Progressive retinal atrophy in the Abyssinian cat: studies of the DC-recorded electroretinogram and the standing potential of the eye

Alternative splicing in CEP290 mutant cats results in a milder phenotype than LCACEP290 patients

PURPOSE

The rdAc cat has an intronic mutation in the centrosomal 290 kDa (CEP290) gene resulting in a frameshift and a premature stop codon (c.6960 + 9 T > G, p.Ile2321AlafsTer3) predicted to truncate the protein by 157 amino acids. CEP290 mutations in human patients cause a range or phenotypes including syndromic conditions and severe childhood loss of vision while the rdAc cat has a milder phenotype. We sought to further characterize the effect of rdAc mutation on CEP290 expression.

METHODS

TaqMan quantitative real-time polymerase chain reaction assays were used to compare wildtype and truncated transcript levels. Relative protein abundance was analyzed by Western blot. Immunohistochemistry (IHC) was performed to detect CEP290 protein.

RESULTS

CEP290 mutant cats show low-level (17.4% of wildtype cats) use of the wildtype splice site and usage of the mutant splice site. Western analysis shows retina from cats homozygous for the mutation has CEP290 protein that likely comprises a combination of both wildtype and truncated protein. IHC detects CEP290 in affected and control retina labeling the region of the interconnecting cilium.

CONCLUSIONS

The comparably milder phenotype of CEP290 mutant cats is likely due to the retained production of some full-length CEP290 protein with possible functional contributions from presence of truncated protein.

Central retinal preservation in rdAc cats

OBJECTIVE

Children with Leber congenital amaurosis (LCA) due to CEP290 mutations show characteristic macular preservation. Spectral domain-optical coherence tomography (SD-OCT) is a noninvasive technique to investigate retinal structural changes. Loss of integrity of the ellipsoid zone (EZ) on OCT in people with retinal disease has been associated with loss of visual function and is a useful measure of retinal disease progression. We hypothesized that rdAc felines with Cep290 mutation would have a similar pattern of degeneration, with relative central retinal preservation associated with maintenance of the EZ.

PROCEDURES

Fundus imaging, confocal scanning laser ophthalmoscopy, and SD-OCT cross-sectional imaging was performed on 11 rdAc cats ranging from 6 months to 10 years of age. Images were collected from the area centralis, visual streak, and the mid-superior and mid-inferior retina. Receptor plus (REC+, encompassing the entire length of photoreceptors) thicknesses were measured. Regional rates of degeneration were determined by regression analysis and compared using unpaired t-tests. The EZ was evaluated for the presence, absence, or loss of definition.

RESULTS

RdAc cats showed REC+ thinning over time in all regions. The area centralis and visual streak had a slower rate of thinning than the mid-peripheral retina. There was loss of integrity of the EZ initially in the more peripheral regions, while its integrity was maintained in the area centralis and visual streak at all ages studied.

CONCLUSIONS

rdAc cats show preservation of the central retina with maintenance of EZ integrity, which recapitulates findings in human patients.

Cats: a gold mine for ophthalmology

Over 200 hereditary diseases have been identified and reported in the cat, several of which affect the eye, with homology to human hereditary disease. Compared with traditional murine models, the cat demonstrates more features in common with humans, including many anatomic and physiologic similarities, longer life span, increased size, and a genetically more heterogeneous background. The development of genomic resources in the cat has facilitated mapping and further characterization of feline models. During recent years, the wealth of knowledge in feline ophthalmology and neurophysiology has been extended to include new diseases of significant interest for comparative ophthalmology. This makes the cat an extremely valuable animal species to utilize for further research into disease processes affecting both cats and humans. This is especially true in the advancement and study of new treatment regimens and for extended therapeutic trials. Groups of feline eye diseases reviewed in the following are lysosomal storage disorders, congenital glaucoma, and neuroretinal degenerations. Each has important implications for human ophthalmic research.

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.

