Biomedical subjects

D P Crabb

Publications and source records attributed to D P Crabb.

14 recordsLinked to original sources

PubMed1999

Severity and stability of glaucoma: patient perception compared with objective measurement.

OBJECTIVE: To elucidate the relationship between the subjective assessment in patients with glaucoma of (1) the severity of their visual loss, and (2) any deterioration in their visual function and their objective visual fields as measured by computed perimetry. DESIGN: First, patients completed a questionnaire relating to perceived visual disability and underwent binocular visual field testing. Second, a separate group of patients answered a question about perceived visual deterioration: their monocular visual field tests were analyzed retrospectively by pointwise linear regression to establish stability or deterioration. SETTING: The Glaucoma Service of a specialist eye hospital, which is a tertiary referral center and serves the local community. SUBJECTS: One hundred twenty-three patients with glaucoma including 62 for the severity arm of the study and 61 for the progression arm. MAIN OUTCOME MEASURES: Questionnaire responses, Esterman binocular disability score, and objective visual field deterioration. RESULTS: Questions strongly associated with Esterman binocular disability scores related to bumping into things, problems with stairs, and finding things that have been dropped. There was a strong association between perceived visual deterioration and measured bilateral visual field deterioration (P<.01). CONCLUSIONS: There is a strong association between some types of perceived visual disability and the severity of binocular field loss. A patient who notices gradual visual deterioration is twice as likely to have bilateral visual field deterioration as not. The findings in this sample of patients with mild-to-moderate glaucoma challenge the belief that glaucoma is an insidious process in which the symptoms do not appear until the end stage of the disease.

Diagnostic Techniques, Ophthalmological↗

PubMed1999

Effects of dipivefrin and pilocarpine on pupil diameter, automated perimetry and LogMAR acuity.

BACKGROUND: A study was carried out to ascertain, in ophthalmologically normal subjects, the short-term effects of dipivefrin hydrochloride 0.1% on visual performance and make comparisons with pilocarpine. METHODS: Twelve normal volunteers aged 20-26 years attended on three occasions. One eye, randomly selected, received one drop of either pilocarpine 2%, dipivefrin or saline 0.9%. High- and low-contrast LogMAR acuity at 6 m and pupil diameter (measured by infra-red pupillometry) were recorded at baseline (T0) and at intervals up to 90 min following instillation of drops. Program 30-2 of the Humphrey Visual Field Analyzer (HFA) was run at T0 and at 60 min after treatment instillation (T60). Saline was always instilled at visit 1, to allow for learning effects. On visits 2 and 3 either pilocarpine or dipivefrin was randomly instilled into the treated eye. RESULTS: Pilocarpine significantly worsened the field global indices mean deviation (P < 0.001) and pattern standard deviation (P < 0.01) compared with T0. There was no significant change with dipivefrin. A significant (P = 0.01) pupil dilation from 5.44 mm (SD 0.79) at T0 to 6.19 mm (SD 1.09) at T90 occurred with dipivefrin. Pilocarpine caused significant miosis. No significant changes in LogMAR values were found with dipivefrin. Pilocarpine significantly (P < 0.01) increased LogMAR values (i.e. reduced acuity) compared with dipivefrin. At T30 the mean increase in LogMAR was 0.76 (SD 0.30) for high and 0.83 (SD 0.11) for low contrast. By T90 recovery of acuity was virtually complete. CONCLUSIONS: In normals dipivefrin causes mydriasis but does not affect the central visual field global indices (as assessed by STATPAC), or high- and low-contrast LogMAR acuity. Pilocarpine adversely affects the visual field and both measures of acuity. Knowledge of these effects is of value in glaucoma therapy and when monitoring the progression of visual loss.

Adult↗

PubMed1999

Characteristics of frequency-of-seeing curves for a motion stimulus in glaucoma eyes, glaucoma suspect eyes, and normal eyes.

This study compared frequency-of-seeing curves for a line displacement test in glaucoma patients and normals. Probit analysis of the frequency-of-seeing curves provided the motion thresholds and the slopes of the frequency-of-seeing curves, represented by the interquartile range. The thresholds and interquartile ranges were significantly elevated in the glaucoma eyes and suspect eyes, compared to controls. A logistic regression model incorporating both the interquartile range and threshold significantly improved the sensitivity of the motion test in the suspects. Abnormal shallowing of the slope of the motion frequency-of-seeing curve may represent one of the earliest changes in glaucoma.

Adult↗

PubMed1998

Simulating binocular visual field status in glaucoma.

