Chinatsu Tosha

Retinoid Content, Visual Responses, and Ocular Morphology Are Compromised in the Retinas of Mice Lacking the Retinol-Binding Protein Receptor, STRA6

PURPOSE We report generation of a mouse model in which the STRA6 gene has been disrupted functionally to facilitate the study of visual responses, changes in ocular morphology, and retinoid processing under STRA6 protein deficiency. METHODS A null mouse line, stra6 -/-, was generated. Western Blot and immunocytochemistry were used to determine expression of STRA6 protein. Visual responses and morphological studies were performed on 6-week, 5-month and 10-month-old mice. The retinoid content of eye tissues was evaluated in dark-adapted mice by high performance liquid chromatography. RESULTS STRA6 protein was not detectable in stra6 -/- null mice, which had a consistent reduction, but not total ablation of their visual responses. The mice also showed significant depletion of their retinoid content in retinal pigment epithelium (RPE) and neurosensory retina, including a 95% reduction in retinyl esters. At the morphological level, a reduction in thickness of the neurosensory retina due to shortening of the rod outer and inner segments was observed when compared to control litter mates with a commensurate reduction in rod a- and b-wave amplitudes. In addition, there was a reduction in cone photoreceptor cell number and cone b-wave amplitude. A typical hallmark in stra6 -/- null eyes was the presence of a persistent primary hypertrophic vitreous, an optically dense vascularized structure located in the vitreous humor between the posterior surface of the lens and neurosensory retina. CONCLUSIONS Our studies of stra6 -/- null mice established the importance of the STRA6 protein for the uptake, intracellular transport, and processing of retinol by the RPE. In its absence, rod photoreceptor outer and inner segment length was reduced, and cone cell numbers were reduced, as were scotopic and photopic responses. STRA6 also was required for dissolution of the primary vitreous. However, it was clear from these studies that STRA6 is not the only pathway for retinol uptake by the RPE.

Visual discomfort and objective measures of static accommodation.

Purpose. Accommodation insufficiency and fatigue have been associated with near work visual discomfort symptoms, but clinical measures of accommodation amplitude suggest insufficiency is uncommon and often not weak enough to cause symptoms. However, recent studies show that the clinical push-up test used to measure amplitude overestimates accommodative function. This study uses an open-field autorefractor to measure accommodative stimulus-response functions objectively in college students with and without near work induced discomfort symptoms. Methods. Using a Grand-Seiko WAM 5500 autorefractor, 2 min recordings were made each at five viewing distances (0 to 5 D) to measure an accommodative response function. Visual discomfort symptoms were assessed using the Conlon survey. Results. A strong and positive correlation between accommodative lag and visual discomfort symptoms was found under near work conditions. The prevalence of accommodative insufficiency was much higher than estimated by clinical measures. Conclusions. Accommodative insufficiency and fatigue should be defined and described by objective methods using extended viewing times to assess function.

Accommodation response and visual discomfort

Although visual discomfort symptoms associated with near work have been correlated with clinical measures of accommodation, studies using objective recordings have not found corresponding deficits in accommodative function. One problem with previous studies is that accommodation measures have been too brief to assess accommodative fatigue. This study examined steady state accommodative responses among a college population with visual discomfort, over a 90‐s time period. Thirty‐one participants were grouped into high (n = 15) or low visual discomfort groups (n = 16) based on their scores on the Conlon Visual Discomfort Survey. Using the WAM‐5500 autorefractor, accommodation responses were recorded at 5 Hz for two consecutive minutes at five viewing distances. The results showed a significant interaction between the high and low discomfort groups over time in accommodation response. The high discomfort group showed an increase in accommodative lag, whereas the low discomfort group had a stable response. Our study suggests that the high visual discomfort group is characterized by accommodative fatigue, with a higher lag of accommodation developing at a near viewing distance over time.

Analysis of autofluorescent retinal images and measurement of atrophic lesion growth in Stargardt disease.

