Biomedical subjects

Shaban Demirel

Publications and source records attributed to Shaban Demirel.

8 recordsLinked to original sources

PubMed2006

Repeatability of normal multifocal VEP: implications for detecting progression.

PURPOSE: To assess the repeatability of the multifocal visual evoked potential (mfVEP) and to compare it with the repeatability of standard automated perimetry (SAP) in the same group of 50 normal controls retested after 1 year. Our second aim was to assess the repeatability of false alarm rates determined previously for the mfVEP using various cluster criteria. METHODS: Fifty individuals with normal vision participated in this study (33 females and 17 males). The age range was 26.7 to 77.9 years and the group average age (+/- SD) was 51.4 (+/- 12.1) years. Pattern-reversal mfVEPs were obtained using a dartboard stimulus pattern in VERIS and two 8-minute runs per eye were averaged. The average number of days between the first and second mfVEP tests was 378 (+/- 58). SAP visual fields were obtained within 17.4 (+/- 20.3) days of the mfVEP using the SITA-standard threshold algorithm. Repeatability of mfVEPs and SAP total deviation values were evaluated by calculating point-wise limits of agreement (LOA). Specificity (1-false alarm rate) was evaluated for a range of cluster criteria, whereby the number and probability level of the points defining a cluster were varied. RESULTS: Point-wise LOA for the mfVEP signal-to-noise ratio (SNR) ranged from 2.0 to 4.3 dB, with an average of 2.9 dB across all 60 locations. For SAP, LOA ranged from 2.4 to 8.9 dB, with an average of 4.0 dB (excluding the points immediately above and below the blind spot). Clusters of abnormal points were not likely to repeat on either mfVEP or SAP. When an mfVEP abnormality was defined as the repeat presence (confirmation) of a 3-point (P < 0.05) cluster anywhere within a single hemifield, only 1 (of 200) monocular hemifield was deemed abnormal. Although the LOA of the mfVEP were similar throughout the field, the limited dynamic range of SNR at superior field locations will limit the ability to follow progression in "depth" at those locations. CONCLUSIONS: Repeatability of the mfVEP was slightly better than SAP visual fields in this group of controls with a 1-year retest interval. This suggests that progression in early stages should be more easily detectable by mfVEP. However, in certain field locations (eg, superior periphery), the relatively more narrow dynamic range of the SNR of the mfVEP may limit detection of progression to just 1 event. Confirmation of a 3-point cluster abnormality is highly suggestive of a true defect on the mfVEP.

Adult↗

PubMed2006

Asymmetries and visual field summaries as predictors of glaucoma in the ocular hypertension treatment study.

PURPOSE: To evaluate whether baseline visual field data and asymmetries between eyes predict the onset of primary open-angle glaucoma (POAG) in Ocular Hypertension Treatment Study (OHTS) participants. METHODS: A new index, mean prognosis (MP), was designed for optimal combination of visual field thresholds, to discriminate between eyes that developed POAG from eyes that did not. Baseline intraocular pressure (IOP) in fellow eyes was used to construct measures of IOP asymmetry. Age-adjusted baseline thresholds were used to develop indicators of visual field asymmetry and summary measures of visual field defects. Marginal multivariate failure time models were constructed that relate the new index MP, IOP asymmetry, and visual field asymmetry to POAG onset for OHTS participants. RESULTS: The marginal multivariate failure time analysis showed that the MP index is significantly related to POAG onset (P < 0.0001) and appears to be a more highly significant predictor of POAG onset than either mean deviation (MD; P = 0.17) or pattern standard deviation (PSD; P = 0.046). A 1-mm Hg increase in IOP asymmetry between fellow eyes is associated with a 17% increase in risk for development of POAG. When threshold asymmetry between eyes existed, the eye with lower thresholds was at a 37% greater risk of development of POAG, and this feature was more predictive of POAG onset than the visual field index MD, though not as strong a predictor as PSD. CONCLUSIONS: The MP index, IOP asymmetry, and binocular test point asymmetry can assist in clinical evaluation of eyes at risk of development of POAG.

