What is a typical optic nerve head?

Whereas it is known that elevated intraocular pressure (IOP) increases the risk of glaucoma, it is not known why optic nerve heads (ONHs) vary so much in sensitivity to IOP and how this sensitivity depends on the characteristics of the ONH such as tissue mechanical properties and geometry. It is often assumed that ONHs with uncommon or atypical sensitivity to IOP, high sensitivity in normal tension glaucoma or high robustness in ocular hypertension, also have atypical ONH characteristics. Here we address two specific questions quantitatively: Do atypical ONH characteristics necessarily lead to atypical biomechanical responses to elevated IOP? And, do typical biomechanical responses necessarily come from ONHs with typical characteristics. We generated 100,000 ONH numerical models with randomly selected values for the characteristics, all falling within literature ranges of normal ONHs. The models were solved to predict their biomechanical response to an increase in IOP. We classified ONH characteristics and biomechanical responses into typical or atypical using a percentile-based threshold, and calculated the fraction of ONHs for which the answers to the two questions were true and/or false. We then studied the effects of varying the percentile threshold. We found that when we classified the extreme 5% of individual ONH characteristics or responses as atypical, only 28% of ONHs with an atypical characteristic had an atypical response. Further, almost 29% of typical responses came from ONHs with at least one atypical characteristic. Thus, the answer to both questions is no. This answer held irrespective of the threshold for classifying typical or atypical. Our results challenge the assumption that ONHs with atypical sensitivity to IOP must have atypical characteristics. This finding suggests that the traditional approach of identifying risk factors by comparing characteristics between patient groups (e.g. ocular hypertensive vs. primary open angle glaucoma) may not be a sound strategy.

A method to estimate biomechanics and mechanical properties of optic nerve head tissues from parameters measurable using optical coherence tomography

Optic nerve head (ONH) tissue properties and biomechanics remain mostly unmeasurable in the experiment. We hypothesized that these can be estimated numerically from ocular parameters measurable in vivo with optical coherence tomography (OCT). Using parametric models representing human ONHs we simulated acute intraocular pressure (IOP) increases (10 mmHg). Statistical models were fit to predict, from OCT-measurable parameters, 15 outputs, including ONH tissue properties, stresses, and deformations. The calculations were repeated adding parameters that have recently been proposed as potentially measurable with OCT. We evaluated the sensitivity of the predictions to variations in the experimental parameters. Excellent fits were obtained to predict all outputs from the experimental parameters, with cross-validated R2s between 0.957 and 0.998. Incorporating the potentially measurable parameters improved fits significantly. Predictions of tissue stiffness were accurate to within 0.66 MPa for the sclera and 0.24 MPa for the lamina cribrosa. Predictions of strains and stresses were accurate to within 0.62% and 4.9 kPa, respectively. Estimates of ONH biomechanics and tissue properties can be obtained quickly from OCT measurements using an applet that we make freely available. These estimates may improve understanding of the eye sensitivity to IOP and assessment of patient risk for development or progression of glaucoma.

Scan quality effect on glaucoma discrimination by glaucoma imaging devices

AIM

To evaluate, within ocular imaging scans of acceptable quality as determined by manufacturers' guidelines, the effects of image quality on glaucoma discrimination capabilities.

METHODS

One hundred and four healthy and 75 glaucomatous eyes from the Advanced Imaging in Glaucoma Study (AIGS) were imaged with GDx-VCC, HRT II and StratusOCT. Quality score (QS>/=8), pixel standard deviation (SD</=50) and signal strength (SS>/=5) were used as quality parameter cut-offs, respectively. GDx nerve fibre indicator (NFI) and HRT Moorfields regression analysis (MRA) classifications and OCT mean retinal nerve fibre layer (RNFL) thickness were used as the discriminatory parameters. Logistic regression models were used to model the dichotomous clinical classification (healthy vs glaucoma) as a function of image-quality parameters and discriminatory parameters.

RESULTS

Quality parameter covariates were statistically non-significant for GDx and HRT but had an inverse effect on OCT in predicting disease (a higher SS had a lower probability of glaucoma). Age was a significant covariate for GDx and HRT, but not OCT, while ethnicity and interaction between the image quality and the institute where scans were acquired were significant covariates in the OCT models.

CONCLUSION

Scan quality within the range recommended as acceptable by the manufacturer of each imaging device does not affect the glaucoma discriminating ability of GDx or HRT but does affect Stratus OCT glaucoma discrimination.

Retinal nerve fibre layer thickness measurement reproducibility improved with spectral domain optical coherence tomography

BACKGROUND/AIMS

To investigate retinal nerve fibre layer (RNFL) thickness measurement reproducibility using conventional time-domain optical coherence tomography (TD-OCT) and spectral-domain OCT (SD-OCT), and to evaluate two methods defining the optic nerve head (ONH) centring: Centred Each Time (CET) vs Centred Once (CO), in terms of RNFL thickness measurement variability on SD-OCT.

METHODS

Twenty-seven eyes (14 healthy subjects) had three circumpapillary scans with TD-OCT and three raster scans (three-dimensional or 3D image data) around ONH with SD-OCT. SD-OCT images were analysed in two ways: (1) CET: ONH centre was defined on each image separately and (2) CO: ONH centre was defined on one image and exported to other images after scan registration. After defining the ONH centre, a 3.4 mm diameter virtual circular OCT was resampled on SD-OCT images to mimic the conventional circumpapillary RNFL thickness measurements taken with TD-OCT.

RESULTS

CET and CO showed statistically significantly better reproducibility than TD-OCT except for 11:00 with CET. CET and CO methods showed similar reproducibility.

CONCLUSIONS

SD-OCT 3D cube data generally showed better RNFL measurement reproducibility than TD-OCT. The choice of ONH centring methods did not affect RNFL measurement reproducibility.

Heidelberg Retina Tomograph 3 machine learning classifiers for glaucoma detection

AIMS

To assess performance of classifiers trained on Heidelberg Retina Tomograph 3 (HRT3) parameters for discriminating between healthy and glaucomatous eyes.

METHODS

Classifiers were trained using HRT3 parameters from 60 healthy subjects and 140 glaucomatous subjects. The classifiers were trained on all 95 variables and smaller sets created with backward elimination. Seven types of classifiers, including Support Vector Machines with radial basis (SVM-radial), and Recursive Partitioning and Regression Trees (RPART), were trained on the parameters. The area under the ROC curve (AUC) was calculated for classifiers, individual parameters and HRT3 glaucoma probability scores (GPS). Classifier AUCs and leave-one-out accuracy were compared with the highest individual parameter and GPS AUCs and accuracies.

RESULTS

The highest AUC and accuracy for an individual parameter were 0.848 and 0.79, for vertical cup/disc ratio (vC/D). For GPS, global GPS performed best with AUC 0.829 and accuracy 0.78. SVM-radial with all parameters showed significant improvement over global GPS and vC/D with AUC 0.916 and accuracy 0.85. RPART with all parameters provided significant improvement over global GPS with AUC 0.899 and significant improvement over global GPS and vC/D with accuracy 0.875.

CONCLUSIONS

Machine learning classifiers of HRT3 data provide significant enhancement over current methods for detection of glaucoma.

Sources of longitudinal variability in optical coherence tomography nerve-fibre layer measurements

AIMS

The purpose of this study was to compare the day-to-day reproducibility of optical coherence tomography (OCT; StratusOCT, Carl Zeiss Meditec, Dublin, CA) measurements of retinal nerve-fibre layer (RNFL) measurements at time points 1 year apart.

