Biomedical subjects

James C H Tan

Publications and source records attributed to James C H Tan.

11 recordsLinked to original sources

PubMed2006

Retinal nerve fiber layer analysis in the diagnosis of glaucoma.

PURPOSE OF REVIEW: The detection of optic disc and retinal nerve fiber layer damage and change is the cornerstone of glaucoma management. Assessment of the retinal nerve fiber layer for localized and diffuse damage has been traditionally based on clinical examination, with documentation of change primarily qualitative. With the latest improvements in optical imaging instruments, objective and quantitative measurements of the retinal nerve fiber layer are now possible. This review summarizes the results from recent cross-sectional studies evaluating the discriminating ability of automated retinal nerve fiber layer measurements to detect glaucoma, and from longitudinal studies assessing the ability to predict and monitor glaucomatous changes. RECENT FINDINGS: Numerous cross-sectional studies have documented good diagnostic accuracy of a scanning laser polarimeter (GDx VCC), the optical coherence tomograph (Stratus), and the Heidelberg Retina Tomograph retinal nerve fiber layer measurements for differentiating between healthy and glaucoma eyes. There are only limited data available on the ability of these retinal nerve fiber layer measurements to document change over time. SUMMARY: It is essential that the clinician understand the specific strengths and weaknesses of each technique so that only good quality retinal nerve fiber layer information will be used in conjunction with careful clinical examination and visual function testing for glaucoma management decisions. Longitudinal studies are needed to evaluate the ability of these instruments to document retinal nerve fiber layer change over time.

Diagnostic Imaging↗

PubMed2005

Stretch-activated channels: a mini-review. Are stretch-activated channels an ocular barometer?

All cells are subject to physical forces by virtue of their position in a dynamically changing environment. This review outlines the various putative 'mechanosensors', or sensors of pressure cells possess, and discusses in particular the role stretch-activated membrane channels play in pressure recognition and transduction. The widespread occurrence of these channels is discussed and these 'mechanosensors' are related to pressure-related diseases, in particular, glaucoma.

Eye Diseases↗

PubMed2004

Magnification changes in scanning laser tomography.

PURPOSE: It is important when evaluating glaucomatous optic disc progression in longitudinal images that image magnification remains unchanged. We studied the effect of changed lens power on magnification in scanning laser tomography. The relative contribution to magnification of axial length, eye-scanner distance, and image-to-image scaling was also assessed. METHODS: A simulated optic disc in a model eye was imaged using the Heidelberg Retina Tomograph. Lens power was alterable by exchanging intraocular lenses (IOL) mounted at the lens plane of the eye to mimic changes in the crystalline lens. IOL power of +20.0D and axial length of 21.5 mm was compatible with emmetropia. The optic disc was imaged through IOLs differing in power (+16.0D to +25.0D) but with axial length kept constant. IOL power was then held constant and imaging was repeated for various axial lengths (17.5-23.5 mm). Model eye-scanner distance was varied with each test sequence. The distances between landmarks on the disc was measured before and after contour lines were exported. RESULTS: Image size varied with IOL power and axial length (r > 0.98; P < 0.0001), with the magnification effect of a +1D increase in lens power equivalent to a third the magnification effect of a 1-mm increase in axial length. Magnification tended to increase with myopia (IOL power > +20.0D) and was accentuated by longer eye-scanner distances. Image-to-image scaling corrected some magnification though this varied with ametropia. CONCLUSIONS: Changed lens power, axial length, and eye-scanner distance can affect the size of the optic disc in scanning laser tomography images. The exported contour line partly compensates for changed magnification.

Disease Progression↗

PubMed2004

Validity of rim area measurements by different reference planes.

PURPOSE: Reference plane description of the neuroretinal rim in scanning laser tomography should correctly represent optic nerve morphology. We evaluated how well rim area analysis by different reference planes agreed with the appearance of rim area in disc images. METHODS: Three expert observers subjectively and repeatedly analyzed rim area in Heidelberg Retina Tomograph (HRT) images so that each optic disc was measured six times in 100 eyes, 50 normal and 50 glaucoma. Rim area was evaluated globally and in 30 degree sectors. Agreement between rim appearance, as subjectively analyzed, and objective analysis by an experimental reference plane, the standard reference plane, and a reference plane fixed 320 microm below the reference ring was assessed in HRT images. RESULTS: Subjective analysis of rim area in HRT images was consistent between expert observers. Their analysis of rim appearance agreed more closely with experimental reference plane analysis than analysis by the standard or 320-microm reference planes; this was true globally and in every region of the nerve (P = 0.000). The experimental reference plane yielded higher estimates of rim area than did the standard or 320-microm reference planes. CONCLUSION: There was closer correspondence between the appearance of the neuroretinal rim in images and description by the experimental reference plane compared with description by the standard and 320-microm reference planes.

