Biomedical subjects

D J Derlacki

Publications and source records attributed to D J Derlacki.

At least 37 records · Page 2Linked to original sources

PubMed1994

Effect of vitamin A treatment on the prolongation of dark adaptation in Stargardt's dystrophy.

BACKGROUND: Prolongation in recovery of rod thresholds has been demonstrated in Stargardt's dystrophy. One possible explanation for this finding includes an impairment of vitamin A transport by the retinal pigment epithelium (RPE). By delivering an increased amount of vitamin A to the RPE, it might be possible to overcome a relative deficiency of vitamin A utilization or transport, and thus improve rod dark adaptation. METHODS: Baseline dark-adapted rod final thresholds were measured for five patients with Stargardt's dystrophy after 60 minutes of dark adaptation. A full dark-adaptation curve was then measured after exposure to a bleaching light for 5 minutes. Time of recovery to within 0.2 log units of the prebleach dark-adapted rod threshold was determined. Each subject then took a 14- to 18-day course of oral vitamin A, 50,000 IU daily. Dark adaptation was then reassessed using the same pretreatment protocol. RESULTS: Before treatment, all five patients had a prolongation of their rod recovery curve. There was no statistically significant difference between subjects in mean time taken to reach prebleach rod baseline thresholds before and after vitamin A treatment. CONCLUSIONS: These findings do not rule out the possibility that a delay in rod dark adaptation in Stargardt's dystrophy results from an inability to transport vitamin A from the RPE to photoreceptor cells. Nevertheless, a high dose of oral vitamin A taken for at least 14 days did not provide any objective improvement in dark-adaptation function in five such patients.

Administration, Oral↗

PubMed1994

Spatial-frequency characteristics of letter identification.

To investigate the spatial-frequency components that govern letter identification we compared contrast thresholds for three types of visual stimulus (1) standard Sloan letters, (2) Sloan letters that were spatially bandpass filtered by cosine log filters, and (3) D6 patterns (sixth spatial derivatives of Gaussians). Stimuli were presented on a gray-scale display screen of a Macintosh computer-based testing system at temporal frequencies primarily of 2 and 16 Hz. Contrast thresholds were measured in two subjects with normal visual acuity with use of forced-choice staircases. Contrast sensitivity functions for standard Sloan letters and D6 patterns were comparable at a temporal frequency of 16 Hz but differed systematically at a temporal frequency of 2 Hz. The measurement of contrast sensitivity for cosine log filtered letters presented at a temporal frequency of 2 Hz indicated that the object spatial frequency of maximum sensitivity shifted to lower frequencies as letter size decreased, whereas the retinal spatial frequency of maximum sensitivity remained relatively constant. When letters were spatially bandpass filtered at a peak object spatial frequency of 2.5 cycles/letter, then contrast sensitivity functions for letter identification were equivalent to those for D6 patterns at both temporal frequencies. These results suggest that spatially filtered letters may provide a more appropriate test of visual function than do standard letter optotypes.

Adult↗

PubMed1993

Clinical subtypes of cone-rod dystrophy.

OBJECTIVE: To determine possible distinct phenotypic subtypes of cone-rod dystrophy. PATIENTS: Thirty-three patients with cone-rod dystrophy (from 25 families) were assessed prospectively on electroretinography, visual field testing, psychophysical threshold profiles, and fundus features. The clinical records of an additional 150 patients with cone-rod dystrophy were examined retrospectively in terms of the classification schema derived from the prospective study. RESULTS: Based on electroretinographic recordings, two major types of cone-rod dystrophy were differentiated. In type 1, cone amplitudes were reduced to a greater degree than were rod amplitudes on electroretinography, while in type 2, cone and rod electroretinographic amplitudes were reduced in equal proportion. These two types were further subdivided on the basis of patterns of visual field loss and threshold elevation. In type 1a, there was a central or paracentral scotoma, and cone thresholds were more elevated centrally than peripherally. In type 1b, there was no central scotoma, and cone thresholds were more elevated peripherally than centrally. In type 2a, there was a central scotoma, cone thresholds were more elevated centrally than peripherally, and rod thresholds were more elevated peripherally than centrally. In type 2b, a partial or complete ring scotoma was present, cone thresholds were more elevated peripherally than centrally, and rod thresholds were more elevated in the midperipheral than in either the central or far peripheral region of the retina. Of the 150 additional patients with cone-rod dystrophy, data sufficient for classification were available for 95 patients, and all but two had findings that were consistent with classification into one of these four subtypes. CONCLUSION: Our results identify four functionally distinct subtypes of cone-rod dystrophy that may be useful for patient counseling and future molecular genetic studies.