METHODS

One eye in each of 11 healthy subjects was examined using the StratusOCT fast RNFL scan protocol. Three fast RNFL scans with signal strength > or =7 were obtained on each of 3 days within a month. This protocol was repeated after 12 months. A linear mixed effects model fitted to the nested data was used to compute the variance components.

RESULTS

The square root of the variance component that was attributed to the differences between subjects was 7.17 microm in 2005 and 7.28 microm in 2006. The square roots of the variance component due to differences between days within a single subject were 1.95 microm and 1.50 microm, respectively, and for within day within a single subject were 2.51 microm and 2.55 microm, respectively. There were no statistically significant differences for any variance component between the two testing occasions.

CONCLUSIONS

Measurement error variance remains similar from year to year. Day and scan variance component values obtained in a cohort study may be safely applied for prediction of long-term reproducibility.

The effect of sildenafil on ocular blood flow

Sildenafil is a potent phosphodiesterase (PDE) 5 inhibitor that is used for patients with erectile dysfunction. Sildenafil induces vasodilation in selected smooth muscle via increased levels of guanosine 3', 5' cyclic monophosphate and increase in nitric oxide. The vasodilatory effects of the PDE 5 inhibitors led us to review its effect on the ocular vasculature. Sildenafil appears to increase blood flow velocity significantly in the retrobulbar and choroidal circulation. Most studies suggest an increase in choroidal blood flow, with a lesser effect on the retinal vasculature.

A comparison of the effects of dorzolamide/timolol fixed combination versus latanoprost on intraocular pressure and pulsatile ocular blood flow in primary open-angle glaucoma patients

PURPOSE

To evaluate the effects of dorzolamide/timolol fixed combination (D/T) compared to latanoprost on intraocular pressure (IOP) and pulsatile ocular blood flow (POBF) in primary open-angle glaucoma (POAG) patients.

METHODS

Thirty patients with POAG were randomized in an open-label, cross-over study. Intraocular pressure reduction was achieved by 4 weeks medical therapy with D/T twice daily or latanoprost 0.005% dosed once in the evening. During a 4-week run-in and a 4-week wash-out period between study arms, patients ceased use of all other glaucoma medications and used timolol maleate 0.5% twice daily. Primary efficacy variables were IOP and POBF.

RESULTS

There was no difference in baseline IOP and POBF parameters between the two study arms. Both D/T and latanoprost statistically significantly reduced IOP by 4.6 mmHg (p < 0.0001) and 3.75 mmHg (p < 0.0001) and increased POBF by 2.048 microl/second (p = 0.0030) and 2.147 microl/second (p = 0.0009), respectively. Repeated measures anova detected significant changes in POBF with treatment (p = 0.0361). Dorzolamide/timolol fixed combination statistically significantly increased pulse volume by 0.767 microl (p = 0.0087), while latanoprost therapy had no significant effect (p = 0.2407).

CONCLUSIONS

Both drugs had similar effects in terms of IOP reduction. Dorzolamide/timolol significantly increased pulse volume while latanoprost had no effect. Further studies are necessary to establish whether the enhancement of choroidal blood flow can prevent glaucoma progression.

Reproducibility of the Heidelberg retinal flowmeter in determining low perfusion areas in peripapillary retina

AIM

To evaluate the interobserver variability and retest reproducibility of confocal scanning laser Doppler flowmeter in measuring capillary perfusion of the peripapillary retina.

METHODS

Blood flow measurements were performed in one eye of 10 normal subjects by two investigators on two different days (visits). Five separate measurements of the peripapillary blood flow parameters were recorded by each observer at each visit. The Heidelberg retina flowmeter was used to record capillary perfusion in a 2560x640 microm area of the superotemporal peripapillary region and pixel by pixel analysis was done from an area adjacent to the optic disc, with a minimum of 1600 pixels. The percentage of pixels with less than 1 arbitrary unit of flow (no flow) and 10, 25, 50, 75, 90th percentiles of flow values was calculated. Interobserver measurements were compared by paired t test. Intraclass correlations (ICC) were used to determine the interobserver variability and retest reproducibility of the measurements. Intrasession coefficients of variations (CV) were also calculated.

RESULTS

There were no statistically significant differences between the two observers for all measurements and between visits for the percentage of pixels with no flow. ICC was 66% (range 57.09%-77.1%) for pixels with no flow. For the 10, 25, 50, 75, 90th percentiles of flow the ICC was 63.07% (53.91%-77.81%), 71.3% (64.23%-80.85%), 72.61% (66.02%-78.96%), 65.86% (58.53%-74.77%), and 60.05% (54.34%-70.06%), respectively. CV was 16.59%, 11.47%, 9.32%, 9.03%, 11.58%, and 16.05% for the percentage of no flow pixels and the 10, 25, 50, 75, 90th percentiles of flow, respectively.

CONCLUSIONS

The Heidelberg retinal flowmeter allows reproducible measurements of all levels of capillary perfusion areas when pixel by pixel analysis is used.

Reproducibility of circadian retinal and optic nerve head blood flow measurements by Heidelberg retina flowmetry

BACKGROUND/AIM

The Heidelberg retina flowmeter (HRF) is designed to measure retinal capillary blood flow. Previous studies however showed weak reproducibility of data. The intraindividual reproducibility of circadian HRF measurements was examined in healthy subjects in three locations of the retina.

METHODS

36 healthy volunteers (27.3 (SD 4.3) years) were examined by HRF seven times a day (t0-t6). Using a default window of 10 x 10 pixels, three consecutive measurements were performed in three precise focusing planes: superficial, intermediate and deep layer, peripapillary retina, neuroretinal rim and cup, respectively. Images of identical tissue locations identified by capillary landmarks of each layer were selected to quantify the retinal microcirculation of each volunteer. Means and standard deviations of all flow results of a given subject were calculated, at t0-t6 and the coefficients of variation as a measure of reproducibility.

RESULTS

The coefficients of variation ranged between 8.4% and 41.0% in the superficial layer (mean 19.8% (SD 8.4%)), 10.6%, and 43.0% in the intermediate layer (mean 24.0% (SD 8.4%)), and 9.9% and 84.0% (mean 29.6% (SD 15.8%)) in the deep layer.

CONCLUSIONS

These data show the best reproducibility of measurements in the superficial layer followed by the intermediate and the deep layer. Clinically, this is an unsatisfactory intraindividual reproducibility of flow values in each studied layer.

Ocular haemodynamic responses to induced hypercapnia and hyperoxia in glaucoma

AIM

To determine the ocular haemodynamic response to gas perturbations in glaucoma.

METHODS

Intraocular pressure (IOP), systemic systolic and diastolic blood pressure (SBP and DBP), and retrobulbar blood flow velocities, measured by colour Doppler imaging (CDI), were recorded at two visits. CDI was used to measure peak systolic and end diastolic velocities (PSV and EDV) and resistance index (RI) in the ophthalmic artery (OA), central retinal artery (CRA), and short posterior ciliary arteries (SPCAs). At the first visit, measurements were taken at baseline (B1: breathing room air) and during isoxic hypercapnia (end tidal PCO(2) increased 15% above baseline) in 16 normal subjects and 12 patients with glaucoma. On another day, measurements were repeated at a second baseline (B2) and during hyperoxia (100% oxygen breathing) for 15 normal subjects and 13 glaucoma patients. Baseline systemic data were compared using paired t tests; REANOVA was performed to compare group differences at baseline and to determine the vessel response to each condition. Fisher's LSD was used for post hoc comparison.

