Assessment of the dark-adaptation time required for recovery of electroretinographic responses in dogs after fundus photography and indirect ophthalmoscopy

OBJECTIVE: To investigate the duration of dark-adaptation time required for recovery of electroretinographic responses after fundus photography or indirect ophthalmoscopy in dogs. ANIMALS: 6 dogs. PROCEDURE: Initially, scotopic-intensity series of electroretinograms (ERGs) were recorded after 20 minutes of dark adaptation. The fundus of the left eye of each dog was photographed (n = 10) or examined via indirect ophthalmoscopy for 5 minutes with moderate- (117 candela [cd]/m2) or bright-intensity (1,693 cd/m2) light; ERGs were repeated after a further 20 or 60 minutes of dark adaptation (6 procedures/dog). RESULTS: Following 20 minutes of dark adaptation after fundus photography, the b- and a-wave amplitudes were reduced in response to brighter stimuli, compared with pretest ERGs; after 60 minutes of dark adaptation, ERG amplitudes had recovered. Following 20 minutes of dark adaptation after indirect ophthalmoscopy (moderate-intensity light), significantly lower b-wave amplitudes were recorded in response to 2 of the brighter flash stimuli, compared with pretest ERGs; after 60 minutes of dark adaptation, ERG amplitudes had recovered. Following 20 minutes of dark adaptation after indirect ophthalmoscopy (bright-intensity light), all ERG amplitudes were significantly decreased and implicit times were significantly decreased at several flash intensities, compared with pretest ERGs; after 60 minutes of dark adaptation, ERG amplitudes and implicit times had returned to initial values, except for b-wave amplitudes recorded in response to dimmer stimuli. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that at least 60 minutes of dark adaptation should be allowed before ERGs are performed in dogs after fundus photography or indirect ophthalmoscopy.     

Oscillating potentials on the B-wave of the ERG in the dog]

The oscillatory potentials (OP) on the b-wave of the canine ERG are characterized. Normal values for an OP-index, implicit times and periodicity are given. The OP-index increases during dark adaptation as well as with increasing stimulus intensities for white light flashes. Scotopic blue and red stimuli, although balanced for the maximum b-wave amplitude, surprisingly result in higher OP-indices for red flashes. Implicit times decrease for all OP with increasing stimulus intensities. Scotopic balanced red light results in OP with markedly reduced implicit times compared to blue light stimulation. The intervals between two OPs remain constant during dark adaptation, however, intervals between OPs with longer implicit times tend to be shorter. In contrast, the b-waves for scotopic balanced stimuli show rhythmic oscillations with constant intervals between them, the intervals being shorter for blue light than for red light. The intervals between OPs tend to become shorter with increasing stimulus intensities. This investigation indicates that OPs are influenced but not generated by photoreceptors. The examination of a dog with optic nerve hypoplasia indicates that the ganglion cells do not contribute to the biogenesis of OPs. The prognostic value of OPs in the early diagnosis for hereditary retinal degenerations in the dog is discussed.     

Effects of thiopentone halothane-nitrous oxide anaesthesia compared to ketamine-xylazine anaesthesia on the DC recorded dog electroretinogram

Eleven ophthal-moscopically healthy dark adapted dogs were examined by DC ERG technique with single flash full field illumination starting with near b-wave threshold blue (tests 1-3) and white (tests 4-6) stimuli of different intensity and ending with 30 Hz photopic flicker smuli (test 7) after light adaptation. All animals were anaesthetized using 2 different anaesthetic methods: Anaesthesia I (A I): Induction with thiopentone sodium, continued with halothane and nitrous oxide in oxygen. Anaesthesia II (A II): Praemedication with xylazine hydrochloride followed by anaesthesia with ketamine hydrochloride. A minimum interval of 1 week was kept between all anaesthesias.The a- and b-wave amplitudes and latencies were determined. Statistical analysis of results indicated that the a- and b-waves were elicited by weaker intensities in A II. In Tests 3-6 the a-wave was highly significantly (P < 0.001), higher in amplitude in AII than in A I. Differencies in b-wave amplitudes were not statistically significant (except Test 1). The b-wave latencies were longer in AI in Test 2 (using low intensity blue light). The a-wave latencies were slightly shorter in AII in Test 6 (using high intensity white light).In additional experiments the selective action of the different agents (except N₂O) used in AI and AII was studied. Thiopentone alone given to 3 dogs seemed to depress the a-wave selectively.Halothane given separately to 3 dogs lowered both the a- and b-wave amplitudes. Ketamine given with a neuromuscular blocking agent to three dogs resulted in responses almost identical to those in AII.Xylazine with vecuronium given to 4 dogs resulted in responses with slighly depressed a- and b-waves in comparison to ketamine with vecuronium.The results indicate that when developing an animal model for the electrophysiologic study of human retinal dystropies, the actions of different anaesthetics upon the ERG components are of great importante.     

