Comparison of two electroretinography systems used in dogs: the HMsERG and the RETIport

The purpose of the study was to compare two different electroretinography (ERG) instruments used on the same animal in a laboratory setting. Retinal function in both eyes of 12 healthy miniature schnauzers was evaluated under general anesthesia. Scotopic and photopic ERGs were recorded by the HMsERG and the RETIport using the Dog Standard Protocol recommended by the European College of Veterinary Ophthalmologists (ECVO). The waveforms of the ERGs obtained by the two ERG units were similar to those described in previous studies. The 90% reference ranges using the multiple of medians (MoM) showed that the changes in ERG parameters obtained by the HMsERG unit were very similar to those of the RETIport for most ERG responses, except for a few. The results demonstrate that the two ERG systems are comparable for evaluating retinal function in dogs. Further, the results also show that it is necessary to establish ERG system-specific limits of normality in each laboratory or clinic in order to obtain a correct diagnosis, for example by using a graphical representation of the 90% reference range.     

Electroretinography recordings using a light emitting diode active corneal electrode in healthy beagle dogs

Electroretinography (ERG) is a well-established diagnostic procedure for objectively evaluating retinal function. In this study, ERG in beagle dogs, which are a popular experimental animal, was performed to determine the normal range of ERG variables and assess differences between the left and right eyes. ERG findings including rod, combined rod-cone, single-flash cone, and 30-Hz flicker responses were recorded with an LED-electrode in 43 sedated beagle dogs. The subjects were divided into young (< 1 year old), adult (1~5 years old), and senile animals (≥ 6 years old). Normal ERG ranges were obtained. Significant differences in b-wave amplitude along with b/a ratio of the combined rod-cone response were found between the young and adult animals as well as young and senile dogs. No significant differences were observed between the left and right eyes. ERG variables in beagle dogs differed by age due to age-related retinal changes. Thus, we propose that normal ERG ranges should be determined according to age in each clinic and laboratory using its own equipment because each institution usually has different systems or protocols for ERG testing.     

Evaluation of short-term increased intraocular pressure on flash- and pattern-generated electroretinograms of dogs

OBJECTIVE: To determine the electrodiagnostic and histologic response of short-term increases of intraocular pressure (IOP) on transient pattern electroretinograms (PERG) and flash electroretinograms (FERG) in the eyes of dogs. ANIMALS: 8 healthy mixed-breed dogs. PROCEDURE: Transient PERG and FERG waveforms were recorded from dogs (while anesthetized) as IOP was increased from baseline (7 to 19 mm Hg) to 90 mm Hg. One hundred mean PERG responses and a single FERG response were recorded at each step during 3 recording sessions. Globes of each dog were enucleated after euthanasia on posttreatment day 7 and evaluated by a pathologist. RESULTS: Increases in spatial frequency resulted in decreased amplitudes of N2 (second negative PERG peak). Increases in IOP resulted in decreases in all 3 PERG waveforms and the FERG waveform. All values began to return to baseline after short-term increases in IOP on day 0, and waveforms were not significantly different on posttreatment days 3 and 7 CONCLUSIONS: Data suggest that short-term increases in IOP affect PERG and FERG waveforms, and PERG waveforms are more sensitive to increases in IOP Differences were not detected between treated and control eyes on histologic examination. Further studies are necessary to determine at what IOP permanent damage to ganglion and photoreceptor cells will develop and whether PERG is a reliable clinical diagnostic technique for use in dogs to reveal retinal damage that is secondary to increased IOP prior to changes in waveforms generated by FERG in dogs.     

