Pupil dilation with tropicamide ophthalmic solution is required to evaluate the posterior segment. Upon dilation, it is possible
to detect inflammatory cells in the anterior vitreous, known as pars planitis. This cellular accumulation occurs secondary to the infiltration of cells from the adjacent pars plana or pars plicata of
the ciliary body.2,3 Dilated examination also allows for the detection of posterior uveitis. Posterior uveitis is often accompanied by retinal
inflammation because of the close anatomical position of the structures.2,3 The breakdown of the blood-ocular barrier located at the retinal blood vessels and the retinal pigment epithelium allows
inflammatory cells to migrate to the area and results in chorioretinitis.3 Clinically, edema, exudation, and hemorrhage within the vitreous, retina, and subretinal space may be observed.2,3
Because of the location of the retina and subretinal space over the tapetum, tapetal reflectivity may be diminished or appear
gray (Figure 6).2,3 Retinal detachments may also occur secondary to severe inflammation.2 Retinal detachments develop when the neurosensory retina separates from the underlying retinal pigmented epithelium. Detachments
typically occur secondary to the accumulation of blood or exudates between these two layers. Further evaluation of the fundus
may reveal vascular tortuosity, hemorrhage, or sheathing of retinal vessels by inflammatory cells, known as perivascular cuffing.
Figure 6. Chorioretinitis in a patient with posterior uveitis. Multifocal dark-gray (hyporeflective) lesions are seen scattered
throughout the tapetal fundus indicating an active inflammatory process. (Photo courtesy of Dr. Ellison Bentley.)
In cats with chronic posterior uveitis, the posterior segment may exhibit changes in pigmentation, retinal vascular attenuation,
and tapetal hyperreflectivity secondary to retinal degeneration and scarring after the resolution of chorioretinitis.2 The globe may become smaller secondary to impairment of aqueous humor production by a chronically inflamed ciliary body.2
In cats with bilateral uveitis, a thorough medical history, physical examination, complete blood count, serum chemistry profile,
and urinalysis are necessary because of the potential for an underlying systemic disorder. Important historical information
to obtain relates to the patient's environment (indoors vs. outdoors), use of flea preventives, travel history, history of
trauma, duration of clinical signs, and the presence of any clinical signs often associated with systemic illness, such as
inappetence and lethargy.
On physical examination, rectal temperature and mucous membrane color should be evaluated, and the cat should be examined
for ectoparasites, ocular or nasal discharge, and lymphadenopathy in addition to undergoing a thorough thoracic auscultation
and abdominal palpation. Additionally, serologic tests are available for many of the infectious disease processes that cause
uveitis (see below).2 Diagnostic testing is also recommended in cases of unilateral uveitis for which a primary ocular cause cannot be identified,
as systemic diseases may not always manifest with bilateral ocular signs.
Further diagnostic tests may be required if a diagnosis is not made with routine testing. Additional testing modalities are
available on aqueous humor samples, including PCR tests for various infectious agents, cytologic examination, and bacterial
culture and antimicrobial sensitivity testing. Samples are collected by aqueous humor paracentesis, which is performed under
general anesthesia. Vitreous humor can also be sampled when other diagnostic test results are unrewarding, but there is a
high risk of ocular hemorrhage and lens or retinal damage. Thus, this procedure is typically limited to patients that are
blind or nearly blind.7 Procedures to acquire aqueous and vitreous humor can carry serious complications since structures within the eye may be
inadvertently damaged. Thus, it is recommended that patients requiring such diagnostic tests be referred to an ophthalmologist.