Trauma

Penetrating injuries (e.g., dog bites, projectiles) or foreign bodies may result in retinal tears or induce intraocular hemorrhage, inflammation, or vitreous infection with subsequent exudative or traction RD. Severe blunt trauma with inflammation and hemorrhage also may cause RD, which may be exudative or rhegmatogenous in nature. It is extremely unlikely for a traumatic incident to cause RD in both eyes. The exception is strangulation, which can lead to bilateral exudative (hemorrhagic) RD.

Ocular Anomalies

RD may be associated with severe retinal dysplasia, optic nerve colobomas, vitreous abnormalities, and retinal nonattachment (i.e., developmental failure of the two retinal layers to unite). Animals with RD present at or soon after birth and often have concurrent abnormalities such as microphthalmia or cataracts. Detachment may be unilateral or bilateral.

Later-onset genetic anomalies such as cataracts or vitreous degeneration may lead to RD. Rhegmatogenous RD also may occur because of vitreous liquefaction or cataract formation, particularly when the cataracts are rapidly forming or hypermature, or when there is lens-induced uveitis. Rhegmatogenous RD may occur as a sequela to cataract surgery (reported incidence, 1% to 4%) or lens luxation or subluxation. Many of the terrier breeds suffer from genetically based primary lens luxation, and retinal tearing and detachment may occur with or without lens surgery in these breeds. Premature and severe vitreal degeneration occurs in the Shih Tzu and can lead to spontaneous retinal tearing and detachment at a young age. Affected animals may be blind with bilateral detachments at presentation. These detachments usually occur one eye at a time, and animals are brought to the veterinarian when the second eye develops a detachment. The author also has seen this type of detachment (related to severe vitreal degeneration) in Boston terriers, Italian greyhounds, Yorkshire terriers, Chihuahuas, and poodles.

When an RD is evident but there is no history of trauma, cataracts, or lens surgery, a thorough diagnostic workup is indicated to identify predisposing systemic diseases.

Hypertension and Hyperviscosity

Systemic hypertension (most often related to renal diseases) and hyperviscosity syndromes (caused by severe hyperlipidemia, hyperglobulinemia, or polycythemia) are relatively frequent causes of RD. These diseases typically damage the vasculature of the choroid and retina, creating hemorrhage in the retina and sometimes in the vitreous. Renal failure with secondary hypertension probably is one of the most common causes of bilateral RD; Cushing’s disease and the adrenal tumor pheochromocytoma also should be considered as potential causes of RD.

Neoplasia

The most common neoplastic causes of RD are multiple myeloma (in which hyperproteinemia and hyperviscosity damage retinal and choroidal vasculature) and lymphosarcoma (which may diffusely infiltrate the retina or choroid). Patients with lymphosarcoma also may present with retinal hemorrhage with or without detachment and anterior uveitis. Each of these neoplasms is likely to affect both eyes but can be unilateral. Large neoplastic masses (e.g., ciliary body iridal tumors) or metastatic lesions may induce traction RDs. If neoplastic tissue is subretinal or choroidal, exudative RD around the subretinal mass may occur. With neoplasms other than lymphoma and myeloma, RD is more likely to be unilateral.

Chorioretinitis/Retinochoroiditis

The term retinochoroiditis is used to imply that retinal tissue was inflamed primarily with choroidal inflammation occurring secondarily, whereas the reverse is true for the term chorioretinitis. Retinitis or chorioretinitis may cause focal or multifocal RDs. As discussed previously, small areas of inflammation are technically RDs, but this type of lesion is best termed chorioretinitis (see Chapters 254 and 255). When choroidal inflammation is severe and diffuse, large segmental or complete RDs may occur. In the dog chorioretinitis with RD may be caused by bacteremia or septicemia (e.g., leptospirosis, brucellosis), rickettsial agents (ehrlichiosis, Rocky Mountain spotted fever, bartonellosis, anaplasmosis), fungal organisms (aspergillosis, blastomycosis, coccidioidomycosis, histoplasmosis, cryptococcosis, and less commonly infection with Acremonium spp. and geotrichosis), algae (protothecosis), and, rarely today, canine distemper virus. Parasitic inflammation of the choroid and retina is more likely to cause smaller areas of detachment (i.e., multifocal chorioretinitis). Causes include ocular larva migrans (due to strongyles, ascarids, Baylisascaris larvae), toxoplasmosis, leishmaniasis, neosporosis, and possibly babesiosis.