Biomedical subjects

Nicola Pritchard

Publications and source records attributed to Nicola Pritchard.

9 recordsLinked to original sources

PubMed2006

Neural and optical limits to visual performance in myopia.

We investigated the relative importance of neural and optical limitations to visual performance in myopia. A number of visual performance measures were made on all or subsets of 121 eyes of emmetropic and myopic volunteers aged 17-35 years. These tests included visual measures that are mainly neurally limited (spatial summation out to +/-30 degrees in the horizontal visual field and resolution acuity out to +/-10 degrees in the horizontal visual field) and central ocular aberrations. We found that myopia affected the neurally limited tests, but had little effect on central higher order aberration. The critical area for spatial summation increased in the temporal visual field at 0.03 log units/dioptre of myopia. Resolution acuity decreased at approximately 0.012 log units/dioptre of myopia. Losses of visual function were slightly greater in the temporal than in the nasal visual field. The observed visual deficit in myopia can be explained by either global retinal expansion with some post-receptor loss (e.g. ganglion cell death) or a posterior polar expansion in which the point about which expansion occurs is near the centre of the previously emmetropic globe.

Adolescent↗

PubMed2005

Peripheral refraction along the horizontal and vertical visual fields in myopia.

Peripheral refractions were measured to 35 degrees eccentricity using a free-space autorefractor in young adult emmetropic and myopic subjects. Refractions were measured along horizontal and vertical visual fields for 116 subjects and a 43 subject subset, respectively. Along the horizontal visual field, peripheral myopic shifts in spherical equivalent M of emmetropes changed to relative hypermetropic shifts in the myopes, there were temporal-nasal asymmetries of 90 degrees to 180 degrees astigmatism J(180) which decreased as myopia increased, and 45 degrees to 135 degrees astigmatism J(45) was linearly related to field angle. Along the vertical visual field, both peripheral myopic shifts in peripheral M and J(180) asymmetry were unaffected by magnitude of myopia, and J(45) changed at three times the rate as for the horizontal visual field. Myopia has more effect on peripheral refraction of adult eyes along the horizontal than along the vertical visual field. The peripheral variations in refraction match well what is known about the shapes of emmetropic and myopic eyes.

Adolescent↗

PubMed2005

Influence of age on peripheral refraction.

To investigate how age affects peripheral refraction we measured objective peripheral refraction for 55 young subjects (24+/-4 years) and 41 older subjects (59+/-3 years) out to 35 degrees eccentricity in temporal and nasal visual fields. Subjects were compared in 1D subgroups based on central spherical equivalent refractions (low hypermetropes +0.54 D to +1.51 D, emmetropes +0.50 D to -0.49 D, low myopes -0.50 D to -1.49 D, moderate myopes -1.50 D to -2.58 D). Overall, young and older subjects with similar refractive corrections had similar peripheral refraction components. Both age groups showed relative hypermetropic shifts in the peripheral fields as myopia increased and also decreases in peripheral astigmatism J180 as myopia increased. J45 varied little across the visual field with linear relationships occurring between J45 and visual field angle for all but one subgroup (older emmetropes). Peripheral refraction in emmetropes to moderate myopes is relatively unaffected by age for healthy eyes of similar refractive errors.

Adult↗

PubMed2005

Shape of the retinal surface in emmetropia and myopia.

PURPOSE: To determine and compare the shapes of the retinas of emmetropic and myopic eyes. METHODS: Nonrotationally symmetrical ellipsoids were mathematically fitted to the retinal surfaces of 21 emmetropic and 66 myopic eyes (up to -12 D) of participants aged 18 to 36 years (mean, 25.5) using transverse axial and sagittal images derived from magnetic resonance imaging. RESULTS: The shapes of the ellipsoids varied considerably between subjects with similar refractive errors. The shapes were oblate (steepening toward the equator) in most of the emmetropic eyes (i.e., the axial dimensions of the ellipsoids were smaller than both the vertical and horizontal dimensions). As myopia increased, all ellipsoid dimensions increased with the axial dimension increasing more than the vertical dimension, which in turn increased more than the horizontal dimension (increases in approximate ratios 3:2:1). The relative difference in the increase of these dimensions meant that as the degree of myopia increased the retinal shape decreased in oblateness. However, few myopic eyes were prolate (flattening toward the equator). Independent of myopia, the ellipsoids were tilted about the vertical axis by 11 degrees +/- 13 degrees , and ellipsoid centers were decentered horizontally by 0.5 +/- 0.4 mm nasally and 0.2 +/- 0.5 mm inferiorly, relative to the fovea. CONCLUSIONS: In general both emmetropic and myopic retinas are oblate in shape, although myopic eyes less so. This finding may be relevant to theories implicating the peripheral retina in the development of myopia.

