Provocative intraocular pressure challenge preferentially decreases venous oxygen saturation despite no reduction in blood flow.

PURPOSE: Ocular disease can both alter the retina's oxygen requirements, and decrease its ability to cope with changes in metabolic demand. We examined the influence of a moderate intraocular pressure (IOP) elevation on three outcome measures: arterial and venous oxygen saturation, blood flow, and the pattern electroretinogram (PERG). METHODS: We increased IOP to ˜30 mmHg in 23 healthy participants (22-39 years) using a mechanical probe applied to the eyelid, thereby lowering ocular perfusion pressure (OPP) by ~30%. The Oxymap retinal oximeter was used to measure oxygen saturation for arteries and veins. Blood flow, volume and velocity were measured using the Heidelberg retinal flowmeter and steady-state PERG waveforms (8.34 Hz) were recorded bilaterally (200 sweeps). For each outcome measure, data was obtained three times: at baseline, 1 min into sustained IOP elevation, and 1 min after the probe was removed. RESULTS: During IOP elevation, changes in oxygen saturation of retinal arteries failed to reach statistical significance [F(1,30) = 3.69, p = 0.05], whereas venous oxygen saturation was significantly reduced [F(1,21) = 27.43, p < 0.01]. Blood flow increased slightly [F(2,40) = 6.28, p < 0.0001], PERG amplitude significantly reduced [F(2,44) = 24.24, p < 0.0001] and PERG phase was significantly delayed [F(2,44) = 17.00, p < 0.0001]. Contralateral eyes were unchanged. OPP reduction correlated little with PERG amplitude, PERG phase or venous oxygen saturation. CONCLUSIONS: Mild, acute IOP elevation increases arterio-venous oxygen saturation differences primarily through lowering venous oxygen saturation, suggesting increased oxygen consumption by healthy neurons when physiologically stressed.

Does the Swedish Interactive Threshold Algorithm (SITA) accurately map visual field loss attributed to vigabatrin?

BACKGROUND: Vigabatrin (VGB) is an anti-epileptic medication which has been linked to peripheral constriction of the visual field. Documenting the natural history associated with continued VGB exposure is important when making decisions about the risk and benefits associated with the treatment. Due to its speed the Swedish Interactive Threshold Algorithm (SITA) has become the algorithm of choice when carrying out Full Threshold automated static perimetry. SITA uses prior distributions of normal and glaucomatous visual field behaviour to estimate threshold sensitivity. As the abnormal model is based on glaucomatous behaviour this algorithm has not been validated for VGB recipients. We aim to assess the clinical utility of the SITA algorithm for accurately mapping VGB attributed field loss. METHODS: The sample comprised one randomly selected eye of 16 patients diagnosed with epilepsy, exposed to VGB therapy. A clinical diagnosis of VGB attributed visual field loss was documented in 44% of the group. The mean age was 39.3 years ± 14.5 years and the mean deviation was -4.76 dB ±4.34 dB. Each patient was examined with the Full Threshold, SITA Standard and SITA Fast algorithm. RESULTS: SITA Standard was on average approximately twice as fast (7.6 minutes) and SITA Fast approximately 3 times as fast (4.7 minutes) as examinations completed using the Full Threshold algorithm (15.8 minutes). In the clinical environment, the visual field outcome with both SITA algorithms was equivalent to visual field examination using the Full Threshold algorithm in terms of visual inspection of the grey scale plots , defect area and defect severity. CONCLUSIONS: Our research shows that both SITA algorithms are able to accurately map visual field loss attributed to VGB. As patients diagnosed with epilepsy are often vulnerable to fatigue, the time saving offered by SITA Fast means that this algorithm has a significant advantage for use with VGB recipients.

Test-retest reliability of retinal oxygen saturation measurement.