Adolescent↗

PubMed1993

The effect of contrast polarity on letter identification.

To determine the effect of contrast polarity on the spatial characteristics of letter identification, we measured contrast sensitivity for individual Sloan letters that were presented either as luminance increments or luminance decrements relative to a continuously presented adapting field. The temporal mode of presentation consisted of either a rapid onset with a Gaussian offset, or the reverse. When contrast was specified in terms of Rayleigh (Michelson) units, the contrast sensitivity function for letters of positive contrast extended to smaller letter sizes than the function for letters of negative contrast. However, when contrast was defined in Weber terms, letter contrast sensitivity functions were identical for letters of positive and negative contrast, indicating that letter identification was equivalent for luminance increments and decrements that had equal absolute magnitude. Onset-offset characteristics had no differential effect on letter contrast sensitivity by either contrast definition. These findings provide a basis for predicting the effect of contrast polarity on tasks that involve letter identification.

Adaptation, Ocular↗

PubMed1993

Visual acuity in patients with best vitelliform macular dystrophy.

PURPOSE: Forty-seven patients with Best vitelliform macular dystrophy were evaluated in a cross-sectional fashion for visual acuity loss with age. METHODS: The authors assessed only patients who had at least one eye with a recognizable phenotype of Best vitelliform macular dystrophy. Patients with absent foveal changes or with only minimal foveal pigment mottling and hypopigmentation in each eye were excluded. RESULTS: A significant difference was noted between the visual acuities of the two eyes of the patients (2 lines or greater in the majority [64%] of patients). Nevertheless, for both eyes a significant correlation was noted between patient age and visual acuity, with older patients tending to have worse visual acuities. In the eyes with the best visual acuity, the majority of patients younger than 40 years of age (76%) had a visual acuity of 20/40 or better. In patients older than 30 years of age, a substantial percentage (74%) had a visual acuity of 20/100 or worse in at least one eye. CONCLUSION: The authors' findings indicate that although patients with Best vitelliform macular dystrophy who show characteristic macular lesions may retain good visual acuity in at least one eye, an appreciable number can lose substantial visual acuity, at least monocularly. In this population, no patient older than 50 years of age fulfilled the visual acuity criterion of 20/40 in at least one eye, the requirement in most states for an unrestricted driver's license, and only 20% of patients older than 40 years of age fulfilled this visual acuity criterion.

Adolescent↗

PubMed1993

Temporal properties of letter identification in retinitis pigmentosa.

The effect of stimulus duration on the visual acuity of individuals with retinitis pigmentosa (RP) was assessed by using Sloan letters of 100% contrast and durations ranging from 15 ms to 3.8 s. In addition, contrast thresholds for identifying Sloan letters (0.7 log minimum angle of resolution; 20/100 Snellen equivalent) were measured over the same range of durations in the same subjects with RP. Compared with results from a control group of subjects with normal vision, the subjects with RP showed losses in visual acuity at all stimulus durations, with a slightly though significantly greater reduction in visual acuity at short durations. The letter-contrast thresholds of the subjects with RP were elevated above the normal range to the same degree at all durations. Analysis of the results in the format of letter contrast sensitivity functions indicates that temporal summation for letter identification was normal for these subjects with RP and that their relatively greater acuity loss at short exposure durations was most likely related to their elevated letter contrast thresholds.

