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Diagnostic Test🐕 Dogs🐈 Catsophthalmicneurological

Tonometry (Intraocular Pressure Measurement)

🗣️ Pronounced:toh-NOM-eh-tree
In One Sentence (Plain English)

An ophthalmic diagnostic procedure measuring intraocular pressure (IOP) in millimeters of mercury (mmHg) to screen for glaucoma and uveitis.

Editorial Status: Verified from peer-reviewed veterinary sources (v1.0)
Updated: 2026-01-15

💬 In Plain English

Tonometry is the medical test used to measure the fluid pressure inside your pet’s eyes. Using a gentle handheld veterinary instrument (most commonly a Tono-Pen or TonoVet), the veterinarian applies a tiny topical numbing eye drop and gently taps the instrument against the cornea for a fraction of a second. Normal eye pressure in dogs and cats is between 10 and 25 mmHg. If the reading is dangerously high (> 25–30 mmHg), it diagnoses glaucoma—an urgent crisis threatening blindness. If the reading is abnormally low (< 10 mmHg), it indicates uveitis (painful internal eye inflammation).

🔬 Clinical & Technical Depth

For Vets & Technicians

Formal Clinical Definition

Ophthalmic instrumental diagnostic measuring the physical tension or compliance of the fibrous tunics of the globe, quantifying intraocular fluid pressure in millimeters of mercury (mmHg) via applanation (Tono-Pen) or rebound (TonoVet) methods.

Why Veterinarians Use It

Veterinarians perform tonometry on red, painful, or cloudy eyes to diagnose glaucoma and uveitis before permanent optic nerve damage occurs.

Clinical Context Encountered

Checked during ophthalmic exams, emergency triage for cloudy eyes, or routine screening in high-risk breeds (Cocker Spaniels, Huskies).

Clinical Reference Guide (Diagnostic Test)

Sample RequiredDirect physical assessment using applanation or rebound tonometer; 1 drop proparacaine 0.5% (if applanation).
Patient PreparationGentle muzzle restraint without pressure on jugular veins or eyelids.

Normal vs. Abnormal Results

Normal reference range: 10 to 25 mmHg in dogs and cats with < 4-5 mmHg discrepancy between eyes. Abnormal: > 25–30 mmHg (glaucoma); < 10 mmHg (anterior uveitis).

Next Steps If Abnormal

High pressure: immediate topical prostaglandin analogues/carbonic anhydrase inhibitors; referral to veterinary ophthalmologist. Low pressure: anti-inflammatory drops.

🚨 Red Flag Warnings (When to Contact an Emergency Vet)
  • •Never press tightly around a pet’s neck during tonometry: jugular vein compression artificially inflates intraocular pressure and can distort diagnostic readings.

❓ Questions to Ask Your Veterinarian

Take these actionable questions with you to your next clinic visit or appointment:

  • 1What was the measured intraocular pressure (IOP) in each eye today (normal 10–25 mmHg)?
  • 2Was a tight collar or leash pulling avoided during testing (which falsely elevates eye pressure)?
  • 3If pressure is elevated, what immediate topical pressure-lowering drops (latanoprost, dorzolamide) are needed?

Academic & Clinical References

  • ACVO Standards for Clinical Tonometry in Dogs and Cats
    Organization: American College of Veterinary Ophthalmologists (ACVO) • Accessed: 2026-01-10
    https://www.acvo.org