Retinal artery and vein pressures in the dog and their relationship to aortic, intraocular, and cerebrospinal fluid pressures

A machine learning approach in the non-invasive prediction of intracranial pressure using Modified Photoplethysmography

The ideal Intracranial pressure (ICP) estimation method should be accurate, reliable, cost-effective, compact, and associated with minimal morbidity/mortality. To this end several described non-invasive methods in ICP estimation have yielded promising results, however the reliability of these techniques have yet to supersede invasive methods of ICP measurement. Over several publications, we described a novel imaging method of Modified Photoplethysmography in the evaluation of the retinal vascular pulse parameters decomposed in the Fourier domain, which enables computationally efficient information filtering of the retinal vascular pulse wave. We applied this method in a population of 21 subjects undergoing lumbar puncture manometry. A regression model was derived by applying an Extreme Gradient Boost (XGB) machine learning algorithm using retinal vascular pulse harmonic regression waveform amplitude (HRWa), first and second harmonic cosine and sine coefficients (an1,2, bn1,2) among other features. Gain and SHapley Additive exPlanation (SHAP) values ranked feature importance in the model. Agreement between the predicted ICP mean, median and peak density with measured ICP was assessed using Bland-Altman bias±standard error. Feature gain of intraocular pressure (IOPi) (arterial = 0.6092, venous = 0.5476), and of the Fourier coefficients, an1 (arterial = 0.1000, venous = 0.1024) ranked highest in the XGB model for both vascular systems. The arterial model SHAP values demonstrated the importance of the laterality of the tested eye (1.2477), which was less prominent in the venous model (0.8710). External validation was achieved using seven hold-out test cases, where the median venous predicted ICP showed better agreement with measured ICP. Although the Bland-Altman bias from the venous model (0.034±1.8013 cm water (p<0.99)) was lower compared to that of the arterial model (0.139±1.6545 cm water (p<0.94)), the arterial model provided a potential avenue for internal validation of the prediction. This approach can potentially be integrated into a neurological clinical decision algorithm to evaluate the indication for lumbar puncture.

Linear interactions between intraocular, intracranial pressure, and retinal vascular pulse amplitude in the fourier domain

Purpose

To compare the retinal vascular pulsatile characteristics in subjects with normal (ICP_n) and high (ICP_h) intracranial pressure and quantify the interactions between intraocular pressure, intracranial pressure, and retinal vascular pulse amplitude in the Fourier domain.

Materials and methods

Twenty-one subjects were examined using modified photoplethysmography with simultaneous ophthalmodynamometry. A harmonic regression model was fitted to each pixel in the time-series, and used to quantify the retinal vascular pulse wave parameters including the harmonic regression wave amplitude (HRW_a). The pulse wave attenuation was measured under different ranges of induced intraocular pressure (IOP_i), as a function of distance along the vessel (V_Dist). Intracranial pressure (ICP) was measured using lumbar puncture. A linear mixed-effects model was used to estimate the correlations between the Yeo-Johnson transformed harmonic regression wave amplitude (HRW_a-YJt) with the predictors (IOP_i, V_Dist and ICP). A comparison of the model coefficients was done by calculating the weighted Beta (β_x) coefficients.

Results

The median HRW_a in the ICP_n group was higher in the retinal veins (4.563, interquartile range (IQR) = 3.656) compared to the retinal arteries (3.475, IQR = 2.458), p<0.0001. In contrast, the ICP_h group demonstrated a reduction in the median venous HRW_a (3.655, IQR = 3.223) and an elevation in the median arterial HRW_a (3.616, IQR = 2.715), p<0.0001. Interactions of the pulsation amplitude with ICP showed a significant disordinal interaction and the loss of a main effect of the Fourier sine coefficient (b_n1) in the ICP_h group, suggesting that this coefficient reflects the retinal vascular response to ICP wave. The linear mixed-effects model (LME) showed the decay in the venous (HRW_a-YJt) was almost twice that in the retinal arteries (−0.067±0.002 compared to −0.028±0.0021 respectively, p<0.00001). The overall interaction models had a total explanatory power of (conditional R²) 38.7%, and 42% of which the fixed effects explained 8.8%, and 5.8% of the variance (marginal R²) for the venous and arterial models respectively. A comparison of the damping effect of V_Dist and ICP showed that ICP had less influence on pulse decay than distance in the retinal arteries (β_ICP = -0.21, se = ±0.017 compared to βVDist=-0.26, se = ±0.019), whereas the mean value was equal for the retinal veins (venous βVDist=-0.42, se = ±0.015, β_ICP = -0.42, se = ±0.019).

Conclusion

The retinal vascular pulsation characteristics in the ICP_h group showed high retinal arterial and low venous pulsation amplitudes. Interactions between retinal vascular pulsation amplitude and ICP suggest that the Fourier sine coefficient b_n1 reflects the retinal vascular response to the ICP wave. Although a matrix of regression lines showed high linear characteristics, the low model explanatory power precludes its use as a predictor of ICP. These results may guide future predictive modelling in non-invasive estimation of ICP using modified photoplethysmography.

Retinal arterial pulsation as an indicator of raised intraocular pressure

Ophthalmic emergencies are invariably challenging for the non-specialist to identify and evaluate, and may be complicated by occult but vision threatening raised intraocular pressure. We present a case of hypertensive uveitis accompanied by the finding of retinal arterial pulsation, which when visualised by direct ophthalmoscopy allows the non-specialist to identify significantly raised intraocular pressure requiring urgent evaluation by an ophthalmologist.

