Non-Inflammatory Disease of the Uvea in Horses

Take Home Message  

Abnormalities of the equine uvea are relatively uncommon and often of minimal consequence to  vision or comfort. However, some innocuous conditions can mimic serious disease and therefore proper assessment is important to avoid misinterpretation and inappropriate clinical action. The  purpose of this lecture is to review noninflammatory diseases of the equine uvea.  

Congenital Diseases  

Aniridia  

While the term aniridia infers a complete absence of an iris, it has been used to describe a  clinical condition reported in horses where only a rudimentary, non-functional ridge of iridal  tissue is present. Aniridia has been reported in a Belgian Draft stallion (along with 65 of his  offspring),¹ a Quarter Horse stallion,^2,3 a Thoroughbred colt⁴ and a Welsh/Thoroughbred cross  filly.⁵ Aniridia is thought to be heritable via an autosomal dominant mode.^1-3 Horses affected  with aniridia are usually presented within the first few months of age because the client has  noticed such signs as an unusual appearance of both eyes, squinting excessively when in bright  sunlight, and/or overreacting to flashes of light.⁶ On ophthalmic examination, the pupils are  widely dilated and non-responsive. The extent of the pupillary dilation is so marked that the lens  equator and ciliary body processes are visible. The corpora nigra is absent. Posterior segment examinations in horses with aniridia have been uniformly unremarkable.⁶  

Anterior Uveal Cysts  

The cysts discussed here may not be congenital, but may be present as very small cysts at birth  only to enlarge and become apparent later in life. Anterior uveal cysts may be found in four  locations: 1) the corpora nigra, 2) along the margin of the pupil or free floating in the anterior  chamber, 3) attached to the ciliary body, and 4) within the iris stroma (Fig. 1).  
 

When the corpora nigra becomes cystic, the normally roughened appearance of the corpora nigra becomes smooth and spherical. The condition may be unilateral or bilateral, and the size of the cysts can vary markedly. Most are sufficiently small that they do not cause significant  interference with vision. However, depending on the specific location, even moderate-sized cysts  can partially inhibit the visual field, especially when the horse is in bright light and the pupil is  miotic. Differential considerations include inflammatory or neoplastic changes to the iris in the  area of the corpora nigra. Although ultrasound could be used to differentiate cystic corpora nigra  from inflammatory or neoplastic infiltrates, this is rarely necessary as the clinical appearance is usually sufficiently characteristic. Treatment is not ordinarily necessary, the most effective and  non-invasive treatment is deflation of the cystic corpora nigra with a laser.⁷  

Iridal cysts may be found distinctly separate from the corpora nigra along the margin of the pupil or free floating in the anterior chamber. These cysts are thought to develop similarly to iridal  cysts in dogs and represent a failure of the two layers of neuroectoderm to completely fuse, thus  allowing fluid to accumulate in areas between the 2-layered posterior iris epithelium. Because  iridal cysts can enlarge, it has been theorized that some of the posterior iridal epithelial cells that  comprise the lining of the cyst retain secretory ability.⁶

Ciliary body cysts (Fig. 2) are a feature of multiple congenital ocular anomalies of Rocky Mountain horse⁸ and are thought to originate in a manner similar to that of iridal cysts, except in this instance the failure of the two layers of the neuroectoderm to fuse is in the area of the ciliary  body. Other potential causes of ciliary body cysts include inflammatory processes, traction from zonules, anterior segment dysgenesis, and age. They tend to be oval and relatively large, often  extending from the pars plicata to the ora ciliaris retinae. Their most common location is  ventrolaterally and they can be difficult to detect without pharmacologic mydriasis. As with the other anterior uveal cysts discussed, differential considerations would include inflammatory and  neoplastic disease. While ocular ultrasound can be used to confirm the diagnosis of ciliary body  cyst, the shape, contour, and ability to transilluminate make this differentiation straightforward on the basis of clinical signs alone. These cysts do not routinely cause pain or visual impairment and treatment is not necessary.⁶ 
 

Iris Hypoplasia  

Historically, dark and bulging areas in the anterior surface of the iris have been interpreted as  cysts within the iris stroma.⁹ These dark, bulging areas are seen predominately in the dorsal  region of eyes with blue irides, although they can be present in the ventral aspect of the iris and  in eyes with brown irides. These lesions represent areas of iridal stromal hypoplasia and that the resulting protrusion is a result of aqueous pressure pushing the relatively weaker portion of iris forward.¹⁰ The affected eyes are not painful and there is no discernable effect on vision. The fact that these protruding areas actually represent thinning of the iris stroma is confirmed by passing  a bright, focal beam of light through the pupil and observing the retroillumination of the fundic reflection through the lesion. When the pupil is subsequently dilated either by the use of mydriatic agents or dark adaptation, the bulging area disappears, taking on a wrinkled  appearance. The most important rule out for this clinical presentation is iridal neoplasia,  specifically melanoma. Ocular ultrasound can aid in making this differentiation, but careful examination under conditions of both miosis and mydriasis as described above should allow for an accurate diagnosis.⁶  [...]