Adult↗

PubMed1992

Light adaptation and the luminance-response function of the cone electroretinogram.

Cone electroretinograms are typically isolated by presenting stimulus flashes against rod-desensitizing adapting fields. To investigate the manner in which adapting-field luminance affects cone electroretinogram response properties, we measured cone electroretinogram luminance-response functions of two normal subjects, with stimuli presented against adapting fields that ranged in luminance from -1.2 to 2.1 log cd/m2. A flicker rate of 31.1 Hz was used to isolate cone electroretinograms under all adaptation conditions. A hyperbolic equation of the form (R/Rmax) = Ln/(Ln + Kn) was fitted to each luminance-response function by a least-squares criterion. As adapting field luminance increased, the best-fit values of the variables K and n increased, which is in general agreement with results of electrophysiologic studies of light adaptation in retinal neurons. However, Rmax values also increased with adapting field luminance. The change in all three of these variables with adapting field luminance must be considered in the interpretation of cone electroretinogram luminance-response functions from patients with retinal disorders.

Adaptation, Ocular↗

PubMed1992

Light adaptation, rods, and the human cone flicker ERG.

During the course of light adaptation, the amplitude and implicit time of the human cone ERG change systematically. In the present study, the effect of adapting field luminance on these ERG changes was assessed, and the hypothesis that light adaptation of the rod system is the primary determining factor was evaluated. Cone ERG responses, isolated through the use of 31.1-Hz flicker, were obtained from two visually normal subjects, initially under dark-adapted conditions and then repeatedly for 30 min following the onset of each of a series of ganzfeld adapting fields with luminances that ranged from -1.2 to 2.1 log cd/m2. The increase in flicker ERG amplitude and decrease in implicit time during light adaptation were greatest at the highest adapting field luminances. Photopically equivalent achromatic and long-wavelength adapting fields induced comparable increases in flicker ERG amplitude, while scotopically equivalent adapting fields had considerably different effects. This latter finding demonstrates that the rod system is not a major determinant of the adaptation-induced increase in cone ERG amplitude.

Adaptation, Ocular↗

PubMed1992

Grating, vernier, and letter acuity in retinitis pigmentosa.

Grating, vernier, and letter acuities were compared in 25 patients with retinitis pigmentosa (RP), whose Snellen visual acuities were better than 20/40, to address the mechanism of visual acuity loss. For these patients with RP, all three types of visual acuity were reduced to an equivalent degree from those of a control group of 10 age-similar, visually normal subjects. The findings indicate that the visual acuity losses of these subjects with RP did not result from cone spatial undersampling (due, for example, to a random loss of foveal cones), from cone sampling irregularities (due to random alterations in foveal cone position), or from a selective loss of sensitivity to high spatial frequencies (as might result from changes in media transmission characteristics or a gain reduction in high spatial frequency mechanisms). In addition, previous studies have indicated that acuity losses in such patients with RP do not result from reductions in the quantum-catching ability of foveal cones. The most likely explanation for the equivalent losses in all three acuity types in these patients with RP appears to be an alteration in foveal spatial scale, consistent with a generalized increase in foveal intercone spacing.

Adult↗

PubMed1992

Contrast thresholds for letter identification in retinitis pigmentosa.

To assess mechanisms of foveal vision loss in retinitis pigmentosa (RP), contrast thresholds were measured for the identification of Sloan letters at four adapting field luminances (0.4, 1.4, 2.4, and 3.4 log td) in a group of 16 patients with RP who had best-corrected Snellen visual acuities of 20/30 or better, minimal or no posterior subcapsular cataracts, and no atrophic or cystic-appearing foveal lesions. Letter contrast sensitivities of the patients with RP were reduced below those of a group of ten subjects with normal vision for all letter sizes and at all adapting field luminances. The overall pattern of these results indicated that neither a reduced quantal absorption by foveal cones nor spatial undersampling from a loss of foveal cones accounted for the reductions in letter contrast sensitivities. The findings were most consistent with a uniform increase in intercone spacing in the foveas of this group of patients with RP and mild visual acuity loss.