Partial Masking of the Canine Electroretinogram by Oscillatory Potentials

Recording the electroretinogram (ERG) in dogs has become a standard procedure in clinical ophthalmology. The ERG provides highly objective information about retinal function that is otherwise unobtainable from dogs. However, problems may be encountered in measuring the latencies amplitudes of the a and b waves, depending upon the amplifier bandwidth used to record the potential. Superimposed on the canine ERG are other complex retinal potentials, some of which have higher frequency spectra than those that give rise to a and b waves. Whereas an amplifier bandwidth of 1.0 to 100 Hz or more insures that the slower components of the ERG are recorded, it also allows higher frequency oscillatory potentials (OP) to be recorded on the b wave. The OP cause a notching in the peak of the b wave thus necessitating a decision about how to measure the b wave. Bandwidth characteristics of the ERG and OP are presented and some possible methods for measuring the OP-contaminated ERG are discussed.     

Clinical findings in early onset cone-rod dystrophy in the Standard Wire-haired Dachshund

OBJECTIVE: To describe the clinical findings and the age of onset of cone-rod dystrophy (crd) in the Standard Wire-haired Dachshund (SWHD) and to evaluate which clinical tests could be used to obtain a reliable diagnosis. ANIMALS: Sixty-eight SWHD and SWHD-derived dogs were used, including 23 affected with crd and 45 controls, respectively. PROCEDURES: The dogs were subjected to behavioral testing, examination of pupillary light reflexes (PLRs), indirect ophthalmoscopy and bilateral full field electroretinography (ERG). RESULTS: The majority of affected puppies (5-10 weeks) displayed pin-point sized pupils upon examination with focal light. All dogs in the control group, except one, displayed normal PLRs upon examination. In all crd-affected dogs there was a great variation both in age of onset and in clinical appearance of retinal changes upon fundoscopy. Two siblings displayed panretinal degeneration at the age of 10 months while other affected dogs showed early changes at the age of 3 years. Generalized bilateral retinal atrophy was the end stage of the disease. The maze test revealed no obvious differences among affected and unaffected groups. ERG recordings showed only slightly reduced rod, and mixed rod-cone responses, but severely reduced cone single flash a- and b-wave amplitudes, and cone flicker amplitudes were observed in all affected dogs. CONCLUSION: Presence of pin-point sized pupils in young SWHDs was found to be an important indicator of early onset crd. Fundoscopic changes and progression of disease at later stages resembled those previously described in the majority of progressive retinal atrophies in dog. ERG was found to be the most reliable diagnostic procedure to clinically diagnose crd in the SWHD.     

Electrophysiologic differentiation of homozygous and heterozygous Abyssinian-crossbred cats with late-onset hereditary retinal degeneration

OBJECTIVE: To develop a method to electrophysiologically differentiate heterozygous-carrier Abyssinian-crossbred cats from homozygous-affected Abyssinian-crossbred cats before clinical onset of inherited rod-cone retinal degeneration. ANIMALS: 14 back-crossed Abyssinian-crossbred cats of unknown genotype (homozygous or heterozygous) for inherited rod-cone retinal degeneration, 24 age-matched mixed-breed control cats, 6 age-matched heterozygous Abyssinian-crossbred cats, and 6 homozygous Abyssinian cats. PROCEDURE: Electroretinography (ERG) of heterozygous and homozygous cats revealed differences, especially for scotopic recordings. Frequent ophthalmoscopy and ERG (2 to 5 times; at intervals of 3 to 6 months) of back-crossed cats were performed. Amplitudes and implicit times were analyzed by use of a graphic representation of results. Ratios for amplitudes of the b-waves to amplitudes of the a-waves (b-wave:a-wave) were compared. RESULTS: 8 back-crossed cats had decreased a-wave amplitudes, increased b-wave implicit times, and abnormal ERG waveforms. Values for the b-wave:a-wave for the highest scotopic light intensity were significantly higher for those same 8 cats. CONCLUSIONS AND CLINICAL RELEVANCE: The 8 back-crossed Abyssinian-crossbred cats with abnormal results developed fundus changes over time consistent with disease. A graphic representation of ERG results can be used to differentiate between genotypes prior to funduscopic changes. Values for the b-wave:a-wave ratio provide confirmation. These ERG analyses may be applied clinically in the diagnosis of retinal degenerations in various species. IMPACT FOR HUMAN MEDICINE: Cats with hereditary rod-cone degeneration may be a useful model for comparative studies in relation to retinitis pigmentosa in humans. Similar evaluations of ERG results could possibly be used for humans with suspected generalized retinal degeneration.     

