Brightness alters Heidelberg retinal flowmeter measurements in an in vitro model.
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PURPOSE: The Heidelberg Retinal Flowmeter (HRF), a laser Doppler flowmetry device, has captured interest as a research and clinical tool for measurement of ocular blood flow. Concerns remain about the range and accuracy of the values that it reports. METHODS: An in vitro blood-flow model was constructed to provide well-controlled laminar flow through a glass capillary for assessment by HRF. A change in material behind the glass capillary was used to simulate changing brightness conditions between eyes. RESULTS: Velocities reported by the HRF correlated linearly to true velocities below 8.8 mm/sec. Beyond 8.8 mm/sec, HRF readings fluctuated randomly. True velocity and HRF reported velocities were highly correlated, with r = 0.967 (P < 0.001) from 0.0 mm/sec to 2.7 mm/sec mean velocity using a light background, and r = 0.900 (P < 0.001) from 2.7 mm/sec to 8.8 mm/sec using a darker background. However, a large change in the y-intercept occurred in the calibration curve with the background change. CONCLUSIONS: The HRF may report velocities inaccurately because of varying brightness in the fundus. In the present experiment, a darker background produced an overreporting of velocities. An offset, possibly introduced by a noise correction routine, apparently contributed to the inaccuracies of the HRF measurements. Such offsets vary with local and global brightness. Therefore, HRF measurements may be error prone when comparing eyes. When used to track perfusion in a single eye over time, meaningful comparison may be possible if meticulous care is taken to align vessels and intensity controls to achieve a similar level of noise correction between measurements. (+info)
Analysis of blood flow in the long posterior ciliary artery of the cat.
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PURPOSE: Experiments were undertaken to use a new technique for direct on-line measurement of blood flow in the long posterior ciliary artery (LPCA) in cats and to evaluate possible physiological mechanisms controlling blood flow in the vascular beds perfused by this artery. METHODS: Blood flow in the temporal LPCA was measured on a continuous basis using ultrasonic flowmetry in anesthetized cats. Effects of acute sectioning of the sympathetic nerve and changes in LPCA and cerebral blood flows in response to altered levels of inspired CO2 and O2 were tested in some animals. In others, the presence of vascular autoregulatory mechanisms in response to stepwise elevations of intraocular pressure was studied. RESULTS: Blood flow in the temporal LPCA averaged 0.58+/-0.03 ml/min in 45 cats anesthetized with pentobarbital. Basal LPCA blood flow was not altered by acute sectioning of the sympathetic nerve or by changes in low levels of inspired CO2 and O2, although 10% CO2 caused a modest increase. Stepwise elevations of intraocular pressure resulted in comparable stepwise decreases of LPCA blood flow, with perfusion pressure declining in a linear manner throughout the perfusion-pressure range. CONCLUSIONS: Ultrasonic flowmetry seems to be a useful tool for continuous on-line measurement of LPCA blood flow in the cat eye. Blood flow to vascular beds perfused by this artery does not seem to be under sympathetic neural control and is refractory to modest alterations of blood gas levels of CO2 and O2. Blood vessels perfused by the LPCA show no clear autoregulatory mechanisms. (+info)
Evaluation of focal defects of the nerve fiber layer using optical coherence tomography.
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OBJECTIVE: To analyze glaucomatous eyes with known focal defects of the nerve fiber layer (NFL), relating optical coherence tomography (OCT) findings to clinical examination, NFL and stereoscopic optic nerve head (ONH) photography, and Humphrey 24-2 visual fields. DESIGN: Cross-sectional prevalence study. PARTICIPANTS: The authors followed 19 patients in the study group and 14 patients in the control group. INTERVENTION: Imaging with OCT was performed circumferentially around the ONH with a circle diameter of 3.4 mm using an internal fixation technique. One hundred OCT scan points taken within 2.5 seconds were analyzed. MAIN OUTCOME MEASURES: Measurements of NFL thickness using OCT were performed. RESULTS: In most eyes with focal NFL defects, OCTs showed significant thinning of the NFL in areas closely corresponding to focal defects visible on clinical examination, to red-free photographs, and to defects on the Humphrey visual fields. Optical coherence tomography enabled the detection of focal defects in the NFL with a sensitivity of 65% and a specificity of 81%. CONCLUSION: Analysis of NFL thickness in eyes with focal defects showed good structural and functional correlation with clinical parameters. Optical coherence tomography contributes to the identification of focal defects in the NFL that occur in early stages of glaucoma. (+info)
Monitoring patients with diabetes mellitus: an application of the probit model using managed care claims data.
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The primary objective of this study was to estimate the likelihood of the use of either a glycosylated hemoglobin (HbA1c) test or an eye examination, or both, among a cohort of patients diagnosed with diabetes mellitus. A secondary objective was to provide a step-by-step discussion of the applicability of an econometric model to managed care organizations. The study used medical and pharmacy claims data from a managed care organization for the calendar year 1995. A probit regression model was specified to estimate the probability of occurrence for either an HbA1c test or an eye examination among patients with insulin-dependent, non-insulin dependent, or atypical/unclassified diabetes. Data were available only for patients under 65 years of age due to data truncation for patients covered by Medicare, resulting in a study sample size of 6,841. Results indicate that age, presence of hypertension, hyperlipidemia, multiple cardiovascular comorbidities, ophthalmic disease, and combinations of multiple commonly observed comorbidities were positively correlated with the probability of either HbA1c testing or eye examination. Gender and the type of benefit plan were not statistically significant as predictors of disease monitoring. A total of 1,860 patients with diabetes mellitus were predicted by the model to have undergone one of the two monitoring procedures; but in actuality, these patients were not monitored in 1995. They could be considered as high-risk patients who were not getting recommended monitoring. The probit model shows a predictive power of 64.48%. (+info)
A new video pachometer.