RESULTS

Baseline OA PSV was lower for the glaucoma than for the normal group (p = 0.047); the groups were otherwise similar at baseline. IOP demonstrated no response to hypercapnia, but reduced during hyperoxia for both the normal subjects (p<0.0001) and glaucoma patients (p = 0.04). During hypercapnia, SBP increased in normal subjects (p = 0.03) and glaucoma patients (p = 0.01); DBP increased in normal subjects (p = 0.021). There was a corresponding increase in ocular perfusion pressure (OPP) for normal subjects (p = 0.01) and glaucoma subjects (p = 0.028), and as a result OPP was included as a covariate in the REANCOVA model. Hypercapnia resulted in increased PSV in the CRA of normal subjects (p = 0.035) and increased PSV and EDV in the SPCAs of glaucoma patients (p = 0.041 and p = 0.030 respectively). Hyperoxia resulted in reduced PSV and EDV in the ophthalmic arteries of normal subjects only (p = 0.001 and 0.031 respectively).

CONCLUSIONS

These findings suggest the presence of relative vasoconstriction in glaucoma patients, which is at least partially reversed by hypercapnia.

The effects of dorzolamide on choroidal and retinal perfusion in non-exudative age related macular degeneration

AIM

To comprehensively evaluate the effects of dorzolamide on the choroidal and retinal circulation in patients with age related macular degeneration (AMD).

METHODS

In this randomised, double masked, parallel study, 36 non-exudative AMD patients were randomised in a 2 to 1 fashion to placebo versus topical dorzolamide and underwent assessment of their choroidal and retinal circulation. Scanning laser ophthalmoscope indocyanine green angiograms (ICGA) were analysed by a new area dilution analysis technique. Four areas in the perifoveal region and two areas in the temporal peripapillary region were evaluated by plotting intensity of fluorescence of each area over time. The means of the choroidal filling times and the heterogeneity of the filling times were assessed. Scanning laser ophthalmoscope fluorescein angiography (FA) was evaluated for retinal arteriovenous passage (AVP) times by plotting intensity of fluorescence of retinal vessels over time. Assessment was performed at baseline and at 4 months.

RESULTS

Compared to placebo, AMD patients treated with dorzolamide showed a significantly increased rapidity of choroidal filling in the superior and inferior peripapillary regions (p=0.007, p=0.02, respectively). No significant difference in choroidal filling times was found in any of the perifoveal areas (p=0.9). Also, on FA assessment, treatment with dorzolamide showed no statistical differences in AVP times (p=0.19).

CONCLUSIONS

Dorzolamide may increase peripapillary choroidal perfusion in non-exudative AMD patients. Further studies are merited.

Blood flow per unit retinal nerve fibre tissue volume is lower in the human inferior retina

AIM

To determine if perfusion per unit tissue volume of retinal nerve fibre layer and optic nerve head in the inferior sector is lower than in the superior sector.

METHODS

Heidelberg retinal tomogram (HRT) for topographic measurement of optic nerve head and retinal nerve fibre layer and Heidelberg retinal flowmeter (HRF) for retinal blood flow were performed on 19 normal healthy subjects. Measurements from the superior and inferior sectors were compared. The perfusion/nerve fibre ratio (PNR); the blood flow per unit retinal nerve fibre tissue volume, was calculated in each sector with a formula; HRF flow measurements divided by HRT measurements.

RESULTS

Retinal nerve fibre layer thickness in the inferior retina was significantly higher than in the superior retina (p<0.05). There were, however, no differences in retinal blood flow between the superior and inferior retinal sectors. The PNR in the inferior sector were significantly lower than in the superior sector (p=0.047 for HRF mean flow/rim volume and p = 0.0282 for HRF 75th percentile flow/rim volume).

CONCLUSIONS

The inferior sector of retinal nerve fibre layer and optic nerve head may have lower blood flow per unit nerve tissue volume compared to the superior sector. This result suggests that the inferior sector is more vulnerable to elevated intraocular pressure (IOP) and ischaemic insults in glaucomatous optic neuropathy.

The first technique for non-invasive measurements of volumetric ophthalmic artery blood flow in humans

AIM

To validate the first non-invasive measurements of volumetric ophthalmic artery blood flow in humans.

METHODS

The ophthalmic arteries of healthy normal adults were examined by Advanced Technology Laboratories (ATL, a subsidiary of Phillips Medical Systems Inc) high definition imaging (HDI) 5000 colour Doppler imaging ultrasound with a 5-12 MHZ probe. A group of 14 subjects for experiment 1 and a group of 10 subjects for experiments 2 and 3 were selected, with the examined eye chosen randomly. Peak systolic velocities (PSV) and end diastolic velocities (EDV) of the ophthalmic artery and central retinal artery were measured and recorded. Cineloops (cinegraphic videos) of the ophthalmic arteries were then recorded with the ATL HDI 5000 and values for ophthalmic artery blood flow were produced offline using experimental analysis software. Multiple regression analysis was used to compare blood flow measurements with PSV and EDV measurements in the ophthalmic artery. In two follow up experiments, intraobserver variation in obtaining cineloops and the interanalyser variability in cineloop analysis were studied.

RESULTS

Volumetric flow correlated with ophthalmic artery PSV and EDV (p = 0.02, r(2) = 0.5). There was no correlation with the cental retinal artery. The intraobserver coefficient of variation in obtaining cineloops was 29.89% for blood flow, 19.07% for diameter, and 22.27% for velocity. The coefficients of variation of the measurements of the two cineloop analysers were 40.21% for blood flow, 22.71% for diameter, and 26.34% for velocity.

CONCLUSION

Cineloop analysis produces ophthalmic artery flow measurements which correlate with PSV and EDV, suggesting validity. The intraobserver variation and cineloop analyser variation were found to be in the acceptable range.

Effect of intravenous droperidol on intraocular pressure and retrobulbar hemodynamics

PURPOSE

Topically-applied dopamine antagonists reduce intraocular pressure (IOP) and inrease retinal blood flow in animal models. We examined the acute effects of intravenous infusion of a dopamine blocker (droperidol) on these parameters in healthy humans.

METHODS

Sixteen subjects free from ocular or systemic disease (mean age 33 +/- 10 yrs) received either 5 mg i.v. droperidol over 5 minutes, or i.v. saline placebo in double-masked fashion. IOP was determined 30 and 60 minutes later, while color Doppler imaging was used to determine flow velocities in the ophthalmic, central retinal, and nasal and temporal posterior ciliary arteries 60 minutes after drug infusion.

RESULTS

30 minutes after drug infusion, IOP was reduced 6.0 mmHg as compared with baseline (p<0.001); after 60 minutes, IOP remained reduced by 3.7 mmHg (p<0.001). Placebo had no effect on IOP. While droperidol slightly elevated blood pressure and increased the calculated ocular perfusion pressure, the drug reduced visual acuity and contrast sensitivity (p<0.05). Droperidol elevated peak systolic velocity in the central retinal and nasal posterior ciliary arteries, without changing end-diastolic velocity or the resistance index in either of these vessels. Droperidol had no effect on flow velocities in the ophthalmic artery or the temporal posterior ciliary artery.

