Determination of the effect of a single hemodialysis session on retrobulbar blood hemodynamics by color Doppler ultrasonography

The Haemodialysis Session Effect on the Choroidal Thickness and Retinal and Choroidal Microcirculation-A Literature Review

Haemodialysis (HD) is currently the most commonly used method of renal replacement therapy. The process of dialysis involves numerous changes that affect many systems, including the eye. The changes occurring in the course of HD may affect the ocular parameters, such as intraocular pressure, central corneal thickness, retinal thickness, retinal nerve fibre layer thickness, and choroidal thickness (CT). The choroid, being one of the most vascularized tissues, is characterized by the highest ratio of blood flow to tissue volume in the entire body, may be particularly susceptible to changes occurring during HD, and at the same time reflect the microcirculatory status and its response to HD. Patients with end-stage renal disease subjected to dialysis are highly susceptible to systemic microvascular dysfunction. Moreover, it is considered that the process of HD itself contributes to vascular dysfunction. Nowadays, thanks to the development of imaging techniques, the widely available optical coherence tomography (OCT) tests allow for the assessment of CT, while OCT-angiography allows for a quick, non-invasive, and repeatable assessment of the condition of retinal and choroidal microcirculation, which significantly expands our knowledge regarding the reaction of ocular microcirculation due to HD. The assessment of both retinal and choroidal circulation is even more attractive because retinal circulation is autoregulated, while choroidal circulation is mainly controlled by extrinsic autonomic innervation. Thus, assessment of the choroidal response to an HD session may provide the possibility to indirectly evaluate the functions of the autonomic system in patients subjected to HD. At a time when the importance of microcirculation in systemic and renal diseases is becoming increasingly evident, the assessment of ocular microcirculation appears to be a potential biomarker for assessing the condition of systemic microcirculation. In this work, we present a review of the literature on the effect of the HD session on CT and the retinal and choroidal microcirculation.

Pre- and Post-Hemodialysis Cerebral Blood Flow Velocity in Patients With End-Stage Renal Disease

BACKGROUND

This study assessed the consequences of hemodialysis (HD) on hemodynamic parameters of cerebral circulation by measuring middle cerebral artery blood flow velocities using transcranial Doppler ultrasound before and after a single dialysis session in patients with end-stage renal disease (ESRD).

MATERIALS AND METHODS

Fifty clinically stable patients with ESRD undergoing HD and 40 healthy controls were recruited for the study. Blood pressure, heart rate, and body weight were measured. Transcranial Doppler ultrasound evaluations and blood analyses were performed immediately before and after a single dialysis session.

RESULTS

The mean cerebral blood flow velocities (CBFVs) in the ESRD patients before HD was 65 ± 17 cm/second and did not differ from that of the normal controls (64 ± 14 cm/s) ( P = 0.735). The postdialysis CBFV also did not differ from that of the controls ( P = 0.054).

CONCLUSION

Compensatory cerebral autoregulation and chronic adjustment to therapy may be the reason for the nondeviation of the CBFV values from normality observed in both sessions.

Comparison of peripapillary retinal nerve fiber layer and macular thickness in non-diabetic chronic kidney disease and controls

Objective

This study aimed to compare the peripapillary retinal nerve fiber layer (pRNFL) thickness and macular thickness (MT) between patients with non-diabetic chronic kidney disease (NDCKD) and controls, as well as between different stages of NDCKD. We also evaluated the correlation between pRNFL thickness and MT with duration of NDCKD.

Methods

This was a comparative cross-sectional study. Subjects were divided into NDCKD and control groups. Both pRNFL thickness and MT, including center subfield thickness (CST), average MT as well as average ganglion cell-inner plexiform layer (GC-IPL) were measured using spectral-domain optical coherence tomography. One-way ANCOVA test was used to compare the differences in pRNFL and MT between NDCKD and controls, as well as between the different stages of NDCKD. Spearman rank-order correlation coefficients were employed to determine the effects of NDCKD duration on pRNFL thickness and MT.

