S (blue) cone pathway vulnerability in retinitis pigmentosa, diabetes and glaucoma

A variety of retinal disease lead to a decrease in the sensitivity of the S (blue) cone pathways. To determine the possible sites and mechanisms of this loss we compared the sensitivities of an S (blue/pi-1) and an M (green/pi-4) cone pathway in patients with retinal diseases that differ as to their primary locus of sensitivity loss. The sensitivities of an S and an M cone pathway were assessed in patients with retinitis pigmentosa, insulin-dependent diabetes mellitus and open-angle glaucoma using Stiles two-color increment threshold technique. A greater loss in sensitivity of an S than an M cone pathway was found for all three disease groups; however, the diabetic patients showed a more selective loss. The results suggest that multiple sites are involved and that the combined effects of metabolic abnormalities and hypoxia contribute to the selective loss.

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

Extended vitamin A deficiency in the cat led to an abnormal appearance in the tapetal fundus with the formation of a dark brown streak centered on the area centralis. At this time rod sensitivity, as measured by the b-wave of the electroretinogram, was reduced by more than two log units; the level of rod visual pigment was reduced by about 90% throughout the paracentral retinal region and was essentially absent from the area centralis. Following oral supplementation with vitamin A there was a rapid partial recovery of both rhodopsin levels and rod sensitivity. Further recovery continued over more than 18 days to levels that were not substantially below normal. This recovery was absent from the area centralis, in which measured visual pigment levels remained very low. In supplemented cats, the brown color in the fundus faded but there remained a small hyper-reflective zone at the area centralis. Morphological examination of the central retina in a supplemented cat showed an outer nuclear layer reduced to one or two rows in the small zone with low rhodopsin levels. Cone but not rod photoreceptors were present in this zone and they appeared to lack outer segments. During recovery, the increase in rod sensitivity was approximately linearly related to the recovery of rhodopsin levels. Thus, in these conditions reduction in sensitivity resulting from previous vitamin A deficiency was limited by the ability of the photoreceptors to absorb incident quanta. The time course of the recovery of rhodopsin and sensitivity suggests that at least two processes were involved. The faster of these may be the regeneration of rhodopsin from existing opsin molecules in the outer segments, while the slower may depend on the renewal of the outer segments themselves.

[ERG characteristics of congenital stationary night blindness]

The ERG of 9 cases (18 eyes) of congenital stationary night blindness with normal fundi or myopia were tested. All eyes showed absence of rod ERG and cone ERG with normal a-wave. Scotopic mixed ERG were of the negative type in 7 eyes and the subnormal type in 11 eyes. The b/a ratio was reduced in all eyes. The ERG characteristics are useful for the classification, estimation of the probable location of the lesion, and differential diagnosis of the disease.

A family with the Bardet-Biedl syndrome and diabetes mellitus

We studied a family with the Bardet-Biedl syndrome and diabetes mellitus. Two affected brothers and one affected sister were examined. Two older sisters with stigmata of the syndrome had died of unclear causes. The 18-year-old brother was obese, was mentally retarded, and had pigmentary retinopathy and insulin-dependent diabetes mellitus. The 16-year-old sister, who died in a diabetic coma during the course of the investigation, had polydactyly, hypogenitalism, obesity, mental retardation, and pigmentary retinopathy. The 8-year-old brother had all the features of the syndrome, but no overt diabetes mellitus. Electroretinography showed severe cone and rod dysfunction. Patients with the Bardet-Biedl syndrome should be screened for the presence of abnormalities in glucose metabolism.

Perceptual evidence for an anomalous distribution of rods and cones in the retinas of dyslexics: a new hypothesis

The present study tested the hypothesis that from the ability of dyslexics to read peripheral letters one may infer the presence of cones, those visual receptors specialized for acuity, in the peripheral retina. The cones are also the receptors for color discriminations. The method of retinal perimetry was used to map color-sensitive zones of the retina for 14 dyslexic and 14 control subjects. A statistically significant difference showed that the 14 dyslexics reported colors at more peripheral positions than did the 14 normal readers. These data are consistent with the premise that dyslexics have an anomalous distribution of retinal receptors. These findings perhaps may help to explain the erratic eye movements of some dyslexics. It is suggested that retinal perimetry might be one component of a diagnostic battery for the differential diagnosis of dyslexics.

