Vitrectomy

Information about equine recurrent uveitis and its treatment
(internal eye inflammation, periodic eye inflammation, moon blindness)

As of January 2020

Cause

Recurrent uveitis in horses is caused in over 95% of cases by a leptospirosis infection in the eye. Leptospires are bacteria that are primarily transmitted by mice and rats, which excrete them in their urine. Horses can become infected with these bacteria in paddocks or stables, where they can enter the bloodstream via mucous membranes and minor injuries. Leptospires can survive for long periods, especially in damp soil or stagnant water. Almost every horse comes into contact with leptospira at some point. A few are unlucky enough to have the pathogens enter not only their bloodstream but also one or both eyes. Since leptospira are not very aggressive germs, they can survive in the eye for months or even years before causing eye inflammation.

The inflammation in the eye gradually damages the structures of the inner eye, and almost all eyes become blind as the disease progresses. There are different courses of this eye inflammation.

Some horses suffer from inflammation that mainly affects the front part of the eye. This is very painful and is therefore noticed by the owner at an early stage. For this reason, patients with acute inflammation are usually treated promptly and intensively. The inflammation can occur at intervals of days, months, or years. As a rule, the inflammatory episodes become more severe and the intervals between them become shorter over time.

Other horses suffer from inflammation that occurs more in the rear section of the eye and is hardly painful. In these horses, the inflammation can progress unnoticed and insidiously. It is often only noticed by chance during purchase examinations or when the horses become blind.

After several episodes of inflammation, or in some horses from the outset, the entire eye may be affected by the inflammation. The inflammation can cause the iris to stick to the lens. These adhesions and inflammatory products in the vitreous cavity, which accumulate on the posterior parts of the lens capsule, can cause lens opacities. The lens has no blood supply and is nourished by the surrounding fluid. If the lens capsule is so severely damaged that nourishment is no longer guaranteed, lens opacities (cataracts) develop. Inflammation causes clouding in the vitreous cavity and thick, membrane-like deposits can form, which significantly impair vision. The retina normally lies against the back of the eye. This is where light signals are received and transmitted to the brain via the optic nerve. The inflammation can loosen the connection between the retina and the back of the eye, leading to retinal detachment (comparable to damp wallpaper coming away from the wall). If extensive retinal detachment occurs, this usually means irreversible blindness for the horse. In rare cases, increased intraocular pressure (glaucoma) can develop.

Treatment

The treatment for acute eye infections is always the same, provided there is no corneal defect: atropine eye ointment to dilate the pupil, several times a day if necessary; dexamethasone-containing eye ointments several times a day; plus an anti-inflammatory and pain-relieving preparation via the oral cavity (e.g., phenylbutazone twice daily).

Despite intensive and consistent medication, recurrences (renewed inflammation) usually occur. The only way to stop further inflammation and thus the progressive destruction of the eye is vitrectomy (vitrectomy). In this procedure, the vitreous body is cut into small pieces and suctioned out. At the same time, the inflammatory products and opacities are removed from the vitreous body, which in many cases improves vision after the operation. Due to the removal of bacteria associated with the irrigation, the eye inflammation is permanently eliminated after the operation with a probability of over 95%.

Surgery

The horses are usually in the clinic for preparatory treatment approximately three days before the operation. After the operation, they remain in the clinic for approximately four to five days for follow-up treatment and monitoring. Currently, around 100 vitrectomies are performed each year at the LMU Horse Clinic.

Vitrectomy can only be performed under general anesthesia. If both eyes are affected, there should be a gap of around five days between the operations on each eye. It is not possible to operate on both eyes under one anesthesia, as the eye that was operated on first would otherwise be positioned under the heavy horse's head during the operation on the second eye. During the operation, the eyeball is rotated downward to allow access to the vitreous cavity via the white of the eye (in the “pars plana” area). After opening the conjunctiva above the sclera, two openings are made in the sclera using a laser. An infusion line is inserted into the eye through the first opening to maintain constant pressure in the eye during the operation. The surgical instrument is inserted into the eye through the second opening to cut up and suction out the vitreous body containing the inflammatory products and bacteria. After the operation, the openings in the sclera and conjunctiva are closed with a thin, self-dissolving suture. Normally, the operated eyes are pain-free immediately after the operation and the eyelid slits are open normally.

After the operation, the infused fluid is quickly replaced by aqueous humor in the eye. The vitreous body does not regenerate. The vitreous body is only necessary for embryonic development; it no longer serves any purpose in adults. The vitreous body structure removes the “hiding place” for bacteria, and after the operation, there is better fluid circulation in the vitreous body space filled with aqueous humor, meaning that bacteria cannot settle there again. It makes no difference to vision or visual acuity whether vitreous body material or aqueous humor is present in the vitreous body space.

