Rhodopsin levels and retinal function in cats during recovery from vitamin A deficiency

Pet Wellness and Vitamin A: A Narrative Overview

The health of companion animals, particularly dogs and cats, is significantly influenced by nutrition, with vitamins playing a crucial role. Vitamin A, in particular, is indispensable, with diverse roles ranging from vision to immune modulation and reproduction. Despite its importance, the metabolism and dietary requirements of vitamin A in companion animals remain complex and not fully understood. This review provides a comprehensive overview of the historical perspective, the digestion, the metabolism, the physiological roles, the deficiency, the excess, and the interactions with other micronutrients of vitamin A in companion animals. Additionally, it highlights future research directions and gaps in our understanding. Insights into the metabolism of vitamin A in companion animals, personalized nutrition strategies based on genetic variability, longitudinal studies tracking the status of vitamin A, and investigations into its immunomodulatory effects are crucial for optimizing pet health and wellness. Furthermore, understanding the stability and bioavailability of vitamin A in pet food formulations is essential for ensuring the provision of adequate micronutrients. Overall, this review underscores the importance of vitamin A in companion animal nutrition and the need for further research to enhance our understanding and to optimize dietary recommendations for pet health and well-being.

Comparison of Visual Function in Older Eyes in the Earliest Stages of Age-related Macular Degeneration to Those in Normal Macular Health

PURPOSE

To compare the ability of several visual functional tests in terms of the strength of their associations with the earliest phases of age-related macular degeneration (AMD), which bears on their potential to serve as functional endpoints in evaluating treatments for early AMD and prevention strategies.

MATERIALS AND METHODS

Eyes from adults ≥60 years old were identified as being in normal macular health or in the earliest stages of AMD (steps 2, 3 or 4) through grading of color stereo-fundus photos by an experienced grader masked to all other study variables who used the 9-step Age-Related Eye Disease Study (AREDS) classification system for AMD severity. Visual function was assessed using the following tests: best-corrected visual acuity, low luminance visual acuity, spatial contrast sensitivity, macular cone-mediated light sensitivity and rod-mediated dark adaptation.

RESULTS

A total of 1260 eyes were tested from 640 participants; 1007 eyes were in normal macular health (defined as step 1 in AREDS system) and 253 eyes had early AMD (defined as steps 2, 3 or 4). Adjusting for age and gender, early AMD eyes had two times the odds of having delayed rod-mediated dark adaptation than eyes in normal macular health (p = 0.0019). Visual acuity, low luminance acuity, spatial contrast sensitivity and macular light sensitivity did not differ between normal eyes and early AMD eyes.

CONCLUSIONS

Eyes in the earliest phases of AMD were two times more likely to have delayed rod-mediated dark adaptation, as assessed by the rod-intercept, as compared to older eyes in normal macular health, whereas there was no difference in early AMD versus normal eyes in tests of visual acuity, low luminance acuity, macular light sensitivity and spatial contrast sensitivity.

Associations between abnormal rod-mediated dark adaptation and health and functioning in older adults with normal macular health

PURPOSE

Delayed rod-mediated dark adaptation (DA) is characteristic of early age-related macular degeneration (AMD) and also can be observed in some older adults in normal macular health. We examine cross-sectional associations between rod-mediated DA and risk factors for AMD in older adults in normal macular health.

METHODS

The sample consisted of adults aged ≥60 years old in normal macular health per grading of fundus photos using an established disease classification system. Rod-mediated DA was measured psychophysically following a photobleach using a computer-automated dark adaptometer with targets centered at 5° on the inferior vertical meridian. The speed of DA was characterized by the rod-intercept value, with abnormal DA defined as rod-intercept ≥ 12.3 minutes. We assessed several health and functional characteristics that the literature has suggested increase AMD risk (e.g., smoking, alcohol use, inflammatory markers, apolipoproteins, low luminance visual acuity, chronic medical conditions, body mass, family history).