AIMS: To simulate the central binocular visual field using results from merged left and right monocular Humphrey fields. To assess the agreement between the simulation and the binocular Humphrey Esterman visual field test (EVFT). METHOD: 59 consecutive patients with bilateral glaucoma each recorded Humphrey 24-2 fields for both eyes and binocular EVFT on the same visit. EVFT results were used to identify patients exhibiting at least one defect (< 10 dB) within the central 20 degrees of the binocular field. This criterion is relevant to a patient's legal fitness to drive in the UK. Individual sensitivity values from monocular fields are merged to generate a simulated central binocular field. Results are displayed as a grey scale and as symbols representing defects at the < 10 dB level. Agreement between patients failing the criterion using the simulation and the EVFT was evaluated. RESULTS: Substantial agreement was observed between the methods in classifying patients with at least one defect (< 10 dB) within the central binocular field (kappa 0.81; SE 0.09). Patients failing this criterion using the EVFT results were identified by the binocular simulation with high levels of sensitivity (100%) and specificity (86%). CONCLUSIONS: Excellent agreement exists between the simulated binocular results and EVFT in classifying glaucomatous patients with central binocular defects. A rapid estimate of a patient's central binocular field and visual functional capacity can be ascertained without extra perimetric examination.

Adult↗

PubMed1997

Effect of surgery on visual field progression in normal-tension glaucoma.

PURPOSE: The effect of intraocular pressure-lowering surgery on the rate of visual field progression in normal-tension glaucoma (NTG) was studied. METHODS: Seventeen patients with NTG who underwent trabeculectomy in one eye for worsening visual field loss were included in the first part of the study. All patients had Humphrey 24-2 visual fields at the rate of 2-3 fields per year. Pointwise linear regression analysis of the visual field data was done separately for the preoperative and postoperative periods. This was performed for both operated and fellow eyes. The mean slope (MS), which indicates rate of visual field progression, was calculated. Change in MS was correlated with change in intraocular pressure (IOP). For the second part of the study, 11 patients who had a minimum of 4 visual fields and 18 months of follow-up before surgery were identified. Using the preoperative fields, the rate of sensitivity loss for each visual field location in the operated eye was ascertained for every patient. This rate of loss was extrapolated to generate the expected visual fields, assuming an unchanged rate of progression. The mean sensitivity of the expected visual field was compared with that of the actual field at the last follow-up. RESULTS: The MS in the operated eyes improved from -2.97 +/- 3.21 (mean +/- SD) in the preoperative period to 0.53 +/- 3.83 (P < 0.005; Student's t test) postoperatively. In the fellow eyes the MS changed from -1.78 +/- 2.55 to -1.43 +/- 3.01 (P = 0.754). There was a weak correlation between change in MS and percentage IOP decrease (correlation coefficient 0.39). The difference in mean sensitivity between the expected and actual visual fields was -3.72 dB (P = 0.002), and was better in the actual field. CONCLUSIONS: In this study, surgical lowering of IOP resulted in a slower rate of visual field loss in the operated eye.

Aged↗

PubMed1997

Improving the prediction of visual field progression in glaucoma using spatial processing.

PURPOSE: The authors show how the predictive performance of a method for determining glaucomatous progression in a series of visual fields can be improved by first subjecting the data to a spatial processing technique. METHOD: Thirty patients with normal-tension glaucoma, each with at least ten Humphrey fields and 3.5 years of follow-up, were included. A linear regression model of sensitivity against time of follow-up determined rates of change at individual test locations over the first five fields (mean follow-up 1.46 years; standard deviation = 0.08) in each field series. Predictions of sensitivity at each location of the field nearest to 1 and 2 years after the fifth field were generated using these rates of change. Predictive performance was evaluated by the difference between the predicted and measured sensitivity values. The analysis was repeated using the same field data subjected to a spatial filtering technique used in image processing. RESULTS: Using linear modeling of the unprocessed field series, at 1 year after the fifth field, 72% of all predicted values were within +/- 5 dB of the corresponding measured threshold. This prediction precision improved to 83% using the processed data. At the 2-year follow-up field, the predictive performance improved from 56% to 73% with respect to the +/- 5 dB criterion. CONCLUSIONS: Predictions of visual field progression using a pointwise linear model can be improved by spatial processing without increased cost or patient time. These methods have clinical potential for accurately detecting and forecasting visual field deterioration in the follow-up of glaucoma.

Adult↗

PubMed1997

High spatial resolution automated perimetry in glaucoma.

