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Biomedical subjects

B C Chauhan

Publications and source records attributed to B C Chauhan.

At least 37 records · Page 2Linked to original sources
PubMed1997

Intra- and interrater agreement with cumulative defect curves.

PURPOSE: To examine intra- and interrater agreement when analyzing cumulative defect curves. Cumulative defect (Bebié) curves provide a graphical representation of the visual field and allow a subjective classification of diffuse and localized loss. METHODS: We used 75 Humphrey 30-2 visual field tests, randomly chosen from a database of 782 fields of 113 patients with open-angle glaucoma. Cumulative defect curves were generated and randomly arranged into five sets, with each set containing the 75 curves in a unique sequence. Five raters (two experienced and three inexperienced) rated each set and classified each curve as showing diffuse loss, localized loss, both diffuse and localized loss, or no loss. The intra- and interrater agreement in rating the curves was then analyzed. RESULTS: Intrarater agreement ranged from 73.3-88.0% for perfect rater agreement, with 5 identical ratings. Agreement for experienced versus inexperienced raters gave similar results (means: 84.0% and 77.8%, respectively, for perfect agreement). Interrater agreement for each set, evaluated by the kappa statistic, was substantial for all 5 sets (0.65-0.71). Kappa values for each set were comparable for experienced and inexperienced raters (0.72-0.83 and 0.59-0.69 respectively). CONCLUSIONS: Analyzing the nature of visual field loss using the cumulative defect curve is simple to learn and provides high intrarater agreement as well as substantial interrater agreement.

Glaucoma, Open-Angle↗
PubMed1997

Confocal scanning laser Doppler flowmetry: experiments in a model flow system.

PURPOSE: We conducted this study toward validating confocal scanning laser Doppler flowmetry (SLDF), a new noninvasive technique for measuring retinal and optic nerve head hemodynamics. METHODS: We designed a model flow system using a glass capillary coupled to a microlitre syringe driven by an infusion pump. Eleven capillaries with parallel walls (internal diameters ranging 705 to 25 microm) were used. The capillaries were perfused with skim milk over a range of pump flow rates. At each flow rate, measurements were made with the Heidelberg Retina Flowmeter (HRF) to study the relationship between HRF-measured flow and actual flow. The initial experiments (n = 2) were conducted to establish the approximate velocity operating range of the HRF with single HRF measurements across a wide range of flow rates, whereas the subsequent experiments (n = 9) were concentrated within this operating range with five HRF measurements at each flow rate. RESULTS: When pump flow rates were converted to actual velocity at the measurement point for the initial experiments, the velocity operating range of the HRF was approximately 0.08 to 1.0 mm/s. For velocities of >1 mm/s, HRF measured velocity was not linearly related to actual velocity. Within the operating range, there was a highly significant linear relationship between HRF-measured flow and actual flow (0.935 < or = r < or = 0.990, p < 0.001). When the curves of HRF-measured velocity versus actual velocity for the different experiments were plotted, they largely superimposed. The variability of the HRF measurements was between 3.57% and 4.05% and was independent of flow rate. CONCLUSIONS: SLDF measures reliably and linearly within a given operating range.

Blood Flow Velocity↗
PubMed1996

Influence of fixation accuracy on threshold variability in patients with open angle glaucoma.

PURPOSE: To evaluate the contribution that fixation errors make to the overall variability of perimetric responses in patients with glaucoma. METHODS: Frequency of seeing curves were established, with and without fixation error correction, at two locations in each of 14 patients with glaucoma and good visual acuity. One location corresponded to a relatively normal region of the visual field, whereas the second corresponded to a region in which there was a sensitivity deficit. All patients had an acuity of better than 20/63 (except one whose acuity was 20/100). The locations of the first and fourth Purkinje images of a collimated infrared source were used to give a measure of eye position, during each stimulus presentation (accuracy 10 minutes of arc). RESULTS: Considerable variation was found in patient fixation accuracy. In the worst case, fixation was within 30 minutes of the target in only 7% of presentations whereas in the best, it was within this range in more than 60%. No relationship was found between accuracy of fixation and extent of loss. The gradient of the frequency of seeing curve was found to be shallow at regions of reduced sensitivity, a finding that supports the recognized relationship between variability and sensitivity deficit. A recalculation of the frequency of seeing curves, using only those responses in which the patient's fixation was within a specified range ( < 60 minutes of arc), did not show a meaningful reduction in variability at either location. CONCLUSIONS: It is concluded that fixation errors, though contributing to variability, are not the major cause of the increased variability seen at locations with reduced sensitivity.