The effects of cataract stage, lens-induced uveitis and cataract removal on ERG in dogs with cataract

OBJECTIVE: The purpose of this study was to determine the effects of cataract stage, lens-induced uveitis and cataract removal on the electroretinogram (ERG) of dogs with cataract. ANIMALS STUDIED: Fifty-seven dogs diagnosed with unilateral or bilateral cataract whose ERG was recorded at Rakuno Gakuen University Teaching Animal Hospital from 2001 to 2004. PROCEDURES: Four responses were recorded during the ERG: rod ERG, standard combined ERG, single-flash cone ERG and 30-Hz flicker ERG. Cataracts were divided into four stages: incipient, immature, mature and hypermature, and with or without lens induced uveitis (LIU). Noncataractous eyes of dogs with unilateral cataract were used as the control. We compared ERG amplitude, implicit time, and the b- to a-wave amplitude ratio of cataractous vs. noncataractous eyes, preoperative vs. postoperative cataractous eyes, and cataractous eyes with and without LIU. RESULTS: No significant difference was found in ERG amplitude between incipient, immature and hypermature cataractous eyes, while in mature cataractous eyes decreased amplitude was confirmed in all responses compared with control eyes. However, no significant difference in b/a ratio was found at any stage of cataract. In postoperative eyes, increased amplitude was recorded in all responses compared to preoperative values. In eyes with LIU a decreased amplitude in the rod ERG and b-wave of standard combined ERG was recorded and, furthermore, a significant decline was confirmed in b/a ratio. CONCLUSION: ERG values were influenced by cataract stage and LIU. LIU was associated with a reduction in the b/a ratio.     

Clinical, electrophysiological and morphological changes in a case of hereditary retinal degeneration in the Papillon dog

An autosomal recessive retinal disease with a late onset in Swedish Papillon dogs has recently been described. A 7-year-old Papillon dog showed no obvious signs of visual impairment and only minor ophthalmoscopic changes. Cone ERG b-wave amplitudes were within normal limits, while rod responses were nonrecordable or severely abnormal. Ultrastructural examination showed a generalized retinal degenerative disease, most prominent in the peripheral areas. The inferior retina was more severely affected than the superior areas. Both rods and cones showed morphological changes. The Papillon dog is another dog breed affected by progressive rod-cone degeneration, with similarities to the canine retinal disease given the gene symbol prcd .     

Electroretinography using contact lens electrode with built-in light source in dogs

Electroretinography (ERG) is an effective method for the diagnosis of retinal disease. In the dog, dependable ERG recording is difficult without the use of an expensive device like a Ganzfeld full-field stimulator. The International Society for Clinical Electrophysiology of Vision has defined the standard flash stimulus condition (SF) and evaluation of the retina using the b/a ratio in humans. In dogs, evaluation using the b/a ratio has not been reported, whereas the intensity of SF has been defined. In this study, we performed a convenient ERG recording method using a contact lens electrode with a built-in light source (LED-electrode), and confirmed SF as reported previously. ERG recordings were performed on 15 healthy beagle dogs under sedation. We performed bilateral ERG at 12 different intensities after 30 min dark adaptation. After 10 min light adaptation, we recorded single flash cone and flicker cone response using the SF determined in this study. In this study, SF of 3.0 cd/m(2)/sec (6,000 cd/m(2), 0.5 msec) resulted in b/a=2. The intensity for rod response that recorded only the b-wave was 0.0096 cd/m(2)/sec (80 cd/m(2), 0.12 msec). We could achieve ERG for each response easily and smoothly under sedation, and without general anesthesia. Using an LED-electrode, we could perform more quantitative and reproducible ERG examinations than with traditional methods. We propose that the b/a ratio is the most useful parameter in ERG reporting for evaluating retinal function.     

