Biomedical subjects

K R Alexander

Publications and source records attributed to K R Alexander.

At least 37 records · Page 2Linked to original sources

PubMed1996

Visual acuity impairment in patients with retinitis pigmentosa.

PURPOSE: The authors evaluated visual acuity impairment in 906 patients from 742 families with either isolated or various identifiable genetic subtypes of retinitis pigmentosa (RP) to determine the severity of their visual acuity impairment. Emphasis was placed on the prevalence of total blindness and visual acuity of 20/200 or worse in this group of patients. METHODS: This cross-sectional retrospective study included all patients with RP who met certain entrance criteria and were examined by one of the authors (GAF). The authors analyzed the eye of each patient with the best-corrected visual acuity on their most recent visit. RESULTS: Seventeen patients with a sector form of RP were excluded from the authors primary analysis. In the remaining group of 889 patients, 710 (80%) had a visual acuity of better than 20/200, 648 (73%) showed a visual acuity of 20/70 or better, and 489 (55%) had a visual acuity of 20/40 or better in at least 1 eye. Seventy-five patients (8%) had visual acuity of count fingers or worse in their best eye. There was only one patient with no light perception in each eye. Patients with autosomal dominant RP, as a group, had the least severe and those with X-linked recessive RP had the most severe impairment in visual acuity. Those with autosomal recessive disease were intermediate in severity of visual impairment. CONCLUSIONS: Analysis of visual acuity in this large group of patients with RP, which genetically is representative of patients with RP seen in the United States by those who specialize in retinal disease, showed that it was rare for the patients to lose all visual acuity from the disease itself. Further, legal blindness from visual acuity loss, defined as best-corrected visual acuity that is no better than 20/200 in at least one eye, occurred in a relatively small percentage (20%) of our patient population, whereas approximately half of all patients and 42% of those older than 60 years had a visual acuity of 20/40 or better in at least one eye. The extent of impairment in visual acuity was associated with the genetic subtype of the disease.

Adolescent↗

PubMed1996

Acquired unilateral night blindness associated with a negative electroretinogram waveform.

PURPOSE: The authors performed clinical, electrophysiologic, psychophysical, and immunologic studies in a patient who presented with an acquired night blindness in one eye to better define the clinical and functional changes in this rare disorder. METHODS: In addition to an ophthalmologic examination, the patient underwent the measurement of electroretinogram responses, dark-adapted thresholds using a Tübingen perimeter (Oculus, Tubingen, Germany), color vision assessment, kinetic visual-field testing using a Goldman perimeter, and immunologic testing to determine if the serum contained autoantibodies to retinal bipolar cells. RESULTS: Fundus examination showed no clinically apparent abnormality in either eye. The patient showed a selective reduction in the b-wave amplitude of the rod electroretinogram and an abnormality of the cone electroretinogram ON response in the affected left eye, whereas the rod and cone electroretinograms of the right eye were normal. Rod thresholds in the affected eye were elevated markedly, whereas rod thresholds in the right eye were normal centrally and slightly elevated in the far periphery. Immunologic testing did not show circulating autoantibodies to retinal cells. CONCLUSIONS: The patient examined in this study showed phenotypic similarities to patients with congenital stationary night blindness and to patients with an acquired form of night blindness associated with cutaneous melanoma (MAR syndrome). The electroretinogram findings from the patient are consistent with an acquired defect in signal transmission from photoreceptors to ON-type bipolar cells. However, the etiology of this unique form of unilateral night blindness remains obscure.

Adult↗

PubMed1995

Visual acuity vs letter contrast sensitivity in retinitis pigmentosa.

This study examined the quantitative relationship between foveal visual acuity and contrast sensitivity for large-letter optotypes in a group of patients with retinitis pigmentosa (RP), in order to assess more completely the extent of foveal vision loss in this group of hereditary retinal dystrophies. High-contrast visual acuity and large-letter contrast sensitivity were measured with a computer-based testing system and with commercially available letter charts (Lighthouse Distance Visual Acuity Test; Pelli-Robson Contrast Sensitivity Chart). Findings from 20 patients with typical RP or Usher syndrome were compared with those from 15 age-similar control subjects with normal vision. On both the computer-based test and the chart tests, the patients with RP showed approximately equal reductions in visual acuity and large-letter contrast sensitivity. However, intersubject controls was greater for contrast sensitivity than for visual acuity on both test protocols. As a result, the patients with RP required a greater reduction in contrast sensitivity than in acuity to exceed the normal range, indicating that visual acuity was the more sensitive index of the loss of foveal visual function.

Adult↗

PubMed1995

Effect of spatial sampling on grating resolution and letter identification.