Aged↗
PubMed1996

The psychometric function and reaction times of automated perimetry in normal and abnormal areas of the visual field in patients with glaucoma.

PURPOSE: To study the relationship of reaction time to the psychometric function in normal subjects, normal sensitivity test locations in patients with glaucoma, and test locations with 10 to 20 dB loss in patients with glaucoma. METHODS: The authors tested 10 patients with glaucoma and 10 age-matched normal volunteers with the Humphrey perimeter, first with program 24-2 and then with the method of constant stimuli to generate frequency of seeing curves. At two widely separated visual field locations on the program 24-2 grid, they presented stimuli in 2-dB intervals, 10 dB either side of the program 24-2 threshold, at 0 dB and 60 dB (15 repetitions per intensity). For the patients with glaucoma, they chose a visual field location with normal sensitivity and a location in an area of 10 to 20 dB loss. RESULTS: Analysis of variance with post hoc t-tests showed that reaction time (RT) at the 0-dB intensity was prolonged by approximately 90 msec in the abnormal sensitivity test location of patients with glaucoma compared to the control and the glaucoma normal sensitivity groups (P<0.0001). However, this difference was accounted for by only 4 of the 10 patients with glaucoma, reaching 100% of stimuli seen with the brightest stimulus at the moderately damaged test location. Reaction time at the frequency of seeing 50% estimated threshold showed no significant differences among the groups. Prolongation of RT from the 0-dB value was analyzed as a function of increasing attenuation of stimulus intensity. The results fit the equation RT = a + b(Intensity3) for all groups. CONCLUSIONS: There is no significant difference in RT between normal subjects and patients with glaucoma either at threshold or to suprathreshold stimuli. Reaction time increases after a power function with increasing attenuation of stimulus intensity up to the threshold.

Glaucoma, Open-Angle↗
PubMed1996

Comparison of psychophysical and electrophysiological testing in early glaucoma.

PURPOSE: To compare the sensitivity and specificity of a wide range of psychophysical and electrophysiological tests in the detection of early glaucomatous damage. METHODS: Forty-three normals and 43 patients with early glaucoma, some still without field defects, were tested with differential light threshold perimetry, short-wavelength automated perimetry, high-pass resolution perimetry, motion detection, flicker contrast sensitivity, flickering and isoluminantly matched letter tests, and pattern and flash electroretinography, including photopic, scotopic, oscillatory potentials, and 30 Hz flicker. Receiver operating characteristic analysis was applied to continuous variables derived from each of the tests. RESULTS: Most parameters reflected glaucomatous loss to some degree, even though only single variables were analyzed separately in the receiver operating characteristic analysis. The pattern electroretinogram and some of the letter acuity tests had the best sensitivity and specificity, followed by short-wavelength automated perimetry and high-pass resolution-perimetry. Motion detection, flicker contrast, and flash electroretinogram parameters scored poorly. Six patients with normal results on the Humphrey field test had abnormal results on many of the other tests. CONCLUSIONS: Applying different psychophysical and electrophysiological tests may add to our ability to detect early glaucomatous damage.

Contrast Sensitivity↗
PubMed1995

Effect of the cardiac cycle on topographic measurements using confocal scanning laser tomography.