Retinal degeneration in nine Swedish Jämthund dogs

The Jämthund is the fourth most common breed in Sweden with approximately 1600 pups registered each year. Although it has been known that some adult dogs go blind, so they cannot hunt, the Jämthund dog has historically not been screened for hereditary eye diseases. This report describes nine Swedish Jämthund dogs with retinal degeneration. These dogs represent all Jämthund dogs diagnosed with progressive retinal atrophy (PRA) by the Swedish Eye Panel and registered with the Swedish Kennel Club from January 1998 to September 2008. The dogs were examined with indirect opthalmoscopy and slitlamp biomicroscopy. Additionally, electroretinograms (ERGs) following ECVO guidelines were performed in two dogs (one affected and one normal) and the eyes from three affected dogs were examined by light‐microscopy postmortem. Typical findings were bilateral symmetric generalized retinal degeneration with tapetal hyper‐reflectivity, attenuation of blood vessels and pigment clumping in the nontapetal fundus. These retinal findings progressed with time in two dogs after re‐examination. Visual impairment, especially under dim light conditions, was observed in the affected dogs. ERG from one affected dog showed profoundly reduced rod responses, whereas cone responses were better preserved. Microscopic changes in the eyes from three dogs were characterized by a severe diffuse predominantly outer retinal degeneration and atrophy. Re‐sequencing of the prcd‐gene for eight of the nine investigated dogs revealed that none of the individuals carried disease allele that has been associated with prcd‐PRA in other breeds. In conclusion, ophthalmoscopic, electroretinographic, and light‐microscopic alterations observed in nine Jämthund dogs were compatible with PRA. The prcd mutation was excluded as a cause of this retinopathy.     

Ophthalmic and cone derived electrodiagnostic findings in outbred Miniature Long-haired Dachshunds homozygous for a RPGRIP1 mutation

Objective To investigate ophthalmic and cone‐derived electrodiagnostic findings in outbred Miniature Long‐haired Dachshunds (MLHD) homozygous for a mutation in the RPGRIP1 gene previously associated with cone‐rod dystrophy 1 (cord1). Animals A total of 36 MLHD homozygous for the RPGRIP1 mutation and 23 dogs clear of the mutation (control group). Procedures The dogs underwent ophthalmic examination and photopic electroretinogram (ERG) recordings. Results None of the control dogs presented with clinical or ophthalmic signs consistent with cord1. Amongst the dogs homozygous for the mutation one presented with bilateral symmetrical total retinal atrophy. None of the other dogs in this group showed signs consistent with cord1. Photopic ERG recordings were available in 23 control dogs and 34 dogs homozygous for the mutation. Photopic a‐ and b‐waves following four light stimuli (3 cdS/m²) at a rate of 5.1 Hz were not significantly different between groups. The amplitudes of the 30 Hz flicker (128 flashes, 3 cdS/m²) response were significantly reduced in the dogs homozygous for the PRGRIP1 mutation. The difference in age between the two groups did not significantly affect the difference. Conclusion Homozygosity of the RPGRIP1 mutation does not invariably result in early onset cord1. However, cone derived ERG recordings show evidence of a reduced cone or inner retinal function in homozygous but clinically normal MLHD. Modifying genes that have yet to be identified may influence an individual dog’s risk of developing the blinding cord1 and also the age of onset and rate of progression.     

Acute blindness in dogs: Sudden acquired retinal degeneration syndrome versus neurological disease (140 cases, 2000–2006)

