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Electroretinography - Are We Misusing an Excellent Diagnostic Tool? Gustavo D University of Pennsylvania ScholarlyCommons Departmental Papers (Vet) School of Veterinary Medicine 1995 Electroretinography - Are We Misusing an Excellent Diagnostic Tool? Gustavo D. Aguirre University of Pennsylvania, [email protected] Follow this and additional works at: https://repository.upenn.edu/vet_papers Part of the Animal Diseases Commons, Eye Diseases Commons, Ophthalmology Commons, and the Veterinary Medicine Commons Recommended Citation Aguirre, G. D. (1995). Electroretinography - Are We Misusing an Excellent Diagnostic Tool?. Veterinary & Comparative Ophthalmology, 5 (1), 2-3. Retrieved from https://repository.upenn.edu/vet_papers/112 Veterinary & Comparative Ophthalmology is not Veterinary Ophthalmology. At the time of publication, author Gustavo D. Aguirre was affiliated with the James A. Baker Institute. Currently, he is a faculty member of Penn Vet at the University of Pennsylvania. This paper is posted at ScholarlyCommons. https://repository.upenn.edu/vet_papers/112 For more information, please contact [email protected]. Electroretinography - Are We Misusing an Excellent Diagnostic Tool? Disciplines Animal Diseases | Eye Diseases | Medicine and Health Sciences | Ophthalmology | Veterinary Medicine Comments Veterinary & Comparative Ophthalmology is not Veterinary Ophthalmology. At the time of publication, author Gustavo D. Aguirre was affiliated with the James A. Baker Institute. Currently, he is a faculty member of Penn Vet at the University of Pennsylvania. This journal article is available at ScholarlyCommons: https://repository.upenn.edu/vet_papers/112 I h< IEditorial! that a •Tho equi • Stric Electroretinography- • Gen Are We Misusing an Excellent Diagnostic Tool? •Use • Pro] • Elin • Use •Use • Sign It was nearly 30 years ago that I first experienced the eration (erdt in Elkhounds; progressive rod-cone degen­ The power and many of the frustrations associated with the eration (prcd) in Poodles, Cocker Spaniels/ and Labrador metw 10 2 use of the electroretinogram (ERG) to evaluate the func­ Retrievers; photoreceptor dysplasia (pd) in Schnauzers; new u 11 tion of the retina in normal and diseased eyes. At that hereditary cone degeneration in Malamutes; congenital more 12 time I was carrying out a research project at the Univer­ stationary night blindness in Briards; and progressive or use 13 14 sity of Pennsylvania examining the effect of Vitamin A retinal atrophy in Tibetan Terriers, Dachshunds, and In fac1 15 deficiency on retinal function in dogs. In that study, Portuguese Water Dogs. is an< weaned puppies were placed on purified diets deficient Ironically, the availability of new and powerful tech­ equipt in Vitamin A and then evaluated by sequential ERGs. I nology for eliciting and recording the ERG has increas­ mid-1~ performed the ERG using an electroencephalograph ma­ ingly become the rationale for its misuse in veterinary instrm chine to measure retinal responses elicited with a Grass ophthalmology. I am concerned about a trend toward fortun• PS-2 photic stimulator. After many months on the defi­ modifying the ERG procedure in ways that disregard ba­ as rep cient diet, the serum and liver Vitamin A levels reflected sic retinal physiology and are therefore unlikely to yield In l< a profound deficiency state; oddly, though, I found the any meaningful or reliable results. The reasoning for this useful ERG responses between the deficient and the Vitamin trend appears to be based on the principle that the (less t A-supplemented controls to be indistinguishable. Com­ greater the cost or the signal-averaging capabilities of a rieed t mon sense did not suggest that this lack of distinction piece of equipment, the less the need to worry about the tractic was because of a yet unknown resistance of the dog retina testing protocols. Taking this argument further, some po­ its use to Vitamin A deprivation. The problem lay instead in my sit that anesthesia is not required because it affects the spons( improper understanding of the physiology of the retina ERG unpredictably and because the ability to signal­ wave J and of the stimulating conditions needed to elicit the averageeliminates the muscle potential, 60-Hz electrical cor de( ERG. I set about to modify the Grass stimulator-to the interference, and all other artifacts that interfere with the ary llll dismay of the machinist in the biomedical electronics proper interpretation of the ERG. Some inhalation anes­ ofthe1 workshop- by attaching a large, metal gasoline funnel to thetics do affect the ERG amplitude and the rate of or us< the front housing of the stimulator and placing a neutral dark-adaptation of the rods, but this effect is predict­ identi filter holder in the narrowed exit pathway of the light. able10 and is not a limiting factor. Others argue that op­ diseru With this very crude device I was then able to detect a tical benches, with or without fiber optic guides, or mod­ Ar 2-log-unit difference in dark-adapted threshold between ified