Amax to scotopic Imax diagnoses feline hereditary rod cone degeneration more efficiently than any other combination of long protocol electroretinogram parameters

AIM

To evaluate two recent methods for detecting feline hereditary rod cone degeneration with maximum efficiency from a long full-field flash ERG protocol. One combines 12 of these measures in an equation that is derived from iterative principal components factor analysis. The other uses the amplitude of the a-wave to the brightest available flash alone.

METHODS

We tested the original 12-parameter equation, by applying it to 50 new ERG series in 23 backcrossed cats. They were necessarily either heterozygous or homozygous for hereditary rod cone degeneration. A masked observer compared the ERG score and fundus examinations. We reanalyzed the old, new and combined data sets. Data sets with only one session per animal were analysed to avoid problems from non-random sampling. A two factor linear model of the a-wave was evaluated.

RESULTS

The prior equation, applied to the new data, discriminated the groups as well as it had initially. In the reanalysis, group separation continued to increase with even fewer measures compared to the previously reported study. Eventually, one measure, the amplitude of the a-wave (amax) to the brightest scotopic flash (Imax) discriminated the groups better than any other measure or combination of measures in all analyses, including data sets using only one session for each animal and in a two factor linear model of the a-wave.

CONCLUSION

Amax to Imax alone proved to be the best diagnostic criterion in all analyses. No linear model is likely to discriminate affected from unaffected animals more effectively because additional variables increased variance more rapidly than they increased discrimination. Amax to Imax may detect other rod and rod/cone dystrophies equally efficiently.

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.

From basic to clinical research: a journey with the retina, the retinal pigment epithelium, the cornea, age-related macular degeneration and hereditary degenerations, as seen in the rear view mirror

PURPOSE

This Acta Ophthalmologica Award and Gold Medal Honorary Lecture (the Lundsgaard Gold Medal Honorary Lecture) reviews some of the work I have carried out with my mentors and many of my wonderful collaborators and research students over more than 40 years, also including related work by other groups. It concentrates on the basic electrophysiology and ultrastructure of the retina and the retinal pigment epithelium (RPE), as well as covering basic and clinical aspects of the cornea, contact lenses, age-related macular degeneration (AMD) and hereditary diseases.

METHODS

The review describes research performed using light and electron microscopy, basic and clinical electrophysiology, genetics and biochemistry in animal experiments and in research on patients. It also outlines clinically used techniques, such as laser and photodynamic treatment and scanning laser ophthalmoscopy.

RESULTS

The paper reports on the following subjects: the mechanisms behind some of the electrical potentials originating in the retina and the RPE and the use of these potentials in hereditary diseases; corneal receptors for lectins and presumably for bacteria; the turnover of the photoreceptor outer segment and the formation of lipofuscin, including the relation of these processes to AMD; certain treatments for AMD, and hereditary degenerations in animal models, such as the RPE65 gene mutation in Briard dogs, which makes them a model of Leber's congenital amaurosis. The dogs are now treated successfully with gene therapy in the USA, and a clinical trial is in preparation.

CONCLUSIONS

During the last 40 years we have had the good fortune to experience a dramatic growth in knowledge and understanding within ophthalmic science of basic mechanisms. Huge progress has been made in diagnostics and clinical ophthalmological treatments, much to the benefit of our patients. Even a small contribution made by my group to these developments has been well worth the effort, particularly as scientific work is not just deeply satisfying: it is also fun!

Morphological changes in the anterior segment of the Abyssinian cat eye with hereditary rod-cone degeneration

PURPOSE

The purpose of this study was to investigate morphological changes of the anterior segment of the eye in Abyssinian cats with progressive rod-cone degeneration and to correlate them with blood flow data obtained in the same animals.

METHODS

Sections of the left eyes of six normal cats and of eight cats with different stages of hereditary retinal degeneration were prepared for transmission and scanning electron microscopy. Tangential and sagittal sections were also stained with antibodies against substance P, tyrosine hydroxylase, panneuronal marker PGP9.5, nitric oxide synthase, synaptophysin, and smooth muscle alpha-actin.