Antihypertensive Agents↗

PubMed2006

Assessment of false positives with the Humphrey Field Analyzer II perimeter with the SITA Algorithm.

PURPOSE: To evaluate the effects of false-positive (FP) response errors on mean deviation (MD), pattern standard deviation (PSD), glaucoma hemifield test (GHT), and test duration in the Humphrey Field Analyzer's (HFA II) Swedish Interactive Threshold Algorithm (SITA; Carl Zeiss Meditec, Inc., Dublin, CA). METHODS: Five individuals with glaucoma (ages 52, 63, 69, 77, and 78 years) and five individuals with normal, healthy eyes (ages 25, 34, 43, 45, and 52 years), participated in the study. Each subject was experienced in automated perimetry and performed multiple, monocular baseline SITA-standard (SITA-S) 24-2 visual field tests. In addition, normal subjects completed SITA-S 24-2 field examinations in which known frequencies of FP error were introduced (0%, 5%, 10%, 20%, or 33% frequency). Likewise, the subjects with glaucoma completed visual field examinations with 0%, 20%, and 33% error introduced during the test. RESULTS: Reported FP errors were significantly lower than the introduced frequency of error. The SITA algorithm more accurately identified FP errors when the MD and PSD diverged from normal. Test duration increased as introduced error frequencies increased. The Statpac single-field analyses indicated that two thirds of the tests with introduced errors produced a "low-patient-reliability" determination. CONCLUSIONS: HFA II SITA-S underestimates patients' FP errors, particularly among normal patients. High FP error frequencies can have adverse effects on MD and PSD, leading clinicians and researchers to an inaccurate determination of the amount and severity of visual field loss.

Adult↗

PubMed2005

Early detection of glaucomatous visual field loss: why, what, where, and how.

This article provides a basis for understanding modern visual field analysis techniques. It describes some of these tests, including their advantages and disadvantages, and the settings in which different tests might be used most successfully. It also includes an examination of the value of visual field assessment from a public health standpoint with particular attention paid to morbidity, cost/benefit ratios, and quality of life correlations.

Glaucoma↗

PubMed2004

Normative ranges and specificity of the multifocal VEP.

PURPOSE: To describe a normative database for the multifocal VEP (mfVEP) and to evaluate specificity for a range of cluster criteria. METHODS: One hundred persons (62 females and 38 males) with normal visual fields and ranging in age from 21.6 to 92.4 years participated in this study. Self-reported race in 80 of these 100 persons was 'White or Caucasian,' eight were 'Black or African-American,' eight were 'Asian,' and four were 'Hispanic or Latino.' Pattern-reversal mfVEPs were obtained using a dartboard stimulus pattern in VERIS and two 8-min runs per eye were averaged. A bootstrap technique was used to estimate the normal range of mfVEP response signal-to-noise ratio (SNR) and inter-ocular amplitude ratio at each location. Specificity (1 - false alarm rate) was evaluated for a range of cluster criteria, whereby the number and probability level of the points defining a cluster were varied. RESULTS: There was no overall effect of age on SNR (r2 = 0.16, p = 0.22) nor was the interaction between age and location significant (F = 0.83, p = 0.82, ANOVA). The location with the largest age effect had an r2 of only 0.13. There was a small but significant effect of sex (t = 2.1, p = 0.04) such that SNR was slightly (11%) larger in females than males, but there was no significant interaction between sex and age (t = 0.82, p = 0.41). There was a slight trend toward higher SNR in the Asian group and lower SNR in the African-American group, but the overall effect of race was not significant (F = 1.99, p = 0.12). Specificity depended on the number and probability level of the points defining a cluster. Specificity did not vary by age group in a simple monotonic manner. False positive rates were slightly higher in females than males, and slightly higher in the African-American group as compared with the Asian group. CONCLUSIONS: Excellent specificity can be achieved for the mfVEP by using particular cluster criteria for monocular and inter-ocular tests. The effects of age, sex, and race were all very small and only the effect of sex was statistically significant. This normative database can be used for analyses of mfVEP results from individual patients with little risk that demographic factors such as age and sex will confound diagnostic accuracy.