Downregulation of Retinal Connexin 43 in GFAP-Expressing Cells Modifies Vasoreactivity Induced by Perfusion Ocular Pressure Changes

Purpose

Glia and their communication via connexin 43 (Cx43) gap junctions are known to mediate neurovascular coupling, a process driven by metabolic demand. However, it is unclear whether Cx43 mediated glial communication intermediates classical autoregulation. Here we used viral transfection and a glial fibrillary acidic protein (GFAP) promoter to downregulate glial Cx43 to evaluate its role in retinal vascular autoregulation to ocular perfusion pressure (OPP) reduction.

Methods

Adult rats were intravitreally injected with the viral active construct or a control. Three weeks after the injection, eyes were imaged using confocal scanning laser ophthalmoscopy before and during a period of OPP decrease induced by blood draw to lower blood pressure or by manometric IOP elevation. Vessel diameter responses to the OPP decrease were compared between Cx43-downregulated and control-injected eyes. The extent of Cx43 downregulation was evaluated by Western blot and immunohistochemistry.

Results

In control eyes, the OPP decrease induced dilatation of arterioles, but not venules. In Cx43-downregulated eyes, Cx43 expression in whole retina was decreased by approximately 40%. In these eyes, the resting diameter of the venules increased significantly, but there was no effect on arterioles. In Cx43-downregulated eyes, vasoreactivity evoked by blood pressure lowering was significantly compromised in both arterioles (P = 0.005) and venules (P = 0.001). Cx43 downregulation did not affect the arteriole responses to IOP elevation, whereas the responses of the venules showed a significantly greater decrease in diameter (P < 0.001).

Conclusions

The downregulation of retinal Cx43 in GFAP–expressing cells compromises vasoreactivity of both arterioles and venules in response to an OPP decrease achieved via blood pressure lowering or IOP elevation. The results also suggest that Cx43-mediated glial communication actively regulates resting venular diameter.

Response of the Trilaminar Retinal Vessel Network to Intraocular Pressure Elevation in Rat Eyes

Purpose

The purpose of this study was to test the hypothesis that the superficial, intermediate, and deep retinal vascular plexus show different responses to intraocular pressure (IOP) elevation.

Methods

Anesthetized adult Long Evans rats (n = 14) were imaged using optical coherence tomography angiography (OCTA; Spectralis) at baseline (IOP 10 mm Hg) and in follow-up mode to examine the vasculature during IOP elevation (10 to 110 mm Hg, 10 mm Hg steps, each step 3 minutes). A 20° × 10° field was imaged. Vessel density within a 2D projection image was determined (%) for the superficial vascular complex (SVC), intermediate capillary plexus (ICP), and deep capillary plexus (DCP). Comparisons were made between layers using 2-way repeated measures ANOVA (layer versus IOP) following normalization to baseline (% relative to 10 mm Hg).

Results

The three vascular layers responded differently to IOP elevation. For IOPs between 40 and 60 mm Hg, DCP and ICP capillaries were significantly more resistant to IOP elevation than those in the SVC. When IOP was elevated above 70 mm Hg, all layers showed reduced vessel density. IOP induced change in SVC vessel density closely followed reductions in thickness of the inner retinal layers (nerve fiber, ganglion cell, and inner plexiform layer). This close relationship between reductions in tissue thickness and vessel density was less apparent for the ICP and DCP.

Conclusions

These data show that the intermediate and deep vascular plexus in the rat retina have a greater capacity for autoregulation against mild IOP elevation but are more affected at high IOP.

Objective Quantification of Spontaneous Retinal Venous Pulsations Using a Novel Tablet-Based Ophthalmoscope

Purpose

Dynamic assessment of retinal vascular characteristics can aid in identifying glaucoma-specific biomarkers. More specifically, a loss of spontaneous retinal venous pulsations (SVPs) has been reported in glaucoma, but a lack of readily available tools has limited the ability to explore the full potential of SVP analysis in glaucoma assessment. Advancements in smart technology have paved the way for the development of portable, noninvasive, and inexpensive imaging modalities. By combining off-the-shelf optical elements and smart devices, the current study aims to determine whether SVPs can be detected and quantified using a novel tablet-based ophthalmoscope in glaucoma and glaucoma suspects.

Methods

Thirty patients, including 21 with confirmed glaucoma (9 men; average age 75 ± 8 years) and 9 glaucoma suspects (5 men; average age 64 ± 9 years), were studied. All patients had intraocular pressure measurements, Humphrey visual field assessment, optical coherence tomography, and a 10-second videoscopy of the retinal circulation. The retinal vasculature recordings (46° field of view at 30 frames per second) were analyzed to extract SVP amplitudes.

Results

SVPs were detected and quantified in 100% of patients with glaucoma and those with suspected glaucoma using the novel device. The average SVP amplitudes in glaucoma and glaucoma suspects were 42.6% ± 10.7% and 34% ± 6.7%, respectively.

Conclusions

Our results suggest that a novel tablet-based ophthalmoscope can aid in documenting and objectively quantifying SVPs in all patients.

Translational Relevance

Outcomes of this study provide an innovative, portable, noninvasive, and inexpensive solution for objective assessment of SVPs, which may have clinical relevance in glaucoma screening.

