Hemianopia (hemianopsia) is a defect of one half of the visual field: this may be vertical or horizontal (= altitudinal field defect).
Hemianopic defects may be congruent (homonymous) or noncongruent (heteronymous), and may be detected by standard confrontational testing of the visual fields or automatically (e.g., Goldman perimetry). These tests of the visual fields are an extension of the tests for visual acuity which assess areas away from the fovea. Because of the strict topographic arrangement of neural pathways within the visual system, particular abnormalities of the visual fields give a very precise indication of the likely site of pathology.
- Homonymous hemianopia:
Reflects a post-chiasmal lesion. It is important to assess whether the vertical meridian of a homonymous hemianopia cuts through the macula (macula splitting), implying a lesion of the optic radiation; or spares the macula (macula sparing), suggesting an occipital cortical lesion. Incongruous defects may be found with lesions of the optic tract. Commonly, homonymous hemianopias result from cerebrovascular disease causing occipital lobe infarction, or intraparenchymal tumor, but they may be "false-localizing" due to raised intracranial pressure if temporal lobe herniation causes posterior cerebral artery compromise.
- Heteronymous hemianopia:
Reflects a chiasmal lesion. The most common of these is a bitemporal hemianopia due to chiasmal compression, for example by a pituitary lesion or craniopharyngioma. Tilted optic discs may also be associated with bitemporal field loss but this extends to the blind spot and not the vertical meridian as in chiasmal pathology ("pseudobitemporal hemianopia"). Binasal defects are rare, suggesting lateral compression of the chiasm, for example from bilateral carotid artery aneurysms; binasal hemianopia is also described with optic nerve head lesions. Unilateral (monocular) temporal hemianopia may result from a lesion anterior to the chiasm which selectively affects only the ipsilateral crossing nasal fibers (junctional scotoma of Traquair).
Unawareness of visual field loss, anosognosic hemianopia, occurs principally with right-sided brain lesions.
Bilateral homonymous hemianopia or double hemianopia may result in cortical blindness.