Many electrophysiological studies have been performed in animals as a means to describe basic physiological response mechanisms of the retina to light stimulation. The recordings have served to expand our knowledge, not only of the normal anatomy and physiology of the retinal cells and their interactions, but also of pathological and disease processes of the eye. For the latter purpose, there have been a number of animal retinal diseases to study. In addition, there are now genetically modified animals, that is, knock-out animals, that are used to examine the role of single genes and amino acids or as direct models of human retinal diseases.

Electroretinogram (ERG) testing in animals larger than rats and mice has been performed for research purposes in guinea pigs,¹¹ pigeons,²⁵ ground squirrels,²⁶ chickens,^21,74 rabbits,^73,75,79 cats,^{38,56,67,76,77} dogs,^{4,28,32,58,69,84} sheep,²⁹ and monkeys,^20,24,68,78 as well as other species. It appears that animals larger than these, such as pigs, cows, and horses, are seldom used for pure research, owing to impracticalities, such as the higher housing costs and the increased risks for both investigators and animals in conjunction with the testing procedures.

Virtually all kinds of objective ERG studies that are routinely done in humans can be performed in larger animals. One major difference is the need for anesthetics in these animals,^1,52,71 which can have a direct effect on the configuration of the ERG recordings. Depending on the choice of anesthetics, the effects range from minimal, when using only hypnotics, to more severe when using barbiturates or volatile anesthetics such as halothane.^33,41

Another difference in larger animals (mammals), except primates, is that they are dichromats. Two types of cones are prevalent, varying in numbers and topographic location in the retina.⁶⁴ Rods dominate most retinas of larger animals, although cones dominate for lizards, birds, and squirrels. These variations in photoreceptor types and distribution between species cause the resultant normal ERG curves to vary markedly between species. Another anatomic variable is the “extra” refractile cell layer located in the inner part of the choroid of many domestic species. It is called tapetum lucidum (dogs and cats) or tapetum cellulosum (horse and cattle) and is found in the superior half of the fundus. This layer increases the effect of incident light on the photoreceptors and results in increased retinal illuminance.⁶⁰ It also increases stray light, effects that significantly affect ERG recordings.⁶⁶

Today, the application for electroretinography in larger animals has broadened, due to the objective nature of the test. This is true especially in the field of veterinary ophthalmology, in which electrophysiological studies in domesticated animals such as dogs, cats, and horses are more or less routinely performed. Although more sophisticated studies such as pattern,⁵⁸ focal, multifocal,⁶⁶ bright-flash, double-flash, and ON and OFF¹⁷ and d.c. ERGs^31,33 are used only in a few specialized centers across the world in larger animals, most veterinary ophthalmology specialty clinics today have equipment to record flash ERGs.⁵⁷

Flash ERGs are most commonly used in dogs in veterinary medicine. For example, ERGs are used as a routine screening procedure for quick evaluation of retinal function, prior to cataract surgery, and in the evaluation of acute blinding diseases or trauma.^2,52 There is an increasing need for flash ERGs to be performed more frequently in horses as well.³⁰ Complete cataracts, especially in foals, is a frequent indication for equine ERGs. Also in trauma cases, ERGs are indicated and in conjunction with intraocular inflammatory disease entities such as uveitis, the most common cause of blindness in the horse.

ERGs are also used for the screening of hereditary eye disease, mainly in dogs, sometimes in cats, and less frequently in Appaloosa horses. In the pedigree dog population, there is a high incidence of generalized, hereditary photoreceptor disorders, collectively termed progressive retinal atrophy (PRA) (table 83.1). Among photoreceptor disorders that have been more specifically studied and documented in dogs are rod-cone dysplasias,^12,37,82 rod dysplasia,⁵ early rod degeneration,² photoreceptor dysplasia,⁵⁹ progressive rod-cone degeneration,^7,45 cone degeneration,⁴ and congenital retinal dystrophy.⁵⁵

Some of these photoreceptor disorders are congenital and cause early-onset severe visual impairment or blindness.⁵⁵ Others do not cause blindness until the animal is several years old and are often not diagnosed until late in the disease process.^1,4,13,54 Early diagnosis of these bilateral, generalized hereditary retinal diseases is advocated. It is recommended that such studies be used prior to breeding, thus reducing the frequency of affected animals with genetic defects prevalent in the population.^1,50

Owing to an increasing awareness among clinicians and pedigree dog and cat owners and breeders, it is likely that diagnostic ERGs will be performed more frequently in the future. In this regard, there has been debate regarding the use and misuse of ERGs in dogs,⁶ and ways have been proposed to resolve this problem: the establishment of referral centers with specific competency in ERG procedures where more comprehensive procedures are routinely performed, enabling researchers and clinicians a more focused approach to diagnostics of retinal disease processes in companion animals.⁴⁶