RESULTS

Among 381 participants (mean age, 68.5 years; SD, 5.5), 78% had normal and 22% had abnormal DA, with the prevalence of abnormal DA increasing with age. After age-adjustment, abnormal DA was associated with increased odds of elevated C-reactive protein (CRP), heavy use of or abstention from alcohol, high blood pressure, and drop in visual acuity under mesopic conditions.

CONCLUSIONS

Despite having normal macular health according to accepted definitions of AMD presence, approximately one-quarter of older adults recruited from primary eye care clinics had abnormal DA, which was associated with known risk factors for AMD, including elevated CRP.

Photopigments in central serous chorioretinopathy

PURPOSE

To investigate functional abnormalities in eyes with central serous chorioretinopathy (CSC).

DESIGN

Observational case series.

METHODS

Sixteen eyes with CSC were enrolled. Autofluorescence densitometry was performed to measure the optical density of the photopigments. Serial fundus autofluorescence (FAF) images were obtained by Heidelberg Retina Angiogram 2. We calculated the autofluorescence optical density difference from the FAF images. To compare the distribution pattern of autofluorescence optical density difference to the findings of outer retina, spectral-domain optical coherence tomography (SD-OCT) was performed in the acute phase and after resolution of CSC.

RESULTS

The autofluorescence optical density difference decreased at the serous retinal detachment (SRD) in all 16 eyes. After resolution, the photoreceptor inner and outer segment junction (IS/OS) was irregular in 13 eyes and defective in 3 eyes on SD-OCT. The autofluorescence optical density difference did not improve in any eyes. Five eyes were reexamined 3 month after resolution. In 4 of the 5 eyes, SD-OCT showed that the IS/OS was well delineated and 1 eye defective. The autofluorescence optical density difference improved in 2 of the 4 eyes, but not in the other 2 eyes. In the 1 eye without well-delineated IS/OS, the autofluorescence optical density difference did not improve.

CONCLUSION

In eyes with CSC, the photopigment density decreased at the SRD. The density remained decreased immediately after resolution and showed delayed recovery. The photopigments decreased even in eyes with morphologic recovery of the outer retina.

[The eye and anorexia nervosa. A case report]

INTRODUCTION

Vitamin A deficiency occurs in the poor in developing countries and is one of the main causes of blindness by perforative corneal complications. It is a rare pathology in industrialized countries and it is associated with an absorption syndrome. The authors report the first case of hypovitaminosis A in a patient suffering from chronic and severe anorexia nervosa.

CASE REPORT

The patient suffered from epiphora, photophobia, and hesperanopia. The ophthalmologic findings were keratoconjunctival xerosis with bilateral corneal ulcerations. The visual field showed a concentric bilateral restriction of isopters with tubular central vision, a similar aspect to retinitis pigmentosa. The ERG was modified with a b2 reduction and normal photopic and impaired scotopic responses. The fluorescein angiography was normal. The serum concentration of retinol confirmed the diagnosis of hypovitaminosis A. Corneoconjunctival improvement was obtained with vitamin supplementation, but no campimetric improvement was observed.

DISCUSSION

The corneoconjunctival signs result from direct destruction of goblet cells, whereas the campimetric deficit is explained by a dysfunction of rod cells. Rhodopsin, necessary to the survival of the cell, cannot be renewed if retinol is not present, which causes a permanent bright light stimulation that is lethal for the photoreceptor.

CONCLUSION

Vitamin A deficiency is rarely caused by psychiatric disease. Even if the main clinical finding is xerophthalmia with a high risk of keratomalacia, the visual prognosis can also be engaged by dysfunction of photoreceptors.

Cone- and rod-mediated dark adaptation impairment in age-related maculopathy

OBJECTIVE

To examine impairment in cone- versus rod-mediated dark adaptation in the parafovea of persons with age-related maculopathy (ARM).

DESIGN

Cross-sectional.