Results

A total of 132 subjects were recruited, 66 with NDCKD and 66 controls. There was a statistically significant difference in superior (110.74 ± 23.35 vs 117.36 ± 16.17 μm, p = 0.022), nasal (65.97 ± 12.90 vs 69.35 ± 10.17 μm, p = 0.006), inferior quadrant (117.44 ± 23.98 vs 126.15 ± 14.75 μm, p = 0.006), average pRNFL (90.36 ± 14.93 vs 95.42 ± 9.87 μm, p = 0.005), CST (231.89 ± 26.72 vs 243.30 ± 21.05 μm, p = 0.006), average MT (268.88 ± 20.21 vs 274.92 ± 12.79 μm, p = 0.020) and average GC-IPL (75.48 ± 12.44 vs 81.56 ± 6.48, p = 0.001) values between the NDCKD group and controls. The superior quadrant (p = 0.007), nasal quadrant (p = 0.030), inferior quadrant (p = 0.047), average pRNFL (p = 0.006), average MT (p = 0.001) and average GC-IPL (p = 0.001) differed significantly between different stages of NDCKD. There was no correlation between pRNFL thickness and MT with duration of NDCKD.

Conclusion

CST, average MT, average GC-IPL thickness, average pRNFL and all quadrants of pRNFL except the temporal quadrant were significantly thinner in NDCKD patients compared to controls. These changes were associated with the severity of CKD, but not its duration.

Hemodialysis Effect on the Composition of the Eye Fluid of Cataract Patients

Numerous reports have proven that dialysis patients experience disturbances in the levels of elements in biological fluids. Disturbances in the homeostasis of essential elements or the appearance of highly toxic elements are serious problems also in clinical ophthalmology. The purpose of this study was to investigate the influence of hemodialysis (HD) on the elemental composition of anterior chamber aqueous humor (AH) in patients undergoing cataract surgery. The study involved 22 patients. The control group enrolled 16 patients (age 75.68 ± 9.67, female 54.55%, male 45.45%) with cataract and normal kidney function (control), and the second group included six patients (age 70.33 ± 12.74, female 33.33%, male 66.67%) with cataract undergoing HD treatment. The elements quantification was established using an inductively coupled plasma optical emission spectrometer (ICP-MS). In the eye fluid of dialysis patients, there were increased levels of manganese (Mn) and mercury (Hg) and decreased levels of vanadium (V) and zinc (Zn). In addition, a statistically significant increase in the Hg/Zn and Hg/selenium (Se) ratios and a lowering of the iron (Fe)/Mn ratio were observed in the studied group in comparison to the control. The obtained results indicated the need for Zn and Se supplementation in order to eliminate the hazards caused by Hg toxicity. A lower level of V in the eye fluid of dialysis patients may have a positive effect on maintaining a calcium and phosphorus homeostasis. Our study gives a deep insight into changes of elements concentrations in AH induced by HD.

Effect of Hemodialysis on Peripapillary Choroidal Thickness Measured by Swept-Source Optical Coherence Tomography

PURPOSE

The aim was to investigate the effects of hemodialysis (HD) on peripapillary choroidal thickness (PCT) by swept-source optical coherence tomography and on other ophthalmologic parameters in patients with end-stage kidney disease.

MATERIALS AND METHODS

This was a prospective observational study. The authors evaluated 29 patients who underwent HD for end-stage kidney disease. Detailed ophthalmologic examinations and swept-source optical coherence tomography were performed immediately before and after HD. PCT was measured using the modification tool in the built-in OCT image viewer program. Changes in PCT before and after HD were statistically analyzed.

RESULTS

The average PCT significantly decreased from 127.3±49.2 μm before HD to 117.1±50.9 μm after HD (P<0.001). A significant correlation was found between changes in PCT and macular choroidal thickness (ρ=0.547, P=0.002). Changes in mean ocular perfusion pressure did not significantly correlate with changes in PCT (ρ=-0.049, P=0.803).

CONCLUSIONS

PCT significantly decreased after HD. HD could influence the optic nerve head and its surrounding structures.

The impact of hemodialysis on retinal and choroidal thickness in patients with chronic renal failure

BACKGROUND

Patients with chronic renal failure are commonly cared using a blood filtration mechanism like hemodialysis. Little information is available regarding ocular changes following hemodialysis for people with chronic renal failure. Accordingly, this study intended to estimate the pre- and post-hemodialysis thickness of retina and choroid and other ocular findings in patients with chronic renal failure.