PURPOSE: To determine intrasession and intersession repeatability of retinal vessel oxygen saturation from the Oxymap Retinal Oximeter using a whole image-based analysis technique and so determine optimal analysis parameters to reduce variability. METHODS: Ten fundus oximetry images were acquired through dilated pupils from 18 healthy participants (aged 22 to 38) using the Oxymap Retinal Oximeter T1. A further 10 images were obtained 1 to 2 weeks later from each individual. Analysis was undertaken for subsets of images to determine the number of images needed to return a stable coefficient of variation (CoV). Intrasession and intersession variability were quantified by evaluating the CoV and establishing the 95% limits of agreement using Bland and Altman analysis. Retinal oxygenation was derived from the distribution of oxygenation values from all vessels of a given width in an image or set of images, as described by Paul et al. in 2013. RESULTS: Grouped in 10-µm-wide bins, oxygen saturation varied significantly for both arteries and veins (p < 0.01). Between 110 and 150 µm, arteries had the least variability between individuals, with average CoVs less than 5% whose confidence intervals did not overlap with the greater than 10% average CoVs for veins across the same range. Bland and Altman analysis showed that there was no bias within or between recording sessions and that the 95% limits of agreement were generally lower in arteries. CONCLUSIONS: Retinal vessel oxygen saturation measurements show variability within and between clinical sessions when the whole image is used, which we believe more accurately reflects the true variability in Oxymap images than previous studies on select image segments. Averaging data from vessels 100 to 150 µm in width may help to minimize such variability.

Retinal oxygen saturation: novel analysis method for the oxymap.

PURPOSE: To use a novel image analysis approach to consider how oxygen saturation changes as a function of vessel width and distance from the nerve and between superior and inferior retinal hemifields. METHODS: Ten images were acquired from one eye of 17 participants (mean [standard deviation] age, 28 [4] years; range, 22-38 years) using the Oxymap T1 retinal oximeter. Every pixel identified by the detection algorithm was extracted, and frequency histograms of retinal vessel oxygen saturation were plotted for each vessel diameter (70-170 µm). Histograms were fitted with two Gaussian models to identify peak arteriole and venule oxygen saturation. Mean (±standard error of the mean) arteriole and venule oxygen saturation at each vessel width were calculated. Data were also analyzed in (1) annuli of 100 µm centered on the optic nerve or (2) upper and lower hemifields demarcated by the center of the optic nerve. RESULTS: Venous oxygen saturation was higher in smaller vessels than in larger vessels. Arterial oxygen saturation remained relatively constant with vessel width. Oxygen saturation was lower in veins nearer the optic nerve. The upper retinal hemisphere showed higher venous oxygen saturation compared with the lower hemifield. CONCLUSIONS: The current objective analysis approach provides a more complete picture of retinal oxygen saturation at the posterior pole as a function of vessel width and retinal location.

Investigation of ocular hemodynamics in Sturge-Weber syndrome.

PURPOSE: Sturge-Weber syndrome (SWS) is a condition often associated with facial cutaneous angioma, vascular malformations in the brain, and ocular anomalies such as glaucoma. Reduced cerebral blood flow and ischemia have been well documented. Less is known about ocular blood flow despite the frequent associations between altered hemodynamics and the mechanisms underlying glaucomatous optic neuropathy. The aim of this research was to investigate retrobulbar hemodynamics in patients diagnosed with SWS. METHODS: The sample comprised 16 patients diagnosed with SWS and 16 age- and gender-matched normal control subjects. Four patients were diagnosed with both SWS and primary open-angle glaucoma (mean age 34.3 years; SD 26.9 years), three patients with both SWS and closed-angle glaucoma (mean age 23.3 years; SD 18.0 years), and nine patients with SWS and no glaucoma (mean age 17.2 years; SD 9.1 years). Systemic blood pressure and intraocular pressure were measured to determine the mean arterial pressure and ocular perfusion pressure. All patients and subjects underwent ultrasonography of the ophthalmic artery, central retinal artery, and short posterior ciliary arteries. RESULTS: No significant difference between groups for mean arterial pressure or ocular perfusion pressure (p > 0.05) was recorded. Participants diagnosed with SWS and primary open-angle glaucoma showed significantly reduced end-diastolic velocity (mean 0.036 m/s; SD 0.005 m/s) in their central retinal artery (p = 0.016) when compared against their age-matched normal controls (mean 0.054 m/s; SD 0.010 m/s). Participants diagnosed with SWS and no glaucoma also showed significantly reduced end-diastolic velocity (mean 0.038 m/s; SD 0.015 m/s) in their central retinal artery (p = 0.046) when compared against their age-matched normal controls (mean 0.054 m/s; SD 0.014 m/s). CONCLUSIONS: Retrobulbar hemodynamics appear to be altered in participants diagnosed with SWS irrespective of their diagnosis of glaucoma. Further research is needed to ascertain whether there are any long-term consequences of such changes to ocular physiology.