Follow-up treatment

After discharge on the 4th or 5th day following the operation, the horses should only be exercised at a controlled walk for 5 days. After that, free running and progressive training are possible again. Follow-up treatment usually consists of applying eye ointment twice a day for a week. After that, no further treatment is normally required.

Suitable eyes

In principle, it is highly likely that vision can be preserved in eyes that have not yet suffered damage to the lens and retina (no or only minor adhesions between the iris and lens; no or only minor inflammatory products adhering to the rear surface of the lens; no visible retinal detachment). If the lens is already more severely affected, early lens opacity may progress even if there is no longer any inflammation. If early (extensive) retinal detachment is already apparent, there is an increased risk of the retinal detachment progressing. It is not yet possible to effectively fix the retina in horses using laser treatment. In the case of lens opacity, only light-dark vision may be possible later on, but no more object vision. Extensive retinal detachment means irreversible blindness for the animals. In more severely damaged eyes, vision cannot always be preserved, but further painful eye inflammation can still be effectively prevented.

The less damage there is to the eyes, the better the prognosis for preserving vision. However, it should be confirmed that the eyes are affected by uveitis, as the operation has no effect on painful and recurrent corneal inflammation, for example. In addition, in individual cases, uveitis may subside and no further inflammation may occur. For these reasons, and because every anesthesia and every operation involves risks, the timing of the operation should be chosen carefully.

Only in cases of doubt (!!!) (uncertainty as to whether the painful eye inflammation is caused by uveitis or, for example, corneal inflammation) can a small amount of fluid be removed from the anterior chamber of the eye under short anesthesia and tested for antibody titers against leptospires. If no antibody titer is present and no leptospira components are detectable, there is a 95% probability that the uveitis is not suitable for surgery. If antibodies against leptospira (using MAR or ELISA) or leptospira components (using PCR) are detectable, there is an indication for vitrectomy.

In rare cases, horses with uveitis may be prone to developing increased intraocular pressure (glaucoma) (blockage of the channels that drain fluid from the eye with inflammatory products or pressure on the drainage channels due to a clouded and displaced lens). In these horses, the course of the disease is more difficult to influence than in animals whose eyes tend to become smaller and who are prone to developing reduced intraocular pressure. Horses with glaucoma therefore have a poorer prognosis and, in the event of surgery, laser treatment of the fluid-producing tissue in the eye (laser cyclocoagulation) should be performed in addition to vitreous surgery.

Known complications

In horses that have been treated with cortisone-containing eye ointments for a long time and in horses that tend to rub their eyes, corneal defects can complicate the course of treatment and prolong the necessary stay in the clinic, as the cornea must be intact for the operation.

As at tournaments and other events where many unfamiliar horses come together, there is an increased risk of (viral) infections (e.g., influenza) in clinics, especially for unvaccinated horses.

As with any other anesthesia, there is a risk involved in this operation. Injuries can occur when the animals are laid down or stood up, and in the worst case, leg fractures or other bone fractures, e.g., to the head or neck, can occur. Incidents can occur during anesthesia (e.g., respiratory or cardiac arrest) that can lead to the loss of the horse. These complications are very rare in planned procedures on healthy horses, but the risk is not zero. In a study conducted at our clinic between 2006 and 2011, excluding “high-risk anesthesia,” 0.3% of a total of 1,989 horse anesthesias resulted in fatal incidents.

Other complications that can occur with any anesthesia/surgery include colitis X (an inflammation of the large intestine that can be accompanied by life-threatening diarrhea) and life-threatening pneumonia and pleurisy. Both diseases can be exacerbated by stressful situations (e.g., change of stable, anesthesia, long transport).

Lens opacities and retinal detachments can occur in the eye, especially in eyes that have been severely damaged. In rare cases, these complications can also occur in eyes that have been minimally damaged. Bleeding rarely occurs during or after surgery, and retinal detachments are very rare. In the case of bleeding, it depends on the amount of blood whether it will be broken down in the following period or whether it will need to be flushed out in a second operation. Complete retinal detachment means irreversible blindness in the eye. In isolated cases (< 1 in 500 horses), a purulent infection of the inner eye may occur after the operation. In these cases, vision cannot usually be preserved. In the worst case, it may also be necessary to remove the affected eye.

Other rare or previously unknown complications cannot be ruled out.