Current retinal imaging techniques using scanning laser ophthalmoscopy (SLO) provide a powerful mechanism for characterizing the topographical distribution of lipofuscin fluorophores and atrophic lesions (ALs) in retinal disease. In this paper we describe a novel Edge-Flow-Driven Variational Image Segmentation analysis to measure and evaluate progressive change in the area of ALs as well as regions of hyperfluorescence (HF). The algorithm is embedded in a series of almost completely automated image processing steps that allow rapid comparison of serial images. The sensitivity of the methodology to detect change was evaluated by measuring progression of AF lesion size in a cohort of Stargardt Macular Dystrophy (STGD) patients. Fifty-two STGD subjects (mean age = 41.0 +/- 16.6 years, range 9-78 yrs) at varying stages of disease participated in this prospective study. Twenty-four of the 52 subjects presented with atrophic lesions in one or both eyes on first evaluation. For this subgroup of subjects, the mean (+/-1 sd) follow-up time was 2.92 (+0.26) years (range 0.57-3.26 years) and the mean (+/-1 sd) rate of change was found to be approximately 0.94 (+/-0.87) mm(2)/year (range 0.2-2.13 mm(2)/yr). With this methodology, progressive enlargement of AL area was detectable in as little as one year, while regions of HF generally decreased, although there was considerable variability in the appearnce of HF, presumably reflecting the combined effects of the creation or expansion of lipofuscin deposits and resorption and loss associated with retinal cell death. Our findings suggest that this methodology is sufficiently sensitive to detect change and provides a clinically relevant tool to monitor progression not only with regards to natural history, but also to evaluate the efficacy of potential therapeutic interventions in STGD. Finally, we evaluated the association between AL area and measures of rod- and cone-mediated retinal function, as assessed with electroretinography (ERG). In general, the larger the AL, the poorer the ERG response, with a greater impact of lesion size on cone- rather than rod-mediated retinal function, a finding that was expected on the basis of the location and size of the AL and the distribution of rod- and cone-photoreceptors.

Longitudinal study of visual discomfort symptoms in college students.

Purpose. To determine the variation of visual discomfort symptom reporting in a group of college students over a 1 year period. Methods. Subjects were screened for visual acuity and uncorrected refractive error before participating in the study. A survey of visual discomfort developed by Conlon et al., Vis Cogn 1999;6:637–666, and the Convergence Insufficiency Symptom Survey (CISS) were administered to a group of 23 college students twice with approximately 1 year between administrations. All subjects also completed two clinical assessments of accommodation and vergence. Results. The mean time between administrations of the symptom surveys was 13 months. There was no clinically significant mean difference between the first and second administration of both visual discomfort surveys. The intraclass correlation coefficient was 0.82 for the Conlon et al., Vis Cogn 1999;6:637–666 survey and 0.85 for the CISS. The 95% limits of agreement for the Conlon et al., Vis Cogn 1999;6:637–666 survey was −18.44, 17.92 and for the CISS was −14.36, 13.36. The intraclass correlation coefficient values for the optometric tests ranged from 0.38 to 0.83. Conclusions. Visual discomfort symptoms were found to be stable in the majority of young college students over a 1 year period. However, a minority of students showed large variability between the two administrations of the surveys.

Measuring near-induced transient myopia in college students with visual discomfort.