Pulse Pressure and Diabetic Eye Disease

See Article Yamamoto et al

Retinal capillary perfusion: Spatial and temporal heterogeneity

The central role of the cardiovascular system is to maintain adequate capillary perfusion. The spatially and temporally heterogeneous nature of capillary perfusion has been reported in some organs. However, such heterogeneous perfusion properties have not been sufficiently explored in the retina. Arguably, spatial and temporal heterogeneity of capillary perfusion could be more predominant in the retina than that in other organs. This is because the retina is one of the highest metabolic demand neural tissues yet it has a limited blood supply due to optical requirements. In addition, the unique heterogeneous distribution of retinal neural cells within different layers and regions, and the significant heterogeneity of intraretinal oxygen distribution and consumption add to the complexity. Retinal blood flow distribution must match consumption of nutrients such as oxygen and glucose within the retina at the cellular level in order to effectively maintain cell survival and function. Sophisticated local blood flow control in the microcirculation is likely required to control the retinal capillary perfusion to supply local retinal tissue and accommodate temporal and spatial variations in metabolic supply and demand. The authors would like to update the knowledge of the retinal microvessel and capillary network and retinal oxidative metabolism from their own studies and the work of others. The coupling between blood supply and energy demands in the retina is particularly interesting. We will mostly describe information regarding the retinal microvessel network and retinal oxidative metabolism relevant to the spatial and temporal heterogeneity of capillary perfusion. We believe that there is significant and necessary spatial and temporal heterogeneity and active regulation of retinal blood flow in the retina, particularly in the macular region. Recently, retinal optical coherence tomography angiography (OCTA) has been widely used in ophthalmology, both experimentally and clinically. OCTA could be a valuable tool for examining retinal microvessel and capillary network structurally and has potential for determining retinal capillary perfusion and its control. We have demonstrated spatial and temporal heterogeneity of capillary perfusion in the retina both experimentally and clinically. We have also found close relationships between the smallest arterioles and capillaries within paired arterioles and venules and determined the distribution of smooth muscle cell contraction proteins in these vessels. Spatial and temporal heterogeneity of retinal capillary perfusion could be a useful parameter to determine retinal microvessel regulatory capability as an early assay for retinal vascular diseases. This topic will be of great interest, not only for the eye but also other organs. The retina could be the best model for such investigations. Unlike cerebral vessels, retinal vessels can be seen even at the capillary level. The purpose of this manuscript is to share our current understanding with the readers and encourage more researchers and clinicians to investigate this field. We begin by reviewing the general principles of microcirculation properties and the spatial and temporal heterogeneity of the capillary perfusion in other organs, before considering the special requirements of the retina. The local heterogeneity of oxygen supply and demand in the retina and the need to have a limited and well-regulated retinal circulation to preserve the transparency of the retina is discussed. We then consider how such a delicate balance of metabolic supply and consumption is achieved. Finally we discuss how new imaging methodologies such as optical coherence tomography angiography may be able to detect the presence of spatial and temporal heterogeneity of capillary perfusion in a clinical setting. We also provide some new information of the control role of very small arterioles in the modulation of retinal capillary perfusion which could be an interesting topic for further investigation.

Spaceflight-Induced Intracranial Hypertension and Visual Impairment: Pathophysiology and Countermeasures

Visual impairment intracranial pressure (VIIP) syndrome is considered an unexplained major risk for future long-duration spaceflight. NASA recently redefined this syndrome as Spaceflight-Associated Neuro-ocular Syndrome (SANS). Evidence thus reviewed supports that chronic, mildly elevated intracranial pressure (ICP) in space (as opposed to more variable ICP with posture and activity on Earth) is largely accounted for by loss of hydrostatic pressures and altered hemodynamics in the intracranial circulation and the cerebrospinal fluid system. In space, an elevated pressure gradient across the lamina cribrosa, caused by a chronic but mildly elevated ICP, likely elicits adaptations of multiple structures and fluid systems in the eye which manifest themselves as the VIIP syndrome. A chronic mismatch between ICP and intraocular pressure (IOP) in space may acclimate the optic nerve head, lamina cribrosa, and optic nerve subarachnoid space to a condition that is maladaptive to Earth, all contributing to the pathogenesis of space VIIP syndrome. Relevant findings help to evaluate whether artificial gravity is an appropriate countermeasure to prevent this seemingly adverse effect of long-duration spaceflight.

Phase and amplitude of spontaneous retinal vein pulsations: An extended constant inflow and variable outflow model

The constant inflow and variable outflow (CIVO) theory correctly predicts that spontaneous pulsation of the retinal veins will be visible close to the point where the vein exits the eye at the lamina cribrosa but will decrease rapidly in amplitude and become too small to see only a short distance upstream. However, the phase of vein oscillation relative to the oscillation of the intraocular pressure (IOP) predicted by CIVO has been unclear and controversial. We show that the CIVO model is indeterminate in predicting such phase relations. We propose a simple extension of the CIVO model that retains its basic equations but applies them to a larger domain that includes not only the intraocular (pre-laminar) portion of the vein but also the retrobulbar (post-laminar) portion of the vein behind the eye. We show that this extended CIVO model makes definite predictions about the phase of vein oscillation relative to the oscillation of IOP. This phase relationship is determined by the relative amplitude and phase of pulsations of the IOP and of the cerebrospinal fluid pressure (CSFP). If IOP and CSFP oscillate in phase, then the pre-laminar vein oscillates in phase with IOP when the amplitude of CSFP exceeds the amplitude of IOP but oscillates in counter phase with IOP when the amplitude of IOP exceeds that of CSFP. These relationships are modified when there is a phase difference between the oscillations of IOP and CSFP. When CSFP leads IOP, the phase of vein oscillation is advanced if the amplitude of CSFP exceeds that of IOP and is delayed if the amplitude of IOP exceeds that of CSFP. The result in each case is that maximum vein size occurs during the rising phase of IOP (ocular systole). We conclude that the driving force of vein oscillation is the difference between the oscillations of IOP and CSFP. The phase of this difference determines the phase relationships above. We show that additional delays in the phase of venous pulsation relative to that of IOP are induced by constriction of the vein within the lamina cribrosa and by recording the vein pulsations upstream from the lamina cribrosa. The amplitude of vein oscillation is proportional to the amplitude of the driving force and to the venous capacitance. Loss of spontaneous retinal vein pulsation with increase in mean CSFP is determined primarily by reduced venous capacitance. Increased amplitude of pulsation may occur when IOP is increased. It is the result of increased venous capacitance and possibly increased driving force of the pulsation. However, in chronic glaucoma the increase in capacitance may be counteracted by venous outflow obstruction, and the increase in driving force may be counteracted by reduced ocular blood flow. As a result retinal vein oscillation may be reduced in amplitude.

