Pituitary disease. Most losses in the bitemporal pattern - partial or complete - originate with tumors of the pituitary gland pressing on the optic nerve chiasm. There are a few rare cases with this pattern from vascular malformations and tumors like meningioma.
Causes of bitemporal. Hemianopsia include pituitary lesions which can put pressure on the optic chiasm. These lesions include adenomas, meningiomas, craniopharyngiomas, sarcoidosis, primitive neuroectodermal tumors (pnet), aneursymal enlargement of the anterior communicating artery, and sphenoid sinus destructive lesions with extension. I hope this helps. Well wishes.
Proximity to optic. The pituitary gland sits immediately behind the optic chiasm which is where the optic nerves from each eye cross. A pituitary tumor or other swelling there of can press on the optic chiasm and cause one to lose vision to each side. Anuerysms of the anterior communicating artery can also do this. Very cool question. One would usually need an MRI of the brain to furhter investigate.
Optic chiasm. The pituitary sits near the optic chiasm. This area is where the optic nerve tracts cross. When there is enlargement of the pituitary or an adenoma, this can put pressure on the chiasm and cause bilateral hemianopsia.
Pituitary tumors. Hi. the optic chiasm (optic nerves; some of them cross sides) is right above the pituitary. Big pituitary tumors can push up out of the pituitary's bony pocket (the sella) and push against the parts of the optic nerves that cross (they see lateral/temporal visual fields) and cause bitemporal hemianopsia; the parts of the optic nerves that see the medial visual fields are spared.
Nerve fiber pattern. Bitemporal hemianopia is the classic later stage vision pattern in advancing pituitary tumor. Generally the central fibers, which carry the best acuity are affected later in the condition. A great deal of vision loss would have to be ignored by someone with this condition before the nerve fibers for the central acuity start to drop out.