Dark adaptation in hyperprolactinemic women

In a study to test the hypothesis that hyperprolactinemia is caused by subclinical zinc deficiency, dark-adaptation curves of seven women with primary hyperprolactinemia and seven normal women were measured to assess tissue zinc status. Although plasma zinc levels, final dark-adapted thresholds, and the time courses of rod dark-adaptation did not differ significantly between patients and normal subjects, the median cone plateau of the hyperprolactinemic patients was significantly higher (0.66 log units) than that of normal subjects. It appears unlikely that derangements of vitamin A metabolism, for which zinc is a cofactor, explains this unanticipated and subtle abnormality in dark adaptation of the hyperprolactinemic women.

Losses of temporal modulation sensitivity in retinal degenerations

Sensitivity losses in patients with retinitis pigmentosa (RP) have been attributed to a decrease in photopigment density, to a reduction in the number of photoreceptors, and also to a change in temporal response properties of the receptors. The sensitivity losses in patients with macular degeneration have also been attributed to a loss of photoreceptors. To test these explanations for sensitivity loss we obtained electrophysiological and psychophysical temporal modulation transfer functions (MTFs) on normal subjects in response to varying stimulus luminances and retinal loci. These stimulus manipulations did not duplicate the changes observed in the temporal MTFs of patients. The temporal response properties of the receptors were tested electrophysiologically by manipulating stimulus presentation interval. The results provided evidence for sensitivity losses in RP patients being due to alterations in the temporal response properties of the receptors.

Interocular asymmetry of visual function in heterozygotes of X-linked retinitis pigmentosa

Heterozygotes of X-linked retinitis pigmentosa were studied with full field rod and cone electroretinography and light adapted kinetic perimetry. Twelve parameters from the electroretinograms (ERGs) and two parameters from the kinetic visual fields of both eyes of 22 heterozygotes were measured and statistical comparisons made with results from female control subjects. Rod and cone ERG amplitude parameters were significantly lower and cone timing delayed in the heterozygotes. Most of the ERG parameters that were abnormal in measured value also showed significantly greater interocular differences compared with controls. Kinetic visual fields with both V-4e and I-4e test targets were smaller in heterozygotes than in controls. Only with the I-4e target, however, were interocular differences significantly larger in the heterozygotes. For the I-4e target and many of the ERG parameters, using the interocular difference in conjunction with the measured parameter value from a single eye significantly increased the efficacy of discrimination between heterozygotes and controls; for some ERG parameters, the interocular difference alone provided the best separation of the two groups.

[Nyctometric studies in argon laser coagulation in patients with diabetic retinopathy]

Forty-two patients (57 eyes) with diabetic retinopathy at various stages have been examined by nyctometry before and over the course of the follow-up period after argon laser coagulation of the retina. The findings evidence a marked reduction of the adaptation ability of the central retinal area even in high vision acuity in the patients with diabetic retinopathy vs. normal subjects. A stable rise of the nyctometric parameters has been observed after argon laser coagulation; these parameters characterize the function of the pigmented epithelium and of the retinal cone, and these data may be considered as an evidence of the improved trophics of these tissues, resultant from such therapy.

Autosomal dominant retinitis pigmentosa: a log quotient analysis of the photopic and scotopic b-wave amplitude

The relationship of the photopic and the scotopic b-wave amplitudes of the electroretinogram was studied in 85 normal subjects and 25 patients with autosomal dominant retinitis pigmentosa, in which one amplitude was at least 20 microvolts. The log quotient of their b-wave amplitudes--that is log of the photopic b-wave amplitude divided by the scotopic b-wave amplitude--was considered to represent the activity of cones relative to rods. The log quotient values had a normal gaussian distribution in the normal control eyes, while they formed two groups in the patients with autosomal dominant retinitis pigmentosa. In the first group (type 1), the scotopic b-wave was non-recordable while the photopic b-wave amplitude was larger than 20 microvolts in all cases, indicating that the log quotient is larger than 0.5 and that the rod system is much more severely affected than the cone system. The second group (type 2) had a log quotient smaller than 0.5 and its distribution almost overlapped the normal one, indicating more symmetrical damage in the cone and rod systems. The mean final rod threshold at 45 minutes for type 1 was significantly higher than that for type 2. The log quotient proved to be a useful index for analysing the cone and rod involvement and consequently provides a better understanding of autosomal dominant retinitis pigmentosa.