CONCLUSIONS

The rapid and marked ocular hypotension resulting from intravenous droperidol suggests that this agent may prove useful in the management of acute ocular hypertension. The retrobulbar changes consequent to the ocular tension reduction likely represent autoregulatory responses to altered ocular perfusion pressure.

Transpupillary thermotherapy for subfoveal occult choroidal neovascularization: effect on ocular perfusion

PURPOSE

To perform a descriptive analysis of the effects on ocular blood flow of transpupillary thermotherapy (TTT) for occult subfoveal choroidal neovascular membranes (CNVMs) in age-related macular degeneration (AMD).

METHODS

Eleven subjects with occult subfoveal CNVM due to AMD were assessed in a masked fashion by color Doppler imaging (CDI) within 24 hours before, 24 hours after, and 1 month after undergoing TTT.

RESULTS

In the posterior ciliary arteries (PCAs), there were no statistically significant changes observed in the peak systolic velocity (PSV), end diastolic velocity (EDV), or resistive index (RI) at 24 hours. At 1 month, the mean EDV decreased 36% (P = 0.0105) and the mean RI increased 3.8% (P = 0.0305) in the nasal PCA. Although there was a similar trend in the temporal PCA, the differences did not reach statistical significance. In the central retinal artery (CRA), the mean PSV decreased 16% (P = 0.0137), and the mean EDV decreased 21% (P = 0.0222) at 24 hours after treatment. There were no statistically significant differences in the CRA blood flow indices at 1 month after treatment. In the ophthalmic artery, there were no statistically significant differences observed in the mean PSV, EDV, or RI at 24 hours or 1 month after treatment.

CONCLUSIONS

TTT is associated with transiently decreased volumetric blood flow in the retinal circulation 24 hours after treatment. In the posterior ciliary arteries that supply the choroid, there were no changes observed at 24 hours, but at 1 month, there was a decrease in the mean EDV and an increase in the RI in the nasal and temporal PCAs, reaching statistical significance in the nasal PCA only. This study suggests that TTT could lead to alterations in choroidal blood flow, as assessed by CDI. Further study is warranted.

Effect of dorzolamide timolol combination versus timolol 0.5% on ocular bloodflow in patients with primary open-angle glaucoma

PURPOSE

Addition of dorzolamide to timolol in primary open-angle glaucoma shows augmented reduction of intraocular pressure. It is unknown as yet if addition of dorzolamide will alter hemodynamics.

METHODS

Fifteen patients with primary open-angle glaucoma were placed on a medication-dependent 1-week to 4-week washout that included maintenance on timolol. After washout, baseline measurements were taken (timolol). They were studied after a month on timolol or dorzolamide-timolol (Cosopt; Merck, Inc, Whitehouse Station, New Jersey), with the second drug preceded by another month of timolol maintenance and second baseline measurements. At each visit, visual function, intraocular pressure, and ocular hemodynamics were monitored, including indocyanine green and fluorescein angiography and color Doppler imaging.

RESULTS

Cosopt significantly reduced intraocular pressure (14.7 to 13.4 mm Hg, P <.05) and increased arteriovenous passage time (superior temporal artery) of fluorescein dye (2.13 to 1.76 seconds, P =.01) but had no effect on visual function.

CONCLUSIONS

When compared with timolol in primary open-angle glaucoma, Cosopt augments ocular tension reduction and reduces the amount of time required for blood to pass through the superior retinal vasculature.

Simultaneous management of blood flow and IOP in glaucoma

Factors other than intraocular pressure (IOP) elevation must be involved in initiation and progression of glaucoma. An additional element in disease causation may be ischemia in the retina and optic nerve head. Ischemic damage to neurons in the CNS is similar mechanistically and histopathologically to changes seen in glaucoma. Further, glaucoma patients with normal IOP show clear evidence for cerebral and ocular ischemia. Aging and atherosclerosis reduce the ability of the eye to autoregulate blood flow when ocular perfusion pressure changes: the dependence of blood flow on perfusion pressure links ischemia to IOP. Consequently, neuroprotective treatments for glaucoma should be designed to both reduce IOP and improve ocular nutrient delivery.

Detector sensitivity influences blood flow sampling in scanning laser Doppler flowmetry

PURPOSE

To establish the effect of photodiode sensitivity on the DC (brightness) value and the resultant blood flow measurements of retina and rim tissue using a scanning laser Doppler flowmeter (SLDF).

METHODS

The sample consisted of one eye of each of 15 healthy subjects (mean age 27.8 +/- 6.1 years). Using the Heidelberg Retina Flowmeter (HRF), three 10-deg images of the superior temporal retina and three further images of the superior temporal rim were acquired for each of five DC bands: band 1: 30-70; band 2: 70-110; band 3: 110-150; band 4 150-190; band 5: 190-230. Retinal blood volume, flow and velocity were determined for each image using a 10 x 10 pixel square grid located at a predetermined location on the retina and rim for each subject. Following image acquisition, the DC values corresponding to each pre-assigned retinal or rim location were determined. The mean and standard deviation were determined for the blood flow parameters within each DC band for each subject in both locations. Analysis of variance was used to identify significant change in the data as a function of the DC value (P<0.05).

RESULTS

Analysis of variance revealed that retinal blood flow measures acquired within DC band 5 resulted in significantly lower measures of blood flow and velocity (P=0.035 and P=0.049 respectively) than at lower DC values. Band 5 values of flow, volume and velocity in the neuroretinal rim were also significantly low (P=0.016, P= 0.003 and P=0.026 respectively). Peak neuroretinal rim blood flow was recorded when the DC value was between 70 and 110. For blood flow measurement at the retina and neuroretinal rim the DC value should not exceed 190.

CONCLUSION

Photodiode sensitivity as indicated by the DC value affects measurements of ocular blood flow using the HRF.

Ocular blood flow measurements and their importance in glaucoma and age-related macular degeneration

This survey of methods for assessing ocular hemodynamics in glaucoma and age-related macular degeneration is not complete, but it does cover those likely to be encountered in the literature. A fundamental problem in getting to grips with the ocular blood flow literature is the difficulty in comparing the results of similar studies employing different assessment techniques. As evident from the discussion above, each technique evaluates a portion of the ocular circulation in a distinct way. Some of the methods overlap with regard to the tissues that can be used for examination, while others are directed at entirely different parts of the ocular vasculature. Despite these difficulties, hemodynamic studies of glaucoma and AMD are likely to grow in importance. On the basis of accumulating epidemiological and clinical evidence, it is becoming apparent that intraocular pressure is not the sole etiological factor in glaucoma, and retinal pigment epithelium senescence is not the sole etiological factor in AMD. Circumstantial evidence of vascular involvement in glaucoma and AMD has now been bolstered by experimental evidence. If the current pace of refinement of newly established technologies for evaluating ocular blood flow is maintained, they will soon be ready for deployment in the clinic. The only problem is the availability of expensive instruments and trained personnel. The ultimate beneficiaries of work in this area will not be researchers, but patients.

