Low eye pressure in dogs, called ocular hypotony, most often results from inflammation inside the eye. Normal eye pressure in dogs averages around 19 mmHg, with a healthy range falling between roughly 11 and 29 mmHg. When pressure drops below that range, something is disrupting the balance between the fluid produced inside the eye and the fluid draining out.
The eye constantly produces a clear fluid called aqueous humor, which fills the front chamber and maintains the eye’s shape and pressure. Low pressure means either too little fluid is being produced or too much is escaping. Several conditions can tip this balance, and identifying the underlying cause is essential because prolonged low pressure can permanently damage the eye.
Uveitis: The Most Common Cause
Uveitis, or inflammation of the inner structures of the eye, is by far the most frequent reason a dog develops low eye pressure. The inflammatory process interferes with the active transport mechanisms that produce aqueous humor, effectively slowing down fluid production. With less fluid being made but drainage continuing at its normal rate, pressure drops.
Uveitis itself has a long list of triggers. Infections are a major category: tick-borne diseases like ehrlichiosis, fungal infections such as blastomycosis, and leptospirosis can all inflame the eye from within. In a study of 73 dogs with ocular blastomycosis, low intraocular pressure was a common finding, and some of those eyes eventually shrank (a condition called phthisis bulbi) after prolonged inflammation. Immune-mediated conditions, where the body’s own defenses attack eye tissue, are another frequent cause. Lens-related problems, including cataracts that begin to leak protein into the eye, can spark inflammation too.
You might notice your dog squinting, tearing up, or having a reddened eye. The pupil on the affected side often appears smaller than normal, and the eye may look cloudy or hazy. These are all classic signs of uveitis, and any of them alongside a low pressure reading points your vet toward this diagnosis.
Trauma and Injury
A direct blow to the eye, a scratch from another animal, or any penetrating wound can cause low pressure in two ways. First, the physical damage may tear or compromise the tissues that produce aqueous humor, reducing output. Second, and more immediately, a wound can create an opening for fluid to leak out of the eye faster than it’s replaced. Even blunt trauma that doesn’t break the surface can trigger enough internal inflammation to suppress fluid production, essentially causing a traumatic uveitis.
Trauma-related drops in pressure are sometimes temporary. Once the injury heals and inflammation resolves, pressure often recovers. But severe or untreated injuries can lead to lasting damage.
Surgery-Related Pressure Drops
Low eye pressure is a recognized complication after eye surgery in dogs, particularly cataract removal and glaucoma procedures. After cataract surgery, aqueous humor can leak through the surgical incision site, whether or not sutures were placed. This excessive outflow lowers pressure until the wound seals completely.
Glaucoma filtration surgery is specifically designed to increase fluid drainage from the eye, so some degree of pressure reduction is intentional. When drainage is more aggressive than expected, though, pressure can fall too low. Post-surgical inflammation also contributes by dampening fluid production, compounding the effect of any wound leak. In most cases, post-operative low pressure resolves within days to weeks as the eye heals, but your vet will monitor it closely during recovery visits.
End-Stage Glaucoma
This one surprises many pet owners. Glaucoma is known for dangerously high eye pressure, but in its most advanced stages, pressure can actually drop to normal or even subnormal levels. This happens because the ciliary body, the tissue responsible for making aqueous humor, becomes damaged and atrophied after prolonged exposure to high pressure. The eye essentially loses its ability to produce fluid. At this point the eye is typically blind and visibly enlarged. A low reading in a previously glaucomatous eye is not a sign of improvement; it signals severe, irreversible damage.
How Vets Measure Eye Pressure
Veterinarians use handheld devices called tonometers to check eye pressure quickly and with minimal discomfort. The two most common types in veterinary clinics work differently but both give reliable readings.
Rebound tonometers (sold under the name TonoVet) fire a tiny, lightweight probe that briefly touches the cornea and bounces back. The device calculates pressure based on how fast the probe decelerates. This method requires no numbing eye drops and causes very little stress, making it popular for dogs that are anxious or difficult to examine. Applanation tonometers (such as the Tono-Pen) gently flatten a small area of the cornea and measure the force needed to do so. These require a drop of topical anesthetic first. Both types are accurate and portable, and the reading takes only seconds per eye.
Your vet will typically measure both eyes for comparison, since a significant difference between the two can be just as telling as an abnormally low number on one side.
Signs You Might Notice at Home
Low eye pressure itself doesn’t cause obvious pain the way high pressure does, so the symptoms you see at home are usually tied to whatever is causing the pressure drop. A dog with uveitis may squint, avoid bright light, paw at the affected eye, or have visible redness. The eye might appear cloudy, and the pupil may look noticeably smaller than the other side. Tearing or discharge is common.
If the pressure has been low for a long time, the eye can start to look physically different. It may appear sunken or smaller than the other eye. In severe, chronic cases, the globe can shrink permanently, a condition called phthisis bulbi, where the eye essentially collapses in on itself. This is an end-stage outcome and typically means the eye has lost function entirely.
How Low Eye Pressure Is Treated
Treatment targets the underlying cause, not the low pressure reading itself. There’s no drop you can give a dog to simply “raise” eye pressure the way glaucoma medications lower it. If uveitis is the culprit, the focus is on controlling the inflammation with anti-inflammatory eye drops and identifying what triggered the uveitis in the first place. When an infection is responsible, treating that infection is the priority. If the inflammation stems from an immune-mediated process, longer-term management with immunosuppressive therapy may be needed.
For post-surgical cases, treatment usually involves watching and waiting while the incision heals, with anti-inflammatory support to manage any associated swelling. Traumatic cases follow a similar pattern: stabilize the eye, control inflammation, and allow healing.
The urgency of treatment depends on how low the pressure is and how long it’s been that way. Short-lived, mild drops often resolve once the cause is addressed. Chronic low pressure is more concerning because the longer the eye stays soft, the higher the risk of structural changes that can’t be reversed. In cases where the eye has already progressed to phthisis bulbi and is blind, surgical removal of the eye may be recommended to prevent ongoing discomfort or infection, though some dogs live comfortably with a shrunken, non-painful globe that doesn’t require surgery.