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PURPOSE: Many studies of the cornea would benefit from a simple, objective method to measure corneal thickness. In this study, a new optical pachometer based on video and computer technology was designed and tested. METHODS: The slit beam of a photographic slit lamp was monitored with a video camera through one half of the biomicroscope. When the slit was properly aligned with the cornea, the operator triggered a flash, and one video frame that included the flash was captured. A custom software package detected epithelial and endothelial edges. Corneal thickness was calculated from the median corneal image width and image widths from similar measurements of contact lenses with known thicknesses. Corneal thickness was measured in 25 subjects by using this new instrument and was compared to thickness measured by using a conventional Haag-Streit pachometer. RESULTS: Corneal thickness in the 25 subjects measured on the new instrument was 512+/-20 microm and 515-/+21 microm in the right and left eyes, respectively (mean+/-SD). Thickness of the same corneas measured on the Haag-Streit pachometer was 530+/-22 microm and 534+/-20 microm in right and left eyes, respectively. The average SD of 10 consecutive measurements was 6.6 microm and 6.7 microm on the video and Haag-Streit pachometers, respectively (n = 50 corneas). CONCLUSIONS: The video pachometer provides a fast, objective means of measuring corneal thickness. It is simple to use and provides precision equal to that of the Haag-Streit pachometer. (+info)
Compliance with methodological standards when evaluating ophthalmic diagnostic tests.
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PURPOSE: To draw attention to the importance of methodological standards when carrying out evaluations of ophthalmic diagnostic tests by reviewing the extent of compliance with these standards in reports of evaluations published within the ophthalmic literature. METHODS: Twenty published evaluations of ophthalmic screening/diagnostic tests or technologies were independently assessed by two reviewers for compliance with the following methodological standards: specification of the spectrum composition for populations used in the evaluation, analysis of pertinent subgroups, avoidance of work-up (verification) bias, avoidance of review bias, presentation of precision of results for test accuracy, presentation of indeterminate test results, and presentation of test reproducibility. RESULTS: Compliance ranged from just 10% (95%CI, 1%-32%) for presentation of test reproducibility data and avoidance of review bias to 70% (95%CI, 46%-88%) for avoidance of work-up bias and presentation of indeterminate test results. Only 5 of the 20 evaluations complied with four or more of the methodological standards and none with more than five of the standards. CONCLUSIONS: The evaluations of ophthalmic diagnostic tests discussed in this article show limited compliance with accepted methodological standards but are no worse than previously described for evaluations published in general medical journals. Adherence to these standards by researchers can improve the study design and reporting of evaluations of new diagnostic techniques. Limited compliance, combined with a lack of awareness of the standards among users of research evidence, may lead to the inappropriate adoption of new diagnostic technologies, with a consequent waste of health care resources. (+info)
Digital image capture and automated analysis of posterior capsular opacification.
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PURPOSE: To develop and validate a digital imaging and analysis technique for assessing the extent of posterior capsular opacification after cataract surgery. METHODS: Retroillumination images of the posterior capsule were obtained by using a digital camera mounted on a slit lamp. The images were analyzed using an available image analysis software program. The image acquisition and analysis techniques were tested for face validity, reproducibility, and the ability to detect progression of capsular opacity over time. RESULTS: Digital retroillumination images were obtained without patient discomfort. Automated analysis of images correlated well with clinical grading both at slit lamp examination and when looking at the images themselves (Spearman's correlation coefficient >0.7). Analysis of images taken at different times showed high reproducibility (intraclass correlation >0.9), and the system was able to identify progression of capsular opacity over a 2-year period with a mean increase of 15.8% in progressors versus an increase of 0.6% in nonprogressors (P < 0.05). CONCLUSIONS: Digital retroillumination images of the posterior capsule can be obtained reliably, and automated analyses correlate well with clinical assessment. The system presented here uses commercially available instruments and software, and it is practical for use in longitudinal studies of posterior capsule opacification. It is reliable, easy to use, and can detect small changes in the percentage area covered by posterior capsule opacification over time. (+info)
Measuring geographic atrophy in advanced age-related macular degeneration.
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PURPOSE: To present a method developed for measuring areas of geographic atrophy (GA) in advanced age-related macular degeneration, METHODS: A microfilm reader projected the 30 degrees fundus photograph of the macula. Retinal landmarks, atrophic areas, and spared areas within the atrophy were traced, without access to drawings of other years. The total atrophic area was calculated, as was the atrophy within a four-disc-area circle entered on the estimated foveal center. The configuration of the atrophy was documented. RESULTS: Avoidable sources of discrepancy included variability in peripapillary atrophy seen on the photograph, and variability seen in the extent of the field. Reproducibility studies found a median absolute difference of 0.19 Macular Photocoagulation Study disc areas (DA) in total atrophy between repeat drawings, with 75% of repeat drawings having a difference of less than 0.33 DA. For central atrophy measures, there was a median difference of 0.08 DA, with 75% of pairs having a difference of less than 0.18 DA. Features making the definition of borders of GA difficult include the presence of drusen and pigmentary alteration, a fundus in which choroidal vessels are easily visible, and variation in the appearance of GA within a single area of atrophy. CONCLUSIONS: This method provides a reliable means of measuring the size of atrophic areas in GA and will be useful for measuring longitudinal change. It may be difficult to determine whether central spared areas are present, and correlation with visual acuity and macular perimetry may be helpful. (+info)