Nasal hemianopia in one eye

Loss of the nasal half of one eye’s visual field, the half nearer the nose, with the other eye untouched.

After a complete lesion of the right edge of the optic chiasm: right nasal 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, outer edge
Left eye loses: Nothing
Right eye loses: Nasal half, 50% of the plotted field
Pattern: Monocular: one eye only, so the lesion is on that side, in front of the chiasm or at its outer edge
Lost to both eyes at once: 0% of the central 30°
Fibers cut: 2 of the 8 fiber groups the model draws

Try in: Visual Field Defects Explorer →

Put the lesion on the right edge of the chiasm to see only the right eye’s uncrossed fibers cut.

What causes a nasal hemianopia in one eye?

Pressure on the outer edge of the optic chiasm on the same side, classically from an aneurysm or a dilated, hardened internal carotid artery. The uncrossed fibers from that eye’s temporal retina run along the edge, and they see its nasal field, so that eye loses its nasal half.

Why only one eye is affected

Temporal fibers are the ones that do not cross. The other 47 percent of each optic nerve, they run back along the outer edge of the chiasm on their own side and into the optic tract on that side. Nothing from the other eye passes that edge, so a lesion confined to it reaches one eye’s temporal fibers and no others.

The loss is monocular, like that of an optic nerve lesion, but it stops at the vertical meridian, which the loss from a complete lesion of the nerve does not.

The fiber groups cut by a complete lesion of the right edge of the optic chiasm
Fibers from	See	Cross at the chiasm	Then run in
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

2 of the 8 fiber groups the model draws are cut; the other 6 reach the cortex.

Reading the chart

The affected eye keeps its blind spot in view, 15° into its temporal field, and the blind spot is always on the temporal side, which is how a chart shows which eye has lost its nasal half.

Both edges at once

Pressure on both outer edges of the chiasm takes the nasal half from both eyes, a binasal hemianopia, which is rare. The crossing fibers in the middle of the chiasm are the ones a pituitary tumor reaches, and they give the opposite defect, a bitemporal hemianopia.

Left-sided and right-sided lesions

The same site on the other side gives the mirror image, named for the half of the field it takes.

The defect from a complete lesion at this site on each side
Lesion	Defect	Left eye loses	Right eye loses
Left edge of the optic chiasm	Left nasal hemianopia	nasal half	nothing
Right edge of the optic chiasm	Right nasal hemianopia	nothing	nasal half

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.