METHODS

This research is a "before and after" clinical experiment, without control. This prospective study was conducted on patients with chronic renal failure who were in Imam Khomeini Hospital for hemodialysis in 2017. The sample size of this study was estimated to be 67 (134 eyes). In this study, after acquiring consent (by the associated assistant) from all patients, thorough ocular examinations including IOP control, VA and fundus examination, reflection, and macula and choroid OCT were performed 30 min before and after hemodialysis and entered in SPSS V. 16 software by the project manager. Finally, these data were analyzed practicing T-pair tests or their nonparametric equivalent, the Wilcoxon test.

RESULTS

Based on the results of this examination, studying 67 participating patients, 134 eyes were examined, of which 80 eyes (59.8%) belonged to men, and 54 eyes (40.2%) belonged to women. The mean and standard deviation of participants' age in the study was reported to be 57.3 ± 15 years, and the dialysis duration was 3.0 ± 2.11 h. According to the Wilcoxon test, the average rank in terms of weight, patient temperature, pulse, blood sugar, urea, potassium, ocular pressure, 500-micron nasal choroidal thickness, and myopia in diopter vary significantly before and after dialysis (p < 0.05). However, according to the Wilcoxon test, choroidal thickness in the subfoveal area of the eye (p = 0.600), the retinal thickness in the subfoveal area (p = 0.839), the amount of astigmatism in diopter (p = 0.757) and the amount of hypermetropia in diopter (p = 0.068) before and after dialysis do not have a significant difference. Based on the t test, it was reported that the average creatinine score, the best corrected vision, and the 500 and 1000-micron temporal and nasal choroidal thickness had a significant difference (p < 0.05).

CONCLUSION

Based on the results of the current study, it was settled that the corrected vision, ocular pressure, as well as the 500-micron nasal choroidal thickness, and myopia in diopter and the 1000-micron temporal and nasal choroidal thickness of patients before and after hemodialysis vary. This difference shows the impact of hemodialysis on changes in ocular characteristics in patients with renal insufficiency.

The systemic microcirculation in dialysis populations

In a rapidly expanding population of patients with chronic kidney disease, including 2 million people requiring renal replacement therapy, cardiovascular mortality is 15 times greater than the general population. In addition to traditional cardiovascular risk factors, more poorly defined risks related to uremia and its treatments appear to contribute to this exaggerated risk. In this context, the microcirculation may play an important early role in cardiovascular disease associated with chronic kidney disease. Experimentally, the uremic environment and dialysis have been linked to multiple pathways causing microvascular dysfunction. Coronary microvascular dysfunction is reflected in remote and more easily studied vascular beds such as the skin. There is increasing evidence for a correlation between systemic microvascular dysfunction and adverse cardiovascular outcomes. Systemic microcirculatory changes have not been extensively investigated across the spectrum of chronic kidney disease. Recent advances in non-invasive techniques studying the microcirculation in vivo in man are increasing the data available particularly in patients on hemodialysis. Here, we review current knowledge of the systemic microcirculation in dialysis populations, explore whether non-invasive techniques to study its function could be used to detect early stage cardiovascular disease, address challenges faced in studying this patient cohort and identify potential future avenues for research.

Vision Impairment in CKD Patients: Epidemiology, Mechanisms, Differential Diagnoses, and Prevention

Eyes and kidneys have numerous structural, developmental, physiologic, and pathogenic pathways in common, suggesting that many kidney and eye diseases may be interlinked. Studies suggest that the prevalence of eye diseases and vision impairment are higher among persons with end-stage kidney disease and earlier stages of chronic kidney disease (CKD) than in those without. Ocular morbidity in persons with CKD and end-stage kidney disease may be due to the following risk factors: (1) underlying conditions and risk factors for CKD such as diabetes or hypertension, (2) metabolic disorders associated with CKD, (3) uremia and anemia, and (4) CKD treatment. Among the chief eye diseases, diabetic retinopathy and age-related macular degeneration are most consistently associated with CKD. Further research for eye diseases such as glaucoma and cataract is needed to determine their relationships with CKD. Despite the high prevalence and burden of vision impairment among persons with CKD, eye screening in patients with CKD is not currently recommended as standard practice. This review suggests that patients with CKD should be encouraged to undergo a complete eye examination. Furthermore, physicians should be aware that patients undergoing dialysis may develop acute eye problems such as acute glaucoma, and appropriate referral to ophthalmologists should be considered in those with a history of glaucoma or recent ocular surgery. Interdisciplinary collaboration between nephrologists and ophthalmologists will ensure enhanced and appropriate management of patients with CKD.

