When the visual field defect occurs in one eye only, the cause can be localized to that eye. Or to the corresponding optic nerve which exits from the eyeball, travels in the orbit and the cranial cavity, before it meets the optic nerve of the other eye in the structure called optic chiasm.
When they reach the optic chiasm, the optic nerves from the two eyes meet and exchange some of the nerve fibers, hence the term nerve fiber crossing. As a consequence of this crossing, the manifestation of the visual loss involves both eyes. Therefore, bilateral hemianopia reflects disease processes at the optic chiasm or farther back in the brain.
The specific condition where visual field defects are present in corresponding halves of the right or left eye fields is called homonymous hemianopia. The affected individual must, therefore, turn the head from side to side to compensate for the defect.
Whereas defects involving either the outer or the inner halves of both visual fields are called heteronymous hemianopia and result in a patient presenting a blind bi-temporal field or a blind bi-nasal field. In other words, affected individuals with bi-temporal hemianopia, for example, will not be able to perceive the objects located in the outer half of both the right and left visual field.
Causes of visual field defects are numerous; they include retinal, orbital and brain diseases, inflammatory processes and vascular diseases. However certain patterns of visual field loss help to pinpoint to a specific cause or location along the visual pathway.
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