Objective  To evaluate dogs with amaurosis and compare signalment, history, ophthalmic examination and neurologic abnormalities between dogs diagnosed with sudden acquired retinal degeneration syndrome (SARDS) versus neurological disease (ND). Animals Studied-140 dogs with acute vision loss and ocular abnormalities insufficient to account for visual deficits. An electroretinogram (ERG) was performed on each dog.
Procedures  Medical records were reviewed and information was collected for all dogs meeting the inclusion criteria. Dogs diagnosed with SARDS were compared to those with ND based on signalment, duration of clinical signs, past medical problems, clinicopathologic findings, and ophthalmic and physical examination abnormalities.
Results  120 dogs were diagnosed with SARDS and 20 dogs with ND based on ERG results. Mixed-breed dogs were most commonly diagnosed with SARDS as well as ND. Pure breed dogs frequently diagnosed with SARDS included the Miniature Schnauzer and Dachshund. Dogs with SARDS did not differ significantly from those with ND based on age or sex distribution. Cushing's-like symptoms were reported more frequently in SARDS dogs as well as conjunctival hyperemia and retinal vascular attenuation. Papilledema and asymmetric visual deficits were observed more frequently in dogs with ND. Dogs with ND were no more likely than SARDS dogs to have additional neurological deficits.
Conclusions  Appreciable overlap of clinical signs exists between dogs with SARDS and dogs with ND resulting in acute vision loss. As a significant portion of dogs (14%) in the present study were diagnosed with ND, an ERG to rule out ND is indicated in dogs with amaurosis.     

The DC-recorded dog electroretinogram in ketamine-medetomidine anaesthesia

A new selective alpha 2-adre-noreceptor agonist, medetomidine hydrochloride was combined with low dosage ketamine hydrochloride and vecuronium bromide for d.c. (direct current) recordings of fast electroretinographic (ERG) components in nine ophthalmoscopically healthy dark adapted dogs. The dogs were tracheally intubated and manually ventilated. They were given full field single flash stimuli of different intensities starting with near b-wave threshold blue light (tests 1-3), followed by white light (tests 4-6) and 30 Hz photopic flicker (test 7). The a- and b-wave amplitudes and flicker responses were measured from the base line. The latencies were measured from the stimulus moment to the highest point of the different waves.Statistical analysis of results gave individual differencies which had a good constancy. This showed that the dogs had an individual ERG profile according to the standardized method. The latencies varied very little as expected, but the amplitudes differed individually and showed a good constancy as seen by reproducibility tests made nine to ten days later on three of the dogs’ ipsilateral eyes. The combination of drugs used in this study was considered suitable for short term (10-12 minutes) stable d.c.–ERG recordings in dogs as the rod and cone responses had higher amplitudes when compared to an identical examination made with other anaesthetic combinations on the same dogs.Involuntary eye movements and other involuntary muscular activity caused by ketamine in dogs were negligible when using medetomidine premedication and was completely absent when using vecuronium.The anaesthetic method described can be recommended for ambulatory ERG recordings in dogs because of the above mentioned advantages.     

Neuronal ceroid lipofuscinosis: clinical and morphologic findings in nine affected Polish Owczarek Nizinny (PON) dogs

OBJECTIVE: The aim of this study was to characterize the clinical and morphologic features of neuronal ceroid lipofuscinosis (NCL) in the Polish Owczarek Nizinny (PON) breed of dog. ANIMALS: Nine Swedish PON dogs of both sexes were included in the study. PROCEDURE: All dogs underwent a detailed clinical evaluation, with emphasis on ophthalmic exams. Histopathology and electron microscopy were performed on the eyes, brain and various internal organs. Immunohistochemical staining for detection of sphingolipid activator proteins (SAPs) and mitochondrial ATP synthase (SCMAS) was performed on the eyes and brain. RESULTS: The dogs showed behavioral abnormalities, motor disturbances and visual impairment or blindness. Pupillary responses were abnormal while fundus changes varied from normal to severe retinal atrophy. Electroretinography (ERG) showed variable changes, from slight alterations in the process of dark adaptation to severely reduced or nonrecordable ERG a- and b-wave amplitudes. Histopathology revealed intracytoplasmic storage bodies within neurons of the brain and in retinal cells, especially the retinal pigment epithelium (RPE). Round to oval granular type of inclusion bodies, known as granular osmiophilic dense deposits (GRODS), were found in neuronal cells in the brain and in the retina. Immunohistochemistry identified the storage material in the brain and retina as consisting of SAPs. CONCLUSION: The presently described NCL disease in PON dogs shows similarities to previously recorded cases in the Miniature Schnauzer. The closest human equivalent to this disease is infantile NCL (CLN1), in which the major stored proteins are SAPs and the ultrastructure of the inclusion bodies of neuronal cells is granular.     