Optical coherence tomography angiography analysis of changes in the retina and the choroid after haemodialysis

The purpose of this study is to evaluate the effect of haemodialysis on perfused vessel density, choroidal thickness (CT), and retinal thickness in end-stage renal disease (ESRD) using swept-source optical coherence tomography angiography (SS-OCTA). We studied twenty-nine eyes of 29 ESRD patients by ophthalmologic examination and SS-OCTA before and after haemodialysis. The colour-coded perfusion density maps were generated and perfused vessel density was calculated. Changes in systemic and other ocular parameters such as retinal and choroidal thickness were measured and analysed. Total perfused vessel density decreased significantly after haemodialysis in the choriocapillaris; it was not significantly different in the superficial capillary plexus (SCP) and the deep capillary plexus (DCP). Total CT decreased significantly, but total retinal thickness was not significantly different. There was no significant correlation between choriocapillaris perfused vessel density and CT. The reduction in choriocapillaris perfused vessel density correlated with the decrease in systolic and mean arterial blood pressures. The decrease in CT correlated with the ultrafiltration volume. There were no significant systemic and ocular factors affecting change in retinal thickness and perfused vessel density of SCP and DCP. This is the first study to assess the effect of haemodialysis on blood flow changes using SS-OCTA; changes may be more prominent in the choroidal compared to the retinal layer.

Effect of Hemodialysis on Eye Coats, Axial Length, and Ocular Perfusion Pressure in Patients with Chronic Renal Failure

Purpose

To investigate changes in eye coats, axial length, and ocular perfusion pressure (OPP) in hemodialysis (HD) patients with chronic renal failure (CRF).

Methods

We included HD patients who were diagnosed with CRF in our hospital from January to December 2015. Fifty-two patients met the inclusion criteria; all right eyes were used for observation. Systolic and diastolic blood pressures were recorded to calculate OPP. Approximately 30 minutes before and after HD, we recorded multiple parameters, including lens thickness (LT), axial length of vitreous (VAL), axial length of eye (EAL), ciliary body thickness (CBT), choroidal thickness (CT), and retinal thickness (RT).

Results

After HD, OPP significantly decreased (P < 0.01, F = 7.023) and CBT became significantly thinner (P < 0.01, t = 3.461). CT was significantly thinner and differed among measurement locations (P < 0.01, t = 6.240; P < 0.01, t = 6.169; P < 0.01, t = 3.405, respectively, fovea, nasal, and temporal side 1500 μm beside the fovea). Further, RT thickened and differed among measurement locations (P < 0.05, t = −2.265; P < 0.05, t = −2.624; P < 0.05, t = −2.220, respectively, fovea, nasal, and temporal side 1500 μm beside fovea), whereas LT, VAL, and EAL significantly increased after HD (P < 0.05, t = −2.076; P < 0.01, t = −3.817; P < 0.01, t = −4.010).

Conclusions

HD impacts the thickness of the eye coats and VAL, particularly affecting CBT, CT, and RT. OPP transiently decreases during HD, which may contribute to an ischemic state.

Effect of hemodialysis on visual acuity, intraocular pressure, and macular thickness in patients with chronic kidney disease

BACKGROUND

The aim of this study was to evaluate the effects of hemodialysis (HD) on visual acuity, intraocular pressure (IOP), and central foveal thickness (CFT) in patients with chronic kidney disease.

MATERIALS AND METHODS

Forty-nine eyes from 49 chronic kidney-disease patients were analyzed. Causes of chronic kidney disease included diabetes mellitus (n=9 patients), hypertensive nephrosclerosis (n=15 patients), and other causes (n=25 patients). All patients underwent HD in the Dialysis Unit of Hôtel-Dieu de France Hospital. Best-corrected visual acuity, CFT, and IOP were evaluated before and after HD. CFT was measured with spectral domain optical coherence tomography, and IOP was measured with Goldmann applanation tonometry.

RESULTS

Neither decimal best-corrected visual acuity (pre-HD 0.71±0.32, post-HD 0.72±0.31; P=0.877) nor CFT (pre-HD 251.39±39.29, post-HD 253.09±39.26; P=0.272) significantly changed after HD. However, mean IOP significantly decreased from 13.99±2.48 before HD to 12.65±2.41 mmHg after HD (P=0.001). IOP change was significantly correlated with serum albumin levels (P=0.008) and weight changes (P=0.047).