BACKGROUND: Automated perimetry is of fundamental importance in assessing visual function in glaucoma. A technique was evaluated to perform high spatial resolution automated perimetry to allow a more detailed assessment of the luminance sensitivity in selected regions of the visual field than is possible with conventional perimetry. METHOD: High spatial resolution perimetry was performed using a Humphrey automated perimeter by measuring luminance sensitivity across a 9 by 9 degree custom grid of 100 test locations with a separation between adjacent locations of 1 degree. Quantitative analysis of the raw and Gaussian filtered thresholds was performed to assess the repeatability of the technique in normals, glaucoma suspects, and glaucoma patients. RESULTS: The testing protocol was well tolerated by all subjects. High spatial resolution perimetry in glaucomatous eyes demonstrated fine luminance sensitivity loss not suspected with conventional perimetry. High spatial resolution perimetry also demonstrated reproducible areas of sensitivity loss in some glaucomatous eyes in areas of the visual field which appear normal with conventional programmes. The repeatability of the technique correlated with mean threshold sensitivity and was substantially improved to clinically acceptable levels by Gaussian filtering the thresholds. CONCLUSION: This technique of high spatial resolution perimetry allows the practical assessment of selected regions of the visual field at higher resolution than conventional perimetry, and may be clinically useful in glaucoma.

Case-Control Studies↗

PubMed1996

Visual field progression: comparison of Humphrey Statpac2 and pointwise linear regression analysis.

BACKGROUND: Humphrey Statpac2 "glaucoma change probability analysis' is a widely available analysis technique to aid the clinician in the diagnosis of glaucomatous visual field deterioration. A comparison of this technique with the more recently described pointwise linear regression analysis (PROGRESSOR) is given. METHODS: Series of visual field data from a group of nine eyes of nine patients with normal-tension glaucoma were selected. Each series had 16 fields with mean follow-up of 5.7 years (SD 0.6 years). Statpac2 "glaucoma change probability analysis' was used to define test locations that had unequivocally deteriorated in the last three fields of each series. The accuracy of both Statpac2 and PROGRESSOR in providing early detection of these deteriorated locations was assessed. RESULTS: The sensitivity and specificity of the two techniques in predicting deteriorated locations were similar when a rate of luminance sensitivity loss of faster than 1 dB/year (2 dB/year for outer locations beyond 15 deg of eccentricity) with a slope significance of P < 0.10 was used as the regression definition of deterioration. The difficulties of comparing two techniques in the early diagnosis of field progression without a true external standard for field loss are illustrated. CONCLUSIONS: PROGRESSOR closely emulates the performance of Statpac2 in detecting sensitivity deterioration at individual test locations. This new technique, which uses all available data in a field series and gives the rate of sensitivity loss at each location, may provide a clinically useful method for detecting field progression in glaucoma.

Disease Progression↗

PubMed1996

Analysis of visual field progression in glaucoma.

BACKGROUND: Despite the widespread use of computerised perimetry the diagnosis of visual field deterioration in following glaucoma patients over time remains particularly difficult. A new method of analysis using a novel graphical display of longitudinal field data is presented. METHODS: A linear regression model of the luminance sensitivity at each stimulus location against time of follow up transforms the quantitative data from a series of fields into a colour coded form which illustrates the spatial configuration of change to aid the interpretation of field loss. The method of analysis and the developed computer software (PROGRESSOR) is described. Comparison with STATPAC-2 glaucoma change probability analysis is given including levels of agreement between the techniques using series of fields of 10 eyes from patients with normal tension glaucoma. RESULTS: Examples of this new method compare well with STATPAC-2 analysis. The level of agreement between the techniques to separate progressing from stable retinal locations is good (kappa = 0.62; SE = 0.04). CONCLUSIONS: This new technique, which combines the change in perimetric sensitivity over time with colour coding of significant change into one image may provide an efficient method to detect true progression in glaucomatous field loss.

Diagnosis, Computer-Assisted↗

PubMed1995

Modelling series of visual fields to detect progression in normal-tension glaucoma.

BACKGROUND: Use of statistical modelling techniques to identify models that both describe glaucomatous sensitivity decay and allow predictions of future field status. METHOD: Twelve initially normal fellow eyes of untreated patients with confirmed normal tension glaucoma were studied. All had in excess of 15 Humphrey fields (mean follow-up 5.7 years). From this cohort individual field locations were selected for analysis if they demonstrated unequivocal deterioration at the final two fields. Forty-seven locations from five eyes satisfied this criterion and were analysed using curve-fitting software which automatically applies 221 different models to sensitivity (y) against time of follow up (x). Curve-fitting was then repeated on the first five fields, followed by projection to the date of the final field to generate a predicted threshold which was compared to the actual threshold. Competing models were therefore assessed on their performance at adequately fitting the data (R2) and their potential to predict future field status. RESULTS: Models that provide the best fit to the data were all complex polynomial expressions (median R2 0.93). Other simple expressions fitted fewer locations and exhibited lower R2 values. However, accuracy in predicting future deterioration was superior with these less complex models. In this group a linear expression demonstrated an adequate fit to the majority of the data and generated the most accurate predictions of future field status. CONCLUSIONS: A linear model of the pointwise sensitivity values against time of follow-up can provide a framework for detecting and forecasting glaucomatous field progression. Linear modelling allows the clinically important rate of sensitivity loss to be estimated.