RESULTS

In Abyssinian cats with hereditary rod-cone degeneration, significant changes were observed in the iris consisting of irregularities in the vascular wall of smaller arteries without changes in their innervation pattern. The ciliary processes were shorter than in normal cats, and their structure appeared more compact and retracted. Slight changes were also observed in the anterior part of the ciliary epithelium. The anterior chamber angle region did not appear to be affected.

CONCLUSIONS

Clear morphological correlations to the physiological blood flow data were observed in the anterior eye segment, pointing not only to functional but also morphological vascular abnormalities in this animal model for retinitis pigmentosa.

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.

Optimal discrimination of an Abyssinian cat recessive retinal degeneration: a short electroretinogram protocol is more efficient than a long one

AIM

To determine the diagnostic efficiency of scores in a long protocol ectroretinogram (ERG) for Abyssinian cat slow recessive rod/cone dystrophy.

METHODS

Kittens (n = 22) were bred from homozygous,affected and heterozygous normals. Ophthalmoscopy was regularly performed and disease signs noted. Cats (age > or = 8 months, 40 sessions, 1-3 repeats) were dark-adapted overnight, anaesthetized and simultaneous binocular ERG recorded using a long protocol. Conventional a- and b-amplitudes and peak implicit times were measured and b/a ratios calculated, initially only for ERG to the maximum photopic and scotopic stimulus. Principal components factor analysis was applied to various subsets of these scores plus age at testing.

RESULTS

Six cats with ophthalmoscopic change were classed as affected. Three cats, one tested three times, were suspect. The rest were considered normal. The first analysis, of 80 eyes and 37 parameters, showed that the first factor was the only effective one. Using it, the groups overlapped 5%, scotopic amplitudes and b/a ratios loaded higher than peak times, the eyes were very similar, and age and photopic b/a ratios loaded poorly. The groups were discriminable with all the data. A second analysis, with eyes averaged and the 20 measures loading over 0.5 on the first factor, showed better group separation on factor I alone. An iterative search with varying data sets found that factor I was optimal, with eight ERG measures to the three brightest scotopic and one brightest photopic response. It produced a large absolute separation and classified the suspects consistently. With b/a ratios on these four ERG also included, 12 parameters gave better separation.

CONCLUSION

Twelve scores on four ERG separate affected from normal cats with a wide gap and consistently classify suspects. It may work for earlier ages. Additional data probably adds noise. This combination of optimal scores needs confirmation in new data.

Abnormal dark-adapted ERG in cats heterozygous for a recessively inherited rod-cone degeneration

PURPOSE

To study retinal function in cats homozygous and heterozygous for a recessively inherited rod-cone degeneration.

METHODS

Dark-adapted electroretinograms (ERGs) were performed on early affected, heterozygous (ophthalmoscopically normal), and clinically normal, nonrelated cats. Responses to blue stimuli over a 3.9-log unit range were recorded.

RESULTS

Lower b-wave amplitudes than normal were observed in heterozygotes and early affected cats. The amplitudes of the heterozygotes took an intermediate position between normal and early affected cats. Normalized amplitude/intensity data suggest a normal dynamic range in carriers. B-wave implicit times in carriers were comparable to those of normal cats.

CONCLUSIONS

These results show that heterozygotes have an altered retinal function, although they are ophthalmoscopically normal. It is difficult to electrophysiologically differentiate heterozygotes from affected cats with the very early stage of retinal degeneration.

Clinical electrophysiology in veterinary ophthalmology--the past, present and future

The aim of this review is to introduce the reader to the world of clinical veterinary electroretinography. An important indication for ERG recordings in the dog is the early diagnosis of progressive retinal atrophy, an inherited form of photoreceptor degeneration, analogous to retinitis pigmentosa in humans. In most of the 20 canine breeds in which the disease has been studied electrophysiologically, changes in the ERG appear long before the appearance of clinical signs. This early diagnosis is a vital tool in efforts to eradicate the disease through preventive breeding. Pre-operative screening of canine cataract patients is another common indication for electroretinography in the dog. The ERG is also used to diagnose inherited and nutritional photoreceptor degenerations in the cat, and retinal disorders in a number of other animal species. The abundance of animal species (and breeds) seen by the veterinary ophthalmologist lends additional importance to the problem of a harmonized ERG recording protocol. The European College of Veterinary Ophthalmologists has set up a special committee to formulate guidelines for such a protocol. International meetings and wetlabs are also being organized as part of an effort to improve the quality of electrophysiological diagnosis that veterinary ophthalmologists provide their patients.