Central retinal artery pressure and carotid artery stenosis

The central retinal artery (CRA), which can be non-invasively examined with ophthalmoscopy, may be regarded as an extracranial part of the cerebrovascular system. Assessment of CRA pressure may be of help in assessing the impediment of the intracranial blood circulation in patients with a carotid artery stenosis (CAS). The aim of this study was to explore the potential associations between diastolic central retinal artery pressure (diastCRAP) and CAS. The prospective longitudinal clinical observational study included patients with CAS and a control group without CAS. diastCRAP was assessed using ophthalmodynamometry. The study group consisted of 95 patients with CAS (50 of whom had >75%CAS and underwent surgery; the surgical study group) and a control group of 64 individuals without CAS. In all study participants, a lower diastCRAP was significantly associated with a higher degree of CAS (P<0.001). Multivariate analysis indicated that a higher CAS degree was significantly (correlation coefficient: r=0.75) associated with a higher brachial diastolic blood pressure (P<0.001) and lower diastCRAP (P<0.001). Within the surgical study group at the baseline of the study, diastCRAP was significantly lower at the surgical side than at the contralateral side (P=0.02). The diastCRAP on the surgical side increased significantly (P<0.001) after surgery. In the surgical study group at baseline, diastCRAP on the surgical side was not significantly associated with brachial diastolic blood pressure (P=0.22), whereas after surgery, diastCRAP was significantly associated with brachial diastolic blood pressure (P=0.001). DiastCRAP was found to be significantly and linearly correlated with the degree of CAS in intra-individual inter-eye, inter-individual and intra-individual follow-up comparisons. The strong and linear association between diastCRAP and the degree of CAS suggest that diastCRAP should be explored further for use as an indicator of cerebrovascular status.

Retinal venous pressure: the role of endothelin

The retinal venous pressure (RVP) can be measured non-invasively. While RVP is equal to or slightly above intraocular pressure (IOP) in healthy people, it is often markedly increased in patients with eye or systemic diseases. Beside a mechanical obstruction, the main cause of such an elevation is a local dysregulation of a retinal vein, particularly a constriction induced by endothelin-1 (ET-1). A local increase of ET-1 can result from a high plasma level, as ET-1 can diffuse from the fenestrated capillaries of the choroid into the optic nerve head (ONH), bypassing the blood retinal barrier. A local increase can also result from increased local production either by a sick neighboring artery or retinal tissue. Generally, the main factors increasing ET-1 are inflammations and hypoxia, either locally or in a remote organ. RVP is known to be increased in patients with glaucoma, retinal vein occlusion (RVO), diabetic retinopathy, high mountain disease, and primary vascular dysregulation (PVD). PVD is the major vascular component of Flammer syndrome (FS). An increase of RVP decreases perfusion pressure, which heightens the risk for hypoxia. An increase of RVP also elevates transmural pressure, which in turn heightens the risk for retinal edema. In patients with RVO, a high level of RVP may not only be a consequence but also a potential cause of the occlusion; therefore, it risks causing a vicious circle. Narrow retinal arteries and particularly dilated retinal veins are known risk indicators for future cardiovascular events. As the major cause for such a retinal venous dilatation is an increased RVP, RVP may likely turn out to be an even stronger predictor.

Correlation of retinal nerve fibre layer thickness and spontaneous retinal venous pulsations in glaucoma and normal controls

Purpose

To study the relationship between amplitude of spontaneous retinal venous pulsatility (SRVP) and retinal nerve fibre layer (RNFL) thickness in glaucomatous eyes, and to determine if this parameter may be a potential marker for glaucoma severity.

Method

85 subjects including 50 glaucoma (21 males, 67±10 yrs) and 35 normals (16 males, 62±11 yrs) were studied. SRVP amplitude was measured using the Dynamic Vessel Analyser (DVA, Imedos, Germany) at four regions of the retina simultaneously within one disc diameter from the optic disc—temporal-superior (TS), nasal-superior (NS), temporal-inferior (TI) and nasal-inferior (NI)). This was followed by RNFL thickness measurement using spectral domain optical coherence tomography (Spectralis OCT). The correlation between SRVP amplitude and corresponding sectoral RNFL thickness was assessed by means of non-linear regression (i.e. logarithmic). Linear regression was also applied and slopes were compared using analysis of covariance (ANCOVA).