Effect of brimonidine tartrate on ocular hemodynamics in healthy volunteers

While alpha2-adrenergic agonists, such as brimonidine tartrate, significantly reduce the intraocular pressure (IOP), the presence of vasoconstrictor postsynaptic alpha2 receptors on vascular smooth muscle raise the possibility that brimonidine could potentially compromise ocular blood flow. Consequently, the ocular hemodynamic effects of brimonidine were studied in normal subjects. Twelve healthy volunteers were included in this prospective, double-masked, placebo controlled, crossover-designed clinical trial. They received either brimonidine tartrate 0.2% or placebo b.i.d. for 2 weeks. Goldmann tonometry and color Doppler imaging (CDI) were performed at baseline, at 2 hr, 1 week, and 2 weeks after the treatment. Fundus angiography using a scanning laser ophthalmoscope was performed at baseline and 2 weeks after treatment to determine retinal arteriovenous passage time. Brimonidine lowered IOP at 2 hr, 1 week, and 2 weeks (p = 0.058, p = 0.031, and p = 0.022, respectively). Brimonidine did not affect the retrobulbar arterial velocities measured by CDI, nor retinal arteriovenous passage time. In conclusion, two-week treatment with brimonidine reduces IOP and does not reduce the bulk retinal or retrobulbar arterial perfusion in young healthy volunteers.

New neuroretinal rim blood flow evaluation method combining Heidelberg retina flowmetry and tomography

AIM

Accurate Heidelberg retina flowmeter (HRF) measurements require correct manual setting of the HRF photodetector sensitivity. The neuroretinal rim produces a weak signal relative to the peripapillary retina. A newly developed HRF alignment and sensitivity protocol, capable of accurate rim measurement, was investigated.

METHODS

18 eyes of nine healthy volunteers were examined by HRF. Three images of each eye were taken using three different imaging methods. Method 1: a conventional image (optic nerve head centred image with photodetector sensitivity optimised for the strong signal from the peripapillary retina); method 2: the setting of method 1 with photodetector sensitivity optimised for the weak signal from the rim; and method 3: the setting of method 2 with the temporal rim margin tangent to the lateral image border to remove the overpowering signal from the temporal peripapillary retina. The neuroretinal rim was defined by the Heidelberg retina tomograph (HRT). Blood flow and reflectivity values (DC component) in the rim area were compared for the three methods using pointwise analysis. Coefficients of variation of repeated measurements in 12 subjects have been calculated for method 3.

RESULTS

The neuroretinal rim area measured by method 1 had a significantly lower brightness compared with method 2 and 3 (p=0.0002 and p=0.0002, respectively). Method 2 provided proper sensitivity for the weak signals of the rim area based on rim tissue DC values; however, this sensitivity setting was too high for the strong signal from the peripapillary retina. Method 3 avoided the strong peripapillary signal with the proper signal from the rim and provided significantly higher flow values of the rim area at 75 and 90 percentile pixels (p=0.0065 and p=0.0038 respectively) compared with method 2. Interobserver repeatability ranged from 16.85% to 21.96% for the different parameters.

CONCLUSIONS

Method 3 provides an accurate and reproducible flow measurement of the neuroretinal rim area through proper sensitivity for the weak rim signal, alignment, and removal of the strong temporal signal from the image. This new method is recommended to improve accuracy of blood flow measurement in the neuroretinal rim.

Photodetector sensitivity level and heidelberg retina flowmeter measurements in humans

PURPOSE

In vitro models suggest that Heidelberg retina flowmeter (HRF) measurements are affected by changes in photodetector sensitivity. We measured blood flow in a single volume of human retinal tissue in vivo at various sensitivity (DC) levels.

METHODS

The peripapillary retinal regions of 12 normal subjects were examined by HRF under five different sensitivity settings: (1) average DC range below 100; (2) average DC range below 125; (3) average DC range near 150 (normal sensitivity); (4) average DC range above 175; and (5) average DC range above 200 or extremely overexposed. The distributions of flow values were examined by pointwise analysis. All pixels from a common tissue location were analyzed, and the effect of their brightness on the flow measurement was evaluated by ANOVA with Fisher's protected least significant difference model.

RESULTS

ANOVA analysis of image DC level showed that significantly different DC levels were achieved for each of the five sensitivity settings (P < 0.0001). Flow values decreased with increasing DC for each of the 25th percentile, 50th percentile (P: < 0.0001 for each), 75th percentile (P: = 0.0026), 90th percentile (P: = 0.0216), and mean (P: = 0.0004) flow values. The percentage of pixels with values of zero (avascular tissue) increased with increasing photodetector sensitivity (P< 0.0001).

CONCLUSIONS

Improper sensitivity settings alter the detected percentage of avascular tissue and the blood flow measurements in tissue containing capillaries. Consistent assessment of retinal blood flow requires consistent photodetector sensitivity settings between longitudinal images.

Intraocular pressure and photorefractive keratectomy: a comparison of three different tonometers

PURPOSE

To evaluate the intraocular pressure (IOP) with three different instruments, Goldmann applanation tonometer (GAT), noncontact tonometer, and Tono-Pen after photorefractive keratectomy (PRK) for myopia.

METHODS

A prospective case series study to evaluate preoperative and postoperative IOP measurements of 149 eyes at 12 months. We performed GAT, noncontact tonometry, Tono-Pen central, and Tono-Pen temporal periphery measurements. We also performed measurements of the central corneal thickness (CCT) by ultrasonic pachymetry and keratometry. Pre-operative IOP reading served as control for all studies.

RESULTS

After PRK, IOP reading was significantly reduced in the treated eyes when compared with the control measurements (11.87+/-1.73 vs. 13.37+/-1.52 mm Hg, p<0.0001 with GAT; 12.07+/-1.6 vs. 13.51+/-1.59 mm Hg, p<0.0001 with noncontact tonometer; 12.18+/-1.6 vs. 13.48+/-1.55 mm Hg, p<0.0001 with Tono-Pen central; 13.48+/-1.65 vs. 13.71+/-1.56 Hg, p<0.0104 with Tono-Pen temporal periphery). There was also a significant correlation between IOP reading changes measured by GAT, noncontact tonometer, Tono-Pen central, and change of CCT and between reduction of IOP reading and keratometry (r2>0.39, p<0.0001 for each). The correlation between IOP reading change by Tono-Pen temporal periphery and CCT was also significant but r2 value was only 0.034. Tono-Pen temporal periphery postoperative IOP measurements had the best correlation with preoperative GAT IOP (r2 = 0.57, p<0.0001).

CONCLUSIONS

PRK reduced IOP reading as measured by GAT, noncontact tonometer, and Tono-Pen central; less so when measured by Tono-Pen temporal periphery. Early detection of glaucoma and IOP follow-up in glaucoma patients may be done best by peripheral Tono-Pen measurements over the nonablated cornea.

Optic nerve head blood flow measurements in non-arteritic anterior ischaemic optic neuropathy

PURPOSE

To determine whether a decrease in blood flow, measured by the Heidelberg retiinal flowmeter, can characterise a disc at risk and predict the occurrence of non-arteritic anterior ischaemic optic neuropathy (NAION).

METHODS

Blood flow, volume and velocity were measured in the optic nerve heads of 14 unaffected fellow eyes of patients with unilateral NAION, and compared with those of the affected eyes and of the eyes of 7 age-matched healthy controls.

RESULTS

The affected eyes were found to differ from the unaffected eyes in all three haemodynamic parameters. Flow (measured in arbitrary units) is the most important parameter to be taken into consideration. Blood flow was significantly lower in affected than in unaffected eyes (upper rim, p < 0.05; lower rim, p < 0.025). It was also significantly lower in the unaffected eyes than in the healthy control eyes (p < 0.005, upper and lower rims), and in the affected eyes than in the control eyes (p < 0.00005, upper and lower rims).

