVI are not totally blind. The term neurological visual impairment (NVI) covers both CVI and total cortical blindness. Delayed visual maturation, another form of NVI, is similar to CVI, except the childs visual difficulties resolve in a few months. Though the vision of a person with CVI may change, it rarely if ever becomes totally normal. The major causes of CVI are as follows: asphyxia, hypoxia (a lack of sufficient oxygen in the bodys blood cells), or ischemia (not enough blood supply to the brain), all of which may occur during the birth process; developmental brain defects; head injury; hydrocephalus (when the cerebrospinal fluid does not circulate properly around the brain, and collects in the head, putting pressure on the brain); a stroke involving the occipital lobe; and infections of the central nervous system, such as meningitis and encephalitis. Visual and Behavioural Characteristics
Visual and Behavioural Characteristics of CVI are individual and may include several (but not necessarily all) of the following:

The person with CVI may exhibit what at first appears to be frequent variable changes in vision.
It can also be difficult for some people with CVI to look at an object and reach for it at the same time. Looking and reaching are sometimes accomplished as two separate acts: look, then look away and reach. People with CVI can sometimes benefit from a form of blindsight, which manifests itself as a kind of awareness of ones surroundings that cannot consciously be explained (for example, the person correctly guesses what they should do in order to avoid an obstacle but does not actually see that obstacle). However, this cannot be relied on to work all the time. In contrast, some people with CVI exhibit spatial difficulties and may have trouble moving about in their environment. Approximately one third of people with CVI have some photophobia. It can take longer than usual to adjust to large changes in light level, and flash photography can be painful. On the other hand, CVI can also in some cases cause a desire to gaze compulsively at light sources, including such things as candle flames and fluorescent overhead lights. The use of good task lighting (especially low-temperature lamps which can be placed at very close range) is often beneficial. Although people (with or without CVI) generally assume that they see things as they really are, in reality the brain may be doing a certain amount of guessing and "filling in", which is why people sometimes think they see things that turn out on closer inspection not to be what they seemed.