Diseases of the Uvea

Chapter 136 Diseases of the Uvea

David A. Wilkie

ANATOMY AND PHYSIOLOGY

The uvea is the middle or vascular tunic of the eye. It is covered externally by the fibrous tunic and provides much of the blood supply to the inner nervous tunic (retina) (see Chapter 131).

The uvea is composed of three regions: the iris, the ciliary body, and the choroid (see Chapter 138).

Iris

• The iris is the anterior most portion of the uvea. It forms an incomplete diaphragm between the anterior and the posterior chambers and regulates the amount of light entering the posterior portions of the eye.

• The color of the iris varies among species, within species, and even within the same animal. Heterochromia is a different color within or between irides.

• The iris contains a sphincter muscle controlled by parasympathetic fibers that travel from the Edinger-Westphal nucleus with the oculomotor nerve (cranial nerve 3) to the ciliary ganglion, where they synapse. The action of the sphincter muscle causes miosis. Intraocular inflammation will also result in contraction of the sphincter muscle. This results in the miosis and pain seen in anterior uveitis. The sphincter muscle can be paralyzed with parasympatholytic agents, causing dilation of the pupil. This is used clinically to better see the posterior portions of the eye and to treat anterior uveitis.

The diagnostic parasympatholytic agent of choice is tropicamide (Mydriacyl), whereas the therapeutic parasympatholytic agent of choice is atropine 1%.

• The iris dilator muscle fibers are radially oriented and lie along the base of the posterior epithelium. The dilator muscle is under sympathetic control in mammals. The sympathetic fibers course along the spinal cord to T1 to T3, leave in the ventral roots, course through the anterior mediastinum, and run anteriorly with the internal carotid to the cranial cervical ganglion, where they synapse. They then travel to the eye in association with the ocular vascular supply (see Chapter 141). The action of the dilator muscle is to produce mydriasis.

Ciliary Body

The ciliary body is divided into the pars plicata and pars plana and is located posterior to the iris. The ciliary processes are folds of the ciliary epithelium with a vascular core and are the source of aqueous humor. The ciliary body is also the origin of the lenticular zonules, which serve to anchor the lens. Through these zonules and the ciliary musculature, accommodation occurs.

Choroid

The choroid is continuous anteriorly with the ciliary body and extends posteriorly to the optic nerve. The choroid lies external to the retina and supplies nutrition to the outer portions (rods and cones) of the retina in the dog and cat. The tapetum is located in the dorsal portion of the fundus and lies within the choroid (see Chapter 138).

CONGENITAL ABNORMALITIES

Persistent Pupillary Membranes

Etiology

These are persistent remnants of fetal blood vessels in the anterior chamber that originate from the collarette region of the anterior iris face. They may be seen alone or in association with other congenital abnormalities. Although persistent pupillary membranes (PPMs) are felt to have an inherited component (especially in basenjis), the mode of inheritance is not established in most affected breeds.

Clinical Signs

The clinical signs vary depending on where the PPM strands attach. In general, the color of the PPM will be similar to that of the iris. PPMs that bridge iris to iris generally result in no clinical signs, whereas those attaching to the anterior lens capsule or corneal endothelium are associated with focal opacities of these structures. Differentiate PPMs from anterior and posterior synechiae when attaching to the cornea or lens, respectively.

Diagnosis

Base the diagnosis on the clinical signs and breed. Perform a complete ophthalmic examination to detect other congenital ocular abnormalities.

Treatment

There is no treatment required and the lesion is non-progressive. Council breeders about possible inheritance and discourage breeding of affected basenjis and corgis.

Coloboma

Etiology

A coloboma is a developmental anomaly in which a portion of the tissue, in this case the uvea, is absent. Colobomas can affect all three portions of the uvea and are often associated with other congenital anomalies such as PPMs, lens coloboma, or retinal detachment. Colobomas can be inherited or the result of a toxic insult to the developing eye during gestation.

Clinical Signs

Depending on which portion of the uvea is affected, clinical signs will vary. Colobomas of the iris result in dyscoria and may be associated with PPMs or lenticular abnormalities. Colobomas of the ciliary body or choroid are generally not noted by the owner and are seen on a routine ophthalmic examination or are present as a result of other congenital abnormalities or from secondary changes such as retinal detachment. Posterior segment colobomas are seen as part of the collie eye anomaly (CEA) (see Chapter 138).

Diagnosis

Base the diagnosis on the clinical signs and breed. Perform a complete ophthalmic examination to detect other congenital ocular abnormalities.

Treatment

There is no treatment required and the lesion is non-progressive. Council breeders about possible inheritance and discourage breeding of affected dogs.

Uveal Hypoplasia or Aniridia

A lack of complete development or absence of the affected portion of the uvea, iris hypoplasia, or aniridia is rare and usually associated with other congenital ocular abnormalities. Choroidal coloboma occurs in several breeds of dogs and is seen as part of CEA.