stroboscopic stimulators are not necessary, and indic< the deficient and supplemented dogs and this difference place the emphasis on the hardware associated \Vith re­ const increased as the study continued. cording and signal"averaging the responses rather than in do The ERG equipment that is now available to veterinary on controlling how those responses are generated. Are infon ophthalmologists is certainly more refined than a home­ we forgetting the time-tested computing principle of"gar­ thesi: made optical stimulator held together with a shoestring bage in/garbage out?" It is important to remember that to im and a prayer. ERG responses can be elicited using a var­ the ERG response to light, as evident in a V/log I curve, the I 42 iety of stimulators among which are optical benches or is a saturating response. To obtain some measure of sen­ SUmll modified Grass stimulators? When used with appropriate sitivity in assessing the degree of outer-retinal damage, these testing protocols, the ERG has been found to be ex­ it is essential to reduce the light intensity of most photic for tl tremely reliable and essential for the early diagnosis of stimulators by several log units. In this way, the stimulus prot( many hereditary retinal degenerations. Examples are rod­ elicits a response in the linear range and not the saturat­ publ cone dysplasia types land 2 (rcdl; rcd2) in Setters and ing end of the V/log I curve, and it is easier to assess with 7 Collies;4-6 rod dysplasia (rd)/· and early retinal degen- damage to the ERG generators. teste Continued PAGE Z e Vol. 5, No. 1 • VETERINARY & COMPARATIVE OPHTHALMOLOGY Editorial: Electroretinography I have been asked several times to list those factors and published in peer-reviewed scientific journals, we that are essential for doing ERG in animals. These are: have to take the cautious approach and continue to fol­ • Thorough knowledge of retinal physiology and of the low protocols that have been shown to be effective and equipment being used. reliable. This is only fair to our patients, to the owners • Strict adherence to a reproducible protocol. and breeders who depend on our skill and judgment, and • General anesthesia or very deep sedation. to our specialty. • Use of optical bench or modified photic stimulator. Veterinary ophthalmology is not alone in facing issues • Proper control of light/dark adaptation status. raised by the variability of ERG testing methods and in­ • Elimination of 60-Hz interference. terpretation of results. This has been an area of intense • Use of reliable ERG electrodes. debate in medical ophthalmology. With respect to retin­ • Use of appropriate recording equipment. itis pigmentosa, there has been a move to standardize the • Signal averaging. terminology and the methods of examination.18 More re­ [egen­ The low priority I give to recording hardware is usually cently, the International Society for Clinical Electrophys­ rador met with disbelief. While very sophisticated and powerful iology of Vision (ISCEV) has established a protocol for uzers; new instruments are available for recording the ERG, standardizing the clinical ERG.19 This protocol estab­ enital more modest recording equipment made up from new lishes standardized methods that must be followed for essive 4 or used components can be used with equal reliability. ERG testing. This protocol does not set limits on what and In fact, my favorite oscilloscope for electroretinography can be recorded, but does set minima, a move that helps is an old Tektronix 564B storage tube oscilloscope. This standardize results obtained between different institu­ tech­ equipment was at the cutting edge of technology in the tions and laboratories. It is high time that veterinary oph­ :reas­ mid-1960s. To make sure that we are able to use this thalmology followed the initiative created by our sister inary instrument well into our retirement years, we have been specialty and began to set standards for ERG testing in ward fortunate to stockpile several used models that can serve animals. This initiative should be supported by the ACVO, dba­ as replacements as needed. ASVO, ESVO, ISVO, and all other organizations that yield In last place on my list is signal averaging. It is very represent the field of veterinary ophthalmology. • r this useful for recording and evaluating very low-amplitude Gustavo Aguim the (less than 5 ,uvolt) signals, or complex responses that James A. Baker Institute of a need to be separated into individual components by sub­ Ithaca, New liJrk t the traction following signal averaging. While we have found : po­ its use necessary for examining the receptor-mediated re­ REFERENCES ; the sponses in erd-affected Elkhounds,8 and to analyze the 1. AguirreGD and Rubin LF: Progressive Retinal Atrophy (Rod Dysplasia)in the ND<Wegian ~a)­ Etkbound. JAVMA 158:208, 1971. wave form and components of the very small signal re­ 2:. Parshall C. et al: Photoreceptor Dysplasia: An inherited Progressive Retinal Atrophy of tical Miniature Schnauzer Dogs. Prog Vet Comp Opbtbalmot 1:18'1, 1991.
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