Sudden Blindness in Dogs: How PLR, Fundus Examination, and ERG Fit Together
Turn “cannot see” into a testable problem
Sudden blindness in dogs often pushes the clinician toward premature labels. One common error is to rely only on the fundus. Another is to assume that a residual pupil response rules out retinal disease. SARDS, retinal detachment, optic neuritis, glaucoma, severe uveitis, and central disease can all present as acute visual change [1,2,9].
The first visit should answer three questions: is the dog truly visually impaired, is the problem more likely ocular, retinal, optic nerve, or central, and is there a painful or high-pressure eye that needs immediate treatment. PLR, fundus examination, and ERG divide that work.
PLR gives pathway information
PLR is fast, noninvasive, and suitable early in the examination. A markedly reduced or absent PLR raises concern for severe retinal, optic nerve, or reflex pathway disease. A retained PLR does not rule out outer retinal disease, because melanopsin-related inner retinal pathways may still drive part of the pupil response [6,7,8].
This matters in SARDS. Affected dogs may become blind suddenly or rapidly, the fundus can be unremarkable early, and ERG commonly shows severe outer retinal dysfunction. Reviews emphasize that a normal early fundus or residual light reflex should not be used to dismiss serious retinal dysfunction [9].
The fundus shows structure, not all function
Fundus examination can reveal vascular attenuation, increased tapetal reflection, optic disc pallor, retinal detachment, hemorrhage, exudate, hypertensive change, or chorioretinitis. Late PRA, detachment, and inflammatory disease often leave visible clues [1,2,3].
However, a normal-looking fundus is not proof of normal function. Early SARDS, acute retinal dysfunction, some optic nerve disorders, and subtle inflammation may not produce enough visible change at the first visit. If behavior and reflex findings do not match the fundus, keep going.
When ERG becomes the next step
ERG directly evaluates retinal electrophysiologic function. It is particularly useful when cataract, vitreous opacity, or early retinal disease makes fundus interpretation insufficient. Cataract surgery candidates also often need ERG to evaluate whether retinal function supports surgery planning [1,2].
When sudden blindness has an atypical fundus, PLR does not match visual behavior, or SARDS or inherited retinal degeneration is suspected, ERG is a strong next step. It does not replace neuroimaging, but it answers the outer retinal function question much more directly.
A practical sequence for general practice
Observe pupil size and symmetry in dim light. Test direct and consensual PLR, adding red and blue stimuli when available. Record latency, amplitude, and pupil escape. Then assess menace, dazzle, obstacle navigation, or cotton ball tracking. Measure IOP, examine the anterior segment, and evaluate the fundus. Based on the result, decide on ERG, blood pressure, infectious disease testing, neurologic examination, or referral.
Document the reasoning rather than only a suspected name. Onset, drinking and urination changes, pain, IOP, fundus findings, PLR pattern, and the reason for ERG or referral should all be clear.
Related Product Information
In these cases the CPRL Tester can provide repeatable red and blue stimuli for documenting pupil strength, latency, and escape. It does not replace ERG, but it can make referral records more complete.
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.