In bitemporal hemianopsia, vision is missing in the outer (temporal or lateral) half of both the right and left visual fields. Information from the temporal visual field falls on the nasal (medial) retina. The nasal retina is responsible for carrying the information along the
optic nerve, and crosses to the other side at the
optic chiasm. When there is compression at the optic chiasm, the visual impulse from both nasal retina are affected, leading to inability to see the temporal, or peripheral, field of vision. This phenomenon is known as bitemporal hemianopsia. Knowing the neurocircuitry of visual signal flow through the
optic tract is very important in understanding bitemporal hemianopsia. Bitemporal hemianopsia most commonly occurs as a result of tumors located at the mid-optic chiasm. Since the adjacent structure is the pituitary gland, some common tumors causing compression are
pituitary adenomas and
craniopharyngiomas. Also, another relatively common neoplastic cause is
meningiomas. A cause of vascular origin is an
aneurysm of the
anterior communicating artery which arises superior to the chiasm, enlarges, and compresses it from above. ==Diagnosis==