Ocular blood flow measurements in healthy human myopic eyes.

BACKGROUND: To evaluate the haemodynamic features of young healthy myopes and emmetropes, in order to ascertain the perfusion profile of human myopia and its relationship with axial length prior to reaching a degenerative state. METHODS: The retrobulbar, microretinal and pulsatile ocular blood flow (POBF) of one eye of each of twenty-two high myopes (N = 22, mean spherical equivalent (MSE) ≤-5.00D), low myopes (N = 22, MSE-1.00 to-4.50D) and emmetropes (N = 22, MSE ± 0.50D) was analyzed using color Doppler Imaging, Heidelberg retinal flowmetry and ocular blood flow analyser (OBF) respectively. Intraocular pressure, axial length (AL), systemic blood pressure, and body mass index were measured. RESULTS: When compared to the emmetropes and low myopes, the AL was greater in high myopia (p < 0.0001). High myopes showed higher central retinal artery resistance index (CRA RI) (p = 0.004), higher peak systolic to end diastolic velocities ratio (CRA ratio) and lower end diastolic velocity (CRA EDv) compared to low myopes (p = 0.014, p = 0.037). Compared to emmetropes, high myopes showed lower OBFamplitude (OBFa) (p = 0.016). The POBF correlated significantly with the systolic and diastolic blood velocities of the CRA (p = 0.016, p = 0.036). MSE and AL correlated negatively with OBFa (p = 0.03, p = 0.003), OBF volume (p = 0.02, p < 0.001), POBF (p = 0.01, p < 0.001) and positively with CRA RI (p = 0.007, p = 0.05). CONCLUSION: High myopes exhibited significantly reduced pulse amplitude and CRA blood velocity, the first of which may be due to an OBF measurement artefact or real decreased ocular blood flow pulsatility. Axial length and refractive error correlated moderately with the ocular pulse and with the resistance index of the CRA, which in turn correlated amongst themselves. It is hypothesized that the compromised pulsatile and CRA haemodynamics observed in young healthy myopes is an early feature of the decrease in ocular blood flow reported in pathological myopia. Such vascular features would increase the susceptibility for vascular and age-related eye diseases.

Reproducibility-repeatability of choroidal thickness calculation using optical coherence tomography.

PURPOSE: To evaluate the repeatability and reproducibility of subfoveal choroidal thickness (CT) calculations performed manually using optical coherence tomography (OCT). METHODS: The CT was imaged in vivo at each of two visits on 11 healthy volunteers (mean age, 35.72 ± 13.19 years) using the spectral domain OCT. CT was manually measured after applying ImageJ processing filters on 15 radial subfoveal scans. Each radial scan was spaced 12° from each other and contained 2500 A-scans. The coefficient of variability, coefficient of repeatability (CoR), coefficient of reproducibility, and intraclass correlation coefficient determined the reproducibility and repeatability of the calculation. Axial length (AL) and mean spherical equivalent refractive error were measured with the IOLMaster and an open view autorefractor to study their potential relationship with CT. RESULTS: The within-visit and between-visit coefficient of variability, CoR, coefficient of reproducibility, and intraclass correlation coefficient were 0.80, 2.97% 2.44%, and 99%, respectively. The subfoveal CT correlated significantly with AL (R = -0.60, p = 0.05). CONCLUSIONS: The subfoveal CT could be measured manually in vivo using OCT and the readings obtained from the healthy subjects evaluated were repeatable and reproducible. It is proposed that OCT could be a useful instrument to perform in vivo assessment and monitoring of CT changes in retinal disease. The preliminary results suggest a negative correlation between subfoveal CT and AL in such a way that it decreases with increasing AL but not with refractive error.

Oxygenation state and mesopic sensitivity to dynamic contrast stimuli.

