Electroretinography, which involves recording of an electrical response generated by retinal neurons and supporting cells when the eye is stimulated by light, is performed routinely in veterinary ophthalmology. The light stimulation creates ionic changes within the intra- and extracellular spaces, which can be measured as electrical potentials1,2 that assume a predictable waveform. The initial negative deflection of the waveform is basically an indicator of photoreceptor hyperpolarization and is referred to as the a-wave. The positive spike that follows is largely a measure of changes in bipolar and Müller cell polarity and is referred to as the b-wave.
Additional analysis of an ERG recording can be performed through assessment of the c-wave, which is generated by the retinal pigment epithelial cells. Clinically, however, the a-wave and b-wave amplitudes and the implicit timing of these amplitudes (interval from stimulus onset to the first negative trough [a-wave] and first positive peak of the wave [b-wave]) are the most useful measurements. The amplitudes and implicit timings of the a- and b-waves vary on the basis of any retinal disease present, recording protocol used, and consciousness state (awake, sedated, or anesthetized) as well as other variables such as species, breed, and age. Electroretinography is therefore an objective test of photoreceptor function and is essential in veterinary ophthalmology for evaluating retinal function.
Many ERG protocols exist, including brief protocols in which overall retinal function is assessed and longer protocols considered more diagnostic for rod and cone disease. The 2 most common indications for ERG in a veterinary ophthalmology practice are to assess retinal function prior to cataract surgery and to distinguish among distal optic neuritis, central blindness, and sudden acquired retinal degeneration syndrome in patients with amaurosis with a clinically normal fundus. In both of these instances, the clinician is simply looking for a brief yes-or-no response to indicate the prognosis for vision following cataract surgery or to rule out sudden acquired retinal degeneration syndrome. In such situations, a brief protocol with a mean duration of approximately 10 min/eye, including dark adaptation, is used to assess overall retinal function.
Published standard ERG protocols suggest that all animals must be anesthetized, particularly for long ERG sessions typically used in the diagnosis of inherited photoreceptor diseases such as progressive rod-cone degeneration or achromatopsia.1,3,4 The reasons for this recommendation are to minimize noise and artifacts associated with blinking or other movement, to facilitate patient handling, and to maintain the electrodes in their proper position. Despite these recommendations, many veterinary ophthalmologists perform ERG in awake or sedated dogs.
The reluctance of some clinicians to perform ERG on an anesthetized patient when the yes-or-no method is to be used is likely because of the high financial cost to the client, possible risk to the patient, and increase in time required for the procedure, compared with in nonanesthetized subjects. Although some clinicians may consider sedation an acceptable alternative to anesthesia for ERG, some of the same financial, time, and health concerns can remain. Additionally, use of sedatives and anesthesia alters the ERG a- and b-wave amplitudes and implicit times3–8 and these effects must be taken into consideration when evaluating the ERG response.
The purpose of the study reported here was to quantitatively and qualitatively compare ERG recordings obtained in awake, sedated, and anesthetized dogs. Our hypothesis was that use of anesthesia, and to a lesser extent sedation, would reduce low- and high-frequency noise associated with the ERG recording, compared with noise in ERG performed in awake dogs, thereby confirming the benefits of performing ERG in anesthetized dogs. We also expected that sedation and anesthesia would prolong the a- and b-wave implicit times and decrease the a- and b-wave amplitudes.
Fast Fourier transformation
SL-15 portable slit-lamp biomicroscope, Kowa Optimed Inc, Torrance, Calif.
Volk Optical Pan Retinal 2.2, Dan Scott and Associates Inc, Westerville, Ohio.
TonoVet, Icare VET, Jorgensen Laboratories Inc, Loveland, Colo.
Goniosol, OCuSoft Inc, Richmond, Tex.
Orion Corp, Espoo, Finland.
Fort Dodge Animal Health, Fort Dodge, Iowa.
Littmann Cardiology III, Littmann Stethoscopes, Saint Paul, Minn.
Cardell, CAS Medical Systems Inc, Branford, Conn.
Baxter Healthcare Corp, Deerfield, Ill.
PropoFlo, Abbott Laboratories Inc, North Chicago, Ill.
Hospira Inc, Lake Forest, Ill.
VetOne, MWI, Meridian, Idaho.
Ben Venue Laboratories, Bedford, Ohio.
Model 1000, Ocuscience, Rolla, Mo.
Tropicamide Ophthalmic Solution USP 1%, Alcon Laboratories Inc, Fort Worth, Tex.
Alcon Laboratories Inc, Fort Worth, Tex.
OCuSOFT Inc, Richmond, Tex.
ERG-jet, Ocuscience, Rolla, Mo.
Stata/Intercooled, version 10.1, StataCorp L P, College Station, Tex.
StatXact, version 8.0, Cytel Software Corp, Cambridge, Mass.
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2. Narfstrom K, Petersen-Jones S. Diseases of the canine ocular fundus. In: Gelatt KN, eds. Veterinary ophthalmology. Vol 2. 4th ed. Ames, Iowa: Blackwell Publishing, 2007;944–1025.
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