advances in diagnostics: dr. kim smyth explains electroretinography
Recently, I wrote a blog about echocardiograms, which included my thoughts on how lucky we are as a profession to be able to rely on specialists (in the case of that blog, I was referring specifically to cardiologists). I am equally thankful for the existence of veterinary ophthalmologists, to whom I can refer my most difficult cases involving the eye.
Animal eyes are an often an enigma to me, mostly because our four legged friends can’t talk to us and tell us how they feel. Obvious cases of conjunctivitis and corneal ulcers aside, many of the things that happen to our pets’ eyes happen inside the eye itself. It would be very helpful to everyone involved if our pets could simply tell us that their eyes are painful.
Instead, we must rely on diagnostic tests to tell us what is happening in a dog’s eye. While most veterinarians can perform some ophthalmic tests right in the exam room to rule conditions in or out, some tests can only be performed by ophthalmologists. Electroretinography (ERG) is one of these tests.
ERGs test the function of the retina, the part of the eye that detects light and creates an image for the brain. ERGs are used to diagnose hereditary retinal dystrophies and other causes of retinal degeneration, as well as ruling out generalized retinal disease in patients with a sudden onset of blindness. ERGs are also used to detect retinal response in patients with mature cataracts before they undergo cataract removal surgery.
ERGs are non-invasive, but are generally performed under sedation by an ophthalmologist. A light source is used to expose the retina light, and electrodes pick up the retina’s response. A waveform is created from the retina’s electrical responses to light, and this image is analyzed for a diagnosis.
The ERG is widely used to assess the function of the retina, but it is important to note that it only detects retinal disease. A pet with sudden onset blindness can have a perfectly normal ERG result if the blindness is caused by a lesion in the brain, as in this case, the retina would be perfectly normal.
Another good example of this is a dog who is blind because of mature cataracts. The lens that is affected by mature cataracts is opaque, rendering the dog (or cat) blind, but the retina is normal (unless otherwise affected by another condition). Though this pet is blind, his retina still functions normally, so removal of the affected lens will restore vision.
The ERG is just one tool in a veterinary ophthalmologists arsenal, and while I hope your pet never has to visit this specialist, or any specialist, for that matter, I’m glad they are out there at the ready, just in case.