CONCLUSION

Decreased blood flow in the optic nerve head may indicate a risk for NAION and be considered a characteristic of the disc at risk.

Regional differences in optic disc and retinal circulation

PURPOSE

To determine regional differences in the circulation of the optic disc, the peripapillary choroid and the retinal vessels using the techniques of computerized image analysis and fluorescein angiography.

METHODS

Ten young normal subjects were studied. Fluorescein angiography was performed with a scanning laser ophthalmoscope. The rate of filling of fluorescein or slope was measured in the four quadrants of the optic disc, the peripapillary choroid and the major retinal arteries and veins. A total of 125 points was measured from each image of the fluorescein angiograms for a total of 20 to 30 images. Retinal vessel width was measured using computerized image analysis.

RESULTS

Comparison of the rates of filling of fluorescein or slopes showed significant differences from zero among the four quadrants of the optic disc (p= 0.01 to <0.02), of the peripapillary choroid (p=0.01 to <0.05) and among the major retinal arteries (p=0.01 to <0.02) and retinal veins (p=0.02 to <0.05). No significant differences were noted between the quadrants for retinal arterial width with the retinal veins only showing a significant difference between the inferior nasal and inferior temporal vein (p=0.02 to <0.05).

CONCLUSION

Evaluation of the circulation of the optic disc, retina, and peripapillary choroid should take into account regional differences among these structures.

Relationship between the effect of carbon dioxide inhalation or nilvadipine on orbital blood flow in normal-tension glaucoma

PURPOSE

To investigate the relationship between the effect of carbon dioxide (CO2) inhalation or oral nilvadipine, a calcium-channel blocker, on orbital blood flow in patients with normal-tension glaucoma, as determined by color Doppler imaging.

PATIENTS AND METHODS

Sixteen patients with normal-tension glaucoma (mean age, 55.6+/-9.8 years; male:female ratio, 3:13) underwent color Doppler imaging to measure the resistance index, and peak systolic and end-diastolic blood flow velocities of the ophthalmic artery, central retinal artery, and nasal and temporal short posterior ciliary arteries. Measurements were taken before and during CO2 supplementation sufficient to increase the end-tidal CO2 partial pressure by 10%. The color Doppler imaging measurements were repeated after 2 to 4 weeks of treatment with 2 mg oral nilvadipine, and comparisons were made between the effects of the two treatments.

RESULTS

Both CO2 inhalation and nilvadipine treatment significantly reduced the resistance index in the central retinal artery, nasal short posterior ciliary artery, and temporal short posterior ciliary artery. There was a significant correlation between the effects of the two treatments on the difference in the resistance indexes of the ophthalmic artery and central retinal artery, but not on those of the nasal or temporal short posterior ciliary artery.

CONCLUSIONS

Both CO2 inhalation and oral nilvadipine significantly reduce the resistance index measured by color Doppler imaging in orbital vessels. The effect of oral nilvadipine might be predicted by CO2 inhalation in patients with normal-tension glaucoma.

A comparative study of betaxolol and dorzolamide effect on ocular circulation in normal-tension glaucoma patients

OBJECTIVE

To determine whether dosages of a selective beta-blocking agent (betaxolol) and a topical carbonic anhydrase inhibitor (dorzolamide), sufficient to significantly lower intraocular pressure (IOP), have similar or disparate impact on the retinal and retrobulbar circulation.

DESIGN

Counterbalanced crossover, with open-label use of medications.

PARTICIPANTS

Nine persons with normal-tension glaucoma (NTG).

INTERVENTION

After a 3-week drug washout, NTG patients were studied after 1 month of treatment with either dorzolamide or betaxolol, with determinations of IOP and retinal and retrobulbar hemodynamics.

MAIN OUTCOME MEASURES

At baseline and after treatment with each drug, retinal arteriovenous passage time was determined by scanning laser ophthalmoscopy after fluorescein dye injection, and flow velocities in the central retinal and ophthalmic arteries were measured with color Doppler ultrasonography imaging.

RESULTS

Betaxolol and dorzolamide each lowered IOP significantly, with these changes apparent and maximal after 2 weeks (each P < 0.05). In contrast, dorzolamide (but not betaxolol) accelerated arteriovenous passage of fluorescein dye in the inferior temporal quadrant of the retina (P < 0.05). Neither drug affected arteriovenous passage in the superotemporal retina or any aspect of central retinal or ophthalmic artery flow velocity after either 2 or 4 weeks.

CONCLUSIONS

Although both dorzolamide and betaxolol are effective ocular hypotensive agents and their topical instillation leaves retrobulbar hemodynamics unaltered, dorzolamide alone accelerates inferotemporal retinal dye transit.

Comprehensive assessment of retinal, choroidal and retrobulbar haemodynamics during blood gas perturbation

BACKGROUND

A study was performed to evaluate the effect of isoxic hypercapnia on ocular haemodynamics using colour Doppler imaging (CDI), scanning laser Doppler flowmetry (SLDF) and ocular blood flow (OBF) tonography.

METHODS

Measurements were taken for one eye of each of 14 healthy subjects (mean age 27 +/- 6 years) during breathing of room air and then during isoxic hypercapnia (breathing CO2 and room air). Using CDI, blood flow velocities and resistance indices were determined for the ophthalmic artery (OA), central retinal artery (CRA) and short posterior ciliary arteries (SPCAs). Using SLDF a 10 x 10 pixel frame was used to measure blood flow, volume and velocity in each quadrant of the peripapillary retina. Pulsatile ocular blood flow (POBF) was measured using the OBF tonograph.

RESULTS

Using CDI, peak systolic and end diastolic velocities increased and resistance index decreased significantly in the SPCAs during hypercapnia. Using SLDF, blood flow, volume and velocity increased significantly during hypercapnia in the superior temporal quadrant of the peripapillary retina. No significant difference was observed between baseline and hypercapnia for POBF.

CONCLUSIONS

Isoxic hypercapnia resulted in an increase in peripapillary retinal and SPCA blood flow parameters as determined by SLDF and CDI respectively. This implies the presence of autoregulatory activity in these vasculatures. These findings may be of significance in the pathogenesis of ocular disease such as glaucoma where autoregulation is thought to be compromised.

Progress in measurement of ocular blood flow and relevance to our understanding of glaucoma and age-related macular degeneration

New technologies have facilitated the study of the ocular circulation. These modalities and analysis techniques facilitate very precise and comprehensive study of retinal, choroidal, and retrobulbar circulations. These techniques include: 1. Vessel caliber assessment; 2. Scanning laser ophthalmoscopic fluorescein angiography and indocyanine green angiography to image and evaluate the retinal circulation and choroidal circulation respectively; 3. Laser Doppler flowmetry and confocal scanning laser Doppler flowmetry to measure blood flow in the optic nerve head and retinal capillary beds; 4. Ocular pulse measurement; and 5. color Doppler imaging to measure blood flow velocities in the central retinal artery, the ciliary arteries and the ophthalmic artery. These technique have greatly enhanced the ability to quantify ocular perfusion defects in many disorders, including glaucoma and age-related macular degeneration, two of the most prevalent causes of blindness in the industrialized world. Recently it has become clear, in animal models of glaucoma, that retinal ganglion cells die via apoptosis. The factors that initiate apoptosis in these cells remain obscure, but ischemia may play a central role. Patients with either primary open-angle glaucoma or normal-tension glaucoma experience various ocular blood flow deficits. With regard to age-related macular degeneration, the etiology remains unknown although some theories include primary retinal pigment epithelial senescence, genetic defects such as those found in the ABCR gene which is also defective in Stargardt's disease and ocular perfusion abnormalities. As the choriocapillaris supplies the metabolic needs of the retinal pigment epithelium and the outer retina, perfusion defect in the choriocapillaris could account for some of the physiologic and pathologic changes in AMD. Vascular defects have been identified in both nonexudative and exudative AMD patients using new technologies. This paper is a comprehensive update describing modalities available for the measurement of all new ocular blood flow in human and the clinical use.