Results

Greater SRVP amplitude was associated with thicker RNFL. Global SRVP amplitude was significantly lower in glaucoma eyes compared with normals (p<0.0001). The correlation coefficient of the linear regression between RNFL and SRVP at TS, NS, TI and NI quadrants in the glaucoma group were r = 0.5, 0.5, 0.48, 0.62. Mean SRVP amplitude and RNFL thickness for TS, NS, TI and NI quadrants were 4.3±1.5, 3.5±1.3, 4.7±1.6, 3.1±1 μm and 96±30, 75±22, 89±35 and 88±30 μm, respectively. The ANCOVA test showed that the slope of linear regression between the four quadrants was not significant (p>0.05). Since the slopes are not significantly different, it is possible to calculate one slope for all the data. The pooled slope equals 10.8 (i.e. RNFL = 10.8SRVP+41).

Conclusion

While SRVP was present and measurable in all individuals, the amplitude of SRVP is reduced in glaucoma with increasing RNFL loss. Our findings suggest the degree of SRVP may be an additional marker for glaucoma severity. Further studies are needed to determine the mechanism of reduction in SRVP, and whether changes can predict increased risk of progression.

Optic nerve head blood flow response to reduced ocular perfusion pressure by alteration of either the blood pressure or intraocular pressure

PURPOSE

To test the hypothesis that blood flow autoregulation in the optic nerve head has less reserve to maintain normal blood flow in the face of blood pressure-induced ocular perfusion pressure decrease than a similar magnitude intraocular pressure-induced ocular perfusion pressure decrease.

MATERIALS AND METHODS

Twelve normal non-human primates were anesthetized by continuous intravenous infusion of pentobarbital. Optic nerve blood flow was monitored by laser speckle flowgraphy. In the first group of animals (n = 6), the experimental eye intraocular pressure was maintained at 10 mmHg using a saline reservoir connected to the anterior chamber. The blood pressure was gradually reduced by a slow injection of pentobarbital. In the second group (n = 6), the intraocular pressure was slowly increased from 10 mmHg to 50 mmHg by raising the reservoir. In both experimental groups, optic nerve head blood flow was measured continuously. The blood pressure and intraocular pressure were simultaneously recorded in all experiments.

RESULTS

The optic nerve head blood flow showed significant difference between the two groups (p = 0.021, repeat measures analysis of variance). It declined significantly more in the blood pressure group compared to the intraocular pressure group when the ocular perfusion pressure was reduced to 35 mmHg (p < 0.045) and below. There was also a significant interaction between blood flow changes and the ocular perfusion pressure treatment (p = 0.004, adjusted Greenhouse & Geisser univariate test), indicating the gradually enlarged blood flow difference between the two groups was due to the ocular perfusion pressure decrease.

CONCLUSIONS

The results show that optic nerve head blood flow is more susceptible to an ocular perfusion pressure decrease induced by lowering the blood pressure compared with that induced by increasing the intraocular pressure. This blood flow autoregulation capacity vulnerability to low blood pressure may provide experimental evidence related to the hemodynamic pathophysiology in glaucoma.

Non-invasive cerebrospinal fluid pressure estimation using multi-layer perceptron neural networks

Cerebrospinal fluid pressure (CSFp) provides vital information in various neurological abnormalities including hydrocephalus, intracranial hypertension and brain tumors. Currently, CSFp is measured invasively through implanted catheters within the brain (ventricles and parenchyma) which is associated with a risk of infection and morbidity. In humans, the cerebrospinal fluid communicates indirectly with the ocular circulation across the lamina cribrosa via the optic nerve subarachnoid space. It has been shown that a relationship between retinal venous pulsation, intraocular pressure (IOP) and CSFp exists with the amplitude of retinal venous pulsation being associated with the trans-laminar pressure gradient (i.e. IOP-CSFp). In this study we use this characteristic to develop a non-invasive approach to estimate CSFp. 15 subjects were included in this study. Dynamic retinal venous diameter changes and IOP were measured and fitted into our model. Artificial neural networks (ANN) were applied to construct a relationship between retinal venous pulsation amplitude, IOP (input) and CSFp (output) and develop an algorithm to estimate CSFp based on these parameters. Results show a mean square error of 2.4 mmHg and 1.27 mmHg for train and test data respectively. There was no significant difference between experimental and ANN estimated CSFp values (p>0.01).This study suggests measurement of retinal venous pulsatility in conjunction with IOP may provide a novel approach to estimate CSFp non-invasively.

Retinal oximetry

ABSTRACT.:

PURPOSE

Malfunction of retinal blood flow or oxygenation is believed to be involved in various diseases. Among them are retinal vessel occlusions, diabetic retinopathy and glaucoma. Reliable, non-invasive technology for retinal oxygen measurements has been scarce and most of the knowledge on retinal oxygenation comes from animal studies. This thesis describes human retinal oximetry, performed with novel retinal oximetry technology. The thesis describes studies on retinal vessel oxygen saturation in (1) light and dark in healthy volunteers, (2) central retinal vein occlusion, (3) branch retinal vein occlusion, (4) central retinal artery occlusion, (5) diabetic retinopathy, (6) patients undergoing glaucoma surgery and (7) patients taking glaucoma medication.

METHODS

The retinal oximeter (Oxymap ehf., Reykjavik, Iceland) is based on a fundus camera. An attached image splitter allows the simultaneous capture of four images of the same area of the fundus. Two images are used for further analysis, one acquired with 586 nm light and one with 605 nm light. Light absorbance of retinal vessels is sensitive to oxygen saturation at 605 nm but not at 586 nm. Measurement of reflected light at these wavelengths allows estimation of oxygen saturation in the main retinal vessels. This is performed with custom-made analysis software.