Adaptation, Ocular↗

PubMed1991

Effects of light adaptation on the response characteristics of human oscillatory potentials.

We have examined the response characteristics of the oscillatory potentials (OPs) of the human electroretinogram (ERG) obtained to ganzfeld flash stimuli presented against adapting fields. First, we determined the extent to which the OPs obtained to high luminance flashes change during the course of light adaptation to a cone-isolating adapting field. Regardless of the number of OP wavelets, the last OP wavelet increased in amplitude and decreased in implicit time to a greater extent than did the earlier wavelet(s). In addition, we examined the role of both flash and adapting field luminance in determining the wave form of the OPs. For each adapting field luminance that was tested, the number of OP wavelets increased as flash luminance increased, primarily resulting from the splitting of the last OP into 2 distinct wavelets. While the number of OP wavelets generally decreased as adapting field luminance increased, the amplitude of the last OP became larger. These functional distinctions between the last and the earlier wavelets are consistent with their representing the activity of different retinal generators.

Adult↗

PubMed1991

Acuity-luminance and foveal increment threshold functions in retinitis pigmentosa.

Acuity-luminance functions and foveal increment threshold functions were measured in 20 subjects with retinitis pigmentosa (RP) who had Snellen acuities of 20/40 or better, minimal or no posterior subcapsular cataracts, and no atrophic-appearing foveal lesions. Compared with the results from ten normal subjects, the visual acuities of the RP subjects were reduced at all luminance levels; the acuity deficits were more pronounced at low luminances. Foveal detection thresholds of the RP subjects showed the greatest elevation at low background luminances and approached normal values at high adapting levels. There was a statistically significant correlation (r = 0.79, P less than 0.01) between the visual acuities and absolute thresholds of the RP subjects. The overall pattern of results cannot be explained by a reduced quantal absorption in foveal cones, but it is consistent with the hypothesis that a reduced cone spatial density is the primary mechanism of foveal visual loss in this group of RP subjects.

Adaptation, Ocular↗

PubMed1990

Correlation of electroretinographic findings and peripheral retinal nonperfusion in patients with sickle cell retinopathy.

This study examined the relationship between the proportion of peripheral retinal nonperfusion and electroretinogram measures in one untreated eye of 44 patients with sickle cell retinopathy. The proportion of peripheral retinal nonperfusion was derived from fluorescein angiographic frames covering 180 degrees of the temporal periphery. The extent of peripheral retinal capillary nonperfusion was greater in patients with peripheral retinal neovascularization compared with those without neovascularization. Reductions in electroretinogram a- and b-wave amplitudes were found in patients with and without neovascularization. However, such reductions were of greater frequency and magnitude in patients with neovascularization. Electroretinogram abnormalities were restricted to reductions in amplitude; implicit times were generally normal. In terms of the variables of the b-wave luminance-response function, abnormalities were restricted to reductions in the amplitude variable Rmax; values of logK and n were generally within the normal range. Across all patients, there was a significant negative correlation between electroretinogram amplitude measures and capillary nonperfusion, indicating that the electroretinogram provides information about the functional consequences of peripheral capillary nonperfusion in sickle cell retinopathy.

Adolescent↗

PubMed1990

Spatial properties of rod-cone interactions in flicker and hue detection.

