In hemianopia, half the
visual field is lost from both eyes. This is almost always right or left, rather than top or bottom. The fact that it is lost from both eyes indicates that the problem is neural, rather than to do with the eyes themselves, as it would be remarkable for both
eyes to suffer exactly the same defect like this.
The parts of the nervous system that can be affected to cause hemianopia are usually in the optic chiama, or in the visual centres of the brain.
Hemianopias are classified according to which part of the visual field they affect.
- Homonymous hemianopia: In both eyes the same half is lost.
- Binatal hemianopia: the inner halves of the visual field are lost
- Bitemporal hemianopia: both outer halves of the vision are lost.
- Altitudinal hemianopia: both upper or lower halves of the visual field are lost.(Very rare).