BACKGROUND: This study was carried out to investigate the effect of the cardiac cycle on topographic measurements of the optic nerve head and peripapillary retina with confocal scanning laser tomography. METHODS: The sample comprised 25 healthy subjects (mean age 40.44 years, range 23-67 years). Using a random crossover design, we obtained a set of three images using the Heidelberg Retina Tomograph (Heidelberg Engineering GmbH, Heidelberg, Germany) under each of two conditions. In the first, the images were obtained normally, while in the second, image acquisition was pulse-synchronised using an electrocardiographic signal. We compared the variability of topographic measurements under the two conditions in the whole image, in the optic nerve head and in the peripapillary retina free of visible vessels. RESULTS: Nineteen subjects (76%) showed a decrease in variability in the whole image under the pulse-synchronised condition. The respective numbers for the optic nerve head and peripapillary retina were 20 (80%) and 21 (84%). The decrease in variability ranged widely, with a mean of 13.62% in the whole image, 12.26% in the optic nerve head and 18.51% in the peripapillary retina. These decreases were highly significant. There was no relationship between the decrease in variability and age, intraocular pressure, blood pressure, heart rate or the area of the image occupied by blood vessels. CONCLUSION: Detecting structural change depends on the accurate assessment of each subject's variability. Because the cardiac cycle confounds this assessment by varying and unpredictable amounts, it may be necessary to obtain pulse-synchronised images routinely.

Adult↗
PubMed1994

Technique for determining glaucomatous visual field progression by using animation graphics.

The first two visual field examinations of each of 16 glaucoma suspects and 16 glaucoma patients, who were followed up for a median of 7.46 years with seven examinations, were averaged. Three-dimensional color-coded images of the visual field were then generated. After correcting for test-retest variability, the images, as well as additional interpolated images, were aligned and presented in rapid succession to create an animation sequence. Five glaucoma specialists trained themselves to classify the visual fields as either progressing or not progressing. The interobserver and intraobserver agreement rates were then calculated. Perfect agreement (100% concordance) between the observers was obtained for 18 (56.3%) subjects, and at least 80% concordance was obtained for 27 (84.4%) subjects. Of the eight sequences that were repeated to test intraobserver agreement, four of the five observers had a value of at least 87.5%.

Adult↗
PubMed1994

Test-retest variability of topographic measurements with confocal scanning laser tomography in patients with glaucoma and control subjects.

Using confocal scanning laser tomography, we studied the test-retest variability of topographic measurements of the optic nerve head and parapapillary retina in 30 patients with glaucoma and 30 normal control subjects. We obtained three independent images, separated by between one and six hours, centered on the optic nerve head. We condensed each original 256 x 256-pixel image to a 64 x 64-pixel image, thereby allowing a realistic estimate of the empiric 90% confidence interval of testretest variability at each condensed pixel. Confidence interval maps generated for each subject showed highest measurement variability along the cup border and blood vessels. The mean standard deviation equivalents of test-retest variability in the patients and controls were 31.20 and 25.94 microns, respectively. These differences were statistically significant (P = .010). Variability increased with age (P < .001). When the analysis was repeated for discrete parapapillary areas, the group differences were not statistically significant (P = .100).

Adult↗
PubMed1993

Comparison of reliability indices in conventional and high-pass resolution perimetry.

PURPOSE: The purpose of this study is to compare reliability indices in conventional (Humphrey) and high-pass resolution (Ring) perimetry in healthy subjects followed prospectively at 6-month intervals. METHODS: Of the 146 healthy subjects (mean age, 50.24 years; range, 30-84 years) enrolled in the study, 102 have been tested twice and 71 three times. The authors compared the reliability indices, fixation losses, false-positive rate, and false-negative rate between the two techniques, both cross-sectionally and serially. RESULTS: Fixation losses were slightly higher with high-pass resolution perimetry, whereas false-positive errors were higher with conventional perimetry. False-negative errors were uncommon with either technique. Of 319 fields, 30 (9.4%) conventional and 39 (12.2%) high-pass resolution perimetry fields were unreliable using the current suggested reliability criteria. Nearly all unreliable fields were due to high fixation errors. Using alternative criteria derived from baseline 95th percentile values, unreliable fields were attributed more equally to all three reliability parameters. In subjects tested three times, the reliability indices remained constant. CONCLUSION: The results of this study showed that healthy subjects have comparable reliability indices when tested with conventional and high-pass resolution perimetry.

Adult↗
PubMed1993

Characteristics of frequency-of-seeing curves in normal subjects, patients with suspected glaucoma, and patients with glaucoma.

