Electroretinography (ERG) is the recording of electrophysiological activity in the retina in response to light stimulation. There are several indications for conducting ERG recordings in animal patients:
1. Routine preoperative evaluation of retinal function before cataract extraction. Unfortunately, many dogs may be simultaneously affected with both progressive retinal degeneration (atrophy) and cataract. Regardless of whether these two diseases are related or independent, it is obvious that cataract surgery will not restore vision if the retina is not functioning. Since the cataract prevents a thorough ophthalmoscopic evaluation of the fundus, the retina can not be evaluated for signs of disease. Therefore, an ERG test is required to determine the prognosis of the surgery, and the potential for post-operative vision. It is important to note that even in the presence of cataracts (or a corneal opacity), sufficient light reaches the retina to cause a response, provided that the retina is functional. This is also the reason why pupillary light reaction can be elicited in cataractous patients. If the opposite fundus can be seen ophthalmoscopically, an ERG test is unnecessary, as inherited retinal atrophies are bilateral diseases.
2. Diagnosis of retinal disorders in which no ophthalmoscopic abnormalities are evident. These include early stages of progressive retinal degeneration, hemeralopia in Alaskan malamutes and sudden acquired retinal degeneration (SARD). In all of these diseases, ERG abnormalities may be recorded even though the fundus may seem normal.
3. Differentiating between retinal and post-retinal causes of blindness. For example, cases of SARD and optic neuritis may present similarly with acute loss of vision, a normal-looking fundus, and fixed, dilated pupils. An ERG may be used to differentiate between the two, as it will be extinguished in SARD (which is a retinal disease) but normal in optic neuritis, which is a post-retinal disease.
4. Early diagnosis of inherited photoreceptor atrophies. In some dog breeds, the ERG may detect changes in retinal function long before ophthalmoscopic or behavioral signs are observed. For example, in the standard poodle, the Portuguese water dog, and the American and English cocker spaniel, ERG changes can be recorded years before the appearance of clinical signs. This early detection is invaluable to breeders wishing to screen their dogs for inherited retinal diseases.
Recording the ERG Response
Though the ERG is a non-invasive procedure, the patient is anesthetized to reduce electric noise and movement artifacts. The stimulating light is placed close to the eye, and responses to the light flashes are recorded using 3 electrodes. The active (recording) electrode is usually mounted on a contact lens placed on the cornea. Two more electrodes are placed subcutaneously to reduce electrical noise.
Based on the clinical indication for the ERG, two recording protocols have evolved for performing the test in dogs. The first is the rapid, "yes-no" protocol, used to demonstrate retinal function. It is conducted to rule out SARD or to determine if the patient is a suitable cataract surgery candidate. This is a brief test, which records the responses to 2-3 flashes of light. For early detection and workup of inherited photoreceptor diseases, a more exhaustive recording protocol is required. This protocol involves extensive testing of rod and cone function, based on their different physiological characteristics. Some of the photoreceptor properties that are evaluated in the course of the comprehensive test include:
1. Responses to dim flashes of light (scotopic vision), which is a feature of rod function, as opposed to responses to bright flashes (photopic vision), which indicate cone function.
2. Color stimulation: cones are more responsive to red light, while rods are more responsive to green or blue stimulation.
3. Adaptation: Once the room lights are turned off, a process of dark adaptation begins. With time, the sensitivity of the rods increases, expressed as increased amplitude of the signal recorded in response to dim light. Conversely, turning on the room lights causes light adaptation, as the rods saturate and do not respond to light.
4. Flicker response: the detection of light flashes depends on the frequency at which the stimulus flickers. Rods can detect light flashing at a frequency < 10Hz, while cones can detect flickers of up to 30-70 Hz (depending on species). Beyond these limits, the photoreceptors can not detect the individual flashes, and their responses "fuse" (which is why we see a continuous picture on our TV, even though it is flickering).
These exhaustive tests of retinal function have been used to detect various types of photoreceptor diseases in numerous breeds, including cone degeneration in the Alaskan malamute, prcd in the poodle, American and English cocker spaniel, rcd1 in the Irish setter, rcd2 in the collie, and others. These diseases, and their abnormalities, are discussed elsewhere in these proceedings.
The recorded ERG signal is analyzed by evaluating the amplitude and latency (timing) of two main components: the a wave is the first negative deflection of the signal, and is indicative of photoreceptor response. It is followed by a large, positive peak, the b wave, which is generated in the mid-retina (bipolar and Müller cells).
It is important to recall that the ERG has several limitations. First and foremost, one should remember that the ERG is a test of retinal function, not of vision. Therefore, it may be normal in some cases of blindness. For example, the ERG is normal in cases of mature cataracts, even though the patient is functionally blind. It is also normal in cases of post-retinal blindness, such as optic neuritis or cortical blindness. Such cases are best evaluated by recording Visual Evoked Potential (VEP's), which represent cortical responses to flashes of light; these are recorded by placing the active electrodes on the scalp over the visual cortex. Another limitation of the ERG is the fact that it represents a global retinal response, and therefore can not be used to diagnose focal areas of dysfunction (scotomas). Finally, as the ERG records the response of the photoreceptor cells, it is normal in glaucoma, which is a disease of the inner retina. These last 2 limitations can be overcome by using specialized ERG equipment, the focal ERG and the pattern ERG, respectively. However, this equipment is not in clinical use in veterinary ophthalmology.
Nevertheless, if these limitations are kept in mind, and if the recordings are performed according to a formal protocol, the ERG can be a powerful tool in the diagnosis of retinal disease.