Veterinary Eye Exam Reference (STT, IOP, PLR)
Quick reference for veterinary eye exam: STT, IOP, common findings, and PLR interpretation.
Schirmer tear test I (no anaesthesia)
| Species | Normal | KCS threshold |
|---|---|---|
| Dog 10–14 mm/min is borderline (early KCS). | ≥ 15 mm/min | < 10 mm/min |
| Cat Cat results vary widely - clinical signs matter more than the number. | ≥ 9–10 mm/min | < 5 mm/min |
Tonometry (intraocular pressure)
| Species | Normal IOP |
|---|---|
| Dog | 10–25 mmHg |
| Cat | 10–25 mmHg |
| Horse | 15–30 mmHg |
Common ophthalmic exam abnormalities
| Finding | Differential |
|---|---|
| Buphthalmos (enlarged globe) | Chronic glaucoma (irreversible) |
| Hyphema | Trauma, coagulopathy, hypertension, retinal detachment |
| Aqueous flare | Anterior uveitis - work up systemic disease |
| Conjunctival hyperaemia + 360° injection | Acute glaucoma, severe uveitis |
| Fluorescein-positive corneal lesion | Ulcer - depth determines management urgency |
| Lens cataract | Diabetes (especially recent onset), inherited, traumatic, senile |
Pupillary light reflex (PLR)
| Test | Interpretation |
|---|---|
| Direct PLR | Ipsilateral pupil constricts when light shone |
| Consensual PLR | Contralateral pupil constricts with light to other eye |
| Absent direct + present consensual (when light in affected eye) | Retinal or optic nerve lesion on that side |
| Absent both PLRs | Cranial nerve III lesion or globe lesion |
Always do STT before any topical anaesthetic - it suppresses tear production. Fluorescein stains epithelial defects (corneal ulcers) yellow-green under cobalt-blue light.
Frequently asked questions
When should I do STT before tonometry, or vice versa?
STT first, always. Topical anaesthetic and any saline flush reduce STT values and you'll falsely diagnose KCS. Tonometry uses topical anaesthetic, so it must come after STT.
What's a quick way to differentiate uveitis from glaucoma?
Both cause red eye and squinting. Uveitis: miotic pupil + low IOP + aqueous flare. Glaucoma: mid-dilated unresponsive pupil + high IOP. Tonometry is the single most useful test for differentiating - measure it before assuming.
What's the most missed eye finding in cats?
Aqueous flare (anterior uveitis) - it's subtle and easy to miss without a focal slit beam in a dark room. Feline uveitis is often the first sign of FIV, FeLV, FIP, toxoplasmosis or lymphoma - always work up systemic disease.