Biomedical subjects

A I McNaught

Publications and source records attributed to A I McNaught.

10 recordsLinked to original sources

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

Motion detection threshold and field progression in normal tension glaucoma.

Psychophysical tests may demonstrate abnormalities of visual function before the appearance of conventional visual field loss in glaucoma. Motion detection thresholds (MDT) were measured in the normal fellow eye of 51 patients with confirmed normal tension glaucoma and initially unilateral field loss. Humphrey visual fields from the initially normal eye covering a mean follow up of 3.4 years were assessed using pointwise linear regression analysis. In 22 of the 51 eyes with normal visual fields at presentation, field deterioration occurred at one or more Humphrey locations within a mean of 1.7 (SD 1.6) years. An initially abnormal MDT test showed a sensitivity of 73% and a specificity of 90% in predicting field deterioration within the cluster of four Humphrey locations closest to the original MDT test site. Sensitivity was lower (40%) in predicting progression at retinal locations distant from the MDT test site, though specificity remained high (90%).

Aged↗

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↗

PubMed1995

Clinical comparison of semiconductor diode versus neodymium: YAG non-contact cyclo photocoagulation.

AIMS: The advent of diode lasers has allowed their use in transscleral cyclo photocoagulation for refractory glaucoma. A trial was performed to compare the ocular hypotensive and inflammatory effects of cyclo photocoagulation using a continuous wave diode (810 nm) and a free running neodymium:yttrium aluminium garnet (Nd:YAG) laser (1064 nm). METHODS: Forty patients with refractory glaucoma were randomised to receive either diode or Nd:YAG therapy. The intraocular pressure (IOP) and inflammatory response to treatment were monitored over 3 months. RESULTS: There was no significant laser related difference in the effect on IOP after one treatment. There was, however, a difference in effect in retreatments with the IOP lowering effect significantly less, but equally sustained in diode retreatment patients. Severe postoperative complications such as hyphaema or fibrinous anterior uveitis only occurred in the Nd:YAG group. CONCLUSION: The degree and duration of the ocular hypotensive response to cyclo photocoagulation appears to be related to the available power output of the system used, and the extent of tissue damage.

Adult↗

PubMed1992

Ownership of direct ophthalmoscopes by medical students.

In September 1986 the Department of Education and Science withdrew the equipment grant for clinical medical students. This coincided with a reduction of 50% in the level of ownership of direct ophthalmoscopes amongst a group of 51 medical students who commenced their medical studies after September 1986 (no longer eligible for the equipment grant), when compared with a group of 63 students (in the same clinical year) who started the medical course before 1 September 1986 and who were hence still able to claim the equipment grant. The majority of non-owners gave lack of finance as the reason for non-ownership. Owners of ophthalmoscopes performed ophthalmoscopy more frequently than non-owners, but the difference did not reach statistical significance.

Education, Medical, Undergraduate↗