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.     

Changes in oscillatory potentials in the canine electroretinogram during dark adaptation

Oscillatory potentials (OP) and electroretinograms (ERG) were recorded from clinically normal dogs after 5, 10, 15, 20, 30, 40, 50, and 60 minutes of dark adaptation. At the end of the adaptation period, OP were characterized by 5 distinct positive peaks, O1 through O5, with mean latencies of 14.46, 20.24, 27.38, 35.31, and 44.85 ms, respectively, and with mean amplitudes ranging from 7.20 to 34.84 microV. After 60 minutes of dark adaptation, the ERG had a mean a-wave latency of 12.03 ms and a mean b-wave amplitude of 109.29 microV. Peaks O3 and O4, which partially mask the summit of the b-wave, had mean latencies of 28.66 and 36.83 ms, respectively. The mean amplitude of the b-wave measured to the peak of O3 was 240.06 microV and 230.73 microV when measured to peak O4. Changes in the OP during dark adaptation consisted of significant (P less than 0.05) increases in the latencies of O1, O2, and O3, and significant increases in the amplitudes of O1, O3, O4, and O5. Concurrent ERG changes consisted of significant increases in the amplitudes of the a-wave and b-wave measured from O3 and O4, and significant increases in the latencies of peaks O3 and O4 on the b-wave.     

The electroretinographic phenotype of dogs with Golden Retriever muscular dystrophy

Objective To characterize the flash electroretinogram (ERG) in the Golden Retriever muscular dystrophy (GRMD) dog and to compare the results with those from a control group of Golden Retrievers. To investigate whether similar abnormalities of the ERG as those found in a majority of human patients with Duchenne muscular dystrophy (DMD) are also observed in the GRMD dog, the canine model for DMD. Animals Five GRMD dogs and five age‐matched clinically normal Golden Retrievers. Procedure An ophthalmic examination was carried out prior to performing electroretinography under general anesthesia. Rod, combined rod–cone and oscillatory potentials responses were recorded after dark adaptation. Responses to 30‐Hz‐flicker were recorded after light adaptation. The ERG responses of the GRMD dogs were compared with those of the control dogs by use of a Wilcoxon signed rank test. Results GRMD dogs had significantly reduced a and b‐wave amplitudes after dim white flash stimuli (rod response) and reduced a‐wave amplitude after bright white flash stimuli (rod–cone response). Conclusion and clinical relevance The ERG abnormalities observed in the GRMD dog suggest a dysfunction in the rod signaling pathway. These ERG alterations are different from those observed in human patients with DMD.     

The determination of dark adaptation time using electroretinography in conscious Miniature Schnauzer dogs

The optimal dark adaptation time of electroretinograms (ERG''s) performed on conscious dogs were determined using a commercially available ERG unit with a contact lens electrode and a built-in light source (LED-electrode). The ERG recordings were performed on nine healthy Miniature Schnauzer dogs. The bilateral ERG''s at seven different dark adaptation times at an intensity of 2.5 cd·s/m² was performed. Signal averaging (4 flashes of light stimuli) was adopted to reduce electrophysiologic noise. As the dark adaptation time increased, a significant increase in the mean a-wave amplitudes was observed in comparison to base-line levels up to 10 min (p < 0.05). Thereafter, no significant differences in amplitude occured over the dark adaptation time. Moreover, at this time the mean amplitude was 60.30 ± 18.47 µV. However, no significant changes were observed for the implicit times of the a-wave. The implicit times and amplitude of the b-wave increased significantly up to 20 min of dark adaptation (p < 0.05). Beyond this time, the mean b-wave amplitudes was 132.92 ± 17.79 µV. The results of the present study demonstrate that, the optimal dark adaptation time when performing ERG''s, should be at least 20 min in conscious Miniature Schnauzer dogs.     

