Retinal Dysplasia

Retinal dysplasia is a group of conditions that involve abnormal development of the retina. Retinal dysplasia is classified by both cause and severity. Categorized by cause, retinal dysplasias include acquired lesions, which may result from insults such as in utero infection or irradiation, and hereditary lesions. Complete or total retinal dysplasia is the most severe form and is characterized by blindness due to detachment or congenital nonattachment of the retina. Concurrent congenital abnormalities such as microphthalmos and cataract may be present. Nonocular congenital anomalies, such as skeletal dwarfism, also may be observed with complete or total retinal dysplasia. An example is oculoskeletal dysplasia, also referred to as dwarfism with retinal dysplasia. In the Labrador retriever and Samoyed breeds the causal gene mutations for dwarfism with retinal dysplasia have been identified.

A less severe retinal dysplasia known as geographic retinal dysplasia is seen in many breeds of dog (see Figure 254-1, C). The tapetal retina dorsal to the optic nerve head is affected most frequently. Affected regions may be focally detached or there may be regions of retinal thinning with subsequent pigmentation, which produces an appearance similar to that of a chorioretinal scar. Unless the geographic lesions are very extensive or a complete retinal detachment develops, vision is not noticeably affected.

A third form of retinal dysplasia is known as multifocal retinal dysplasia. In this form the lesions may appear as circular dots, linear streaks, or V– or Y-shaped streaks, most commonly in the tapetal fundus. Typically there is a duplication of the photoreceptor layer within the lesions, which also are called rosettes because of their appearance in histologic sections. The lesions usually have a different color from the surrounding normal fundus and may appear hyporeflective (due to thickening of abnormal retinal tissue). The affected retina also may degenerate, which leaves a hyperreflective spot or streak. Multifocal retinal dysplasia typically does not have any detectable effect on vision.

Canine Multifocal Retinopathy

Canine multifocal retinopathy (cmr) is an autosomal-recessive condition caused by mutations in the canine BEST1 gene (cBEST1). Three different mutations have been identified in cBEST1 in different breeds of dog causing cmr1, cmr2, and cmr3. Mastiff breeds are affected with cmr1, the Coton de Tulear breed with cmr2, and the Lapponian herder with cmr3. Dogs affected with cmr develop multiple small bullous retinal detachments that have tan-colored subretinal fluid. These first develop in young dogs and then may remain unchanged for years. The cmr3 form may have a somewhat later onset than cmr1 and cmr2, with lesions developing at about 1 year of age. Histologically, the lesions show focal loss of photoreceptors, and the underlying retinal pigment epithelium is abnormal. Currently there is no treatment for the condition, although gene therapy is being investigated experimentally. Genetic tests for cmr1, cmr2, and cmr3 have been developed.

Progressive Retinal Atrophy

Progressive retinal atrophy (PRA) is a group of conditions that have a similar clinical presentation, although the age of onset and rate of progression can vary considerably by breed. The majority of forms are inherited in an autosomal-recessive fashion, although dominant and X-linked PRAs have been identified. There are forms of PRA that are breed specific (the causal mutation has been identified in only a single breed), others that occur only in closely related breeds, and one important form that is caused by a mutation in the progressive rod-cone degeneration gene (PRCD) that occurs in many different breeds. In attempts to categorize the different forms, PRA has been divided into categories based on age of onset and the pathologic changes that occur in the retina of affected dogs. Thus there are early, middle-aged, and late-onset forms and forms with descriptive names such as rod dysplasia, rod-cone dysplasia (types 1, 2, and 3), early retinal degeneration, and progressive rod-cone degeneration. Advances in molecular genetics have allowed the underlying gene mutation of several forms to be identified. This allows classification on a molecular basis so that the disorder is described as PRA due to a particular mutation in a certain gene (Table 254-1).

Classically, PRA results in an initial deterioration of vision in dim light followed by a progressive loss of daytime vision and ultimate blindness. Owners also may notice that the pupils are unusually dilated and there may be increased “eye shine” due to a combination of the pupillary dilation and increased tapetal reflectivity. Cataract formation commonly accompanies PRA and may be the initial presenting complaint. Ophthalmoscopic signs of PRA include progressive development of a generalized hyperreflectivity of the tapetal fundus, which is a result of retinal thinning (see Figure 254-1, B). Retinal vasculature becomes progressively attenuated, and atrophy involving the optic nerve head also develops. Pigmentary changes (patchy increase and decrease in pigmentation) in the nontapetal fundus also may become apparent. Electroretinography, a diagnostic method used to measure the retinal electrical response to light stimulation, can be used for the early diagnosis of many forms of PRA and also for assessment of retinal function when ophthalmoscopic examination is not possible because of cataract. DNA tests now are available for many breeds.

Cone-Rod Dystrophy

The term cone-rod dystrophy (CRD) is used for retinal degenerative conditions in which the cone photoreceptors are affected earlier or more severely than the rod photoreceptors. This is in contrast to “classical” PRA in which rods are affected initially or more severely in the early stages of the condition compared with cones. The ophthalmoscopic changes in CRD may be very similar to those in PRA (tapetal hyperreflectivity, retinal vascular attenuation, optic nerve atrophy). Electroretinography is useful in differentiating the two conditions. DNA testing is available for many breeds.