Adolescent↗

PubMed2004

Eye shape in emmetropia and myopia.

PURPOSES: To determine axial, vertical, and horizontal eye dimensions in myopic and emmetropic eyes by using magnetic resonance imaging (MRI) and to relate these to different ocular expansion models of myopia development. METHODS: The internal length (cornea to retina), height and width (both retina to retina) were measured in emmetropic and myopic eyes (up to -12 D) of 88 participants aged 18 to 36 years. Participants were positioned supine in a clinical MRI scanner. The fixation target was imaged straight ahead of the subject by an overhead 45 degrees inclined mirror. Eye images were acquired with a 7.5-cm receive-only radio frequency surface coil. Axial (horizontal through middle of eye) and sagittal (vertical through visual axis) sections were taken with a T(1)-weighted fast spin-echo sequence. RESULTS: With an increase in myopic refractive correction, myopic eyes became much larger in all three dimensions, but more so in length (0.35 mm/D, 95% confidence interval [CI] 0.28-0.40) than in height (0.19 mm/D, 95% CI 0.09-0.29) and more so in height than in width (0.10 mm/D, 95% CI 0.01-0.20). Based on height and length dimensions, 25% and 29% of myopic eyes exclusively fitted global expansion and axial elongation models, respectively. Based on width and length dimensions, 17% and 39% of myopic eyes exclusively fitted the global expansion and axial elongation models, respectively. CONCLUSIONS: Although there are considerable individual variations, in general myopic eyes are elongated relative to emmetropic eyes, more in length than in height and even less in width. Approximately a quarter of the myopic participants fitted each of the global expansion or axial elongation model exclusively. The small proportions are due primarily to the large variability in the dimensions of emmetropic eyes.

Accommodation, Ocular↗

PubMed2003

Subjective and objective measures of corneal staining related to multipurpose care systems.

An objective, digital-imaging method of measuring corneal staining was evaluated in 24 subjects wearing soft contact lenses. The method was used to compare the clinical performance of common multipurpose care systems (MPS) for soft contact lens care. Subjects used three different MPS, one containing polyquaternium-1 (PQ) and two containing polyhexanide (PX1 and PX2), for 2 weeks in a randomised, single-masked (investigator) crossover study. Corneal staining induced with the three MPS was analysed using an image-processing program (ImageTool, UTHSCSA Version 2, University of Texas, USA). Conjunctival hyperaemia and papillae were also evaluated. The intraclass correlation coefficient was similar with image analysis to that of investigator grading (0.876, 0.879, respectively). Significant differences in staining response were detected using the objective method. There was significantly less staining area with polyquaternium-1 (PQ) than polyhexanide (PQ: 0.12 mm(2), PX2: 0.91 mm(2)). Inferior palpebral papillae were significantly greater with PX2 than with PQ (1.0, 0.7 (0-4), respectively). The technique was shown to be an effective method of evaluating different corneal staining responses. Bilateral corneal staining in three or more quadrants is useful in the diagnosis of MPS-related staining.

Journal Article↗

PubMed2002

A multi-centre study of lapsed contact lens wearers.