Dark adaptation time in canine electroretinography using a contact lens electrode with a built-in light source

The purpose of this study was to evaluate the dark adaptation time in canine electroretinography (ERG) using a contact lens electrode with a built-in LED. Twelve eyes of six normal laboratory beagle dogs were used and exposed to steady room light at 500 lux for 30 min for light adaption. ERG was recorded at different time points during dark adaptation in sedated and light-adapted beagles. The stimulus intensity was 0.0096 cd/m²/sec. The b-wave amplitude increased significantly until 25 min of dark adaptation, whereas no significant changes in amplitudes were observed after 30 min. Dark adaptation for more than 25 min would be necessary for accurate ERG in canine ERG using a contact lens electrode with a built-in LED.     

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.     

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.     

Electroretinogram and visual-evoked potential measurements in Holstein cows

The electroretinogram (ERG) and flash visual-evoked potential (FVEP) were recorded from 25 Holstein cows in a clinical setting without dark adaptation. Latencies to the a and b waves of the ERG were measured bilaterally. The ERG recordings were between the lower eyelid of the stimulated eye and the vertex, with ground on the nuchal crest. Stimuli for ERG and FVEP were 1.5 flashes of white light/s. Recorded ERG were highly consistent and repeatable. The average latencies for the a and b waves were 14 and 30 ms; the mean ab amplitude was 43 microV. The FVEP were recorded bilaterally between the middle of the interocular line and the midpoint of the nuchal crest, with ground on the vertex. The FVEP measurements included latencies to 5 alternating positive and negative peaks (P1, N1, P2, N2, and P3) and 4 peak-to-peak amplitudes (P1 to N1, N1 to P2, P2 to N2, and N2 to P3). The P2 peak was consistently the most prominent. Average latencies for the 5 peaks were 46, 64, 86, 106, and 137 ms for P1, N1, P2, N2, and P3, respectively. The FVEP peak-to-peak amplitudes had a high variability.     

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.     

Recording of the full-field electroretinogram in minipigs

Purpose To test a simple electroretinographic protocol on a representative sample of minipigs. Animal studied Minipig. Procedures Electroretinogram recordings were conducted on 162 healthy minipigs (81 males and 81 females) aged 4-6?months. After a 1.5-h light-adaptation period, the animals were anesthetized with general anesthesia. First, binocular full-field photopic electroretinogram recordings were conducted under photopic conditions. Subsequently, scotopic electroretinogram recordings were conducted during dark-adaptation periods every 4?min for a 20-min period. At the end of this period, the maximal combined rod-cone response was recorded by measuring the retinal response to a single high-intensity flash. We used sclerocorneal clip electrodes as active electrodes and needle electrodes as reference and ground electrodes. Results The a-wave and b-wave peak times and amplitudes have been measured and statistically analyzed. For each of the statistical comparisons, normality and homogeneity of variances were evaluated. No significant gender differences?were observed, with the exception of a higher b-wave amplitude for the photopic ERG recordings observed in females when compared to males (48.14?±?12.909?μV vs. 42.88?±?10.666?μV; P?=?0.005). The process of dark adaptation was evaluated, and the maximal combined rod-cone response was measured (a- and b-waves amplitude and peak time). Conclusions We conducted photopic and scotopic electroretinogram recordings from a protocol based on light adaptation followed by dark adaptation using sclerocorneal clip electrodes, which allows quick assembly and examination.     

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.     

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.     

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.     