PARTICIPANTS

Older adults with ARM at various severity levels from early to advanced (n = 83) and in good retinal health (n = 43), as determined by stereo fundus photographs evaluated with the Age-Related Eye Disease Study severity scale.

METHODS

Dark adaptation, both cone- and rod-mediated components, was measured with a modified Humphrey Field Analyzer using a target located 12 degrees in the inferior visual field on the vertical meridian, after exposure to a 98% bleach. Information was collected on self-reported problems for activities at night or under dim illumination (Low Luminance Questionnaire [LLQ]) and for activities during daytime conditions (modified National Eye Institute Visual Function Questionnaire [NEI VFQ]).

MAIN OUTCOME MEASURES

Cone- and rod-mediated parameters of dark adaptation.

RESULTS

Compared with older adults in normal retinal health, ARM patients had significant impairments in rod-mediated parameters of dark adaptation (rod-cone break, rod slope, rod sensitivity) (P<0.0001), which were increasingly abnormal as disease severity increased. Cone-mediated parameters (cone time constant and cone sensitivity) were not impaired. Low Luminance Questionnaire scores and NEI VFQ scores decreased with increased ARM severity (P = 0.0004 and P = 0.0005, respectively); the percent decrease in LLQ scores as a function of disease severity was larger in magnitude than the percent decrease in NEI VFQ scores.

CONCLUSIONS

Disturbances in rod-mediated but not cone-mediated dark adaptation in the parafovea at 12 degrees in the inferior field on the vertical meridian are characteristic of ARM even in its early phases.

Age-related macular degeneration: a target for nanotechnology derived medicines

Despite the fact that the retina is a fairly accessible portion of the central nervous system, there are virtually no treatments for early age-related macular degeneration (AMD). AMD is a degenerative retinal disease that causes progressive loss of central vision and is the leading cause of irreversible vision loss and legal blindness in individuals over the age of 50. Both environmental and genetic components play a role in its development. AMD is a multifactorial disease with characteristics that include drusen, hyperpigmentation and/or hypopigmentation of the retinal pigment epithelium (RPE), geographic atrophy and, in a subset of patients, late-stage choroidal neovascularization (CNV). Drugs that inhibit vascular endothelial growth factor (VEGF) have proven effective in treating late-stage CNV, but optimal means of drug delivery remains to be determined. Microscopic particles, whose size is on the nanometer scale, show considerable promise for drug delivery to the retina, for gene therapy, and for powering prosthetic "artificial retinas." This article summarizes the pathophysiology of AMD stressing potential applications from nanotechnology.

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.

Dark adaptation and the retinoid cycle of vision

Following exposure of our eye to very intense illumination, we experience a greatly elevated visual threshold, that takes tens of minutes to return completely to normal. The slowness of this phenomenon of "dark adaptation" has been studied for many decades, yet is still not fully understood. Here we review the biochemical and physical processes involved in eliminating the products of light absorption from the photoreceptor outer segment, in recycling the released retinoid to its original isomeric form as 11-cis retinal, and in regenerating the visual pigment rhodopsin. Then we analyse the time-course of three aspects of human dark adaptation: the recovery of psychophysical threshold, the recovery of rod photoreceptor circulating current, and the regeneration of rhodopsin. We begin with normal human subjects, and then analyse the recovery in several retinal disorders, including Oguchi disease, vitamin A deficiency, fundus albipunctatus, Bothnia dystrophy and Stargardt disease. We review a large body of evidence showing that the time-course of human dark adaptation and pigment regeneration is determined by the local concentration of 11-cis retinal, and that after a large bleach the recovery is limited by the rate at which 11-cis retinal is delivered to opsin in the bleached rod outer segments. We present a mathematical model that successfully describes a wide range of results in human and other mammals. The theoretical analysis provides a simple means of estimating the relative concentration of free 11-cis retinal in the retina/RPE, in disorders exhibiting slowed dark adaptation, from analysis of psychophysical measurements of threshold recovery or from analysis of pigment regeneration kinetics.

