bitemporal hemianopia

**Definition:** Loss of the temporal half of the visual field in each eye ("tunnel vision" or "blinders" effect). Also called heteronymous (bitemporal) hemianopia.

**Anatomy/pathophysiology:** - Temporal visual fields project onto the NASAL retina. Nasal retinal ganglion cell axons cross (decussate) at the optic chiasm. - A midline chiasmal lesion interrupts these crossing fibers, producing loss of both temporal fields. Uncrossed temporal retinal fibers (nasal field) are spared. - Inferior chiasm fibers (from the inferior retina, carrying superior visual field) are compressed first by a pituitary mass growing from below, so the defect begins as a superior bitemporal quadrantanopia, which later progresses to a full bitemporal hemianopia. - Superior compression (e.g., craniopharyngioma) from above affects the superior chiasm fibers first, giving an inferior bitemporal quadrantanopia first.

**Common causes:** - Pituitary adenoma (most common in adults). Prolactinoma is the most common functional type. Look for galactorrhea or amenorrhea, acromegaly, or Cushing features. - Craniopharyngioma (derived from Rathke pouch remnants). It is calcified and cystic and occurs in children and young adults. It is the most common supratentorial childhood tumor, and the superior chiasm is compressed first. - Meningioma (suprasellar/tuberculum sellae). - Pituitary apoplexy (sudden headache, visual loss, ophthalmoplegia, hypotension). - Internal carotid artery aneurysm (lateral compression can cause binasal hemianopia). - Others: glioma, Rathke cleft cyst, sellar metastases.

**Clinical features:** Often patients are unaware of the deficit until they bump into objects at their sides or have trouble driving. There may be headaches and signs of hypopituitarism or hormone excess. Visual acuity is usually preserved until late in the course.

**Diagnosis:** Confrontation fields and formal perimetry (Humphrey/Goldmann) show a respect for the vertical meridian. MRI of the sella/brain with contrast is the imaging study of choice. Check pituitary hormones (prolactin, IGF-1, cortisol, TSH/free T4, etc.).

**Management:** Treat the underlying cause: dopamine agonists (cabergoline) for prolactinoma, transsphenoidal surgery for most other adenomas with visual compromise, and surgery or radiation for craniopharyngioma. Visual fields frequently improve after decompression.

**High-yield visual pathway lesion localization:** - Optic nerve: monocular vision loss (ipsilateral blindness). - Optic chiasm: bitemporal hemianopia. - Optic tract: contralateral homonymous hemianopia. - Meyer loop (temporal lobe): contralateral "pie in the sky" (superior quadrantanopia). - Dorsal (parietal) optic radiation: contralateral "pie on the floor" (inferior quadrantanopia). - Occipital cortex (PCA stroke): contralateral homonymous hemianopia with macular sparing. - Binasal hemianopia: bilateral lateral chiasmal compression (e.g., bilateral ICA atherosclerosis or aneurysms) or bilateral optic nerve disease. It is rare.

**Mnemonic:** "Pituitary tumor = Pie in the sky" is a common trap. Bitemporal hemianopia is from a chiasm lesion, whereas pie in the sky is from a temporal lobe lesion. A pituitary tumor compresses from below, so it affects the superior fields first.