RESULTS

LIGHT AND DARK: After 30 min in the dark, oxygen saturation in retinal arterioles of healthy volunteers was 92 ± 4% (mean ± SD, n = 15). After 5 min in 80 cd/m(2) light, the arteriolar saturation was 89 ± 5%. The decrease was statistically significant (p = 0.008). The corresponding values for retinal venules were 60 ± 5% in the dark and 55 ± 10% in the light (p = 0.020). Similar results were found after alternating 5 min periods of darkness and light. In a second experiment (n = 19), a significant decrease in retinal vessel oxygen saturation was found in 100 cd/m(2) light compared with darkness but 1 and 10 cd/m(2) light had no significant effect. CENTRAL RETINAL VEIN OCCLUSION: In patients with central retinal vein occlusion, the mean saturation in affected retinal venules was 49 ± 12%, while the mean value for venules in the fellow eye was 65 ± 6% (mean ± SD, p = 0.003, n = 8). The retinal arteriolar saturation was the same in affected (99 ± 3%) and the unaffected (99 ± 6%) eyes. The venous oxygen saturation showed much variation between affected eyes. BRANCH RETINAL VEIN OCCLUSION: Median oxygen saturation in venules affected by branch retinal vein occlusion was 59% (range, 12-93%, n = 22), while it was 63% (23-80%) in unaffected venules in the affected eye and 55% (39-80%) in venules in the fellow eye. The difference was not statistically significant (p > 0.05). There was a significant difference between affected arterioles (median 101%; range, 89-115%) and unaffected arterioles (95%, 85-104%) in the affected eye (p < 0.05, n = 18). CENTRAL RETINAL ARTERY OCCLUSION: In a patient with a day's history of central retinal artery occlusion due to temporal arteritis, the mean arteriolar saturation was 71 ± 9% and 63 ± 9% in the venules. One month later, after treatment with prednisolone, the mean arteriolar saturation was 100 ± 4% and the venous saturation 54 ± 5%. DIABETIC RETINOPATHY: When compared with healthy volunteers (n = 31), patients with all categories of diabetic retinopathy had on average 7-10 percentage points higher saturation in retinal arterioles (p < 0.05 for all categories, n = 6-8 in each category). In venules, the saturation was 8-12 percentage points higher (p < 0.05 for all categories). GLAUCOMA SURGERY: Oxygen saturation in retinal arterioles increased by 2 percentage points on average (p = 0.046, n = 19) with surgery, which lowered intraocular pressure from 23 ± 7 mmHg (mean ± SD) to 10 ± 4 mmHg (p < 0.0001). No other significant changes were found (p ≥ 0.35). DORZOLAMIDE: A significant reduction of 3 percentage points was found in arterioles (p < 0.01) and venules (p < 0.05) when patients with glaucoma or ocular hypertension changed from dorzolamide-timolol combination eye drops to timolol alone (n = 6). No change was found in patients, who started on timolol and switched to the combination therapy (p > 0.05, n = 7).

CONCLUSIONS

Dual wavelength oximetry can be used to non-invasively measure retinal vessel oxygen saturation in health and disease. The results indicate that retinal vessel oxygen saturation is (1) increased in the dark, (2) lower in venules affected by central retinal vein occlusions, (3) variable in branch retinal vein occlusion, (4) lower in retinal arterioles in central retinal artery occlusion, (5) increased in diabetic retinopathy, (6-7) mildly affected by glaucoma surgery or dorzolamide.

Hemodynamic interactions in the eye: a review

The ocular circulation provides readily visible information about the state of the systemic circulation, as well as being potentially of relevance to the pathogenesis of ocular disorders such as glaucoma. The interaction between intraocular pressure, retinal vessels and cerebrospinal fluid pressure located at the retrolaminar portion of the eye has been of great interest for both ophthalmic and neurological clinicians and researchers. Understanding the relationship between these physiological parameters can explain phenomena such as spontaneous retinal venous pulsatility, and characterize the effects of the translaminar pressure gradient. It may be feasible to use measurable changes in venous pulsatility to enhance clinical assessment in different diseases. In this article we review recent findings on ocular hemodynamics and the relevance of these parameters in the diagnosis of ophthalmic and neurological diseases.

Role of cerebrospinal fluid pressure in the pathogenesis of glaucoma

The pathogenesis of normal (intraocular) pressure glaucoma has remained unclear so far. As hospital-based studies showed an association of normal-pressure glaucoma with low systemic blood pressure, particularly at night, and with vasospastic symptoms, it has been hypothesized that a vascular factor may play a primary role in the pathogenesis of normal-pressure glaucoma. That assumption may, however, be contradicted by the morphology of the optic nerve head. Eyes with normal-pressure glaucoma and glaucomatous eyes with high-intraocular pressure can show a strikingly similar appearance of the optic nerve head, including a loss of neuroretinal rim, a deepening of the optic cup, and an enlargement of parapapillary atrophy. These features, however, are not found in any (other) vascular optic neuropathy, with the exception of an enlargement and deepening of the optic cup in arteritic anterior ischaemic optic neuropathy. One may additionally take into account (i) that it is the trans-lamina cribrosa pressure difference (and not the trans-corneal pressure difference, i.e. the so-called intraocular pressure) which is of importance for the physiology and pathophysiology of the optic nerve head; (ii) that studies have shown that the anatomy of the optic nerve head including the intraocular pressure, the anatomy and biomechanics of the lamina cribrosa and peripapillary sclera, retrobulbar orbital cerebrospinal fluid pressure and the retrobulbar optic nerve tissue pressure may be of importance for the pathogenesis of the highly myopic type of chronic open-angle glaucoma; (iii) that studies have suggested a physiological association between the pressure in all three fluid filled compartments, i.e. the systemic arterial blood pressure, the cerebrospinal fluid pressure and the intraocular pressure; (iv) that an experimental investigation suggested that a low cerebrospinal fluid pressure may play a role in the pathogenesis of normal (intraocular) pressure glaucoma; and (v) that recent clinical studies reported that patients with normal (intraocular) pressure glaucoma had significantly lower cerebrospinal fluid pressure and a higher trans-lamina cribrosa pressure difference when compared to normal subjects. One may, therefore, postulate that a low cerebrospinal fluid pressure may be associated with normal (intraocular) pressure glaucoma. A low systemic blood pressure, particularly at night, could physiologically be associated with a low cerebrospinal fluid pressure, which leads to an abnormally high trans-lamina cribrosa pressure difference and as such to a similar situation as if the cerebrospinal fluid pressure is normal and the intraocular pressure is elevated. This model could explain why patients with normal (intraocular) pressure glaucoma tend to have a low systemic blood pressure, and why eyes with normal (intraocular) pressure glaucoma and eyes with high-pressure glaucoma, in contrast to eyes with a direct vascular optic neuropathy, show profound similarities in the appearance of the optic nerve head.