The normal electroretinogram in adult healthy Shih Tzu dogs using the HMsERG

Electroretinography (ERG) is a reliable diagnostic tool for the diagnosis of retinal disease. It measures electric potentials occurring in the retina in response to light stimulation. In this study, we examined the normal electroretinogram using the Handheld Multispecies ERG (HMsERG) in Shih Tzu dogs. ERG recordings were performed in twelve eyes of six healthy Shih Tzu dogs. Dogs were anesthetized with a combination of medetomidine and ketamine. Proparacaine eye drops were also applied as a topical anesthetic. Tropicamide eye drops were applied for mydriasis. After 20 min of dark adaptation, we recorded the amplitudes and implicit times of the b-waves of the rod, standard rod and cone (Std R&C), high-intensity rod and cone (Hi-int R&C), and cone systems, and responses of the cones and inner retina by flicker light stimulation (cone flicker). Results showed that mean the amplitudes of a-waves of Std R&C, Hi-int R&C, and the cone responses were 141.25 µV, 173.00 µV, and 12.92 µV, respectively. The b-waves of the rod responses ranged from 141.58 to 155.25 µV; the Std R&C was 314.75 µV, the Hi-int R&C was 329.42 µV, the cones were 37.75 µV, and the flicker responses were 64.08 µV. The b/a ratios for the Std R&C, Hi-int R&C, and the cone response were 2.29, 1.94, and 3.71, respectively. Mean implicit time of the a-wave of the Std R&C was 15.12 ms, of Hi-int R&C was 13.42 ms, and of the cone response was 7.22 ms. The b-wave of the rod responses ranged from 68.12 to 72.68 ms, of Std R&C were 37.28 ms, of Hi-int R&C were 41.90, of the cone responses were 38.12 ms, and of the cone flicker responses were 22.80 ms. We believe that these parameters can be used as reference "normal" ERGs ranges for Shih Tzu dogs using the HMsERG under medetomidine and ketamine anesthesia.     

Detection of cone dysfunction induced by digoxin in dogs by multicolor electroretinography

It is difficult to detect discrete cone function with the present conventional electroretinography (ERG) examination. In this study, we developed contact electrodes with a built-in color (red (644 nm), green (525 nm), or blue (470 nm)) light source (color LED-electrode), and evaluated an experimental model of digoxin in the dog. First, 17 normal Beagle dogs were used to determine which electrode works well for color ERG measurement on dogs. Then, color ERG was performed on seven normal Beagle dogs at various points during a 14-day period of digoxin administration. A single daily dose of 0.0125 mg/kg/day, which is within the recommended oral maintenance dosage range for dogs, was administered orally for 2 weeks. Ophthalmic examination, measurement of plasma concentration of digoxin, and color ERG examination were performed. On first examination, amplitudes of all responses were significantly (P < 0.01) lower with the red, than with the blue and green electrodes during ERG recording. In ERG using the red electrode, the standard deviation was large. According to these preliminary results, the red electrode was not used in the experimental dog model with digoxin. In the digoxin administrated animals, no significant change was observed in the ophthalmic examination findings. The digoxin level increased steadily throughout the dosing period but was always within the therapeutic range for dogs. In rod ERG, no abnormalities were detected with any electrode. In standard combined ERG, decreased amplitude of the a-wave was detected with every electrode. In single flash cone ERG, prolongation of implicit time was detected by color ERG with the blue and green electrodes. In 30-Hz flicker ERG, decreased amplitude was detected only by color ERG with the blue electrode. The decreased amplitude and prolonged implicit time recovered after termination of digoxin administration. Cone dysfunction induced by digoxin in the dog was revealed by multicolor ERG using blue and green LED-electrodes. Multi-color ERG was useful for detecting cone type-specific dysfunction in the dog.     