PURPOSE: Comparative studies suggest that increasing photoreceptor oxygen consumption in dim light, relative to bright light, may make the outer retina susceptible to hypoxia at light levels relevant to aviation at night. Accordingly, this study investigates effects of relevant oxygenation states on sensitivity to a dynamic contrast stimulus at low photopic and mesopic light levels experienced during night flying. METHODS: Threshold sensitivity to frequency-doubled contrast stimuli was assessed under mild hypoxia (breathing 14.1% oxygen), hyperoxia (100% oxygen), and normoxia (air) using frequency doubling perimetry, viewing at background fields of approximately 10 cd/m2 and approximately 1 cd/m2. Data were analyzed by retinal eccentricity and visual field quadrant. RESULTS: At low photopic luminance (approximately 10 cd/m2), sensitivity was marginally enhanced when breathing 100% oxygen. At mesopic luminance (approximately 1 cd/m2), sensitivity was consistently poorest with hypoxia and greatest with supplementary oxygen at all eccentricities and in all field quadrants, suggesting oxygen-dependent performance. CONCLUSIONS: The known effects of oxygenation state on pupil size are likely to influence frequency doubling perimetry thresholds, but oxygen-dependent changes in mesopic sensitivity are greater than expected from altered retinal illumination alone and support outer retinal (photoreceptor) susceptibility to hypoxia under twilight viewing.

Mild hypoxia impairs chromatic sensitivity in the mesopic range.

PURPOSE: The effect of mild hypoxia on chromatic sensitivity in the mesopic range is poorly documented. This study was conducted to examine the effects of mild hypoxia and hyperoxia on red-green (R-G) and yellow-blue (Y-B) chromatic sensitivity thresholds at low photopic (22.3 cd . m(-2)), borderline upper mesopic (1.67 cd . m(-2)) and mid-mesopic (0.21 cd . m(-2)) luminance. METHODS: The Color Assessment and Diagnosis (CAD) test was used to measure binocular and monocular R-G and Y-B chromatic sensitivity by using dynamic luminance contrast noise to isolate the use of color signals. Mild hypoxia was imposed by breathing 14.1% oxygen and was investigated relative to control exposures breathing air (normoxia) at each light level. Subsequently, hyperoxia, breathing 100% oxygen, was assessed relative to hypoxia under the mesopic conditions. A balanced, repeated-measures design allowed assessment of main effects and interactions of light level, viewing condition, gender, breathing gas, and exposure order by using multivariate analysis of variance (MANOVA), with post hoc analysis employing ANOVA and paired t-tests. RESULTS: Light level, number of viewing eyes, and oxygenation state were significant determinants of chromatic sensitivity. One man and one woman introduced orthogonal sources of gender bias. The CAD test revealed minimal deuteranomaly (R-G deficiency) in the man and loss of Y-B sensitivity in the only woman using hormonal contraception. CONCLUSIONS: In the mesopic range, mild hypoxia impairs chromatic sensitivity progressively with reducing luminance. Binocular summation of chromatic signals is consistent and independent of the luminance channel. The CAD test is highly sensitive to mild congenital and acquired color vision deficiencies.

Comparison of higher order aberrations measured by NIDEK OPD-Scan dynamic skiascopy and Zeiss WASCA Hartmann-Shack aberrometers.