Aged↗

PubMed2004

Reversal of disc cupping after intraocular pressure reduction in topographic image series.

PURPOSE: To identify and characterize 'reversal' of optic nerve cupping following intraocular pressure (IOP) lowering in scanning laser tomography (SLT) longitudinal image series. METHODS: Modification was made to a previously described analytical approach to longitudinally study putatively increased rim area following IOP lowering. Sustained IOP reduction of 25% was by topical medication. Forty SLT image series with equivalent follow up were assessed: 10 with ocular hypertension (OHT), 10 with primary open angle glaucoma (POAG), and as controls, 20 normal. Reproducible rim area reversal was identified by sector and its time-course over 1 year examined. RESULTS: By a 2-of-3 reproducibility criterion, reversal following IOP lowering was confirmed in about a third of treated eyes (POAG and OHT) but not in any controls. Rim sectors showing reversal were mostly nasal, with a few occurring superotemporally. Reversal in a fifth of treated eyes persisted for at least 1 year; all these were in the nasal half of the disc. The number of sectors with persisting reversal affected less than 6% of all treated eyes' rim sectors. CONCLUSION: Rim area is not uncommonly increased after IOP lowering and this 'reversal' may persist for at least a year. Within topically treated eyes having IOP lowering of at least 25%, the proportion of rim sectors with persistent reversal appears small. Nevertheless, the effects of IOP reduction on topography, especially in the short term, should be considered when longitudinally assessing progressive rim loss in SLT images.

Aged↗

PubMed2004

Optimizing and validating an approach for identifying glaucomatous change in optic nerve topography.

PURPOSE: To determine and validate optimal parameters for analysis in a previously described approach for identifying glaucomatous optic nerve progression by scanning laser tomography. METHODS: Thirty-degree sectors of rim area, as defined by an experimental reference plane, were analyzed for change with respect to different statistical limits of variability (80%, 90%, 95%, 98%, 99%, and 99.9%) in the longitudinal image series of 62 eyes from 30 ocular hypertension converters and 32 normal control subjects. A criterion requiring that change is repeatable in two of three consecutive tests (the 2-of-3 criterion) was compared with a single-test strategy not requiring confirmation, and four other plausible criteria. The influence of these various parameters on sensitivity and the false-positive rate was evaluated. The same series were also assessed for change by the known method of computer-generated probability maps. RESULTS: More sectors were identified as progressing in converter eyes than in control eyes at every limit of variability. With stricter limits of variability and a requirement of confirmation, fewer sectors were identified as changing, especially in control eyes. The 2-of-3 criterion had the most favorably balanced sensitivity and false-positive rates: these were, for the 90% limit of variability, 90.0% and 6.2%, respectively, and for the 95% limit, 83.3% and 3.1%, respectively. Confirmed rim loss in converter eyes was most frequent in the disc poles and corresponded with the field hemisphere of conversion in 80%. Probability maps detected significant and repeatable change in 26 (86.7%) of 30 converter eyes and 14 (43.8%) of 32 of control eyes. CONCLUSIONS: This study was conducted to optimize and validate an approach for identifying progression. The method distinguished eyes with glaucomatous change from unchanging control eyes.

Adult↗

PubMed2004

Tomographic identification of neuroretinal rim loss in high-pressure, normal-pressure, and suspected glaucoma.

PURPOSE: To identify progressive rim loss and describe patterns of regional change in various clinical presentations of glaucoma by scanning laser tomography (SLT). METHODS: A previously described analytical approach was used to identify progressive rim area loss in SLT disc images of eyes of people with ocular hypertension (OHT, n = 97), early POAG (OHT converters; n = 30), asymmetric normal-pressure glaucoma (NPG, established and suspected in contralateral eyes; n = 26), and normal control subjects (n = 32). Analysis was performed longitudinally in individual image series, and cross-sectionally within groups at different time points. RESULTS: Reproducibly reduced rim area was detected in 2 (6.2%) of 32 normal control subjects, 11 (11%) of 97 OHT subjects, 27 (90%) of 30 OHT converters, 16 (58%) of 26 of suspected NPG eyes, and 15 (54%) of 26 of established NPG eyes (mean MD = -6.5 dB). Of 5 (19%) of 26 of suspected NPG eyes that converted on visual field testing, rim loss was detected in 3 of 5. In all groups, rim loss was common in the disc poles, especially inferiorly. Patterns of rim loss were similar within high-pressure and normal-pressure groups, whether or not eyes had field defects in each. In high-pressure groups, rim loss was more common nasally than temporally. Normal-pressure groups, unlike high-pressure groups, frequently had rim loss temporally. Suspected NPG eyes had more rim loss temporally and their rim area tended to be less compared with OHT and OHT converters, despite the three groups having equivalent baseline fields. CONCLUSIONS: There were similarities and differences in the pattern of rim loss in SLT disc images of high- and normal-pressure presentations of glaucoma. Progressive rim loss was detected in eyes without visual field defects, eyes that progressed to develop field defects, and eyes with established and more severe glaucoma.