Regional differences in retinal vascular reactivity

PURPOSE

Although glaucomatous visual field defects are more common in the superior field than in the inferior field, microaneurysms are more frequent in the superior than in the inferior retina in diabetic retinopathy. The authors hypothesized that differences in vascular hemodynamics in the two areas might contribute to these phenomena.

METHODS

The blood flow response to hyperoxia and hypercapnia was evaluated in peripapillary retinal tissue superior and inferior to the optic nerve head using confocal scanning laser Doppler flowmetry. In 14 young, healthy persons, blood flow was measured while breathing room air and during isocapnic hyperoxia (100% O2 breathing) and isoxic hypercapnia (PCO2 increased 15% above baseline). Histograms were generated from pixel-by-pixel analysis of retinal portions of superior and inferior temporal quadrants of the entire image.

RESULTS

Baseline blood flow in the inferior temporal quadrant was significantly greater than in the superior temporal quadrant (P < 0.05). However, the inferior region failed to increase in perfusion during hypercapnia and experienced significant mean blood flow reduction; flow reduction in the pixels at the 25th, 50th, 75th, and 90th percentile of flow; and an increased percentage of pixels without measurable flow, during hyperoxia (each P < 0.05). In contrast, in the superior temporal region, hyperoxia failed to reduce blood volume, velocity, or flow, whereas hypercapnia significantly increased mean flow; increased flow in the pixels at the 25th, 50th, 75th, and 90th percentile of flow; and reduced the percentage of pixels without measurable flow (each P < 0.05).

CONCLUSIONS

The inferior temporal quadrant of the peripapillary retina is, in comparison with the superior temporas region, less responsive to vasodilation and more responsive to vasoconstriction. These differences could contribute to different susceptibility to visual field defect or vascular dysfunction in the superior and inferior retina.

Color Doppler imaging discloses reduced ocular blood flow velocities in nonexudative age-related macular degeneration

PURPOSE

To study ocular perfusion defects in age-related macular degeneration.

METHODS

Twenty-five subjects with nonexudative age-related macular degeneration were compared with 25 age-matched control subjects in studies of flow velocities in several retrobulbar vessels. Color Doppler imaging, which was performed by an examiner who was masked to the subjects' assignment to the control or age-related macular degeneration group, measured peak systolic and end diastolic velocity in the ophthalmic, central retinal, and nasal and temporal posterior ciliary arteries of one eye. A resistive index was calculated from the peak systolic and end diastolic velocity.

RESULTS

Subjects with nonexudative age-related macular degeneration showed a consistent trend toward lower peak systolic and end-diastolic velocities in the posterior ciliary arteries. For example, in the nasal posterior ciliary artery, the mean end diastolic velocity measured 1.45 +/- 0.34 cm per sec in the age-related macular degeneration group compared with 1.96 +/- 0.66 cm per sec in the control group, yielding a 26% decrease in the age-related macular degeneration group, which represented the largest difference and was highly statistically significant (P = .0012). The resistive index was not significantly altered in the nasal or temporal posterior ciliary artery. Subjects with nonexudative age-related macular degeneration did not differ from control subjects in peak systolic velocity, end diastolic velocity, or resistive index in the ophthalmic artery. In the central retinal artery, the end diastolic velocity was lower (1.37 +/- 1.95 cm per sec vs 1.95 +/- 0.66 cm per sec), whereas the resistive index was higher (0.83 +/- 0.05 vs 0.76 +/- 0.06 cm per sec), in the age-related macular degeneration group; these results were highly statistically significant (P = .0007 and P < .0001, respectively).

CONCLUSIONS

Retrobulbar vascular changes in nonexudative age-related macular degeneration subjects include reduced flow velocities in the nasal and temporal posterior ciliary arteries. The reduced peak systolic velocity, combined with the reduced end diastolic velocity at a constant resistive index, seen in nonexudative age-related macular degeneration, is consistent with reduced bulk flow in these vessels, suggesting that choroidal perfusion is abnormal in this form of age-related macular degeneration. The changes in the central retinal artery suggest there may be a more generalized perfusion abnormality beyond the choroid in patients with age-related macular degeneration or that the central retinal artery exhibits a secondary autoregulatory response to a primary change elsewhere.

Glaucoma patients demonstrate faulty autoregulation of ocular blood flow during posture change

BACKGROUND/AIMS

Autoregulation of blood flow during posture change is important to ensure consistent organ circulation. The purpose of this study was to compare the change in retrobulbar ocular blood flow in glaucoma patients with normal subjects during supine and upright posture.

METHODS

20 open angle glaucoma patients and 20 normal subjects, similar in age and sex distribution, were evaluated. Blood pressure, intraocular pressure, and retrobulbar blood velocity were tested after 30 minutes of sitting and again after 30 minutes of lying. Retrobulbar haemodynamic measures of peak systolic velocity (PSV), end diastolic velocity (EDV), and resistance index (RI) were obtained in the ophthalmic and central retinal arteries using colour Doppler imaging (CDI).

RESULTS

When changing from the upright to supine posture, normal subjects demonstrated a significant increase in OA EDV (p = 0.016) and significant decrease in OA RI (p = 0.0006) and CRA RI (p = 0.016). Glaucoma patients demonstrated similar changes in OA measures of EDV (p = 0.02) and RI (p = 0.04), but no change in CRA measures.

CONCLUSION

Glaucoma patients exhibit faulty autoregulation of central retinal artery blood flow during posture change.

Dorzolamide, visual function and ocular hemodynamics in normal-tension glaucoma

The purpose of this study was to determine how a topical carbonic anhydrase inhibitor, dorzolamide, alters visual function and ocular blood flow in persons with normal-tension glaucoma. Eighteen normal tension glaucoma patients, after washout of other ocular medications, were treated for four weeks with 2% dorzolamide, three times daily. A control group of eleven other normal-tension glaucoma patients received placebo eye drops. Patients were studied before treatment, and after two and four weeks of treatment. Each study included assessment of central visual function (contrast sensitivity), intraocular pressure (IOP), and several aspects of ocular hemodynamics, including measures of retinal arteriovenous passage time, retinal arterial and venous diameters, and flow velocities in the ophthalmic, central retinal, and posterior ciliary arteries. Dorzolamide significantly reduced IOP at two and four weeks (each p<0.01), and at the same time increased contrast sensitivity at both three and six cycles per degree (each p<0.05). Neither of these variables changed significantly in the control group. Dorzolamide also accelerated retinal arteriovenous passage time of fluorescein dye, at constant retinal arterial and venous diameters (p<0.05), but failed to change flow velocities in any retrobulbar vessel. The ability of dorzolamide to improve contrast sensitivity in persons with normal-tension glaucoma may be related to either IOP reduction or altered ocular perfusion.