Rod-cone interactions in flicker and hue detection were compared to examine the hypothesis that they are mediated by mechanisms with different spatial properties. Flicker and hue thresholds for a 1 deg test stimulus (TS) were measured as a function of background luminance and diameter. Flicker thresholds were reduced from their dark-adapted value by an 11 deg diameter background, but not by a 1 deg background. These results, in agreement with previous work, demonstrate that light adaptation of rods surrounding the TS is necessary to eliminate their effect on cone flicker thresholds. In contrast, hue thresholds were reduced from their dark-adapted value to a comparable degree by 1 and 11 deg backgrounds, indicating that light adaptation of the rods stimulated by the TS is sufficient to abolish the rod-cone hue interaction. Our results support the contention that the rod-cone flicker and hue interactions are mediated by different mechanisms. We also demonstrated that changing the detection task while keeping stimulus parameters constant is sufficient to shift between these two types of rod-cone interactions.

Adaptation, Ocular↗

PubMed1990

A form of congenital stationary night blindness with apparent defect of rod phototransduction.

We report findings obtained from an individual with an unusual form of congenital stationary night blindness (CSNB). Although the rhodopsin density difference of this subject was normal, there was no evidence of rod-mediated visual function. Dark-adapted thresholds were cone-mediated, and dark-adapted electroretinograms (ERGs) represented activity of the cone system exclusively. ERG a- and b-waves obtained under light-adapted conditions were normal. The absence of a rod a-wave but the presence of normal rhodopsin density, in combination with normal cone function, indicates that this form of CSNB likely involves a defect of phototransduction that is limited to the rods. In addition, light-adapted b-wave responses to high luminance flashes were larger than dark-adapted responses, whereas a-wave amplitudes were reduced by light adaptation. These ERG results address proposed mechanisms by which light adaptation might enhance cone system responses.

Adolescent↗

PubMed1989

Letter contrast sensitivity in retinitis pigmentosa patients assessed by Regan charts.

Regan Contrast Sensitivity Letter Charts were used to assess the effects of reduced contrast and luminance on visual acuity in retinitis pigmentosa (RP) patients. Letter acuity was measured monocularly in 17 RP patients and in 14 age-similar normals using charts of three different contrast levels presented at each of four levels of illumination. Despite visual acuities of 20/40 or better under standard clinical test conditions, an appreciable number of RP patients were unable to identify any letters on the intermediate- and/or low-contrast Regan charts. For example, even at the recommended test luminance, 5 patients could not read any letters on the intermediate (11%) contrast chart, and 9 could not be scored on the low (4%) contrast chart. Consequently, the Regan charts were found to have limited usefulness in quantifying the exact extent of visual impairment in this group of RP patients. Nevertheless, our results document the degree of visual acuity loss that can be experienced by RP patients under conditions of low contrast and luminance.

Adolescent↗

PubMed1988

X-linked retinitis pigmentosa. Profile of clinical findings.

An evaluation of 56 patients with X-linked retinitis pigmentosa revealed a profile of findings that include the following: night blindness within the first two decades of life; spherical refractive errors of -2.00 diopters or greater in addition to an increased prevalence of a cylindrical correction of +1.50 diopters or greater; appreciable impairment of central visual acuity to 20/200 or less by the fifth decade of life; characteristic patterns of field loss; presence of a foveal lesion in up to 75% of the study group; posterior subcapsular lens opacities; and nondetectable electroretinographic amplitudes in more than two thirds of the patients (using conventional full-field recording procedures). These observations are of general value in diagnosis of this disease and for counseling of patients afflicted with this severe form of hereditary night blindness.

Actuarial Analysis↗

PubMed1988

Mechanisms of rod-cone interaction: evidence from congenital stationary nightblindness.

The dark-adapted rod system can elevate cone-mediated thresholds for flicker detection as well as thresholds for the detection of hue. We examined these two types of rod-cone interactions in two individuals with congenital stationary nightblindness (CSNB), a retinal disorder in which rod outer segment function is intact, but in which a defect occurs in the transmission of rod signals within the retina. The two types of rod-cone interaction were differentially affected by the retinal pathology; the rod-cone flicker interaction was normal, but the rod-cone hue interaction was absent. These results provide evidence that, despite similarities in the adaptational properties of these two types of rod-cone interaction, they are mediated by different visual mechanisms.

Adolescent↗