Adult↗

PubMed2003

Short-wavelength acuity: optical factors affecting detection and resolution of blue-yellow sinusoidal gratings in foveal and peripheral vision.

Previous studies have indicated that peripheral achromatic grating resolution is limited by the sampling density of the neural array (sampling limited), and largely unaffected by large amounts of optical defocus and significant changes in luminance. Under certain conditions, peripheral short-wavelength sensitive (SWS) grating acuity is also sampling limited. We wished to determine how the sampling-limited nature of SWS-driven grating resolution was affected by changing optical defocus and stimulus luminance. Using SWS-cone isolation techniques, detection and resolution acuity were measured for sinusoidal gratings under varying levels of stimulus mean luminance and optical defocus in the fovea and at 20 degrees eccentricity. From 1.4 down to 0.3 cd/m(2) peripheral detection acuity was superior to resolution acuity, accompanied by observations of aliasing: there was little change in resolution performance throughout this range. For defocus up to 3-4 dioptres, peripheral detection acuity was superior to resolution but fell steadily: resolution performance remained flat throughout the same range. Unlike achromatic acuity, foveal resolution performance displayed some robustness to defocus but to a lesser degree than the periphery. Peripheral SWS-driven resolution remains sampling limited for large changes in stimulus luminance and optical defocus, and should thus be useful as a clinical test of SWS-driven ganglion cell density.

Color Perception↗

PubMed2003

Peripheral resolution for achromatic and SWS gratings in early to moderate glaucoma and the implications for selective ganglion cell density loss.

PURPOSE: To investigate whether there is significant selective reduction in short-wavelength-sensitive (SWS) ganglion cell density in early to moderate glaucoma. METHODS: Peripheral achromatic resolution acuity (an indirect measure of the underlying midget ganglion cell density) and peripheral chromatic resolution acuity under conditions of blue cone isolation (an indirect measure of the underlying small bistratified ganglion cell density) were measured at 13 degrees eccentricity in four oblique meridians in 15 eyes (mean age, 64.6 +/- 9.6 years) with early to moderate glaucoma. The results from the subjects with glaucoma were compared with those in a group of 17 age-matched normal eyes (mean age, 62.5 +/- 6.6 years). RESULTS: Mean achromatic resolution acuity across the four locations was significantly lower in the subjects with glaucoma than in the normal subjects (2.92 vs. 4.01 cyc/deg). Mean chromatic resolution acuity across the four locations was also significantly lower in the subjects with glaucoma than the normal subjects (0.78 vs. 0.99 cyc/deg). There was no selective loss of mean SWS acuity in the subjects with glaucoma. Individual location analysis revealed that the chromatic-achromatic resolution ratio was not significantly different in the subjects with glaucoma who had early glaucomatous damage when compared with the normal subjects. The chromatic-achromatic resolution ratio was lower than normal at certain locations in certain individuals with early glaucoma. CONCLUSIONS: The results indicate that there is no evidence of significant selective reduction in global SWS ganglion cell density in early to moderate glaucoma. However, there may be selective loss of SWS ganglion cell density at individual locations in individual eyes.

Aged↗

PubMed2002

What limits detection and resolution of short-wavelength sinusoidal gratings across the retina?

Peripheral resolution acuity for achromatic gratings is known to be limited by the density of the underlying ganglion cell sampling array. After confirming isolation of the short-wavelength sensitive (SWS) system using chromatic adaptation methods, we wished to determine if resolution is limited by blue/yellow ganglion cell sampling (evidenced by a superiority of detection over resolution acuity and the perception of aliasing) and thus directly related to SWS-driven ganglion cell density. We measured detection and resolution acuity between 0 degrees and 35 degrees, using blue sinusoidal gratings superimposed on a yellow adapting background which ranged in intensity from 2.5 to 4.7logTrolands. At all locations, a break could be observed in the acuity vs. illumination curves followed by a plateau. Detection and resolution acuity were the same for low background intensities, but resolution acuity was lower than detection at higher intensities, accompanied by observations of chromatic aliasing. SWS resolution is sampling-limited across the retina and agrees well with predicted performance based on anatomical estimates of small bistratified ganglion cell density.

Adolescent↗