CONCLUSION

HD can affect various ocular parameters. This is particularly true of IOP, which decreases significantly following HD.

Evaluation of choroidal and retinal thickness measurements using optical coherence tomography in non-diabetic haemodialysis patients

The aim of this study is to evaluate the effects of haemodialysis with a high ultrafiltration rate on the choroidal and retinal thickness of non-diabetic end-stage chronic renal failure (CRF) patients using optical coherence tomography (OCT). Twenty-one eyes of 21 male CRF patients aged between 46 and 80 years were included in this prospective study. Retinal and choroidal thicknesses of the patients were measured using high-resolution OCT line scans with the activated enhanced depth imaging mode before and shortly after haemodialysis. Retinal and choroidal thickness measurements were taken at the fovea and at two points that were 1,500 μm nasal and temporal to the fovea. The relationships between the haemodynamic changes, intraocular pressure (IOP) and central corneal thickness (CCT) were also evaluated. The mean choroidal thicknesses before haemodialysis at the subfoveal, temporal and nasal locations were 232.81 ± 71.92, 212.43 ± 70.50 and 182.14 ± 68.88 μm, respectively. The mean choroidal thicknesses after haemodialysis at the subfoveal, temporal and nasal locations were 210.90 ± 65.53, 195.38 ± 66.48 and 165.19 ± 66.73 μm, respectively. There were significant differences between the choroidal thicknesses before and after haemodialysis (p < 0.001 for all). The mean retinal thicknesses before haemodialysis at the foveal, temporal and nasal locations were 215.86 ± 41.06, 308.86 ± 37.73 and 338.00 ± 33.32 μm, respectively. The mean retinal thicknesses after haemodialysis at the foveal, temporal and nasal locations were 216.90 ± 39.70, 313.86 ± 32.89 and 335.29 ± 36.85 μm, respectively. There was no significant difference between the retinal thicknesses before and after haemodialysis (p > 0.05 for all). The mean CCT decreased insignificantly from 550.48 ± 17.46 to 548.10 ± 21.12 μm (p = 0.411). The mean IOP decreased significantly from 14.09 ± 2.58 to 12.54 ± 2.23 mmHg (p = 0.003), which did not correlate with the CCT [r = (-)0.134, p = 0.562]. Haemodialysis with a high ultrafiltration volume did not alter the retinal thickness but caused a significant choroidal thinning and an IOP decrease in non-diabetic end-stage CRF patients.

Microvascular dilatation after haemodialysis is determined by the volume of fluid removed and fall in mean arterial pressure

BACKGROUND/AIMS

The effects of haemodialysis on the microcirculation are poorly understood. This study examined the changes in small vessel calibre.

METHODS

24 patients (including 12 males, median age 62.5 years, range 30-87) underwent digital retinal photography immediately before and after routine haemodialysis. Arteriolar and venular calibres were measured from the images by a trained grader using a highly reproducible, computer-assisted method.

RESULTS

Patients had an average 2.0 ± 0.3 litres of fluid removed with dialysis, and their mean arterial blood pressure fell by 6.8 mm Hg (CI 13.8-0.2, p = 0.06). Retinal arteriole calibre did not change (mean difference 2.3 µm, CI -1.1 to 5.7, p = 0.17) but the venules dilated (mean difference 12.7 µm, CI 7.3-18.3, p < 0.001). Calibre returned to baseline by 2 h. Venules dilated less in diabetics than non-diabetics (mean difference -6.2 µm, CI -9.6 to -2.9, p < 0.01). Retinal venular dilatation correlated positively with the volume of fluid removed per kilogramme body weight (5.9, CI 0.2-11.5, p = 0.04), and negatively with the fall in mean arterial pressure (-0.36, CI -0.72 to -0.01, p < 0.05) after adjusting for age, gender, diabetes and dyslipidaemia.

CONCLUSION

Haemodialysis is associated with systemic venular dilatation.

Color Doppler indices of orbital arterial flow in end-stage renal disease patients; are the changes related to chronic hemodialysis or chronic renal failure?

