Is Cat Glaucoma the Same as Dog Glaucoma?
Glaucoma — elevated intraocular pressure (IOP) causing damage to the optic nerve and retina — affects both cats and dogs, but the similarities largely end there. Feline glaucoma has a distinct pattern of causes, a subtler clinical presentation, and different treatment considerations. Understanding these differences is essential for cat owners and for vets approaching this condition in their feline patients.
Normal IOP in cats is 10 to 25 mmHg, comparable to dogs. However, cats with glaucoma may show less obvious pain signs than dogs, which means the condition can be further advanced by the time it is recognised. Awareness and routine veterinary eye checks are particularly important in cats.
Primary Glaucoma in Cats: Rare But Present
Primary glaucoma — where elevated IOP develops due to an inherited drainage angle abnormality with no underlying disease — is relatively uncommon in cats. When it does occur, it is most frequently seen in Siamese, Burmese, and Persian breeds, in whom a developmental abnormality called goniodysgenesis has been identified. The drainage angle is malformed, reducing aqueous outflow over time.
Primary glaucoma in cats tends to have a better long-term outlook than the secondary form when caught early, since there is no ongoing destructive underlying disease process. However, both eyes remain at risk and long-term medical management is required.
Secondary Glaucoma: The More Common Picture
The vast majority of glaucoma cases in cats are secondary — meaning that elevated IOP is a consequence of another disease occurring inside the eye. The most important causes are:
Uveitis
Inflammation inside the eye (uveitis) is the single most common cause of secondary glaucoma in cats. Inflammatory cells, protein, and debris block the drainage angle and cause IOP to rise. Uveitis in cats has many potential causes, including:
- Feline infectious peritonitis (FIP)
- FIV and FeLV-associated uveitis
- Toxoplasma gondii infection
- Bartonella species
- Trauma
- Chronic lymphocytic plasmacytic uveitis — an immune-mediated condition often of unknown cause, seen particularly in older cats
Post-inflammatory scarring can form adhesions between the iris and the lens (posterior synechiae) or between the iris and the cornea (anterior synechiae), further obstructing aqueous drainage and contributing to long-term pressure elevation.
Intraocular Tumours
Diffuse iris melanoma is the most common intraocular tumour in cats and a significant cause of secondary glaucoma. It typically begins as flat, dark pigmented areas spreading across the surface of the iris — quite different from the discrete, raised nodular melanomas seen in dogs. The pigmentation gradually expands, the angle becomes infiltrated, and IOP rises as a consequence.
Diffuse iris melanoma has the potential to metastasise, most commonly to the liver, lungs, and abdominal lymph nodes. This is why early recognition and appropriate action are critical. Many veterinary ophthalmologists recommend enucleation (removal of the eye) when diffuse iris melanoma is diagnosed, even before glaucoma develops, to reduce the risk of metastatic spread. A cat with new, spreading dark patches on the iris surface should be assessed by a veterinary ophthalmologist without delay.
Lens Luxation
A dislocated lens can physically block aqueous flow, causing a rapid rise in IOP. While lens luxation is a more frequent cause of glaucoma in dogs than in cats, it does occur in felines and should be considered when pressure elevation develops acutely.
Recognising the Symptoms in Cats
Cats are masters at masking discomfort, which makes glaucoma particularly challenging to detect early. Symptoms may include:
- Buphthalmos — an enlarged, visibly bigger eye compared to the other side; this is often the sign that prompts owners to seek help
- A fixed, dilated pupil that does not constrict normally in bright light
- Corneal oedema — a bluish or cloudy appearance to the cornea
- Changes in iris colour or texture — particularly new dark pigmentation spreading across the iris
- Apparent vision loss — bumping into objects, difficulty navigating in low light
- A dull, sunken, or altered appearance to the affected eye
Unlike dogs, cats may not vocalise pain or show obvious distress. Behavioural changes such as reduced grooming, hiding, or decreased appetite can be the only signs that something is wrong.
Feline Aqueous Humour Misdirection Syndrome
This is a distinct and poorly understood feline syndrome, previously referred to as malignant glaucoma. In this condition, aqueous humour is directed posteriorly into or behind the vitreous rather than flowing through its normal drainage pathway. The result is a forward displacement of the lens-iris diaphragm, collapse of the anterior chamber, and a dramatic rise in IOP.
Feline aqueous humour misdirection syndrome is often bilateral, tends to affect middle-aged to older cats, and responds poorly to conventional glaucoma medications. It represents one of the most challenging presentations in feline ophthalmology and typically requires specialist management.
Diagnosis
Diagnosis requires tonometry to confirm elevated IOP, followed by a full ophthalmic examination including slit-lamp biomicroscopy and fundic examination. A systemic workup is essential in cats to identify underlying causes. This typically includes:
- Blood panel and urinalysis
- FIV and FeLV testing
- Toxoplasma and Bartonella titres where indicated
- Thoracic radiographs to check for metastatic disease if iris melanoma is suspected
- Abdominal ultrasound in selected cases
Treatment Options
Treatment in cats depends on the underlying cause:
- Uveitis is managed with topical or systemic anti-inflammatory medications — steroids where infection has been excluded, or NSAIDs — alongside treatment of any identifiable infectious cause
- Topical pressure-lowering agents such as dorzolamide and timolol are used to reduce IOP medically
- Carbonic anhydrase inhibitors help reduce aqueous production
- Laser cyclophotocoagulation is performed less commonly in cats than in dogs, but may be considered in selected cases
- Enucleation is often the most appropriate option, particularly when diffuse iris melanoma is present, when the eye is chronically painful and blind, or when IOP cannot be controlled
Prognosis
Outlook depends entirely on the underlying cause. Primary glaucoma diagnosed early carries a relatively good prognosis with consistent medical management. Uveitis-associated glaucoma varies according to the cause of the inflammation. Diffuse iris melanoma carries a guarded prognosis because of metastatic potential, though enucleation before spread occurs gives the best chance of a favourable outcome. Any cat with a visibly changed or enlarged eye deserves prompt veterinary assessment — early action significantly improves the chances of preserving comfort and, where possible, vision.