Electroretinogram and visual-evoked potential measurements in sheep.

Electroretinogram (ERG) and visual-evoked potential (VEP) recordings were taken from ten Suffolk-cross sheep. Stimuli for VEP were 1.5 flashes of white light/s; ERG stimuli were single flashes. The ERG measurements of the a and b wave latencies and a-to-b amplitude were measured between the lower eyelid and the vertex, with ground on the nuchal crest. The VEP after monocular stimulation were measured between the nuchal crest and the interorbital line, with ground on the vertex. Measurements consisted of the latencies to seven alternating positive and negative peaks P1, N1, P2, N2, P3, N3 and P4, and six amplitudes, P1-N1, N1-P2, P2-N2, N2-P3, P3-N3 and N3-P4. Average latencies for the a and b waves were 13.6 and 28.2 ms; the mean ab amplitude was 131.68 microV. Average latencies for the seven VEP peaks were 35.0, 43.1, 52.8, 64.1, 74.5, 90.4 and 112.2 ms. Mean amplitudes ranged from 3.90 to 8.29 microV.     

Normal clinical electroretinography parameters for poodle,Labrador retriever,Thai ridgeback,and Thai Bangkaew

The purpose of the present study was to establish normal electroretinogram (ERG) parameters using 56 normal eyes of four dog breeds common in Thailand: poodle, Labrador retriever, Thai ridgeback, and Thai Bangkaew. Standard ERG findings were bilaterally recorded using a handheld multi-species ERG unit with an ERG-jet lens electrode for 28 dogs under preanesthesia with diazepam, anesthesia with propofol, and anesthesia maintenance with isoflurane. There were significant differences in the mean values of ERG amplitudes and implicit times among the four dog breeds (p < 0.05) except for the b-wave implicit time of the photopic 30 Hz flicker response with 3 cd.s/m² (p = 0.610). Out of the four breeds, Thai Bangkaew had the longest implicit time (p < 0.001) of scotopic low intensity responses, b-wave of scotopic standard intensity responses (3 cd.s/m²), a-wave of the higher intensity response (10 cd.s/m²), and a-wave of the photopic single flash response (3 cd.s/m²). For the b/a ratio, only the ratio of the Cone response was significantly different among the different breeds. In this summary, normal ERG parameters for four dog breeds were reported. Data from the investigation supported the hypothesis that determination of breed-specific limits of normality for ERG responses is necessary for individual clinics and laboratories.     

ERG findings in three hypothyroid adult dogs with and without levothyroxine treatment

Purpose To evaluate the effects of levothyroxine (LTh) on the electroretinogram (ERG) of adult dogs. Material and methods Binocular, full field photopic and scotopic ERGs were recorded from an anesthetized Maltese Bichon cross (MB), a Yorkshire Terrier (YT) and a Shetland Sheepdog (SS) affected with hypothyroidism and treated with a daily dose of LTh at 20 µg/kg. The photopic ERGs were evoked to 12 different intensities ranging from 0.81 to –2.19 log cd.s/m² and presented under photopic conditions in order to assess (from the derived luminance-response curves) Vmax and b : a amplitude ratio parameters. Photopic flicker ERGs were obtained at 30 Hz. The scotopic ERGs (intensity: –3.09 log cd.s/m²) were recorded while the retina was dark-adapting and after 32 min of dark adaptation. This procedure was performed on two separate sessions: following a 3-day interruption of LTh treatment (S1) and following 30 days without interruption of LTh treatment (S2). Results The mean photopic a-wave peak times were 9.8 ms at S1 and 5.0 ms at S2, respectively. The mean photopic b-wave peak times were 23.3 ms at S1 and 11.5 ms at S2, respectively, and the mean scotopic b-wave peak times (after 32 min of dark adaptation) were 45.2 ms at S1 and 26.0 ms at S2, respectively. No other significant ERG changes were observed. Conclusion Our results indicate that a dose of 20 µg/kg of LTh given to adult dogs was accompanied by a marked peak time shortening of both photopic and scotopic ERGs, without affecting other ERG parameters.     

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.