PURPOSE: To compare the measurement of wavefront aberrations in non-cyclopleged human eyes with Hartmann-Shack and dynamic skiascopy wavefront analyzers. METHODS: Eighty eyes of 40 healthy young adults (19 men, 21 women; mean age 20.8 +/- 2.5 years) with refractive errors ranging from +1.50 to -9.75 diopters (D) sphere and up to 1.75 D cylinder (mean spherical equivalent refraction -2.12 +/- 2.69 D) were examined with the Zeiss/Meditec WASCA and NIDEK OPD-Scan wavefront analyzers and with the Nippon SRW5000 binocular, open-field autorefractor without the instillation of antimuscarinic agents. Three measurements were taken with each system, in randomized sequence. To avoid differences due to instrument myopia, eyes were excluded if mean spherical equivalent refraction with any of the analyzers exceeded those obtained with the SRW5000 by more than 1.00 D; 13 eyes were excluded. Coefficient of repeatability was determined for the WASCA as the confidence interval (CI) for the differences between the repeated measures. Paired t tests with Bonferroni adjustment for multiple comparisons, correlation analysis, and Bland-Altman plots of difference versus mean were performed. RESULTS: The coefficient of repeatability for the WASCA ranged from 0.008 to 0.022 microm. Data distribution was normal for all Zernike coefficients measured with WASCA, but only for Z(3)(-1), Z(3)(1), and Z(4)(0) measured with the OPD-Scan. Mean differences between instruments in coefficients Z(3)(-3), Z(3)(1), Z(3)(3), Z(4)(0), and Z(4)(2) and higher order root-mean-square (RMS) reached statistical significance (mean difference +/- CI: -0.054 +/- 0.021, -0.056 +/- 0.022, and 0.030 +/- 0.016 microm, respectively, P < .05 in all cases). Correlation coefficients were significant only for higher order RMS (Spearman's rho = 0.777; P > .001). CONCLUSIONS: Although agreement is shown for higher order RMS, aberration values obtained with dynamic skiascopy and Hartmann-Shack systems on non-cyclopleged human eyes are not well correlated with each other, and are therefore not interchangeable.

Oxygenation and gender effects on photopic frequency-doubled contrast sensitivity.

Thresholds to a temporally modulated contrast stimulus were examined across the central visual field, at photopic luminance (100 cd m(-2)), under aviation-related respiratory disturbances. These were mild hypoxia (14.1% oxygen), hyperoxia (100% oxygen), and hypocapnia (voluntary hyperventilation), with control exposures breathing air at rest. Thresholds were analysed by retinal eccentricity and by visual field quadrant. Hypoxia compromised sensitivity away from fixation (p<.001). Gender differences in sensitivity were apparent over the nasal hemifield and in response to 100% oxygen. An unexpected and highly statistically significant effect of oxygen tension (PO2) exposure order (p<.001) implies the existence of short-term retinal 'memory' for recent PO2.

Objective measurement of intraocular forward light scatter using Hartmann-Shack spot patterns from clinical aberrometers. Model-eye and human-eye study.

PURPOSE: To apply software-based image-analysis tools to objectively determine intraocular scatter determined from clinically derived Hartmann-Shack patterns. SETTING: Aston Academy of Life Sciences, Aston University, Birmingham, United Kingdom, and Department of Optics, University of Valencia, Valencia, Spain. METHODS: Purpose-designed image-analysis software was used to quantify scatter from centroid patterns obtained using a clinical Hartmann-Shack analyzer (WASCA, Zeiss/Meditec). Three scatter values, as the maximum standard deviation within a lenslet for all lenslets in the pattern, were obtained in 6 model eyes and 10 human eyes. In the model-eye sample, patterns were obtained in 4 sessions: 2 without realigning between measurements, 1 with realignment, and 1 with an angular shift of 6 degrees from the instrument axis. Three measurements were made in the human eyes with the C-Quant straylight meter (Oculus) to obtain psychometric and objective measures of retinal straylight. Analysis of variance, intraclass correlation coefficients, coefficient of repeatability (CoR), and correlations were used to determine intrasession and intersession repeatability and the relationship between measures. RESULTS: No significant differences were found between the sessions in the model eye (P=.234). The mean CoR was less than 10% in all model- and human-eye sessions. After incomplete patterns were removed, good correlation was achieved between psychometric and objective scatter measurements despite the small sample size (n=6; r=-0.831; P=.040). CONCLUSIONS: The methodology was repeatable in model and human eyes, strong against realignment and misalignment, and sensitive. Clinical application would benefit from effective use of the sensor's dynamic range.

Retinal straylight in patients with monofocal and multifocal intraocular lenses.

