Monocular blindness from an optic nerve lesion

Loss of the whole visual field of one eye, the defect of a complete lesion of one optic nerve.

After a complete lesion of the right optic nerve: right monocular blindness. 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 nerve, in front of the chiasm
Left eye loses: Nothing
Right eye loses: Whole field, 100% 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: 4 of the 8 fiber groups the model draws

Try in: Visual Field Defects Explorer →

Put the lesion on the right optic nerve to see the right eye’s fibers end at the cut.

Where is the lesion in monocular blindness?

In front of the optic chiasm, on the side of the blind eye: in the optic nerve, or in the eye itself. Each optic nerve carries the fibers of one eye and no others, so a complete lesion there takes that eye’s whole field and leaves the other eye’s field full.

Why one nerve takes one eye’s field

The nerve holds both halves of its eye’s retina: the nasal fibers that are about to cross at the chiasm and the temporal fibers that will stay on the same side. A complete lesion cuts both before any sorting by half has happened, so the loss has no vertical edge, and behind the chiasm the cut fibers would have run in both optic tracts, as the table shows.

The fiber groups cut by a complete lesion of the right optic nerve
Fibers from	See	Cross at the chiasm	Then run in
Right eye, nasal retina, upper half	Lower temporal quarter	Yes	Left optic tract, then the parietal radiation
Right eye, nasal retina, lower half	Upper temporal quarter	Yes	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.

The pupils: a relative afferent pupillary defect

Light shone in the blind eye moves neither pupil, because no signal leaves that eye, while light in the seeing eye constricts both. Swung from the good eye to the blind one, the light makes both pupils dilate. That is the swinging light test, and the sign it shows is a relative afferent pupillary defect.

Usual causes, and why most are partial

Optic neuritis, ischemic optic neuropathy, compression and trauma. Most of them damage only part of the nerve: optic neuritis typically leaves a central scotoma, an island of lost vision at fixation, and ischemic optic neuropathy an altitudinal defect, the upper or lower half of one eye’s field, rather than a blind eye.

Telling it apart from a junctional scotoma

A lesion where one optic nerve meets the chiasm can catch that nerve and the crossing fibers from the other eye together. The result is a central scotoma in one eye and an upper temporal defect in the other, the junctional scotoma, so a defect in the second eye, even a small one, points to the junction rather than to the nerve alone. Further back the crossing fibers alone give 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 optic nerve	Left monocular blindness	whole field	nothing
Right optic nerve	Right monocular blindness	nothing	whole field

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.