Pannus in dogs – chronic superficial keratitis

Overview

The cornea is the transparent front of the eye, through which light enters. In pannus, inflammation develops on its surface and in the superficial supporting tissue, the stroma.

The condition is also called CSK or Überreiter's syndrome. Changes usually affect both eyes, but one can be much worse.

Inflammation often begins at the lower outer corneal edge and progresses towards the centre. Some dogs also have inflammation of the third eyelid at the inner eye corner.

Inflammation of the third eyelid, called plasmoma, can also occur without corneal pannus. Pannus has no permanent cure, but the long-term outlook is often good when it is properly controlled.

Causes

The immune system reacts to the cornea

In pannus, the dog's immune system reacts against the eye's own tissues, most commonly the cornea. A presumed inherited susceptibility is involved.

In one study from northern Europe, a particular combination of genes in the DLA region was associated with pannus in German Shepherds. The study compared 30 affected dogs with 25 healthy controls.

In that 2011 study, the odds ratio associated with the gene combination was 2.67. Eight affected dogs carried two copies of the risk combination, while none of the healthy controls did.

A study of inheritance in Greyhounds published in 2016 supported dominant inheritance. This breed-specific finding does not define inheritance of pannus in every breed.

Inflammatory cells, including lymphocytes and plasma cells, migrate from the corneal edge into superficial layers. New superficial blood vessels grow into the transparent tissue alongside these cells.

Accumulation of cells and vessels forms abnormal connective and vascular tissue in the cornea. Pigment may collect in the pink tissue as inflammation progresses.

Pigment-producing melanocytes migrate into the inflamed corneal stroma. Progressive changes may also involve corneal oedema.

UV light and third-eyelid changes

UV radiation is thought to alter corneal structure so that the immune system begins recognising it as foreign. In third-eyelid plasmoma, plasma cells and other inflammatory cells accumulate, particularly along the leading edge.

Inflammation can therefore thicken that edge and reduce its normal pigment.

Risk factors

Pannus is most often diagnosed in German Shepherds, Belgian Shepherds and Greyhounds. Other recognised predisposed breeds include Border Collies, Italian Greyhounds, Siberian Huskies and Australian Shepherds.

However, other breeds and crossbreeds can also develop it. Pannus is also diagnosed in Dachshunds, although typical patients often belong to herding breeds.

Symptoms typically begin at 1–6 years of age. Disease in a young dog may be more aggressive than disease starting later.

Strong UV exposure and living at high altitude can make control more difficult. Inflammatory activity may vary with the season and time spent outdoors.

Typical symptoms

Early changes are often painless and begin at the edge before covering the centre.

• Pink, slightly raised tissue appears at the outer corneal edge.

• Small blood vessels grow from the margins into the clear cornea.

• Brown or dark pigment appears on the cornea.

• The front surface becomes cloudy, making it harder to see through.

• The third-eyelid edge thickens, reddens or loses its dark colour.

• Mucoid discharge may accompany inflammation.

• Vision worsens if dark changes cover a large part of the cornea.

Active inflammation may sometimes cause discomfort, although pannus is usually painless. The initially raised lesion may spread and flatten while darkening and scarring.

Complications and consequences

Scarring and pigment block light

Without treatment, blood vessels, scarring and pigment may make the cornea so dense that light no longer passes through. Severe pannus can cause substantial visual impairment or blindness.

Changes caused by inflammatory cells and new vessels generally respond better to treatment than scarring and pigment deposits. Early treatment can halt progression in many dogs.

Even a brief interruption of medication can cause a severe recurrence and impair vision. Disease that has worsened again may be harder to control than before.

Diagnosis at the vet

Location of changes and ruling out other conditions

The vet assesses the eye symptom history and characteristic corneal changes to make the diagnosis. Superficial vessel growth and inflammatory tissue extending over the cornea are consistent with pannus.

The third eyelid is also examined for edge thickening and pigment loss. A slit-lamp microscope can assess corneal changes and the front of the eye.

Fluorescein staining checks for a corneal surface ulcer. The dye adheres where protective surface cells are missing.

Measuring tear production helps assess whether dryness contributes to symptoms. Eye pressure may be measured to rule out other eye diseases.

A sample from the cornea or conjunctiva may be collected for cell examination when needed.

Monitoring the extent of changes

Thick inflammatory tissue can conceal pigment and make its true extent difficult to assess. A 2021 study from Turkey classified pigment into four grades according to how well the internal eye structures could be seen.

In grade 1, little pigment was present and the internal structures were clearly visible. In grade 2, discolouration was more obvious but the structures remained visible.

In grade 3, heavy pigment made assessment of the internal structures difficult. In grade 4, pigment prevented seeing them through the cornea.

Monitoring assesses reduction of inflammatory tissue and spread of pigment separately. Treatment may be adjusted according to blood vessels, inflammatory tissue and conjunctival redness.