PURPOSE: To determine the differences in retinal straylight perceived by patients with monofocal intraocular lenses (IOLs) and patients with multifocal IOLs. SETTING: Vissum-Instituto de Oftalmológico de Alicante, Alicante, Spain. METHODS: In this prospective study, contrast sensitivity and retinal straylight were measured prospectively 6 months postoperatively in 67 eyes of 40 patients with a monofocal or multifocal IOL. In the monofocal group, a ThinOptX IOL (ThinOptX, Inc.) was implanted in 12 eyes and an Acri. Smart 48 S IOL (Acri.Tec) in 20 eyes. In the multifocal IOL group, a ReZoom IOL (Advanced Medical Optics) was implanted in 13 eyes and a ReSTOR IOL (Alcon Laboratories) in 22 eyes. RESULTS: All measured values were similar between the monofocal and multifocal groups. Contrast sensitivity correlated significantly with the retinal straylight value (r= 0.258; P= .026) and the mean residual spherical equivalent. There was no statistically significant relationship between IOL type and the retinal straylight value (P= .089). Adjusted mean scores suggest that straylight values in the monofocal group were slightly higher than in the multifocal group. In the monofocal group, retinal straylight values were significantly correlated with pupil size (r= -0.377; P= .033). In the multifocal group, a correlation was found between retinal straylight values and contrast sensitivity (r= 0.397; P= .024). After controlling for age differences, there were no significant differences in retinal straylight between IOLs (P= .069). CONCLUSIONS: No significant differences in straylight values were found between multifocal IOLs and monofocal IOLs. Pupil miosis during retinal straylight measurement and neural adaptation after multifocal IOL implantation may overcome differences between IOLs.

A pilot study on the differences in wavefront aberrations between two ethnic groups of young generally myopic subjects.

A comparative population-based cross-sectional study design was used to examine the prevalence of wavefront patterns in two different ethnic groups, and the relationship of these patterns with ocular biometrics and gender. The Shin-Nippon SRW5000 open field autorefractor, the Wavefront Analysis Supported Customized Ablation (WASCA) wavefront analyser and the IOLMaster were used to determine wavefront aberrations, mean spherical equivalent (SE) refractive error and axial length (AL). Seventy-four eyes from 74 young healthy subjects (44 British Asians, 30 Caucasians; 36 men, 38 women; mean age 22.51 +/- 3.89 years) with mean SE averaging -1.90 +/- 2.76 D (range -10.88 to +2.19 D) were examined. Relationships between ethnicity, gender, AL and SE, against the wavefront high-order root mean square, and aberration components up to the fifth order, were assessed by using multiple regression and correlation analysis. AL on its own accounted for 4.7% of the variance in trefoil component Z(-3)(3) (F(1,72) = 4.602; p = 0.035), 13.7% of coma component Z(1)(3) (F(1,72) = 12.536; p = 0.001), 6.1% of trefoil component Z(3)(3) (F(1,72) = 5.705; p = 0.020) and 9.8% of coefficient Z(-2)(4) (F(1,72) = 8.908; p = 0.004). A significant model emerged (F(2,71) = 6.164; p = 0.003) for ethnicity and axial length, accounting for 12.4% of variance in primary spherical aberration with ethnicity accounting for 8.4% of that variance. For Caucasian subjects, a significant correlation was found between axial length and (Pearson's correlation coefficient -0.500; p = 0.005) and Z(0)(4) (Pearson's correlation coefficient -0.423; p = 0.020). For British Asian subjects, AL was only correlated with coefficient Z(-2)(4) (Pearson's correlation coefficient -0.358; p = 0.017). Ethnicity is a factor to be considered in the variability of wavefront aberration, particularly spherical aberration. Relationship between AL and wavefront aberrations seems to vary between ethnicities. If higher order aberrations play a role in the emmetropization process, this may be different for different populations.

Clinical ocular wavefront analyzers.

PURPOSE: To provide a summary of the methods used by clinical wavefront analyzers and their historical, current, and future applications. METHODS: Review of the literature and authors' experience with the various devices. RESULTS: A wide range of clinical wavefront aberrometers, which use different principles, are available to clinicians and researchers. CONCLUSIONS: Applications of wavefront analyzers in vision sciences range from assessment of refractive error, refractive surgery planning, evaluation of outcomes, optimization of contact lenses and IOL designs, evaluation of pathology relating to optical performance of the eye, and evaluation of accommodation alterations.

