The diagnostic algorithm for acute-onset blindness in a dog with no prior ocular history proceeds in three sequential tiers: localize the lesion, characterize retinal function, then pursue advanced imaging.

Begin with a complete neuro-ophthalmic examination to localize the lesion. Assess menace response, pupillary light reflexes (PLRs), dazzle reflex, and direct fundoscopic examination. Acute vision loss arises from three broad anatomic categories: opaque ocular media (e.g., diffuse corneal edema, cataracts), retinal dysfunction, or optic pathway disease (optic nerve, chiasm, tracts, or visual cortex).MSD Vet Manuals True ocular media disease is largely ruled out by anterior segment evaluation and tonometry at this step.Journal of the…

Electroretinography (ERG) and chromatic PLR testing are the pivotal second-tier tests that distinguish retinal from post-retinal disease. Dogs with photoreceptor-layer disease — including sudden acquired retinal degeneration syndrome (SARDS) and immune-mediated retinitis (IMR) — have an absent or markedly attenuated PLR to red light (630 nm, which stimulates photoreceptors) but a preserved PLR to blue light (480 nm, which stimulates retinal ganglion cells).Journal of the…+1 SARDS produces an extinguished ERG tracing, whereas early IMR may produce a normal, high, or low ERG.Journal of the… Ivermectin toxicosis produces markedly attenuated outer retinal responses on both ERG and chromatic PLR and should be considered even without a known exposure history, including in MDR1-negative dogs.Veterinary Reco… When ERG and PLR are both normal despite complete visual deficits, the lesion is post-retinal — as demonstrated in a Welsh Corgi with optic chiasm compression from hydrocephalus, where ERG and PLR were normal bilaterally.Journal of Vete…

When retinal function is intact, MRI with contrast is the definitive third-tier test to characterize the optic pathway and intracranial disease. Retrobulbar and intracranial differentials include optic neuritis, retrobulbar abscess or neoplasia, and disease of the optic chiasm, optic tracts, or visual cortex.Journal of the… Compressive lesions at the optic chiasm — from pituitary masses or metastatic disease — present with acute blindness and localize to the optic nerves, optic tracts, or chiasm on neuro-ophthalmic examination; MRI characterizes the mass and its extrasellar extension.Journal of the… Ischemic optic neuropathy produces prechiasmatic blindness with a normal ERG; MRI diffusion-weighted imaging shows hyperintensity of the affected optic nerve and chiasm with corresponding hypointensity on apparent diffusion coefficient mapping.Journal of Vete… Sphenoid bone neoplasia can produce a lytic, expansile mass with nasopharyngeal extension and concurrent neurologic signs; CSF total protein is elevated above the reference interval of less than 25 mg/dL in such cases.Journal of the…

Blood pressure measurement is a mandatory parallel step at initial presentation. Systemic hypertension causes acute blindness via retinal detachment and, less commonly, ischemic optic neuropathy; primary hypertension has been documented as the sole identifiable cause in dogs with concurrent lacunar infarcts.Journal of Vete… The referring clinician in one case initiated amlodipine besylate at 0.4 mg/kg PO q24h empirically for suspected hypertensive retinal detachment before referral.Journal of the…

The diagnostic sequence in summary: anterior segment exam + tonometry → fundoscopy → ERG + chromatic PLR → blood pressure → MRI with contrast ± CSF analysis. Glaucoma is a common cause of irreversible blindness in dogs and must not be missed at the tonometry step.MSD Vet Manuals

Would you like to work through the treatment approach for immune-mediated retinitis once it's confirmed on ERG and chromatic PLR?