BACKGROUND

Endothelial injury is a well-known complication in chronic kidney disease (CKD) and hemodialysis. One of the sites in which early vascular changes may be detected is the retina. Of course, these flow changes may not be detected in ophthalmologic exams, but it seems that color Doppler sonography of retinal arteries may be helpful in these cases.

OBJECTIVES

In previous studies on CKD patients who underwent chronic hemodialysis,hemodynamic changes were noted in retinal arteries, but no study has been performed to determine which of the two processes (CKD or chronic hemodialysis) can produce these changes. In this study, we tried to answer this question.

PATIENTS AND METHODS

Doppler ultrasonography of the orbital vasculature including the ophthalmic artery and the central retinal artery was carried out in 17 patients (34 eyes) with chronic renal failure (CRF) who underwent hemodialysis, 17 patients (34 eyes)with CRF without a history of hemodialysis and 17 normal patients (34 eyes). The peak systolic velocity (PSV), end diastolic velocity (EDV) and resistance index were measured excluding hypertensive, diabetic patients and patients with cardiovascular disease.

RESULTS

The mean PSV and EDV were lower only in the ophthalmic artery of CRF patients irrespective of the history of hemodialysis (PSV was 35.2 in hemodialysis, 38.8 in CRF and 51.6 in normal patients, P value = 0.001 and EDV was 7.4, 9.4, 11.8, respectively, P value =0.001) with no significant difference in the resistance index of the ophthalmic artery and other parameters [EDV, PSV, Resistance Index (RI)] in the central retinal artery.

CONCLUSIONS

The mean PSV and DSV in the ophthalmic artery were lower only in the ophthalmic artery of CRF patients regardless of the history of hemodialysis. No significant difference in the resistance index of the ophthalmic artery and other parameters (EDV, PSV) of the central retinal artery were noted between different groups. These findings suggest that microvascular disease and endothelial cell dysfunction of the orbital vasculature are related to CRF and not to chronic hemodialysis.

Does ranibizumab (Lucentis®) change retrobulbar blood flow in patients with neovascular age-related macular degeneration?

PURPOSE

To investigate the effects of intravitreal ranibizumab on retrobulbar blood flow in patients with neovascular age-related macular degeneration (AMD).

METHODS

Thirty-one eyes of 30 patients with neovascular AMD were examined prospectively by both color Doppler imaging and fundus fluorescein angiography. Color Doppler imaging was used to measure the maximum and minimum velocities of the central retinal vein, peak systolic/end-diastolic velocities of blood flows, and pulsatility index and resistivity index values in the central retinal artery, nasal/temporal posterior ciliary arteries (NPCA/TPCA) and ophthalmic artery. The t test for paired samples was used for comparing retrobulbar blood flow values before and after intravitreal ranibizumab (Lucentis®) injection in the study and control groups.

RESULTS

There was a statistically significant (p < 0.05) difference between the pre-injection and post-injection end-diastolic velocities of the NPCA and TPCA and resistivity index values of TPCA. The other parameters showed no statistically significant difference.

CONCLUSION

Our results show that intravitreal ranibizumab injection increases retrobulbar blood flow.

Effects of intravitreal bevacizumab (Avastin) therapy on retrobulbar blood flow parameters in patients with neovascular age-related macular degeneration

BACKGROUND

To investigate the effects of intravitreal bevacizumab on retrobulbar circulation in patients with neovascular age-related macular degeneration (AMD).

METHOD

Thirty patients with neovascular age-related macular degeneration were assessed prospectively by both color Doppler imaging and fundus fluorescein angiography. Spectral Doppler analysis allowed the measurement of the maximum velocity (Vmax) and minimum velocity (Vmin) of the central retinal vein (CRV), and peak systolic (PSV), end-diastolic (EDV) velocities of blood flows, and pulsatility index (PI) and resistance index (RI) values in the central retinal artery (CRA), nasal and temporal posterior ciliary arteries (NPCA, TPCA), and ophthalmic artery (OA). The t test for paired samples was used to compare retrobulbar blood flow values before and after intravitreal bevacizumab injection.

RESULT

PSV and EDV of the NPCA and PSV of the TPCA were significantly decreased after intravitreal bevacizumab injection (p < 0.05). There was no statistically significant difference in the other parameters.

CONCLUSION

Our results suggest that intravitreal bevacizumab therapy has a measurable effect on retrobulbar blood flow.