Effect of acute postural variation on diabetic macular oedema

PURPOSE

This study aimed to study the pathophysiology of diabetic macular oedema (DMO) by analysis of concomitant changes in macular volume (MV), mean arterial blood pressure (MABP), intraocular pressure (IOP), and retinal artery and vein diameters in response to acute postural changes in patients with DMO and healthy subjects.

METHODS

Thirteen patients with DMO (13 eyes) and five healthy subjects (five eyes) were examined after resting in a chair for 15 mins using optical coherence tomography to measure MV and fundus photography to assess retinal vessel diameters. The patients then lay down for 60 mins, during which they were examined repeatedly before they were reseated and examined again. Intraocular pressure was measured using pulse-air tonometry, arterial blood pressure by sphygomanometry and fluid columns using rulers and a spirit level.

RESULTS

In healthy subjects, retinal artery (p = 0.02) and vein (p = 0.001) diameters decreased when subjects lay down, whereas MV remained stable. In patients with DMO, no orthostatic variation in retinal vessel diameters could be demonstrated, whereas MV had increased by 2.4 +/- 0.6% (mean +/- standard error of the mean; p = 0.006) 50 mins after assuming a recumbent position. In both healthy subjects and DMO patients, MABP decreased and IOP increased in a recumbent position, with no significant difference between the groups.

CONCLUSIONS

The increase in MV that occurs in DMO when changing from a seated to a recumbent position is associated with a failure of retinal artery contraction, a response seen in healthy subjects that appears to counter-regulate the increase in ocular perfusion pressure caused by assuming a recumbent position.

Ophthalmodynamometric determination of the central retinal vessel collapse pressure correlated with systemic blood pressure

AIMS

To evaluate whether determination of the central retinal artery and vein collapse pressure correlate with systemic blood pressure measurements, using a new Goldmann contact lens associated ophthalmodynamometric device

METHODS

The prospective clinical study included 92 eyes of 92 patients presenting with cataract or refractive problems (n = 40; control study group) or with retinal and orbital pathologies (n = 52). With topical anaesthesia, a Goldmann contact lens fitted with a pressure sensor in its holding ring was placed onto the cornea. Pressure was asserted onto the globe by pressing the contact lens, and the pressure value at the time when the central retinal artery and vein started pulsating were noted as central retinal artery and vein collapse pressure. Additionally, the brachial arterial blood pressure was measured.

RESULTS

In the control study group, central retinal artery collapse pressure was highly significantly correlated with diastolic blood pressure (correlation coefficient r = 0.77; p<0.001) and systolic blood pressure (r = 0.35; p = 0.03). Central retinal vein collapse pressure was statistically independent of diastolic blood pressure (p = 0.11). In eyes with retinal or orbital diseases, the correlation coefficients were lower than in the control study group. In eyes with retinal arterial occlusions, central retinal vessel collapse pressure measurements were not correlated with arterial blood pressure measurements.

CONCLUSIONS

Depending on coexisting retinal or orbital diseases, ophthalmodynamometric estimation of the central retinal artery collapse pressure, performed during a routine Goldmann contact lens ophthalmoscopy, correlates with systemic blood pressure measurements.

Retinal hemorrhage asymmetry in inflicted head injury: a clue to pathogenesis?

OBJECTIVE

To determine whether regional cerebral parenchymal injury patterns correlate with the distribution of retinal hemorrhages after inflicted head injury.

STUDY DESIGN

Retrospective case series of funduscopic photographs and serial computerized tomographic imaging of 14 children with confirmed inflicted head injury.

MAIN OUTCOME MEASURES

Retinal Hemorrhage Score per eye and per subject, visual field examination, regional patterns of parenchymal injury on computerized tomographic scans and necropsy, and retinal/optic nerve sheath hemorrhage distribution at necropsy.

RESULTS

Ten of 14 children had retinal hemorrhages (71%); 90% were asymmetric (mean retinal score, 4.89 vs 2.56; P=.006). Retinal hemorrhages were maximal on the side of greatest cerebral injury in seven of 10 children initially. Subsequent imaging asymmetry predicted retinal hemorrhage distribution in all eight survivors. Children's Coma Scores, apnea or cardiorespiratory arrest, initial hemoglobin, and plasma glucose concentration did not predict laterality. Asymmetry was greatest if dilated ophthalmoscopy was performed during the first 24 hours (P=.03). Visual outcome was poor; three had homonymous hemianopia and four had cortical visual loss, all correlating with parenchymal atrophy patterns.