PURPOSE: The authors performed this study to determine factors that affect the characteristics of frequency-of-seeing curves in normal subjects, patients with suspected glaucoma, and patients with glaucoma. METHODS: The sample consisted of 70 subjects (22 normal subjects, 12 patients with suspected glaucoma, and 36 patients with glaucoma). A program was written to interface with the Humphrey Field Analyzer (Humphrey Instruments, San Leandro, CA) to measure frequency-of-seeing curves. The authors presented stimuli 8 dB either side of the estimated threshold in 1-dB intervals with five repetitions at each stimulus intensity. The authors tested four to six locations in each subject, with randomization of the stimulus intensity and location. Fixation was monitored with the Heijl-Krakau method. Using a probit program, the authors calculated the threshold and slope (estimated by the interquartile range) of each curve. RESULTS: The authors obtained 124 curves from the normal subjects, 71 from the patients with suspected glaucoma, and 183 from the patients with glaucoma. In all three groups, the slope of the frequency-of-seeing curve was correlated highly with the threshold or threshold deviation, although the correlation was significantly higher in the normal subjects compared with the patients with suspected glaucoma and patients with glaucoma, even after controlling for the range of the threshold and threshold deviation. For this reason, the authors found considerably different frequency-of-seeing curves, between subject groups and also within the group of patients with glaucoma, in locations with the same threshold. CONCLUSIONS: There may be fundamental differences in areas of normal subjects and patients with glaucoma with similar thresholds or threshold deviations. These differences also may exist within patients with glaucoma.

Adult↗
PubMed1993

Comparison of high-pass resolution perimetry and pattern discrimination perimetry to conventional perimetry in glaucoma.

We compared two new perimetric techniques, high-pass resolution perimetry and pattern discrimination perimetry, to conventional automated static perimetry in an ongoing prospective study. Forty-eight patients with open-angle glaucoma whose median age was 63.4 (range 15.6 to 89.7) years were included in the study. There was a significant correlation between the results obtained with high-pass resolution perimetry and conventional perimetry for both the whole field and the four quadrants (p < 0.001). However, the correlation between pattern discrimination perimetry and conventional perimetry was poor. The differences between the correlations of the newer techniques with conventional perimetry were statistically significant for the whole field and the superior quadrants (p < 0.01) and were borderline significant for the inferior quadrants. Although the three techniques showed very good concordance in both the extent and location of damage of the visual field in some patients, in others it was poor. Our results show that high-pass resolution perimetry and pattern discrimination perimetry may be measuring different components of the visual response.

Adolescent↗
PubMed1993

Correlation between the optic disc and results obtained with conventional, high-pass resolution and pattern discrimination perimetry in glaucoma.

The purpose of this study was to determine whether results obtained with two new techniques, high-pass resolution perimetry and pattern discrimination perimetry, correlated better to the appearance of the optic disc than results obtained with conventional automated perimetry in patients with open-angle glaucoma. Our sample consisted of 28 eyes of 28 patients with early glaucomatous field damage who are part of a prospective follow-up study. We used the second of two sets of baseline perimetric examinations separated by 1 week in the analysis. Neuroretinal rim area was calculated by planimetry of stereophotographs of the disc and was correlated to the visual field sensitivity measured with each of the techniques. We found a poor correlation between the disc and field measurements (correlation coefficients; -0.048 for conventional perimetry, 0.111 for high-pass resolution perimetry and -0.315 for pattern discrimination perimetry). Although the correlation appeared to be higher with the newer techniques, the differences were not statistically significant.

Adolescent↗
PubMed1992

The relationship between intraocular pressure and visual field progression in glaucoma.

The purpose of this study was to determine the intraocular pressure characteristics in glaucoma suspects and patients whose visual fields were classified as stable or progressing over a long-term follow-up. We present data from 64 patients who received either medical or laser treatment and who were followed up for a median of 7.4 years. The visual fields of 27 patients were classified as stable and 37 as progressing using predetermined criteria on either the Tübinger or Goldmann perimeter. Patients with initially normal and initially abnormal fields were analysed separately to avoid bias. There were no significant group differences in the mean, highest or interquartile range of intraocular pressure in the follow-up. The largely overlapping distributions over a wide spectrum of the pressure variables in patients with stable and progressing fields show that intraocular pressure alone cannot separate these two groups of patients. Our study does not suggest that pressure reduction in glaucoma has no beneficial effect, but that there may be other factors which determine the fate of the visual field in glaucoma.

Aged↗
PubMed1992

Properties of staircase procedures for estimating thresholds in automated perimetry.

