Evaluating and treating retinal disease (Proceedings)
A thorough fundic evaluation is important in diagnosing retinal disease. At the beginning of the exam the menace response, dazzle reflex and the pupillary light reflexes should be assessed. Abnormalities on the neuro-ophthalmic portion of the exam may augment interpretation of fundic findings. Additionally, behavioral testing such as ability to navigate a maze, to follow a dropped cotton ball, etc should be performed. Maze testing should be performed under both photopic and scotopic conditions. Dilation of the pupil, after initial examination and checking intraocular pressures to confirm that glaucoma is not present, should be performed. This allows visualization of the optic nerve as well as the peripheral retina. Visualization of the retina may be performed using an indirect lens and a transilluminator, otoscope, or penlight as a light source. A good quality 20D lens is a basic lens to start an evaluation. Most ophthalmologists use an indirect ophthalmoscope that is binocular and adds 0.5 D of magnification. Additionally, just as a refresher the use of an indirect lens makes the image upside down and backwards – important in localizing the lesion!
If the retina is not visible due to cataract formation or interference by opaque ocular media then other diagnostics need to be utilized. Ocular ultrasound and the electroretinogram (ERG) may be utilized to assess the retina if it cannot be directly visualized. Ocular ultrasound involves the use of a 12mHz probe (most commonly used) for assessing the optic nerve and retrobulbar area. Other available probes range from 20 to 50 mHz and are mostly used for diagnostics of the anterior segment. B mode ultrasound is generally utilized for evaluation of the globe. Very sensitive and lightweight ERG equipment is now available to facilitate retinal evaluation. The ERG may yield valuable information even when the retina is visible. The ERG machine may also be used to perform other electrodiagnostics such as visually evoked potentials, pattern ERG's, and multifocal ERG's. Gross intraocular details may also be noted on MRI and CT, however these are not optimum modalities for diagnosis. Other testing includes angiography, scanning laser ophthalmoscopy, and optical coherence tomography.
DNA testing is becoming more widely available for early diagnosis of many inherited retinal diseases. Mutation detection tests are specific to the actual genetic defect and are thus the most accurate and sensitive. Linkage tests use a DNA marker close to the actual genetic defect and although very informative, they are not as specific and sensitive as testing for the actual mutation. These tests are easily performed on blood or often a cheek swab of oral mucosa. Optigen is one of the many companies that perform these tests. CERF clinics offer basic screening for retinal disease using ophthalmoscopy and are important in detecting new abnormalities and mutations.Inherited and developmental retinal abnormalities are a significant cause of visual disturbance. Collie Eye Anomaly (CEA) may range in severity; choroidal hypoplasia is the least severe manifestation and is often termed grade 1 CEA, optic nerve colobomas, retinal detachment, and hemorrhage are more severe manifestations which may lead to blindness as well as other complications like glaucoma. Merle Ocular Dysgenesis is another inherited and developmental abnormality that may be vision threatening. Choroidal hypoplasia and posterior staphylomas are associated with vision disturbance; microphthalmia and iris/anterior segment abnormalities may also be involved. Iris sphincter dysplasia in Dalmations has been added to the CERF list of inherited abnormalities recently.
Retinal dysplasias may be non-vision compromising or lead to blindness depending on the severity. Retinal folds result from incongruity in the growth of different layers of the retina creating a rosette formation when examining histopathologically. These folds may resolve as the animal grows; this may lead to confusion in the CERF exam with a normal or abnormal exam depending on the age at exam. When this occurs the term "go normal" is often used to describe the situation; breeds that are predisposed to this progression should be examined at 6-8 weeks to try to catch the folds before they resolve. Even if the folds "go normal", the genetic defect is still present and may result in a more serious manifestation in offspring. Often these animals pass their CERF but have a note on their certificate. Folds that do not resolve are considered multifocal retinal dysplasia, but do not usually affect visual acuity. Geographic retinal dysplasia may lead to visual abnormalities and complete dysplasia with retinal detachment is blinding.