Neuro-Ophthalmic Localization Through the Pupil: Retina, Optic Nerve, Third Nerve, and Brain
Keep the pathway in mind
The afferent side of PLR includes retina, optic nerve, and pretectal pathways. Central integration occurs in midbrain reflex centers. The efferent side includes parasympathetic fibers of cranial nerve III, the ciliary ganglion, short ciliary nerves, and the iris sphincter. Clinicians do not need to recite every nucleus, but they must know that the stimulated eye and the moving pupil are different pieces of information [1,2].
Many mistakes happen when the pupil is treated as a single-eye structure. Light into one eye can constrict both pupils, so the consensual response is often the localization clue.
The afferent pattern
A classic afferent problem produces weak responses in both pupils when the affected eye is stimulated, but better responses in both pupils when the normal eye is stimulated. The lesion may be in the affected retina, optic nerve, or a more proximal afferent pathway.
Possible causes include acute blindness, retinal degeneration, retinal detachment, optic neuritis, trauma, or compressive disease. If the fundus does not explain the finding, ERG, neurologic examination, and imaging may be needed [1,3,9].
The efferent pattern
An efferent problem looks more like one pupil that does not move. If the affected pupil fails to constrict regardless of which eye is illuminated, while the other pupil moves, consider the parasympathetic third nerve pathway, ciliary ganglion, iris sphincter, or topical drug exposure.
Look for abnormal globe position, ptosis, fixed mydriasis, intraocular inflammation, iris atrophy, or posterior synechia. Atropine exposure, mydriatic drugs, and toxic plant exposure can mimic neurologic disease.
Why brain disease can spare PLR
Cortical, visual radiation, and some thalamic lesions may cause visual behavior deficits while the basic PLR remains present, because the reflex pathway branches before conscious visual processing. A blind animal with retained PLR is therefore not a contradiction.
Menace response, postural reactions, gait, mentation, and other cranial nerve findings must be assessed. A normal PLR should not be used to dismiss central disease.
How red and blue stimuli fit localization
Red and blue testing does not change localization principles. It standardizes the stimulus and helps clinicians compare outer retinal responses, melanopsin-related sustained responses, and pupil escape [6,7,8,10].
If both red and blue responses are markedly weak, severe afferent pathway disease or media opacity should be considered. If red is weak but blue remains delayed and sustained, combine the finding with history and ERG to assess whether outer retinal and inner retinal light responses may be diverging.
Related Product Information
The CPRL Tester can provide consistent red and blue stimuli for documenting direct response, consensual response, latency, and escape. Localization still depends on a complete ophthalmic and neurologic examination.
References
[1] Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary Ophthalmology. Wiley-Blackwell. Local reference PDF.
[2] Maggs DJ, Miller PE, Ofri R. Slatter's Fundamentals of Veterinary Ophthalmology. Saunders/Elsevier. Local reference PDF.
[3] Gould D, McLellan GJ, eds. BSAVA Manual of Canine and Feline Ophthalmology. BSAVA. Local reference PDF.
[4] Companion Animal Ophthalmology. Local Chinese reference PDF.
[5] Small Animal Ophthalmology. Local Chinese reference PDF.
[6] Hattar S, Liao HW, Takao M, Berson DM, Yau KW. Melanopsin-containing retinal ganglion cells: architecture, projections, and intrinsic photosensitivity. Science. 2002;295(5557):1065-1070.
[7] Berson DM, Dunn FA, Takao M. Phototransduction by retinal ganglion cells that set the circadian clock. Science. 2002;295(5557):1070-1073.
[8] Do MTH, Yau KW. Intrinsically photosensitive retinal ganglion cells. Physiological Reviews. 2010;90(4):1547-1581.
[9] Komaromy AM, Abrams KL, Heckenlively JR, et al. Sudden acquired retinal degeneration syndrome (SARDS): a review and proposed strategies toward a better understanding of pathogenesis, early diagnosis, and therapy. Veterinary Ophthalmology. 2016;19(4):319-331.
[10] Hall CA, Chilcott RP. Eyeing up the future of the pupillary light reflex in neurodiagnostics. Diagnostics. 2021;11(10):1795.
[11] Product files for this week: CPRL Tester, red/blue light manual PDF, and pupil reaction chart image. Product files were used only to verify product name and visible use cues.