Photoreceptor degeneration and dysfunction in aging and age-related maculopathy

The relative rate of rod and cone degeneration is a fundamental characteristic of any disorder affecting photoreceptors, including aging and age-related maculopathy (ARM). The human macula consists of a small cone-dominated fovea surrounded by a rod-dominated parafovea. In aging and early ARM, rods degenerate before cones, a decline in scotopic (rod-mediated) sensitivity is more prominent than a decline in photopic (cone-mediated) sensitivity, and the time course of dark adaptation of rods slows dramatically. The topography of rod dysfunction and loss in early ARM matches the location of pathology in the retinal pigment epithelium (RPE)/Bruch's membrane complex visible in the ocular fundus. Rod dysfunction and loss in aging and ARM may be due to retinoid deficiency at the level of the photoreceptors cause by impaired retinoid translocation across the RPE/Bruch's membrane complex, a hypothesis deserving of further investigation.

Genetically engineered large animal model for studying cone photoreceptor survival and degeneration in retinitis pigmentosa

Patients with retinitis pigmentosa (RP) typically develop night blindness early in life due to loss of rod photoreceptors. The remaining cone photoreceptors are the mainstay of their vision; however, over years or decades, these cones slowly degenerate, leading to blindness. We created transgenic pigs that express a mutated rhodopsin gene (Pro347Leu). Like RP patients with the same mutation, these pigs have early and severe rod loss; initially their cones are relatively spared, but these surviving cones slowly degenerate. By age 20 months, there is only a single layer of morphologically abnormal cones and the cone electroretinogram is markedly reduced. Given the strong similarities in phenotype to that of RP patients, these transgenic pigs will provide a large animal model for study of the protracted phase of cone degeneration found in RP and for preclinical treatment trials.

Night blindness in Sorsby's fundus dystrophy reversed by vitamin A

Sorsby's fundus dystrophy (SFD) is an autosomal dominant retinal degeneration caused by mutations in the tissue inhibitor of metalloproteinases-3 (TIMP3) gene. Mechanisms of the visual loss in SFD, however, remain unknown. In a SFD family with a novel TIMP3 point mutation, we tested a hypothesis that their night blindness is due to a chronic deprivation of vitamin A at the level of the photoreceptors caused by a thickened membrane barrier between the photoreceptor layer and its blood supply. Vitamin A at 50,000 IU/d was administered orally. Within a week, the night blindness disappeared in patients at early stages of disease. Nutritional night blindness is thus part of the pathophysiology of this genetic disease and vitamin A supplementation can lead to dramatic restoration of photoreceptor function.

Rhodopsin levels in the central retinas of normal miniature poodles and those with progressive rod-cone degeneration

Visual pigment in normal miniature poodles and those with progressive rod-cone degeneration (prcd), a late-onset autosomal recessive photoreceptor degeneration, has been studied using imaging fundus reflectometry (IFR). The stage to which the disease had advanced in the animals with prcd was assessed with electroretinography (ERG). Measurements were carried out on seven affected, two heterozygous and three homozygous normal animals. The IFR measurements showed that the in situ difference spectrum of visual pigment measured in the central retina of the normal poodle is typical of vertebrate rhodopsin, with a maximum at about 510 nm. Rhodopsin regeneration following extensive bleaches continues for up to 70 min. In poodles with prcd, rhodopsin is spectrally normal and regenerates at normal rates. In young affected animals under 1 year of age, the final levels of rhodopsin could already be substantially reduced. Serial measurements of visual pigment in these dogs showed differences in the degree and spatial pattern of pigment loss and rate of progression between animals. The extent of visual pigment loss also differed among the older (greater than 4.5 years) affected animals: while in one animal no pigment could be detected, in another a central band of retina was relatively spared, and significant levels of visual pigment were measured within it. Pigment levels measured within the central 25 degrees of the retinas of poodles heterozygous for prcd were lower than those in normal animals, even though their ERGs were within the normal range.

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.