Binasal hemianopia

Loss of the nasal half of the visual field in both eyes, also spelled binasal hemianopsia: the mirror image of a bitemporal hemianopia.

After complete lesions of both outer edges of the optic chiasm: binasal hemianopia. Fields as the patient sees them, central 30° with rings at 10° and 20°. Hatched areas are lost, and the dark ovals are the normal blind spots.

Lesion site: Optic chiasm, both outer edges
Left eye loses: Nasal half, 50% of the plotted field
Right eye loses: Nasal half, 50% of the plotted field
Pattern: Heteronymous: opposite halves in the two eyes, so the lesion is at the chiasm
Lost to both eyes at once: 0% of the central 30°
Fibers cut: 4 of the 8 fiber groups the model draws

Try in: Visual Field Defects Explorer →

The explorer draws one lesion at a time: put it on the left edge of the chiasm, then move it to the right side, and each eye in turn loses its nasal half.

What causes binasal hemianopia?

A pathway lesion that gives it has to press on both outer edges of the optic chiasm at once, cutting the uncrossed fibers from the temporal half of each retina, which see the nasal fields. That is rare, and most binasal field defects come from disease of the eye or the optic disc instead.

Why the two nasal halves go

Each eye’s uncrossed fibers, about 47 percent of its optic nerve, run along the outer edge of the chiasm on its own side. A lesion on one edge takes one eye’s nasal half, a nasal hemianopia; the two edges together take both, drawn here as two complete lesions of the model, one on each side.

Like a bitemporal hemianopia, the defect is heteronymous, the two eyes losing opposite sides of the world, but the sides are reversed: each eye loses the half toward the nose.

The fiber groups cut by complete lesions of both outer edges of the optic chiasm
Fibers from	See	Cross at the chiasm	Then run in
Left eye, temporal retina, upper half	Lower nasal quarter	No	Left optic tract, then the parietal radiation
Left eye, temporal retina, lower half	Upper nasal quarter	No	Left optic tract, then Meyer’s loop
Right eye, temporal retina, upper half	Lower nasal quarter	No	Right optic tract, then the parietal radiation
Right eye, temporal retina, lower half	Upper nasal quarter	No	Right optic tract, then Meyer’s loop

4 of the 8 fiber groups the model draws are cut; the other 4 reach the cortex.

With both eyes open

In the central 30°, each eye’s surviving temporal half covers what the other has lost, so the share of that field lost to both eyes at once is 0%.

Telling a chiasm defect from an eye defect

A binasal defect from the chiasm stops at the vertical meridian, because that is where crossed and uncrossed fibers divide. The defects of eye disease, such as glaucoma, respect the horizontal meridian or no meridian at all.

Tools for the visual pathway

Visualiser · Biology Visual Field Defects Explorer Place a lesion anywhere from optic nerve to visual cortex and see each eye’s field loss, named and explained. Visualiser · Biology Blind Spot Mapper Watch a dot vanish, as Mariotte’s paper did in 1668, then map your own blind spot in degrees and see it as the optic disc on the retina. Visualiser · Biology Brain Explorer 101 brain structures, gyri to ventricles, separable.

Visual Field Defects Explorer puts a lesion anywhere on the pathway and draws every fiber it cuts.
Blind Spot Mapper maps your own blind spot, the dark oval in each chart, in degrees.
Brain Explorer has the optic nerves, chiasm, tracts and geniculate bodies as 3D models.

Sources: Kandel, Principles of Neural Science, 6th edition, chapter 22, for the pathway and its lesions; Purves, Neuroscience, 6th edition, chapter 12, for the pathway and its lesions; Kupfer, Chumbley and Downer, Journal of Anatomy 1967;101:393, for the 53 percent of optic nerve fibers that cross at the chiasm. A teaching diagram of complete lesions: real fields are read with the history, the other eye, the optic discs and imaging.