Immunohistochemical studies of cone photoreceptors and cells of the inner retina in feline rod-cone degeneration

Previous studies using electroretinography and immunohistochemistry have shown normal cone function and structure in early stages of hereditary rod-cone degeneration of Abyssinian cats. To further investigate the cone photoreceptors and the inner retina of dystrophic cats, antibodies against green- and blue-sensitive cones and specific cell types of inner retina were used in seven cats with the recessively inherited rod-cone degeneration, and three normal European short-haired cats. There was a reduction in number of both types of cones early in the disease. Changes at early stages of disease also occurred among horizontal cells in which there was an extension and a thickening of their lateral processes. The regular configuration of bipolar cells was changed in the more advanced stages of disease and their apical dendrites were lost. Abnormalities were not observed in the amacrine cells and in the ganglion cell layer in any of the present cases. This study shows that the cone system is morphologically abnormal in young cats at an earlier stage of disease than previously shown. The present findings also support the assumption that the inner retina is largely preserved throughout the disease process.

Cone positive off-response in normal and dystrophic cats

Light-adapted cone ERG in response to white stimuli with long duration (200 ms) was studied in seven normal cats and six cats with early to moderate, inherited retinal dystrophy. The stimuli typically elicited an ERG consisting of an a- and b-wave in response to light onset, whereas light-offset was followed by a cornea-positive d-wave and subsequent negative dip and occasionally a second positive peak in both normal and dystrophic cats, although b- and d- waves had less distinct peaks in cats with moderately advanced retinal dystrophy. Linear regression models indicated a positive correlation between d-wave amplitude and stimulus luminance, whereas a negative correlation was found between amplitude and background light luminance in normal cats. The d-wave implicit time was independent of both stimulus and background light luminance in normal cats. The d-wave amplitude was not significantly different in dystrophic cats, whereas the implicit time was increased when affected cats were compared to normal cats. The significant increase in implicit time in dystrophic cats could not be explained with a reduced sensitivity of the off-pathway to background or stimulus light. This is supported by the finding that d-wave amplitude was not significantly altered in early to moderately advanced dystrophic cats.

Hereditary and congenital ocular disease in the cat

The aim of this review of hereditary and congenital ocular disease in cats is to present an overview of the most common disorders seen in this species, the pathogenesis of the problems and wherever possible, how they are treated. Several defects are common in breeds such as the Persian, Himalayan and Burmese cats and affect the anterior segment of the eye. Examples are agenesis of the eyelids, dermoids, entropion and corneal sequestrum. Other problems such as cataracts, lens luxation and retinal dysplasia, cause problems of the intraocular structures, but are less common in cats compared to dogs. Finally, various parts of the retina and in some diseases other parts of the eye, are specifically affected by hereditary diseases. Examples of these are lysosomal storage disease, Chediak-Higashi syndrome and progressive rod cone degeneration and rod cone dysplasia. Research of the latter two hereditary diseases, both described in the Abyssinian breed of cat, have made affected individuals important animal models for research into comparable diseases of humans.