Cohort Studies↗

PubMed1995

Image processing of computerised visual field data.

BACKGROUND: Computerised perimetry is of fundamental importance in assessing visual function. However, visual fields are subject to patient response variability which limits the detection of true visual loss. METHODS: A method of improving the repeatability of visual field data was demonstrated by applying techniques used in image processing. An illustrative sample of nine normals and nine patients with field loss was used. Two successive Humphrey fields were selected for each subject. Repeatability was defined as the standard deviation of the pointwise differences between sensitivity values of the reference field and repeat field. The field data were then separately subjected to Gaussian and median image processing filters and the repeatability was compared with the unprocessed field results. RESULTS: Improvement in repeatability, by a factor of approximately 2, was demonstrated by both processes. CONCLUSION: These techniques may improve the reliable detection of loss of visual function using computerised perimetry.

Adult↗

PubMed1995

New approach to estimating variability in visual field data using an image processing technique.

AIMS: A new framework for evaluating pointwise sensitivity variation in computerised visual field data is demonstrated. METHODS: A measure of local spatial variability (LSV) is generated using an image processing technique. Fifty five eyes from a sample of normal and glaucomatous subjects, examined on the Humphrey field analyser (HFA), were used to illustrate the method. RESULTS: Significant correlation between LSV and conventional estimates--namely, HFA pattern standard deviation and short term fluctuation, were found. CONCLUSION: LSV is not dependent on normals' reference data or repeated threshold determinations, thus potentially reducing test time. Also, the illustrated pointwise maps of LSV could provide a method for identifying areas of fluctuation commonly found in early glaucomatous field loss.

Adult↗

PubMed1992

Repeatability, reproducibility and intersession variability of the Allergan Humphrey ultrasonic biometer.

We evaluated repeatability and reproducibility of the Allergan Humphrey model 820 ultrasonic biometer. Sixty eyes (30 subjects) free from ophthalmological abnormality were examined on two separate occasions by two experimenters, one experienced and one inexperienced. The 95% confidence limits for repeatability lie between +0.12/-0.13 mm and reproducibility between +0.15/-0.14 mm for AC depth, between +0.11/-0.13 mm and +0.12/-0.16 mm for lens thickness, between +0.17/-0.19 mm and +0.19/-0.18 mm for right axial length and between +0.26/-0.40 mm and +0.22/-0.25 mm for left axial length. The 40% gain used to measure left axial lengths produced poor retinal echospikes, which may account for reduced accuracy compared to the right eyes using 60% gain. Ten subjects (20 eyes) returned for 3 further visits and the variability over 5 sessions was examined using a two-factor ANOVA with repeated measures on one factor. There were no significant experimenter or session effects and no interaction effect. This biometer gives clinically repeatable and reproducible results and is convenient to use.

Anterior Chamber↗

PubMed1992

Effects of different ocular fixation conditions on A-scan ultrasound biometry measurements.

A-scan readings must be taken along the visual axis for accurate axial length measurement. Various fixation targets, which ideally should not induce accommodation, are used to limit eye movements. In this study results using the Humphrey biometer under different fixation conditions were compared to establish the most suitable target. We also investigated corneal applanation effects by comparing optical and ultrasonic measurements of anterior chamber depth. We examined 12 randomly selected normal eyes of 12 young subjects. Three A-scans were recorded for each of five fixation conditions: spotlight at 6 m; Snellen letter at 6 m; solid probe's built-in green fixation target; Snellen letter at 6 m viewed through a -4.00 DS lens; spotlight at 6 m under cycloplegia (the comparison condition). Variability 'within' each fixation condition, estimated by the standard deviation for each subject, was equal throughout the 12 subjects for each fixation condition. Also variation was independent of size of the ocular dimension. The good agreement between the comparison condition and optical anterior chamber depth measurements indicated that the solid probe produced little applanation. Differences 'between' fixation conditions were investigated. It was concluded that a spotlight or letter at 6 m provides a suitable fixation target for biometry. Built-in fixation targets can produce significant errors.

Adult↗