Uveal Cyst

Etiology

Uveal cysts arise from the posterior iris or ciliary body and can remain attached or break free and float in the anterior chamber. They arise as a primary disorder in certain breeds (golden retriever, Boston terrier) or can be secondary to anterior uveitis.

Clinical Signs

Uveal cysts are pigmented, attached to the posterior iris, or free floating in the anterior chamber. They can rupture, depositing on the ventral corneal endothelium or anterior lens capsule. In dogs, iris cysts will transilluminate, facilitating diagnosis. In cats, however, cysts are more densely pigmented, often remain attached, and may not transilluminate, making diagnosis more difficult.

Diagnosis

Base the diagnosis on the clinical signs and breed. A free-floating, pigmented mass that transilluminates is a cyst. If the mass remains attached or fails to transilluminate, it must be distinguished from a uveal melanoma. Use ocular ultrasound to demonstrate the hollow appearance of a cyst and distinguish between a cyst and a melanoma.

Treatment

If the cyst is not interfering with vision, no treatment is required. Large or multiple cysts may require treatment if vision becomes impaired. In addition, in cats, large cysts can be associated with an increase in intraocular pressure (IOP), requiring intervention. Treatment includes mechanical aspiration of the cyst from the anterior chamber or diode or Nd:YAG laser photoablation of the cyst, resulting in the rupture and/or resorption of the cyst.

IRIS ATROPHY

Etiology

Iris atrophy is seen most often in aged animals, especially toy-breed dogs and cats. Iris atrophy has been classified as primary, senile, and secondary. Primary iris atrophy is seen in Siamese cats, miniature schnauzers, poodles, and Chihuahuas. This is a slow progressive atrophy of the iris. Senile iris atrophy occurs in aged animals of all species and breeds. The distinction between these two forms of atrophy may not be significant. Secondary iris atrophy occurs as a sequela to chronic glaucoma, uveitis, or ocular trauma.

Clinical Signs

If the iris musculature is affected, the pupillary light response may be incomplete or absent and anisocoria may be noted. Atrophy of the pupil margin results in dyscoria. Thinning of the iris stroma appears as a focal color change or, if complete, as a hole in the iris.

Diagnosis

The diagnosis is made based on the clinical appearance and signalment. The differential diagnosis includes other internal or external causes of dyscoria, anisocoria, incomplete pupillary light response, or iris color changes. Measurement of IOP, a complete ophthalmic examination, and determination of afferent-versus-efferent pupil response abnormalities are all indicated with iris atrophy (see Chapter 141).

Treatment

No treatment is required. However, differentiate iris atrophy from other pupillary response abnormalities.

ANTERIOR UVEITIS

Uveitis is inflammation of the uvea and is divided into anterior (iris and ciliary body), posterior (choroid), and panuveitis (all three portions of the uvea). Posterior uveitis is often called chorioretinitis because of the intimate association between the choroid and the retina (see Chapter 138). Anterior uveitis is associated with pain and can be the result of both ocular and systemic factors. Because many etiologies of uveitis are systemic, it is essential to ascertain the reason for the inflammation when presented with an animal with anterior or posterior uveitis (Table 136-1).

If no primary ocular etiology of anterior uveitis can be ascertained, consider systemic disease. This approach applies whether the uveitis is unilateral or bilateral.

Etiology

Corneal Ulceration

Corneal ulceration often results in reflex anterior uveitis. Uveitis occurs due to stimulation of the ophthalmic branch of cranial nerve 5 (CN5) and does not indicate infectious keratitis.

Trauma

Trauma can result in anterior uveitis due to a direct penetration of the globe or a concussive effect. Uveitis may be associated with other ophthalmic abnormalities, such as corneal ulceration, hyphema, lens luxation, retinal detachment, proptosis, and globe rupture. See appropriate chapters for further discussion of these abnormalities.

Infection

Infectious causes of anterior uveitis are numerous and include bacteria, fungi, rickettsia, and protozoal organisms (see Table 136-1). These organisms result in anterior uveitis by direct infections of the eye, immune-mediated responses, or circulating endotoxins. Many of these infectious agents also cause posterior segment (retina, choroid) involvement (see Chapter 138). Direct infection of the eye can occur from penetrating trauma or blood-borne infection. Although anterior uveitis is commonly associated with infectious causes, the organism itself is generally not present within the anterior segment of the eye.

Only gold members can continue reading. Log In or Register to continue

Related

Stay updated, free articles. Join our Telegram channel

Tags: Saunders Manual of Small Animal Practice

Aug 27, 2016 | Posted by admin in SMALL ANIMAL | Comments Off

Full access? Get Clinical Tree

Get Clinical Tree app for offline access

Get Clinical Tree app for offline access