Non-invasive vascular impedance measures demonstrate ocular vasoconstriction during isometric exercise.

AIMS: The calculation of impedance for a vascular network is a common method used in circulation studies. Impedance indices (the ratios of the harmonics of pressure to the harmonics of flow) provide the investigator with a measure of the opposition to blood flow in a pulsatile system and are a proven indicator for vasculopathy. Previous studies investigating the eye's opposition to blood flow have concentrated on simple measures of resistance (the ratio of mean pressure difference to mean flow) which are more appropriate to a steady state or non-pulsatile system. The purpose of this study is to demonstrate a new, non-invasive, method to determine the vascular impedance of the eye during the known physiologic stress of sustained isometric exercise. METHODS: Waveforms of ocular blood flow and carotid arterial blood pressure were measured non-invasively. Ocular blood flow waveforms were calculated using the Langham-Silver method by measuring the small fluctuations in intraocular pressure intraocular pressure over time with a high fidelity pneumatonometer. Carotid arterial blood pressure waveforms were determined using a SphygmoCor electronic tonometer held over the common carotid artery of the neck. Both waveforms were recorded simultaneously in normal volunteers under two conditions: (1) a baseline resting state and (2) during sustained isometric exercise. The components of the two waveforms (the harmonics) were calculated using a Fast Fourier transform and expressed as a ratio in order to determine a set of impedance values for each condition. The first four impedance values were calculated. RESULTS: 12 volunteers (six male: six female) with a mean age of 27 years (range 22-32 years) were recruited to the study. In comparison to baseline resting conditions, mean carotid blood pressure and heart rate both increased significantly during exercise: baseline mean carotid blood pressure, 82.6+/-8.2 mm Hg vs exercise mean carotid blood pressure, 93.8+/-12.8 mm Hg (p<0.001); baseline pulse rate, 64.6+/-9.1 BP(m) vs exercise pulse rate, 71.8+/-9.7 BP(m) (p<0.001). Compared to resting conditions, the first and third impedance values demonstrated significant change during exercise: the first impedance value rose (83.9+/-25.6 mm Hg-s/microl to 117.1 +/- 40.9 mm Hg-s/microl, p = 0.01) and the third impedance value fell (487.9 +/- 294.7 mm Hg-s/microl to 248.3+/-206.8 mm Hg-s/microl, p = 0.01). CONCLUSIONS: The present study demonstrates, for the first time, a practical non-invasive method of calculating an index of impedance moduli for the pulsatile quotient of blood flow to the eye. Furthermore, during controlled isometric exercise, the impedance moduli displayed changes consistent with that known for a vascular system during vasoconstriction. The calculation of impedance moduli for the eye therefore shows promise for future investigations into ocular conditions where vascular obstruction is an aetiological factor.

Operator-induced errors in Hartmann-Shack wavefront sensing: model eye study.

PURPOSE: To evaluate the effects of instrument realignment and angular misalignment during the clinical determination of wavefront aberrations by simulation in model eyes. SETTING: Aston Academy of Life Sciences, Aston University, Birmingham, United Kingdom. METHODS: Six model eyes were examined with wavefront-aberration-supported cornea ablation (WASCA) (Carl Zeiss Meditec) in 4 sessions of 10 measurements each: sessions 1 and 2, consecutive repeated measures without realignment; session 3, realignment of the instrument between readings; session 4, measurements without realignment but with the model eye shifted 6 degrees angularly. Intersession repeatability and the effects of realignment and misalignment were obtained by comparing the measurements in the various sessions for coma, spherical aberration, and higher-order aberrations (HOAs). RESULTS: The mean differences between the 2 sessions without realignment of the instrument were 0.020 microm +/- 0.076 (SD) for Z(3)(-1)(P = .551), 0.009 +/- 0.139 microm for Z(3)(1)(P = .877), 0.004 +/- 0.037 microm for Z(4)(0) (P = .820), and 0.005 +/- 0.01 microm for HO root mean square (RMS) (P = .301). Differences between the nonrealigned and realigned instruments were -0.017 +/- 0.026 microm for Z(3)(-1)(P = .159), 0.009 +/- 0.028 microm for Z(3)(1) (P = .475), 0.007 +/- 0.014 microm for Z(4)(0)(P = .296), and 0.002 +/- 0.007 microm for HO RMS (P = 0.529; differences between centered and misaligned instruments were -0.355 +/- 0.149 microm for Z(3)(-1) (P = .002), 0.007 +/- 0.034 microm for Z(3)(1)(P = .620), -0.005 +/- 0.081 microm for Z(4)(0)(P = .885), and 0.012 +/- 0.020 microm for HO RMS (P = .195). Realignment increased the standard deviation by a factor of 3 compared with the first session without realignment. CONCLUSIONS: Repeatability of the WASCA was excellent in all situations tested. Realignment substantially increased the variance of the measurements. Angular misalignment can result in significant errors, particularly in the determination of coma. These findings are important when assessing highly aberrated eyes during follow-up or before surgery.