Official eye screening also assesses other structures, such as the lids, lens and back of the eye. The pupils are dilated with eye drops before the official examination to assess different structures reliably.

Some official screening schemes require identification before the examination, with the identification checked at the practice. The individual gene change underlying pannus in Greyhounds is not yet known.

Treatment

Lifelong topical treatment

The vet prescribes medication according to the eye examination and inflammatory activity. Usual treatment is an eye drop or ointment that suppresses inflammation and the immune response.

Corticosteroids used may include prednisolone or dexamethasone. Medicines affecting the immune response include ciclosporin and tacrolimus.

These may be combined with a corticosteroid to control disease with as little corticosteroid medication as possible. Once inflammation settles, the vet reduces treatment to the maintenance level the dog needs.

However, topical anti-inflammatory treatment continues for life. Antibiotics are sometimes needed if a secondary infection develops.

What is known about treatment response?

In a retrospective study from Turkey covering 2012–2019, pannus was diagnosed in 104 eyes of 55 dogs. Forty-two dogs were German Shepherds.

Conjunctival redness decreased during the first treatment week, although corneal vessels remained visible. At one month, mainly pigment remained on the corneas.

Pigment disappeared completely in three dogs during six months of follow-up. In photographic follow-up of one German Shepherd, pigment had increased again by the seventh treatment year.

These were treated dogs being followed over time, so the figures do not mean that every dog will follow the same improvement timetable.

Other treatments for severe pannus

A corticosteroid injection beneath the conjunctiva may be considered for severe inflammation. A subconjunctival implant releasing ciclosporin may help control disease in some dogs.

A superficial tissue layer may sometimes be removed from an extensively scarred, pigmented cornea to improve vision. The procedure is called superficial keratectomy.

Disease often recurs after surgery, so it is considered a last resort. Beta radiotherapy has been used in a small number of cases to reduce corneal vessels and inflammatory cells.

Prevention

Reducing UV exposure is part of preventing flare-ups. UV-protective goggles designed for dogs can reduce radiation reaching the eyes.

Avoid strong direct sunlight, particularly when the sun is high. Sun protection does not remove the need for lifelong topical treatment.

A dog diagnosed with pannus should not be used for breeding. An eye made symptom-free with medication does not become unaffected in a breeding assessment.

Continuing medication after symptoms settle reduces the risk of severe recurrence from a treatment interruption. Follow-up visits help match treatment to the disease's current activity.

Important for dog owners

Record medication and whether it can be given as agreed. Tell the practice if eye drops are difficult to administer or treatment has been interrupted.

Watch particularly for tissue and pigment spreading from the edges towards the centre. Photographs taken in the same light can help demonstrate changes at follow-up.

Improved symptoms do not mean a permanent cure, because pannus needs ongoing control.

Also agree with your vet how to manage outings in strong sunshine and suitable eye protection. New squinting, pain or a clear vision change needs reassessment even when pannus has already been diagnosed.

Washing hands before and after giving eye medication reduces product contamination and spread of microbes.

Keep the bottle tip clean and do not let it touch the eye, lids or other surfaces.

Calm rewards may make the next administration easier by keeping the experience positive.

Gentle support from another person may help keep the dog still while eye medication is given.

Discharge around the eyes can be gently removed with a towel dampened with warm water before medication.

You can lightly support your hand on the dog's head to steady the drop bottle.

Blinking spreads the medicine over the eye's surface.

Some drops taste bitter when they pass through the tear ducts to the back of the mouth and may cause drooling or gagging.

If the eye looks more inflamed or red after medication, the vet should assess it.

Frequently asked questions

Is pannus painful for dogs?

Typical pannus is often painless and does not itself involve corneal ulceration. Active inflammation may nevertheless cause discomfort. Severe squinting or keeping the eye closed may involve a corneal ulcer and is a reason for prompt examination.

Can pannus blind a dog?

Severe pannus can scar and pigment the cornea enough to impair or destroy vision. Early treatment halts progression in many dogs, but existing scarring and pigment respond less well than active inflammatory tissue.

Can I stop the eye drops when the eyes become clear?

Pannus treatment is lifelong, even if the eyes look better and inflammation settles. The vet can reduce treatment to an individual maintenance level, but even a brief interruption can cause a severe flare-up that may damage vision. There is no permanent cure.

Do sunglasses help with pannus?

UV-protective goggles designed for dogs can reduce exposure to radiation that worsens pannus. They are used when needed alongside lifelong eye medication, because sun protection does not remove the disease's immune-mediated nature or make it permanently curable.

Why does only one eye look affected?

Pannus usually affects both eyes, but changes can differ greatly in severity. The vet therefore assesses both eyes and the third eyelids, where inflammation may cause thickening and loss of normal edge pigment.