The choroid is the posterior aspect of the uvea or vascular tunic; it contains blood vessels, melanin, and, dorsally, the tapetum. The choroid supplies nutrition and removes waste material from the retina; provides a cooling function, helping to dissipate heat generated by the visual process; and facilitates vision in dim light. To perform these functions, the choroid has a tremendous blood flow, far more than that simply required for oxygenation. Although this ensures the health of the retina, it also results in significant risk to the retina and choroid from vascular abnormalities (eg, hypertension, hyperviscosity, vasculitis), hematogenously disseminated disease (eg, neoplasia, bacteremia, viremia, tick-borne disease, disseminated mycosis, parasitism, algal infection, protozoal infection), and immune disease (eg, uveodermatologic syndrome).1–9 Whereas chorioretinitis is most often bilateral, unilateral disease can occur and does not preclude systemic disease.
Examination of the posterior segment of the eye is indicated in animals with vision disturbance, when anterior uveitis is present, in cases of known systemic disease, and in patients with systemic abnormalities of unknown cause as in fever of unknown origin.
Active chorioretinitis is typically an ocular manifestation of a systemic disease with a hematogenous cause. Baseline data should include a complete history, including travel history and vaccination status, as well as a physical examination, CBC, serum chemistry profile, urinalysis, diagnostic testing for relevant infectious diseases, thoracic and abdominal imaging, and cytology.
Given the predilection for chorioretinitis in many systemic diseases, a dilated fundic examination should also be a routine part of the physical examination in animals with fever of unknown origin and when disseminated neoplasia, mycosis, vasculitis, tick-borne diseases, or similar conditions are considered as possible differentials.