Wavefront analyzers induce instrument myopia.

PURPOSE: To assess the accuracy of three wavefront analyzers versus a validated binocular open-view autorefractor in determining refractive error in non-cycloplegic eyes. METHODS: Eighty eyes were examined using the SRW-5000 open-view infrared autorefractor and, in randomized sequence, three wavefront analyzers: 1) OPD-Scan (NIDEK, Gamagori, Japan), 2) WASCA (Zeiss/Meditec, Jena, Germany), and 3) Allegretto (WaveLight Laser Technologies AG, Erlangen, Germany). Subjects were healthy adults (19 men and 21 women; mean age: 20.8 +/- 2.5 years). Refractive errors ranged from +1.5 to -9.75 diopters (D) (mean: +1.83 +/- 2.74 D) with up to 1.75 D cylinder (mean: 0.58 +/- 0.53 D). Three readings were collected per instrument by one examiner without anticholinergic agents. Refraction values were decomposed into vector components for analysis, resulting in mean spherical equivalent refraction (M) and J0 and J45 being vectors of cylindrical power at 0 degrees and 45 degrees, respectively. RESULTS: Positive correlation was observed between wavefront analyzers and the SRW-5000 for spherical equivalent refraction (OPD-Scan, r=0.959, P<.001; WASCA, r=0.981, P<.001; Allegretto, r=0.942, P<.001). Mean differences and limits of agreement showed more negative spherical equivalent refraction with wavefront analyzers (OPD-Scan, 0.406 +/- 0.768 D [range: 0.235 to 0.580 D] [P<.001]; WASCA, 0.511 +/- 0.550 D [range: 0.390 to 0.634 D] [P<.001]; and Allegretto, 0.434 +/- 0.904 D [range: 0.233 to 0.635 D] [P<.001]). A second analysis eliminating outliers showed the same trend but lower differences: OPD-Scan (n=75), 0.24 +/- 0.41 D (range: 0.15 to 0.34 D) (P<.001); WASCA (n=78), 0.46 +/- 0.47 D (range: 0.36 to 0.57 D) (P<.001); and Allegretto (n=77), 0.30 +/- 0.62 D (range: 0.16 to 0.44 D) (P<.001). No statistically significant differences were noted for J0 and J45. CONCLUSIONS: Wavefront analyzer refraction resulted in 0.30 D more myopia compared to SRW-5000 refraction in eyes without cycloplegia. This is the result of the accommodation excess attributable to instrument myopia. For the relatively low degrees of astigmatism in this study (<2.0 D), good agreement was noted between wavefront analyzers and the SRW-5000.

Aviation-related respiratory gas disturbances affect dark adaptation: a reappraisal.

This study examined the time course of early scotopic threshold sensitivity during dark adaptation under mild to moderate hypoxia, moderate hypocapnia and hyperoxia, measuring detection time displacement relative to normoxia. Cone rod inflection and early rod adaptation were highlighted using progressively dimmer green flash stimuli. Early scotopic sensitivity was significantly delayed by hypoxia and hastened by hypocapnia and hyperoxia. Effects of respiratory disturbance on dark adaptation include temporal shifts of early scotopic sensitivity while human rod photoreceptors appear functionally hypoxic when breathing air at one atmosphere. At night, supplementary oxygen may benefit aircrew visual sensitivity, even at ground level.