Vascular aspects in the pathophysiology of glaucomatous optic neuropathy

Glaucoma remains a major eye illness with unknown etiology. Although elevated intraocular pressure is clearly a major risk factor, vascular deficits may contribute to initiation and progression of glaucoma. When intraocular pressure is acutely elevated in healthy individuals, the resistance index (derived from the peak systolic and end-diastolic velocities and an indirect index of vascular resistance distal to the site of measurement) in the central retinal and posterior ciliary arteries increases progressively. This result implies that mechanical and vascular factors may be coupled in such a way that perfusion of the retina and optic nerve head may be influenced by changes in the intraocular pressure. Further, at night, when ophthalmic artery flow velocities fall as arterial blood pressure falls in glaucoma patients, the risk of disease progression may be increased. The constancy of these same flow velocities in age-matched healthy individuals points to a possible vascular autoregulatory defect in glaucoma. In addition, in normal-tension glaucoma, vasodilation (CO2 inhalation) normalizes retrobulbar arterial flow velocities, hinting that some vascular deficits in glaucoma may be reversible. Finally, Ca2+ channel blockade improves contrast sensitivity in patients with normal-tension glaucoma, who also show increased retrobulbar vessel flow velocities, a result suggesting that visual function loss may be linked to ocular ischemia. Emerging evidence points to a role of ischemia in the pathogenesis of glaucoma, suggesting that treatments designed to improve ocular blood flow may benefit glaucoma patients.

Peripapillary retinal blood flow in normal tension glaucoma

AIMS

To determine if normal tension glaucoma (NTG) patients differ from age matched controls in blood flow to the peripapillary retina, as measured with confocal scanning laser Doppler flowmetry (cSLDF; "Heidelberg retinal flowmetry").

METHODS

12 NTG patients and 12 age matched controls were compared using (a) 10 x 10 pixel boxes (the instrument default sample size), taken from the nasal and temporal peripapillary retina, (b) the average from two of these boxes, and (c) every qualifying pixel within the peripapillary retina.

RESULTS

Patients and controls did not differ in blood flow measured using the default sample from a single 10 x 10 pixel box, placed in either the temporal or nasal peripapillary retina, or expressed as the average from these two boxes. However, in histograms using every pixel from the peripapillary retina, NTG patients displayed significantly higher percentages of minimal flow pixels (defined as less than one arbitrary unit of flow: 30% v 19%, p < 0.01), and significantly lower flow in the 25th, 50th, and 75th percentile flow pixel (each p < 0.05) than did age matched controls.

CONCLUSION

NTG is characterised by reduced blood flow in the peripapillary retina, a result suggesting that blood flow deficits accompany, and perhaps may contribute to, disease development in these patients.

Brightness alters Heidelberg retinal flowmeter measurements in an in vitro model

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.

A new system to supply carbon dioxide safely to glaucoma patients

PURPOSE

To develop a new system for safely supplying carbon dioxide (CO2) to open-angle glaucoma patients.

METHODS

The orbital hemodynamics of 7 glaucoma patients were determined by color Doppler imaging under baseline conditions and during CO2 supplementation sufficient to increase the end-tidal CO2 partial pressure by 10%. Systemic conditions, including oxygen saturation and blood pressure, were monitored throughout the CO2 inhalation.

RESULTS

Our results demonstrate that this new system enables us to supply CO2 in a safe, controlled manner to glaucoma patients.

CONCLUSIONS

This new system will be useful for investigating the effects of vasodilation by CO2 on orbital blood flow.

Assessment of human ocular hemodynamics

Vascular abnormality and altered hemodynamics play important roles in many ophthalmic pathologies. Much of our knowledge of ocular hemodynamics was gained from invasive animal research, although a number of noninvasive methods suitable for in vivo use in humans have been developed. Data from these methods now produce a significant literature of their own. Understanding the origins of the data and appreciating their limitations can be difficult. Modern hemodynamic assessment techniques each examine a unique facet of the ocular circulation. No single facet provides a complete description of the hemodynamic state of the eye. These methods have contributed a great deal to our understanding of normal hemodynamics. More importantly, they continue to add to our understanding of altered hemodynamics found in disease. Some have found their way into limited clinical practice. The predominant ocular hemodynamic assessment techniques are reviewed with the aims of introducing the fundamental principles behind each, highlighting their inherent advantages and limitations, highlighting their contributions to understanding ocular physiology, and considering their potential to provide signs for diagnosis.

Heidelberg retinal flowmetry: factors affecting blood flow measurement

AIMS

To evaluate factors affecting Heidelberg retinal flowmeter (HRF) measurements of retinal and optic nerve head blood flow in human subjects.

METHODS

The angle of incidence between laser beam and fundus, and camera distance from the eye, were evaluated for their effect upon measures of blood volume, velocity, and flow in a single 100 x 100 x 400 microns volume of temporal peripapillary retinal tissue in normal volunteers. Both intra and intersession reproducibility of these measures were studied. Intersession data were obtained by taking one image per week for 4 weeks. Finally, the intersession haemodynamic data were examined in the entire image (640 x 2560 x 400 microns), using histograms of pixel by pixel blood flow.

RESULTS

Measures of blood volume, velocity, and flow from a single anatomical site were unaffected by laser beam to fundus angle of incidence (n = 12). As camera distance from the eye was increased (from 2 to 5 to 7 cm), flow measurements showed increasing individual changes, despite unaltered measured vessel lengths and constant overall mean flow (n = 14). The coefficient of variation for two intersession images of optic nerve head blood flow averaged 7% (n = 20); in contrast, the 4 week intersession coefficient of variation averaged 30% (n = 15). Intersession reproducibility was increased by using flow histograms from the entire image: the coefficients of variation averaged 16% for total flow and 17% for flow in the pixel of median flow.

CONCLUSION

HRF measures of flow are independent of the laser beam to fundus angle of the incidence and dependent upon camera distance from the eye. Intersession reproducibility is best using pixel by pixel analysis of the entire image.

Laser Doppler flowmetry measurement of changes in human optic nerve head blood flow in response to blood gas perturbations

PURPOSE

The objective of this study was to establish the ability of laser Doppler flowmetry to detect relative changes in human optic nerve head hemodynamics caused by physiologic blood gas perturbations.

METHODS

Laser Doppler flowmetry permits the noninvasive assessment of relative blood velocity, volume, and flow (flux) in a sample volume of the nerve head. Such measurements were performed in two groups of healthy subjects. The first group (n = 11) was tested during normal room air breathing and then while breathing 100% oxygen (isocapnic hyperoxia). The second group (n = 10) was also tested under normal conditions as well as during isoxic hypercapnia (+ 15% end-tidal carbon dioxide). Results were analyzed by paired t tests.

RESULTS

Hyperoxia created a significant 25% (p = 0.002) decrease in optic nerve head blood flow, with blood volume decreased by 9% (p = 0.095) and blood velocity reduced by 13% (p = 0.154) compared to the room air condition. During hypercapnia, optic nerve head blood flow was increased by 28% (p = 0.012), with blood volume increased by 22% (p = 0.017) and blood velocity increased by 9% (p = 0.218) as compared to the normal room air condition.

CONCLUSION

Blood flow in the optic nerve head capillaries changes in response to hyperoxia and hypercapnia as demonstrated in the brain and retina. Laser Doppler flowmetry permits the noninvasive assessment of these responses in humans under conditions within the physiologic range.