The purpose of the study was to determine whether spatial sampling has equivalent effects on visual performance for grating and letter stimuli, two optotypes that are frequently used in the clinical assessment of visual function. Test targets consisted of five-bar square-wave gratings, spatial D6 (sixth derivative of a Gaussian) patterns, or individual Sloan letters, presented for 255 ms in the center of an adapting field. Spatial sampling was introduced by replacing random regions of the targets with occluding elements that had the same luminance as the adapting field, so that only random samples of the test target were visible on each trial. The occluding elements consisted of 4 x 4, 8 x 8, or 16 x 16 pixel arrays (subtending 0.67, 1.33, and 2.67 arcmin, respectively), and the percentage of the test target that remained unoccluded ranged from 3% to 100%. Visual acuity and contrast sensitivity for the spatially sampled targets were measured in two visually normal subjects. Results were compared with the predicted effect of the reduction in effective contrast that was introduced by the sampling paradigm. For grating stimuli the effect of spatial sampling was equivalent to the reduction in effective stimulus contrast produced by the sampling, regardless of occluding-element size. For letter stimuli the reduction in effective stimulus contrast predicted most, but not all, of the loss of visual acuity and contrast sensitivity for sampled targets, and the deficit in performance increased when larger occluding elements were used.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1994

Prolonged rod dark adaptation in patients with cone-rod dystrophy.

Thirteen patients with cone-rod dystrophy were assigned into one of four previously described category subtypes according to clinical, electrophysiologic, and psychophysical criteria. The time course of rod dark adaptation was determined for each patient by means of a Goldmann-Weekers dark adaptometer. Nine of the 13 patients showed a normal time to return to their dark-adapted thresholds before bleaching, while four patients showed a prolonged recovery time. The four patients with a prolonged rod-recovery time were all from the same clinical subtype and showed a similar fundus appearance as well as similar electrophysiologic and psychophysical findings.

Adolescent↗

PubMed1994

Dark adaptation in patients with Best vitelliform macular dystrophy.

Psychophysical dark adaptation studies were performed in six patients with Best vitelliform macular dystrophy (BVMD) using a Goldmann-Weekers dark adaptometer. Prebleach thresholds were determined before obtaining a postbleach full recovery curve. Unlike patients with Stargardt macular dystrophy, all patients with BVMD showed a normal time to reach their baseline dark adapted thresholds after bleaching of their rod visual pigment when tested in clinically normal appearing retina. Although a lipofuscin material accumulates within retinal pigment epithelial cells in patients with either Best or Stargardt dystrophy, functional findings pertaining to recovery of rod dark adaptation thresholds as well as electro-oculogram light peak to dark trough ratios are different in these two disorders.

Adolescent↗

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

Evaluation of driving performance in patients with juvenile macular dystrophies.

The driving performance of 20 subjects with central vision impairment due to either Stargardt disease or cone-rod dystrophy (visual acuity, 20/40 to 20/70) was compared with that of 29 control subjects with normal vision who had similar driving histories. Driving performance was defined by accident involvement based on self-report and state records and by an evaluation of performance on an interactive driving simulator. The proportion of individuals involved in accidents in the central vision loss group was comparable to that of the control group. For 13 of the 20 subjects with central vision loss who did not restrict their driving to daylight hours, there was a greater likelihood of involvement in nighttime accidents than in the control group. Visual function measures and simulator indexes did not predict accident involvement for the central visual loss group, although these subjects showed longer braking response times and a greater number of lane boundary crossings than the control group. These findings are in contrast to our previously published report of subjects with retinitis pigmentosa, who were more likely to have been involved in both daytime and nighttime accidents than a control group and for whom visual field extent was significantly related to accident involvement.

Accidents, Traffic↗

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↗

PubMed1993

Autoantibodies against retinal bipolar cells in cutaneous melanoma-associated retinopathy.

PURPOSE: This study's goal was to determine the pathophysiology of the retinopathy that occurs in patients with metastatic cutaneous melanoma and sudden onset of night blindness, the so-called melanoma-associated retinopathy (MAR) syndrome. We tested the hypothesis that sera from two MAR patients contained autoantibodies that reacted with "on" bipolar cells of the human retina. METHODS: Immunofluorescence was performed on cryostat sections of unfixed normal human retinas. Sera and IgG fractions were tested from the two MAR patients and 38 control subjects (28 patients with metastatic melanoma, but no visual symptoms; two patients with non-MAR retinopathy; and eight normal subjects). RESULTS: The sera and IgG fractions from both MAR patients but from none of the control subjects produced heavy immunostaining of bipolar cells, which were identified as rod bipolars by a double labeling procedure using anti-protein kinase C. CONCLUSIONS: We hypothesize that MAR patients generate autoantibodies against a melanoma antigen that cross react with bipolar cells of the retina. These antibodies, by an unknown mechanism, may cause abnormalities of the rod and cone systems that are characteristic of MAR.