Purpose. Visual discomfort is a common problem, and our previous research indicated that 17% of college students experience moderate to high levels of discomfort when reading or studying. There have been several visual factors associated with visual discomfort, and in this study, we focused on measuring the near-induced transient myopia response in a group of college students with significant visual discomfort. Methods. Visual discomfort was evaluated with a survey developed by Conlon et al. (Conlon et al., Visual Cogn 1999;6:637–663). Twelve college students with high visual discomfort (scoring 1 SD higher than the mean value) and 12 college students with low visual discomfort (scoring within 0.5 SD of the mean value) participated in the study. All students had 20/25 or better visual acuity, no strabismus, and no significant uncorrected refractive error. All refractive error and accommodative measurements were made with the WAM-5500 autorefractor. A pretask distance refraction at 6 m was taken for 60 s, and then the students read a story for 10 min at 20 cm. After reading the passage, the posttask distance refraction was measured for 2 min at 6 m. Values for the pre- and posttask measures were averaged in 10-s blocks of time. Results. A mixed analysis of variance comparing discomfort group by pre- and postnear work distance refraction showed a significant interaction (p = 0.05). Comparing the means of the pre- and posttask distance refraction indicated that the high discomfort group showed no change in refractive error, but the low discomfort group showed a myopic shift of 0.13 diopter. Conclusions. A near-induced transient myopia response is not associated with high visual discomfort experienced by college students when reading or doing near work.

Test-retest reliability and inter-ocular symmetry of multi-focal electroretinography in Stargardt disease.

Purpose: To evaluate test-retest reliability and inter-ocular symmetry of multi-focal electroretinography recordings in Stargardt disease and in patients without retinal pathology. Methods: Multi-focal electroretinography were recorded with continuous fundus monitoring using the VERIS multifocal recording system using an array of 103 hexagons spanning 50° of central retina. Recordings were made sequentially from one eye (test-retest) followed by recordings from the fellow eye. A departure score, expressed as a percentage difference from the first recording (or from the fellow eye), was calculated for each comparison and parameter. Results: Multi-focal electroretinography response topographies were similar within and between eyes for a particular individual and parameter. However, local response variability within and between eyes was significantly higher in Stargardt disease than in patients without retinal pathology for both amplitude and timing parameters. Amplitude variability in Stargardt disease decreased with more peripheral targets and with larger stimuli but never to the levels achieved with normal observers. Conclusions: Both test-retest reliability and inter-ocular symmetry of multi-focal electroretinography responses were significantly lower in Stargardt disease compared to normal eyes, a finding attributable primarily to unsteady fixation. The greater variability between eyes in Stargardt disease may also reflect pathological differences in the topography of retinal function. This inherent variability must be taken into consideration if the multi-focal electroretinography is to be sufficiently sensitive to reliably detect small differences in retinal function.

ERGs and psychophysical thresholds in students with reading discomfort.

Purpose. Symptoms of reading discomfort include unpleasant somatic and perceptual effects, such as eye-strain, headache, and blurred text, despite normal visual acuity. Reading discomfort has been proposed to be the result of increased noise in the visual system. Several studies have suggested that the symptoms can be minimized by having the subject wear colored filters. Thus, there may be abnormal neural processing in the cone pathways. This study measured L- and M-cone isolating (1) ERGs and (2) psychophysical thresholds in normal and reading discomfort subjects to determine if cone processing was abnormal in the reading discomfort population. Methods. Twenty-two normal and nineteen reading discomfort college students took part in this study. The normal subjects had Conlon survey scores within 0.5 SD of the mean and the reading discomfort subjects had scores >1.5 SD above the mean. ERGs were determined for a range (5 to 15%) of L- and M-cone contrasts. Slopes were determined for the L- and M-cone ERG amplitudes for each subject. Psychophysical thresholds were determined with a 2AFC technique combined with a 3-up and 1-down staircase procedure that terminated after 18 reversals occurred. The threshold was calculated as the average of the last 8 reversals. Results. The average ERG slopes were not significantly different between the normal and reading discomfort groups (L-Cone, p = 0.086; M-Cone, p = 0.47). The L/M cone ratios for the slopes were not significantly different (p = 0.55). The log of the color contrast thresholds were not significantly different between the normal and reading discomfort groups (L-Cone, p = 0.97; M-Cone, p = 0.92). Conclusions. L- and M- cone ERG contrast gains and psychophysical estimates of color contrast thresholds were not significantly different. These results do not support the noisy visual system hypothesis of reading discomfort.