PURPOSE: Discontinuation from contact lens wear has been identified as a contributing factor in the lack of growth of contact lens use in Europe. The purpose of this study was to determine the proportion of lapsed contact lens wearers that can be refitted successfully with contact lenses and to evaluate the reasons for discontinuation from contact lens wear. METHODS: This was a multi-site clinical study involving 15 UK investigators and 236 lapsed contact lens wearers who had previously tried contact lenses and discontinued. The reasons for discontinuation were assessed by subjects, who were then refitted with contact lenses. Investigators subsequently evaluated factors associated with their initial discontinuation. Short-term success was defined as the successful completion of 1 month of contact lens wear with absence of complications that would prevent further wear. Subjects were interviewed by telephone 6 months after being refitted to determine whether they were still wearing contact lenses. Those who were not wearing lenses were questioned about the reasons for discontinuation and their likelihood of resuming lens wear. RESULTS: A majority of subjects (51%) cited discomfort as the principal reason for having previously given up contact lens wear. Problems with vision were the second most common reason (13%), either with reading (6%) or general vision (7%). The most common type of discomfort was dryness (40%). In almost all cases (97%), the investigator's assessment of the reasons for discontinuation agreed with the subject's own assessment but investigators also cited product-related and practitioner-related factors as contributing to previous failure. The short-term success rate for refitting lapsed wearers was found to be 77% (Cl: 70-82%). The highest short-term success rates were for 2-weekly/monthly soft spherical lenses (91%) and daily disposable lenses (89%). Lower success rates were found for soft toric (69%) and soft bifocal (53%) lenses. At the 6-month stage, 73% of those who continued in lenses after the 1-month visit were still wearing contact lenses and a further 18% indicated that they were either 'very likely' or 'likely' to resume lens wear. CONCLUSIONS: A high proportion of lapsed contact lens wearers can be successfully refitted with contact lenses. The prime reason for previous discontinuation from contact lenses is discomfort and, in particular, dryness-related discomfort. In many cases, previous contact lens failure is product or practitioner-related rather than because of patient-specific problems. Once refitted with contact lenses, lapsed wearers cite vision problems rather than discomfort as the most common reason for discontinuing. This finding suggests that recent advances in contact lens material, design, replacement frequencies and care systems have improved the prospects for avoiding lens-related discomfort and for continuing contact lens wear.

Adolescent↗

PubMed2002

A comparison of three different scales for rating contact lens handling.

PURPOSE: To compare the validity, responsiveness, and reliability of three subjective rating scales applied to soft contact lens (SCL) handling. METHODS: Fifty-four adapted SCL wearers handled three different types of lenses on two occasions and rated the handling with each scale: visual analogue scales (VAS), 20-interval visual analogue scales with descriptors (VAD) and Likert rating scales with five intervals (LRS). RESULTS: There were significant differences between the scales (p < 0.01) and between the subjective ratings of lens handling (p < 0.001). VAS showed the least variability, exhibited the highest construct validity, were the most responsive, and were the most reliable: interclass correlations (0.63), coefficient of repeatability (27.5), and correlation between test and retest (Spearman r = 0.65, [all p < 0.05]). Higher repeatability, because of the fewer intervals of LRS, was not demonstrated and, generally, LRS was the least satisfactory scale. Handling was rated as easiest using VAD and most difficult using LRS. CONCLUSIONS: Although all three scales can be used to provide measures of lens handling, VAS may provide a simple and repeatable tool for measuring subjective responses.

Contact Lenses, Hydrophilic↗

PubMed2002

The ocular response to extended wear of a high Dk silicone hydrogel contact lens.

PURPOSE: A four-month extended wear clinical trial was conducted to compare the ocular effects of a high Dk Balafilcon A silicone hydrogel lens and a low Dk HEMA 38.6 per cent H20 soft lens. METHOD: Twenty-four subjects who were adapted to daily wear of soft lenses wore a high Dk lens in one eye and a low Dk HEMA lens in the other eye for four months on an extended wear basis after one week of daily wear. Thirteen progress evaluations were conducted using standard clinical procedures. RESULTS: Eighteen subjects (75 per cent) completed the study. The high Dk lens induced significantly less bulbar and limbal injection and corneal vascularisation than the low Dk HEMA lens (p < 0.05). Epithelial microcysts were observed only in the eyes wearing the low Dk lens. A significant increase in myopia was found in the eyes wearing the low Dk HEMA lens (mean = 0.50 D, p < 0.01) compared to the insignificant myopic increase of 0.06 D in the eyes wearing the high Dk lens. Three subjects developed small infiltrates in the high Dk lens wearing eyes and significantly more post-lens debris was observed under the high Dk lens. Six subjects developed papillary conjunctivitis in the eye wearing silicone hydrogel lenses but only two of those were discontinued from the study. CONCLUSION: No hypoxia-related effects were observed with extended wear of the high Dk Balafilcon A silicone hydrogel lens.

Adult↗