Light adaptation of the electroretinogram. Diminished in retinitis pigmentosa

The cone electroretinogram has been examined at different levels of ganzfeld adaptation in normals and subjects with retinitis pigmentosa (RP). As light adaptation increases, the amplitude and time course (measured as b-wave implicit time) of the cone ERG decrease in both normal and RP subjects. The same level of light adaptation, however, decreases the amplitude and the implicit time more in normal than in RP subjects. The ineffectiveness of light for adapting the ERG of RP subjects is most easily explained by assuming that RP cones absorb less light than normal cones. By comparing these parameters between normal and RP subjects at different levels of light adaptation, it is possible to estimate this ineffectiveness of cone absorption in RP subjects. The results imply that RP cones can transduce and adapt but fail to absorb light as effectively as normal cones. The quantitative relationship between cone b-wave implicit time and retinal illumination provides a unique method for examining cone function as well for standardizing ganzfeld backgrounds in ERG laboratories.

Identification of a photoreceptor-specific mRNA encoded by the gene responsible for retinal degeneration slow (rds)

Mutant mice homozygous for 'retinal degeneration slow' (rds/rds) are characterized phenotypically by abnormal development of photoreceptor outer segments in the retina, followed by gradual degeneration of photoreceptors. This process of degeneration is complete by one year, with preservation of all other retinal cells. The biochemical defect that leads to the mutant phenotype is not known. Our strategy for cloning the rds gene was based upon three previously reported observations. First, the rds locus maps to chromosome 17. Second, experimental rds/rds----+/+ and rds/+----+/+ tetra-parental mice manifest patchy photoreceptor changes in the retina, suggesting that the wild-type rds locus is expressed within cells of the photoreceptor lineage. Finally, the process of degeneration is specific to photoreceptors. On the basis of these observations, we predicted that the rds mRNA is encoded by a gene on chromosome 17 and is normally expressed exclusively within photoreceptors in the retina. We here present evidence that this is the case.

Transplanted retinal pigment epithelium modifies the retinal degeneration in the RCS rat

Transplantation of dissociated retinal epithelial cells obtained from the retinas of normal, congenic pigmented strain of rats to Bruch's membrane and the subretinal space of dystrophic rats from the Royal College of Surgeon (RCS) strain can prevent photoreceptor cell degeneration in this retina for at least 4 months after transplantation. Host and transplant cells form close apposition with one another but can be distinguished by the presence of both phagosomes and melanin granules in the transplant and the absence of these inclusions in the host retinal epithelium. Transplanted cells show excessive amounts of phagosomal material within 48 hr after transplantation, implying that restoration of phagocytosis is responsible for the photoreceptor survival.

Large rod-like photopic signals in a possible new form of congenital night blindness

A 10-year-old Persian girl has symptoms of congenital stationary night blindness and some drusen-like lesions in the region of the vascular arcades. Her electroretinogram shows no rod response to a weak stimulus, but a large (475 microV) slow scotopic response to a strong stimulus that is unchanged by photopic conditions (15 Fl background illumination). However, the response to flicker had the typical (smaller) amplitude of a cone signal. They may represent a new form of night blindness in which rod sensitivity is reduced so that there is no vision under dim conditions but rod function still persists under photopic conditions.

Probing visual function with psychophysics and photochemistry

New methods using computer based measurements and image analysis techniques can improve and expand our ability to investigate non-invasively the function of the retina in patients. These can provide insight into the underlying mechanism of an abnormality and further our understanding of disease processes.

Progressive peripheral cone dysfunction

A 22-year-old man had a three-year history of progressive day blindness, most notably peripherally, and denied difficulty with central vision or color vision. Visual function studies demonstrated a diffuse dysfunction of the photopic system and normal scotopic function. The central cone function, however, was essentially normal. Visual acuity was 20/20 in each eye, results on AO-HRR and Ishihara color plate testing were normal, color naming visual fields demonstrated color discrimination in the central 10 degrees, and foveal adaptation was normal.