Effects of stimulus intensity for electroretinogram in conscious Miniature Schnauzers

The aim of this study was to determine the most effective light intensity for flash electroretinogram (ERG) examination in conscious dogs using ERG equipment with a contact lens electrode with a built-in LED light source. ERG was performed on the bilateral eyes of ten clinically healthy Miniature Schnauzers at 6 different intensities (0.025, 0.079, 0.25, 0.79, 2.5 and 7.9 cd.s/m2) after dark adaptation for 20 min. With the increase in stimulus intensity, the most significant increase in a and b-wave amplitudes were observed at 2.5 cd.s/m2 (p<0.05). As the intensity of light was increased, the implicit times of both waves significantly decreased. Therefore, the most effective intensity of stimulus was 2.5 cd.s/m2 in the conscious Miniature Schnauzers. This suggests that this procedure would be applicable for evaluation of retinal function in conscious dogs, especially in high-risk patients.     

Day-blindness in three dogs: clinical and electroretinographic findings

A 6-month-old Rhodesian ridgeback-cross, a 6-year-old Chihuahua and a 12-month-old Australian cattle dog were presented to the authors with a history of colliding with obstacles in daylight. Ophthalmic examination was normal and all three dogs successfully negotiated obstacle courses in dim light. In daylight the dogs became suddenly blind and repeatedly collided with obstacles. Elecroretinography (ERG) revealed no retinal activity to high frequency (30 Hz), bright intensity blue light retinal stimulation by any dog, confirming cone dysfunction. Achromatopsia has previously been recorded in Alaskan malamutes and miniature poodles. This clinical case series illustrates the characteristic behavioral presentation and the electroretinographic findings of severe day-blindness and demonstrates that this condition may exist in other breeds of dogs.     

The effects of medetomidine hydrochloride on the electroretinogram of normal dogs

Purpose  To determine the effects of a standardized intravenous dose of an α-2 agonist (Domitor^®, Orion Pharma, distributed by Pfizer Animal Health, Exton, PA) on the electroretinogram (ERG) response in normal dogs.
Methods  Twenty-five normal dogs were used to collect ERG responses including a- and b-wave implicit times (IT) and amplitudes (AMP) before and after administration of medetomidine. Dogs were dark adapted for 20 min and ERGs were obtained using the HMsERG (RetVetCorp Inc., Columbia, MO). The QuickRetCheck protocol (Narfström) was employed to provide the following flash intensities: 10 mcd s/m², 3 cd s/m², and 10 cd s/m². ERGs were repeated after 375 µg/m² of medetomidine intravenously. Statistical analysis of the difference between the responses before and after medetomidine at all flash intensities was performed using a mixed effects model for anova .
Results  The P value for the effect of medetomidine on each of the ERG responses was < 0.01. The estimates of the effect of medetomidine were (+)1.35 ms, (–)23 µV, (+)3.16 ms, and (–)47 µV for the a-wave IT, a-wave AMP, b-wave IT, and the b-wave AMP, respectively.
Conclusions  Medetomidine significantly prolongs the implicit time and lowers the amplitude response of both the a- and b-waves in normal dogs at all flash intensities examined. Clinically, however, medetomidine only minimally affects the retinal responses and is a viable choice for use in dog ERGs.     

Characterization of feline hereditary retinal dystrophies using clinical, functional, structural and molecular genetic studies

Only in recent years have specific mutations been elucidated for feline hereditary retinal dystrophies. Molecular genetic characterization of feline diseases has so far been a slow process but with a full genome sequence for the cat recently completed and the development of a feline single nucleotide polymorphism chip, the characterization of feline monogenic disorders will be significantly simplified. This review summarizes current knowledge with regard to specific hereditary retinal dystrophies in cats and gives an overview of how cats can be used as models in translational research.