The properties of the staircase procedure as applied in automated perimetry were examined. Two computer simulation models were used to vary different test- and patient-related parameters in clinical perimetry. One model was based on the KRAKEN computer simulation program; the other computer simulation was based on stimulus-response data sets from 11 normal subjects. The results were analyzed in terms of efficiency and accuracy. It was found that: (1) in general, there was an efficiency-accuracy trade-off; (2) increases in response fluctuation produced substantially greater errors in threshold estimates; (3) little or no improvements in accuracy were achieved by increasing the number of reversals; (4) the starting position of the staircase relative to the threshold influenced the efficiency of threshold determinations but not their accuracy; (5) a single-response error reduced the efficiency of staircases; (6) the position of a single-response error in a staircase sequence influenced the accuracy and efficiency of the threshold determination; and (7) more than one response error during a staircase sequence always resulted in a marked reduction in accuracy and/or efficiency. Current perimetric strategies appear to be at or near optimal levels, and therefore, strategies in the future may need to depart from a staircase-style procedure to achieve a significant increase in both accuracy and efficiency. Computer simulation studies can provide an effective means of evaluating perimetric test procedures and defining optimum strategies, which then can be verified clinically by subsequent testing in patient populations.

Adolescent↗
PubMed1991

Diffuse and localized glaucomatous field loss in light-sense, flicker and resolution perimetry.

A total of 75 eyes in 75 patients with different types of glaucoma (21 eyes with low-tension glaucoma, 49 with primary open-angle glaucoma and 5 with pigmentary glaucoma) were examined by automated light-sense, flicker and resolution perimetry. All fields were classified in a masked fashion as being "normal" (N) or as having "diffuse loss" (D), "localized loss" (L) or "diffuse plus localized loss" (DL). The frequency distributions for the various field loss categories were plotted against the highest intraocular pressure ever reported in the patients' records. The frequency distribution for the purely localized defects showed a peak at 20 mmHg and were markedly skewed to low pressure values, whereas those for both diffuse plus localized damage and purely diffuse loss peaked at about 30 mmHg. The data suggest that diffuse field loss may be an indicator of pressure-induced damage.

Adult↗
PubMed1991

Effect of the number of threshold determinations on short-term fluctuation in automated perimetry.

The authors studied the effect of multiple threshold determinations on local and global (root-mean-square) short-term fluctuation in 30 subjects (10 controls, 10 glaucoma suspects, and 10 glaucoma patients) with a mean age of 56.70 years. Five custom programs were designed on the Octopus perimeter in which the total number of threshold determinations was equal but in which thresholds at 2 principal locations were determined between 2 and 15 times. One randomly selected eye of each subject was first tested with the standard Octopus programs G1 and 31 followed by the custom programs. Local fluctuation initially increased with the number of threshold determinations and then stabilized after five determinations. Global fluctuation was not influenced by the number of determinations and the values obtained by the shorter custom programs were not significantly different from those obtained by the standard programs. Programs that determine thresholds only twice at a given location may underestimate local fluctuation. Furthermore, local fluctuation is de-emphasized by the "watering-down" effect of estimating global fluctuation.

Adult↗
PubMed1991

Intratest variability in conventional and high-pass resolution perimetry.

The authors studied the effects of threshold, age, and visual field location on intratest variability in 11 normal subjects between the ages of 15 and 50 years (mean, 35.37 years). The subjects were tested with a conventional manual (Tübinger) perimeter and a high-pass resolution (Ring) perimeter at the following locations: (0 degrees, 30 degrees), (0 degrees, 15 degrees), (0 degrees, -15 degrees), and (0 degrees, -30 degrees). Frequency-of-seeing curves were constructed to obtain accurate estimates of threshold and intratest variability. Although location did not have a significant effect on intratest variability in conventional perimetry, intratest variability increased with threshold (P = 0.031) and age (P = 0.012). The results with resolution perimetry contrasted sharply and showed that neither threshold, age, nor location had a significant effect on intratest variability (P greater than 0.225). Although the thresholds with the two types of perimetry were correlated, intratest variability was not. These results show that resolution perimetry may be able to bypass some of the limitations encountered in obtaining thresholds with conventional perimetry, thereby providing more reliable information.

Adolescent↗