Adult↗

PubMed2003

Reasons for rim area variability in scanning laser tomography.

PURPOSE: To determine reasons for rim area variability in scanning laser tomography. METHODS: Regional rim area variability from testing in same and different visits and by same and different observers was characterized in 30 normal and 42 glaucomatous eyes. Variations in (1) optic nerve head (ONH) surface geometry (center of gravity: X, Y, Z), (2) image tilting (horizontally and vertically), and (3) position of the reference plane in relation to the ONH (REF) were analyzed by multiple regression analysis. Whether and how much these factors explain rim area variability was studied in cross-sectional and longitudinal data by using two different reference planes. RESULTS: Variability was higher in glaucoma and in testing by different observers in separate visits. Across a range of eyes, approximately 40% of variability in single-topography images and 60% of variability in mean-topography images was explained. In individual image series, a median 85% of variability was explained, exceeding 90% in at least 25% of eyes. The most frequent contributors to rim area variability were REF (in > or =95%) and Z (in > or =80%); they also usually explained more variability than other factors. The nature of variability differed between reference planes. CONCLUSIONS: A large proportion of rim area variability was explained by variation in the topographical features studied, especially REF and Z. Reference plane definition also influenced variability. Variation in the position of the reference plane in relation to the ONH can affect rim area measurements and should be considered when evaluating the progression of glaucoma.

Aged↗

PubMed2003

Reference plane definition and reproducibility in optic nerve head images.

PURPOSE: To describe and evaluate a new experimental reference plane for measuring rim area in scanning laser tomography. METHODS: The experimental reference plane was positioned so that (1) it always lay entirely below the margin of the optic nerve head (ONH), (2) it remained at a set z-axis distance below the ONH in images of each eye, and (3) it was at a level where variability in rim area is least. Twenty normal control subjects and 20 patients with glaucoma underwent test-retest scanning laser tomographic imaging by same and different operators during same and separate visits. Control subjects had image series spanning at least 3 years. The effect of the positioning of the reference plane on global and regional rim area variability was assessed in intra- and intervisit test-retest images and longitudinal image series and compared with the standard and 320- microm reference planes. RESULTS: Variability in the experimental reference plane was less in test-retest images and longitudinal data (P < 0.05) and more uniform around the ONH than with other reference planes. Variability in the former was not appreciably affected by testing involving different operators and visits, or by the presence of glaucoma. CONCLUSIONS: Variability in rim area by the experimental reference plane was significantly less, more uniform around the ONH, not affected by different operators and visits, and less affected by glaucomatous morphology than other reference planes. This difference was pronounced in sequential data and has implications for detecting progression of glaucoma.

Aged↗

PubMed2003

Approach for identifying glaucomatous optic nerve progression by scanning laser tomography.

PURPOSE: To describe and test an analytical approach for identifying glaucomatous optic nerve change by scanning laser tomography. METHODS: The approach (1). analyzed 30 degrees sectors of rim area by (2). a novel and reproducible experimental reference plane, (3). estimated and accounted for measurement variability in each sector, and (4). required that any change exceeding variability in a single (positive) test should be confirmed as repeatable by a criterion requiring two of three consecutive tests to be positive. The sensitivity and false-positive rate of a single positive test and the two-of-three criterion were assessed in image series of one eye each of 20 ocular hypertension patients who converted to glaucoma (referred to as converters) who had unambiguous disease progression, and in one eye each of 20 normal control subjects. RESULTS: Eighteen of 20 (90% sensitivity) converters and 7 of 20 (35% false-positive responses) control subjects had single positive test results, but with confirmation by the two-of-three criterion, the false-positive rate improved to 5% (1/20) whereas sensitivity was relatively preserved at 85% (17/20). CONCLUSIONS: Estimates of rim area variability in each sector of each nerve allowed change consistent with disease progression to be distinguished from measurement variability. Confirming that change is repeatable by the criterion used in the study resulted in considerably fewer false-positive responses than did testing without confirmation, but with sensitivity not significantly compromised in the former. By this approach, eyes with progressive glaucoma could be distinguished from unchanging normal control eyes.

Diagnostic Techniques, Ophthalmological↗

PubMed2002

Interpreting glaucoma progression by white-on-white perimetry.

Sequential automated static perimetry is commonly used to test whether glaucoma is progressing, but its interpretation depends on analysing complex numerical data and can be complicated. Various methods of analysis - both subjective and objective - can be used, but these methods differ in their interpretation of change. Test fluctuation and media opacities can also confound the evaluation of change. Recently, innovations in perimetric testing and analysis have sought to provide solutions. This article reviews what is known about the nature of visual field progression and examines the usefulness of perimetry in detecting worsening glaucoma.

Disease Progression↗