Autosomal dominant rod-cone dysplasia in the Rdy cat. 2. Electrophysiological findings

Electroretinography was performed on cats affected with autosomal dominant rod-cone dysplasia (gene symbol Rdy). In normal kittens it was found that retinal sensitivity increased and rod thresholds decreased as the animals matured. Electroretinogram (ERG) amplitudes were mature by 4.5 weeks and adult timing was attained by 6 weeks of age, consistent with the findings of other workers. In Rdy-affected heterozygous kittens the ERG was absent or barely recordable using conventional corneal contact lens electrodes. However, the enhanced sensitivity of an intravitreal needle electrode permitted the recording of ERGs from affected kittens aged 4.5 weeks and older. The intravitreally recorded scotopic ERG in Rdy-affected kittens was a very low amplitude, largely negative response with prolonged a- and b-wave times-to-peak (two to threefold longer than in comparable recordings from an age-matched normal kitten). The b-wave lacked oscillatory potentials and was relatively small so that the ERG was a-wave dominated. This was attributed to delayed and defective synaptogenesis in the outer plexiform layer of dystrophic retinas. In contrast to normal kittens, the b-wave threshold was higher than that of the a-wave in affected kittens. Photopic responses were unrecordable. The intravitreal ERG was barely recordable in a 5-month-old Rdy-affected cat and was apparently extinguished by 7 months of age. In vitro electroretinography permitted a comparison of the photoreceptor responses (fast PIII) from the isolated retinas of 6-week-old control and Rdy-affected heterozygous kittens. Maximum fast PIII amplitudes were reduced by about 75% in affected retinas compared with age-matched normal retinas (P less than 0.005). The mean fast PIII time-to-peak, at maximum light intensity, in Rdy-affected retinas was prolonged by about 15 msec and was approximately twofold longer than the time-to-peak of normal retinas (P less than 0.005).4+ steeper slope with relatively greater prolongations in time-to-peak at lower luminances compared with normal retinas (P less than 0.025). These changes in temporal characteristics may be explained either by severe disorganization of photoreceptor outer segments or by altered phototransduction kinetics.

Retinal degeneration in the dog and cat

Retinal degenerations in the dog and cat are an important cause of blindness in these species. Particularly in the dog, many retinal degenerations, collectively called progressive retinal atrophy, seen in clinical practice are inherited. The clinical signs, electrophysiological findings, pathology, and underlying biochemical defects in the retina vary from breed to breed. Specific categories of inherited retinal degeneration are now recognized, and classified into early onset photoreceptor dysplasias, late-onset retinal degenerations, or retinal degenerations secondary to primary RPE dystrophy. As new inherited retinal degenerations are reported in different breeds they can generally be assigned to one these categories. Other causes of retinal degeneration include nutritional deficiencies, glaucoma, inflammation, ischemia, and toxins. Idiopathic retinal degeneration occurs in the dog with some frequency.

Rhodopsin levels and rod-mediated function in Abyssinian cats with hereditary retinal degeneration

Abyssinian cats with different stages of a slowly progressive autosomal recessively-inherited retinal degeneration were studied with imaging fundus reflectometry (IFR) and electroretinography (ERG). Maps of the visual pigment distribution were made in an area of retina extending from the posterior pole to the midperiphery. Rhodopsin levels in the midperipheral retina of a 6-month-old affected cat (stage of suspected disease) were reduced about 20% relative to the mean normal value. The same cat, tested at 2.5 yr of age (now moderately advanced stage), showed a 60% reduction. A 3-yr-old affected cat (also moderately advanced) had a reduction in rhodopsin of about 60%. There was no measurable rhodopsin in a 7-yr-old affected cat (advanced stage). Rhodopsin regeneration kinetics at the different stages of disease were found to be similar to those of normal cats. The rod ERG b-wave threshold in the 6-month-old cat was elevated by 0.26 log units; at 2.5 yr of age, the threshold was elevated by 0.48 log units. A 0.34 log units threshold elevation was found in the 3-yr-old cat. There was no detectable ERG in the 7-yr-old cat. The relationship between the rod ERG threshold elevations and the rhodopsin levels was close to that expected if the dysfunction was caused by decreased quantal absorption.

