Vetnews | Desember 2024 38 « BACK TO CONTENTS The blood supply to the eye is called the uvea or uveal tract and is the“middle layer”of the eye, the outer or“fibrous layer” being the cornea and sclera with the retina the innermost layer. The choroid, ciliary body and iris are the 3 components of the uvea. Anterior uveal cysts are a relatively common finding in horses and may or may not be congenital in origin and have been observed in horses of varying ages. They may initially present as very small structures that enlarge and become apparent only later in life. These cysts may be found in 4 locations: 1) the corpora nigra; 2) along the pupil margin or free-floating in the anterior chamber; 3) attached to the ciliary body; 4) within the iris stroma. The corpora nigra are vacuolated extensions of the pigmented, posterior iris epithelium. They mostly occur along the dorsal border of the pupil margin and less commonly along the ventral pupil margin. Cystic corpora nigra is usually seen in middle-aged to older horses and may thus not be congenital. The exact cause/causes is not known but intraocular inflammation may play a role. Cystic corpora nigra become smooth and lose their usual roughened appearance. The cysts can be unilateral/bilateral and can vary significantly in size. Vision may be affected depending on the location and size of the cysts and may become apparent during bright conditions with related miosis of the pupil. Assessment of animals with uveal cysts and perceived vision issues should be examined in bright conditions without pharmacological dilation initially so that any change in vision and blockage of the pupil aperture can be observed. Iridal cysts are distinctly separate from the corpora nigra and are found along the pupil margin or free-floating in the anterior chamber. In dogs, these cysts represent a failure of the two layers of neuroectoderm to completely fuse and so allow fluid to accumulate between the epithelial layers. These cysts may enlarge with time. These cysts appear clinically as round to oval, smooth structures that may transmit light (transillumination) in some instances. Spontaneous rupture of the cysts may leave a round to oval area of pigment dispersion on the posterior surface of the cornea or anterior lens capsule. Ciliary body cysts arise in a similar manner to iridal cysts, the difference being that the failure of closure of the neuroectoderm occurs at the ciliary body and not the posterior iris thus the non-pigmented epithelium (NPE) is involved. These cysts can usually be transilluminated because they are lined with NPE. These cysts are usually oval and can be relatively large, they are mostly contained in the ventro-lateral posterior chamber (space between the iris and lens). For this reason, they can be difficult to observe without pharmacological mydriasis. These clinical signs make diagnosis of ciliary body cysts relatively straightforward, but ultrasound should always be used to confirm the diagnosis. Treatment of cysts is indicated when they cause visual and/ or behavioural disturbances. We recently had a case of a sport horse with a large iridal cyst that occluded the nasal half of the pupil during normal miosis in sunlight. The horse displayed considerable behaviour disturbances, while working in bright light, that completely resolved when the affected eye was occluded with a complete eye mask. Transcorneal ablation using a diode, argon, or neodymium: yttrium-aluminium-garnet laser is the preferred treatment for uveal cysts affecting vision or behaviour in horses. It is always prudent to distinguish between inflammatory/ neoplastic masses and cysts when considering treatment. Laser ablation can be performed with the aid of standing sedation and has been found to cause minimal postprocedure inflammation, which in turn reduces the risk of complications, minimizes follow-up treatment, and speeds return to work. Other treatment options, such as fine needle aspiration or automated irrigation and aspiration, are more likely to require general anaesthesia and also necessitate entry into the anterior chamber, increasing the risk of complications and intensifying post-procedure intraocular inflammation. Regulars I Ophthalmology Column Equine Uveal Cysts Dr Paul Burger Specialist Veterinary Ophthalmologist Johannesburg and Cape Animal Eye Hospitals www.animaleyehospital.co.za Figure 1: normal corpora nigra in a horse
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