What is the Uveal Tract and Uveitis?
The uveal tract is made up of the anterior uvea and posterior uvea. The anterior uvea includes the iris and ciliary body (which itself is made of the pars plana and pars plicata). The posterior uvea includes the choroid. Inflammation of any of these parts of the uveal tract is termed uveitis. We can for example have iritis or cyclitis (inflammation of the ciliary body), which is then termed anterior uveitis. Similarly choroiditis implies posterior uveitis. Specific diseases can selectively affect specific parts of the uveal tract. For example lymphosarcoma in cats is often manifested as iritis, while uveitis associated with feline AIDS has been associated with pars planitis. Some diseases such as the Vogt Koyonagi Harada Syndrome, an autoimmune disease directed against melanocytes, may cause more of a pan uveitis, involving both the anterior and posterior uvea.
Uveitis causes a breakdown in the blood aqueous barrier (BAB) at the level of the ciliary body. The breakdown in the BAB results from locally acting hormones (autacoids) such as prostaglandins and other inflammatory mediators such as histamine, bradykinins etc. The importance of this is that rather than having a selectively permeable membrane at the level of the ciliary body, the tight junctions of the endothelial cells are leaky. This results in influx of initially low molecular weight compounds and then in time higher molecular weight compounds and even white blood cells into the anterior chamber. These products if left uncontrolled may cause disastrous sequelae in the eye.
Why is Uveitis Bad in the Eye?
Inflammation within the eye may cause functional damage of the affected part. For example, inflammation related to the drainage angle and cornea may cause adhesions (anterior synechiae) resulting in secondary glaucoma, inflammation in the vitreous may result in retinal detachments, while inflammation associated with the lens may cause cataracts. Most of the structures within the eye do not tolerate inflammation at all well to the extent that the actual function of that structure is lost, for example inflammation of the cornea initially results in corneal oedema and vascularisation, and eventually in pigmentation and blindness (as seen in the dry eye syndrome).
What Does the Eye Do To Try To Avoid Inflammation?
The eye is an immune privileged site in the body i.e., the eye has developed elaborate mechanisms to avoid inflammation. Immune privilege in the eye has been known for more than a century, when it was noted that allogenic tissue transplanted into the anterior chamber survived for a much longer time than tissue transplanted to other sites. This unusual response is termed anterior chamber associated immune deviation, or ACAID. ACAID in all likelihood helps to avoid the high level of non specific inflammation associated with clearing an antigen, and some of the undesirable sequelae listed above.
What Causes Uveitis?
Uveitis is not a diagnosis. Uveitis is a symptomatic description, and does not give an idea of the aetiologic agent involved. Often a specific cause of uveitis can not be found. In humans with uveitis only about 50% of cases are diagnosed with an aetiologic agent, even after a barrage of tests. There is still the need to do a work up for uveitis, but there may not be any definitive aetiologic diagnosis found. There are a wide range of exogenous and endogenous causes of uveitis.
Some Causes of Uveitis
Algal--prototheca
Bacterial--Brucella canis, Borrelia burgdorferi (lymes disease), Leptospirosis sp., septicaemia of any cause
Fungal--Blastomyces dermatitidis, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum, other mycoses
Immune mediated--lens trauma (phacoclastic uveitis), other immune mediated diseases such as VKH, immune mediated vasculitis and immune mediated thrombocytopaenia
Neoplastic and paraneoplastic disorders--hyperviscosity syndrome, granulomatous meningoencephalitis, primary and secondary neoplasia
Metabolic--diabetes mellitus, systemic hypertension, hyperlipidaemia syndromes
Miscellaneous--trauma, toxaemia, coagulopathies, idiopathic, pigmentary uveitis, radiation, ulcerative keratitis of any cause
Parasitic--ophthalmyiasis interna, ocular filariasis, ocular larval migrans
Protozoan--Toxoplasma gondii, Ehrlichia canis, Ricketsia rickettsii, Leishmania donovani
Viral--adenovirus, distemper, rabies, herpes virus
Each species and geographic region has specific causes and one must be familiar with thee for testing. In Sydney for example, if a cat presents for uveitis then I will usually suggest a complete blood count, and biochemistry panel, as well as FIV, FeLV and toxoplasmosis titres.
What are the Clinical Signs of Uveitis?
The typical signs associated with uveitis are: conjunctivitis, corneal oedema, low intraocular pressure, small pupil, pus in the anterior chamber, reddened irides, hyphema or blood in the eye, cataract formation, adhesions of the iris to the lens (posterior synechiae) or cornea (anterior synechiae), vision loss, retinal detachments, secondary glaucoma, and if the uveitis is intractable then shrinkage of the eye (phthisis bulbi).
What Treatments are Available for Uveitis
Treatments for uveitis are initially directed at the specific cause of the inflammation. For example toxoplasmosis in cats may be treated with azithromycin at appropriate dose rates. Other specific treatments can be obtained from appropriate texts. In many cases however we may not be able to identify the cause.
After treating the primary cause, secondary complications can be attended to. As a general rule of thumb the use of systemic and / or topical steroids is the mainstay of treatment. The use of non steroidal antiinflammatories may also be used in cases where the inflammation is mild or there is concern that use of systemic steroids, such as in diabetes cases, may have a detrimental effect.
Pain relief is also a priority. Personally I also like to use atropine drops or ointment in mild cases of inflammation as an analgesic. Atropine has an analgesic effect by acting as a cycloplegic (i.e., atropine paralyses the ciliary body muscle spasm which ordinarily causes some of the pain of uveitis). However great care must be used as this may predispose animals to secondary glaucoma. I would say that unless you are confident then don't use it. If severe pain is present, then systemic carprophen at standard doses may be used. Also animals with uveitis are very light sensitive (photophobic), and so keeping such animals in low light is a good idea as well.
What are the Complications of Uveitis?
Unfortunately in some cases, despite aggressive medication, we are unable to control the inflammation in the eye. In these cases, permanent scarring inside the eye, retinal detachments or glaucoma may occur. Vision may also be lost. In the worst case scenario, we may need to remove the eye, or place an intrascleral prosthesis. In cases where the uveitis has been present for some time, the eye may even degenerate (phthisis bulbi).
References
1. Veterinary Ophthalmology 4th Edition, 2007 edited by Kirk Gelatt.