[Blue cone monochromasia: diagnosis, genetic counseling and optical aids]

Cone and rod functions of three blue cone monochromats (age 13-20, male) from three different families were investigated. In contrast to rod monochromats, they lack prominent nystagmus. Color matches as determined using the Nagel anomaloscope came close to those of rod monochromats but the green primary appeared slightly brighter to them. In color discrimination tests (Farnsworth-Munsell 100-hue and Panel D-15 desaturated), maximum confusion followed protan rather than scotopic axes. Measurements of spectral sensitivity revealed action spectra exclusively of blue sensitive cones, even under conditions that should isolate green- or red-sensitive cones. After 20 minutes of dark adaptation, rods determined the spectral sensitivity function. Transient tritanopia, which in normals results from the interaction between cones sensitive to short and long wavelengths, was completely absent in blue cone monochromats. Visual acuity (Snellen charts and contrast gratings) revealed values between 20/200 and 20/60. Recognition of high-spatial low-contrast gratings was improved by blue cut-off filters (Schott BG 28) and considerably worsened by yellow cut-off filters (Schott OG 510). Since alteration of visual acuity induced by cut-off filters was not found in rod monochromats, this two-filter test is a means of differentiating quickly between rod achromats and blue cone monochromats. As the mode of inheritance is autosomal recessive in rod achromats and x-linked recessive in blue cone monochromats, differential diagnosis is important for correct genetic counseling.

Two types of visual dysfunction in autosomal dominant retinitis pigmentosa

Visual thresholds and rhodopsin levels were determined in nine subjects with autosomal dominant retinitis pigmentosa. The subjects fell into two groups, corresponding to two subtypes of the disease revealed by two-color, dark-adapted static perimetry. In the first of these subtypes, rod-mediated function was variably reduced and was accompanied by a corresponding reduction in cone function in the same retinal region. Dark-adapted threshold elevations varied in a way consistent with decreased quantal absorption by the rods as a result of reduced rhodopsin levels. In the second subtype, rod function was greatly reduced or absent throughout the retina, while cone function was much less severely affected. Although the levels of rhodopsin were only about half of normal, they were much too great to account for the visual threshold elevations on the basis of decreased probabilities of absorption by the visual pigment. Rhodopsin regeneration appeared to follow normal kinetics in patients from both groups. The results indicate that the examples of the two psychophysical subtypes of AD RP investigated here have very different disease manifestations.

N-3-pyridylmethyl-N'-p-nitrophenylurea ocular toxicity in man and rabbits

Ingestion of the rat poison N-3-pyridylmethyl-N'-p-nitrophenylurea (PNU) produced ocular toxicity in three humans and in an animal model, the Dutch Belted rabbit. The electroretinogram b wave was especially susceptible to the effects of the rodenticide, and the target tissue appeared to be the retinal pigment epithelium. Injection of PNU itself did not produce ocular toxicity. The poison had to be administered orally. Gentamicin administered orally with PNU prevented the ocular toxicity. Presumably this antibiotic killed those gastrointestinal bacteria responsible for PNU's metabolism into an ocular toxin. L-tryptophan, a known antidote for the lethal effects of PNU, was an antidote for the ocular toxicity when administered orally but not when administered parenterally.

Cone electroretinograms and visual acuities of diabetic patients on sorbinil treatment

Photopic, 30-Hz, and foveal electroretinograms were measured in 19 diabetic patients in an experimental study of the effects of short-term Sorbinil (an aldose-reductase inhibitor) on retinal function. Patients were assigned in double-blind fashion to Sorbinil (250 mg/day) or placebo groups and were tested at the outset and after four weeks of therapy. Comparisons (t-test) between the Sorbinil and placebo groups failed to show significant effects of treatment on electroretinograms, although there was a significant correlation within the Sorbinil group between foveal recordings and red cell sorbitol at the end of treatment. Analysis showed that increased foveal electroretinograms were found in patients with low initial retinopathy but not in those with greater retinopathy. Eight patients continued Sorbinil treatment for one year. Again patients improving their foveal measurements had less initial retinopathy than those not improving. This difference was significant after one year of treatment but not at four weeks.