Adult↗

PubMed1992

Ocular findings associated with rhodopsin gene codon 267 and codon 190 mutations in dominant retinitis pigmentosa.

Two members of a family with autosomal dominant retinitis pigmentosa were found to have a cytosine-to-thymine mutation in the second nucleotide of codon 267 in the rhodopsin gene that resulted in a proline-to-leucine change. Two members of another family with autosomal dominant retinitis pigmentosa showed a guanine-to-thymine mutation in the first nucleotide of codon 190 in the rhodopsin gene that resulted in an aspartate-to-tyrosine change. Three members from a third family with autosomal dominant retinitis pigmentosa were also found to have a mutation in codon 190; however, this guanine-to-adenine mutation in the first nucleotide of codon 190 resulted in an aspartate-to-asparagine change. The relatively less severe functional retinal impairment in our patients with a transmembrane codon 267 rhodopsin gene mutation is generally comparable with that observed in a previously described codon 58 transmembrane mutation. The two families with different intradiscal codon 190 mutations showed a considerable difference in severity of their disease.

Adult↗

PubMed1992

Assessment of driving performance in patients with retinitis pigmentosa.

The driving performance of 21 subjects with retinitis pigmentosa (RP) and varying degrees of peripheral field loss was compared with the performance of 31 normally sighted control subjects who did not differ statistically from the subjects with RP in age, gender, years of driving experience, or miles driven per year. Driving performance was assessed by self-reported accident frequency and by an evaluation of performance on an interactive driving simulator. A significantly greater proportion of individuals had self-reported accidents in the RP group than in the normal group. Likewise, a significantly greater proportion of subjects with RP than normal subjects had accidents on the driving simulator. Logistic regression analyses indicated that binocular horizontal field extent and binocular field area significantly differentiated between those having no self-reported accidents and those subjects with RP having one or more self-reported accidents. Because the simulator indexes were correlated with visual field measures for the subjects with RP, no additional information was incorporated into the regression model by adding the simulator measures. Therefore, our results indicate that visual field loss is a primary correlate of automotive accidents in individuals with RP.

Accidents, Traffic↗

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

Rapid and slow changes in the human cone electroretinogram during light and dark adaptation.

Changes in the response characteristics of the human cone electroretinogram (ERG) during light and dark adaptation were studied in two visually normal subjects. Cone ERG responses were isolated under all adaptation conditions through the use of 31 Hz flicker. To determine the time-course of changes in the cone ERG during adaptation, responses to stimuli of constant luminance were measured repeatedly during 15 min of exposure to an adapting field of 2.0 log cd/m2 and during 30 min of dark adaptation following adapting field termination. In addition, luminance-response functions were obtained before and immediately after adapting field onset, as well as before and immediately after adapting field termination. The results indicate that the human cone ERG is influenced by two major processes. One process has a relatively rapid time-course and serves to reposition the luminance-response function along the luminance axis following changes in ambient light levels. The second process, which has a slow time-course, scales response amplitudes during light and dark adaptation by the same proportion at all stimulus luminances. The results provide a framework for predicting the manner in which the cone ERG will change with alterations in the state of retinal adaptation.

Adaptation, Ocular↗

PubMed1992

Effect of mydriasis on visual field area in retinitis pigmentosa.

PURPOSE: The effect of mydriasis on Goldmann visual field area in patients with retinitis pigmentosa has not been suitably defined. The aim of this study is to determine whether visual field area in these patients varies with pharmacologic mydriasis. METHODS: Fifteen adult patients with retinitis pigmentosa were studied prospectively. Goldmann visual fields with II4e and V4e isopters were obtained in both eyes before and after full pharmacologic mydriasis of the right eye. The isopter areas were quantified and analyzed to determine the effect of mydriasis on visual field area. RESULTS: The difference in the right eye isopter areas was compared with the difference in the left eye isopter areas using paired t tests, where the differences were computed from areas obtained before and after dilation of the right eye. Mydriasis had no significant effect on the visual field in terms of isopter area difference (II4e, P = 0.87; V4e, P = 0.45) and percent change in isopter area (II4e, P = 0.81; V4e, P = 0.24). CONCLUSION: Pharmacologic mydriasis had no appreciable effect on the Goldmann visual field area in a selected group of patients with retinitis pigmentosa. These findings suggest that visual fields of such patients obtained in the dilated or undilated state can be meaningfully compared.

Adult↗