Morphological findings during retinal development and maturation in hereditary rod-cone degeneration in Abyssinian cats

The sequence of structural changes involved in postnatal photoreceptor differentiation, maturation and early degeneration was studied in young Abyssinian cats and kittens with hereditary rod-cone degeneration and compared to maturation in normal controls. In affected cats the earliest change seen was disorientation of outer segment discs in the majority of the rods, while other rods appeared to develop and mature normally. Such disorientation of discs (at oblique angles or parallel to the longitudinal axis of the outer segment, or whorls of discs) is considered as 'immaturity', since controls also showed a substantial number of disoriented rod outer segment discs at this young age. At postnatal day 35 the difference between affected animals and controls was marked with a high frequency of immature appearing rod outer segment discs in affected animals, while all rod outer segment discs were adult-like and arranged in an orderly manner in controls. Cones seemed unaffected at this age. More severe changes in affected rod outer segments in the form of disintegration of discs (vacuolization and clumping of disc material, or formation of debris), which we consider to represent degeneration, were first observed at the time when retinal maturation normally occurs in the cat, i.e. 150 days postnatally. Subsequently a drop-out of rods was seen, primarily of rods with disoriented and disintegrated outer segment discs, followed by a slow, progressive degeneration of rods that had developed and matured normally. Cones appeared normal during the time of retinal development and maturation and it was not until the age of 2-3 years (Narfström and Nilsson, 1986, Incest. Ophthalmol, Vis. Sci. 27, 1569-76) that degenerative changes were seen also in cones.

Retinal sensitivity in hereditary retinal degeneration in Abyssinian cats: electrophysiological similarities between man and cat

The functional and electrophysiological similarities in the changes in the electroretinogram (ERG) of man and cat affected by hereditary retinal degenerative disease were studied. The results of a series of log intensity-amplitude studies in a group of young affected Abyssinian cats were fitted to the Naka-Rushton relationship by means of a mathematical package on the University of London mainframe. The analysis showed that the amplitude of the maximum dark-adapted b-wave was significantly reduced by the end of the period studied but that the value of k, a variable inversely equivalent to retinal sensitivity, was only slightly reduced by the retinal degenerative process. The electrophysiological findings thus are similar to those found in cases of human diffuse dominantly inherited retinitis pigmentosa.

Hereditary retinal degeneration in the Abyssinian cat: developmental studies using clinical electroretinography

Electroretinography was performed in 10 Abyssinian cats, homozygous for a hereditary retinal degenerative disease but still with an ophthalmoscopically normal retina, and in 11 mixed-breed controls, all between the ages of 8 and 104 weeks. A significant reduction of maximum dark-adapted b-wave amplitude was found in affected kittens as young as 8-16 weeks when compared with controls, although there was no major difference in dark-adapted b-wave threshold or implicit time for the b-wave between affected and controls. For cats 33-104 weeks, similar results were obtained except for the b-wave threshold, which was elevated 2.5 log units in one of the affected cats. No significant difference in 30-Hz cone flicker responses were found between affected and controls at any age studied. In the time period 17-32 weeks affected Abyssinian kittens could not be differentiated from controls by means of the electroretinogram. The significant reduction in scotopic b-wave maximum amplitudes in young affected kittens (8-16 weeks) in conjunction with normal thresholds suggests an early drop-out of rods. It is clear that affected kittens can be differentiated electrophysiologically from controls long before there are ophthalmoscopic signs of retinal disease.

Corneal D.C. recordings of slow ocular potential changes such as the ERG c-wave and the light peak in clinical work. Equipment and examples of results

A set-up for D.C. recordings of slow ocular potentials such as the c-wave of the electroretinogram (ERG) as well as the fast oscillation (FO), the light peak (LP) and the dark trough (DT) in both clinical and experimental work is described. It includes matched calomel half-cells connected by saline-agar bridges to a corneal contact lens on the eye and a reference chamber on the forehead, a low-drift differential-input D.C. amplifier, an A/D converter, a computer, a thermoprinter, a flexible disc memory, a plotter, and a device for light stimulation controlled by the computer. Examples of the usefulness of the set-up in clinical work are shown in the form of D.C. c-wave ERGs of normal subjects as well as of patients with vitelliform macular degeneration, choriocapillaris atrophy, and retinitis pigmentosa. The direct corneal recording of the FO and LP is demonstrated as well. The different origins of the standing potential (SP) of the eye, the ERG c-wave, the FO and the LP are reviewed briefly.