CONCLUSION

The distribution of retinal hemorrhages after inflicted head injury correlates with acute and evolving regional cerebral parenchymal injury patterns.

Reproducibility of ophthalmodynamometric measurements of central retinal artery and vein collapse pressure

BACKGROUND

To assess the reproducibility of ophthalmodynamometric measurements using a new, Goldmann contact lens associated, device allowing biomicroscopic visualisation of the optic disc.

METHODS

The prospective clinical study included 87 eyes of 58 subjects presenting with a normal fundus (n=40), or ocular diseases (n=47). With topical anaesthesia, a Goldmann contact lens, fitted with a pressure sensor mounted into the holding ring of the contact lens, was placed onto the cornea. Pressure was applied onto the globe through the contact lens, and the pressure values obtained when the central retinal vessels started pulsating were noted. The measurements were performed 10 times.

RESULTS

The mean coefficients of variation for redeterminations of the collapse pressure of the central retinal vein and artery were 16.3% (SD 11.4%), and 8.5% (4.1%), respectively.

CONCLUSIONS

A simple and new, Goldmann contact lens associated, ophthalmodynamometer allows central retinal artery and vein collapse pressure measurements which are reproducible in a clinical setting.

Retinal blood flow during hyperoxia in humans revisited: concerted results using different measurement techniques

Retinal vasculature shows pronounced vasoconstriction in response to hyperoxia, which appears to be related to the constant oxygen demand of the retina. However, the exact amount of blood flow reduction and the exact time course of this phenomenon are still a matter of debate. We set out to investigate the retinal response to hyperoxia using innovative techniques for the assessment of retinal hemodynamics. In a total of 48 healthy volunteers we studied the effect of 100% O(2) breathing on retinal blood flow using two methods. Red blood cell movement in larger retinal veins was quantified with combined laser Doppler velocimetry and retinal vessel size measurement. Retinal white blood cell movement was quantified with the blue field entoptic technique. The time course of retinal vasoconstriction in response to hyperoxia was assessed by continuous vessel size determination using the Zeiss retinal vessel analyzer. The response to hyperoxia as measured with combined laser Doppler velocimetry and vessel size measurement was almost twice as high as that observed with the blue field technique. Vasoconstriction in response to 100% O(2) breathing occurred within the first 5 min and no counterregulatory or adaptive mechanisms were observed. Based on these results we hypothesize that hyperoxia-induced vasoconstriction differentially affects red and white blood cell movement in the human retina. This hypothesis is based on the complex interactions between red and white blood cells in microcirculation, which have been described in detail for other vascular beds.

Comparative optic nerve physiology: implications for glaucoma, neuroprotection, and neuroregeneration

The axoplasm of optic nerve axons moves bidirectionally at various speeds along an intra-axonal pressure gradient from the retinal ganglion cell (RGC) somata toward its synapse, and from the synapse towards the RGC somata. The axoplasmic flow of optic nerve axons is precarious even at normal intraocular pressures (IOP) as it moves from the intraocular optic nerve through the scleral lamina cribrosa to the intraorbital optic nerve. The scleral lamina cribrosa is not simply a porous region of the sclera but a specialized extracellular matrix of the central nervous system whose movement during fluctuations in IOP can affect optic nerve axoplasmic flow. The abundant optic nerve blood supply maintains adequate optic nerve head perfusion through a process of vascular autoregulation. Glaucoma is associated with reduced optic nerve axoplasmic flow and compromised optic nerve circulation such that RGC death due to glutamate excitotoxicity and neurotrophin deprivation result.

A pilot study of in vivo venous pressures in the pig retinal circulation

Retinal venous pressure was examined in six pigs using a balanced servo-nulling micropuncture technique. The mean transmural venous pressure was 0.95 mmHg (SD 1.50 mmHg). The transmural venous pressure was lower at the optic disc than away from the disc (0.32 +/- 1.46 mmHg vs 1.69 +/- 1.19 mmHg, P < 0.01). At the disc a transmural pressure of zero or less (0 to -0.5 mmHg) was demonstrated in 10/21 (48%) of the disc readings. Over an intraocular pressure range of 15-26 mmHg there was a strong correlation between intraocular pressure and retinal venous pressure (Pearson coefficient r = 0.92). The results are compatible with the Starling resistor theory of venous outflow from the eye.

Relation between pressure determined by ophthalmodynamometry and aortic pressure in the dog

AIMS

Ophthalmodynamometry has been used extensively since the last century; however, controversy surrounds what it actually measures. This study was set up to determine the relation between ophthalmodynamometric (ODP) and systemic blood pressures.

METHODS

Aortic pressure was continuously monitored and altered by phlebotomy in six anaesthetised dogs, while ophthalmodynamometry was performed, by directly altering intraocular pressure. Maxillary artery pressure was monitored in two animals. All pressure transducers were zeroed at eye level.

RESULTS

Mean ODP was 96.6% (1.6%) (95% confidence interval, n = 49) of aortic pressure. Mean maxillary artery pressure was 95.7% (5.5%) (95% CI, n = 16) of aortic pressure. ODP was 1.9 (0.6) mm Hg (95% CI, n = 33) higher than maxillary artery pressures.

CONCLUSION

ODP was only slightly below aortic pressure and not significantly different from maxillary artery pressure, the analogue of the internal carotid artery in humans. These results also suggest a retinal artery collapse pressure of at least 1.9 mm Hg.