December 2009
Volume 50, Issue 12
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Physiology and Pharmacology  |   December 2009
Neuronal Activity Influences Hemodynamics in the Paraoptic Short Posterior Ciliary Arteries: A Comparison between Healthy and Glaucomatous Subjects
Author Notes
  • From the Universitätsklinikum Hamburg-Eppendorf, Klinik und Poliklinik für Augenheilkunde, Hamburg, Germany. 
  • Corresponding author: Oliver Zeitz, Universitätsklinikum Hamburg-Eppendorf, Klinik und Poliklinik für Augenheilkunde, Martinistrasse 52, D-20246 Hamburg, Germany; oliver.zeitz@googlemail.com
Investigative Ophthalmology & Visual Science December 2009, Vol.50, 5846-5850. doi:10.1167/iovs.09-3696
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      Oliver Zeitz, Jeannette Mayer, Doreen Hufnagel, Ralf Praga, Lars Wagenfeld, Peter Galambos, Anne Wiermann, Claudia Rebel, Gisbert Richard, Maren Klemm; Neuronal Activity Influences Hemodynamics in the Paraoptic Short Posterior Ciliary Arteries: A Comparison between Healthy and Glaucomatous Subjects. Invest. Ophthalmol. Vis. Sci. 2009;50(12):5846-5850. doi: 10.1167/iovs.09-3696.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose.: Perfusion of the retina adapts to metabolic demand by neurovascular coupling. The present study was an investigation of the presence of neurovascular coupling in the anterior part of the optic nerve in healthy subjects and patients with glaucoma.

Methods.: Retrobulbar blood flow velocities were determined by color Doppler imaging (CDI). Peak systolic and end diastolic velocities (PSVs and EDVs) in the central retinal artery (CRA) or the short posterior ciliary artery (SPCA) were the primary readout. CDI measurements were performed shortly before, during, immediately after, 60 seconds after, and 120 seconds after a 10-Hz flicker stimulation of the retina.

Results.: Thirty-five patients with glaucoma and 44 healthy control subjects were included in the study. In the SPCA of healthy control subjects, flicker stimulation led to an increase in PSV from 9.7 ± 0.8 to 12.5 ± 0.8 cm/s (P < 0.001; n = 24) and of EDV from 2.4 ± 0.3 to 3.6 ± 0.3 cm/s (P < 0.001; n = 24). This effect was not detectable in patients with glaucoma. In the CRA, flicker light led to an increase in EDV from 2.1 ± 0.2 to 3.0 ± 0.3 cm/s (P = 0.002; n = 20) in healthy volunteers and from 1.3 ± 0.2 to 2.0 ± 0.2 cm/s in patients with glaucoma (P = 0.004; n = 15). PSV was not affected by flicker stimulation in either the healthy volunteers or patients with glaucoma.

Conclusions.: The data indicate the presence of neurovascular coupling in the vascular bed supplied by the paraoptic SPCA. The response pattern to the flicker stimulus differs between healthy subjects and individuals with glaucoma.

Ocular perfusion is regulated by a variety of local and systemic factors. Common diseases like diabetic retinopathy or glaucoma are assumed to be associated with disturbances in the balance of such factors. 1 Methods aimed at visualizing these alterations should include a challenge to the regulatory capacity of ocular perfusion in their protocol (e.g., Refs. 24). Recently, the dynamic retinal vessel analyzer (DRVA) has been used in research and has also been proposed for a variety of clinical applications. 5,6 With this method, the regulatory capacity is challenged by stimulating the retina with a flickering light source. During stimulation the diameter of retinal vessels is continually tracked. DRVA enables novel insights into neurovascular coupling (i.e., the regulation of retinal vessel diameter in response to flicker light-induced changes in local metabolite levels in the retina). Alterations in the neurovascular coupling response pattern have been identified in various pathologic conditions. 5,7 In particular, atherosclerosis-related conditions and diabetic retinopathy are associated with altered vascular reactivity. 8 The limitation of the DRVA method is its inability to measure actual hemodynamic effects, since only the vessel diameter is tracked. In addition, this method is limited to evaluation of funduscopically visible vessels. Disturbances of ocular perfusion in diseases such as primary open-angle glaucoma are assumed to predominantly affect the nonvisible vascular beds around the optic nerve head, in particular the peripapillary choroidal circulation and the circuit of Zinn-Haller. Michelson came up with the idea of combining color Doppler imaging (CDI) measurements of the retrobulbar blood flow with flicker stimulation of the retina and published his report of the method in 2002. 9 His investigations were focused on the central retinal artery. The present study was aimed at extending this method to the investigation of the paraoptic short posterior ciliary artery (SPCA), the main source of blood supply to the optic nerve head. It was not known whether this vessel would also react to flicker light. Furthermore, we hypothesized that patients with glaucoma exhibit a response to flicker light stimulation in this vessel different from the response in healthy control subjects. 
Methods
Subjects
Patients with glaucoma and healthy control subjects were enrolled on a voluntary basis in this trial. Informed consent was obtained from all participants. The study protocol was approved of by the local ethics committee, and the study subjects were treated in accordance with the Declaration of Helsinki. 
Patients with primary open-angle glaucoma including normal-tension glaucoma were enrolled in the present trial. Diagnosis of primary open-angle glaucoma was based on the assessment of an experienced glaucoma specialist (MK). All patients with glaucoma had abnormal thinning of the neuroretinal rim on funduscopic examination with glaucomatous cupping of the optic nerve head (cup/disc ratio ≥0.6). The diagnosis of glaucoma was confirmed in by visual field perimetry (Humphrey 30-2 Visual Field Analyzer; Carl Zeiss Meditec, Oberkochen, Germany) by interpretation of the global indices mean deviation (MD) and pattern standard deviation (PSD), as well as by the detection of characteristic visual field defects in the total and pattern deviation maps according to the classification of Hodapp et al. 10 Intraocular pressure had to be within the normal range (6–21 mm Hg) at the time of enrollment in the trial. Diastolic blood pressure had to be between 60 and 90 mm Hg and systolic blood pressure should not exceed 150 mm Hg. Patients with glaucoma with a change in ocular antihypertensive medication within the last 6 weeks were excluded from the study. Patients with retinal diseases were excluded. Eyes with evidence of secondary glaucoma or closure of the iridocorneal angle, previous surgical interventions other than cataract surgery in both eyes, and a refractive error exceeding ±5 D were also excluded from the study. 
The healthy control group consisted of volunteers with no history of ophthalmic disease resulting in optic neuropathy or vascular impairment, no abnormal findings on ophthalmic examination, and a refractive error below ±5 D. The same restrictions regarding blood pressure as for the glaucoma population were applied (diastolic between 60 and 90 mm Hg and systolic <150 mm Hg). 
Experimental Protocol
To ensure comparability, all investigations were performed between 3 and 6 PM. The specific time for the examination was chosen on organizational grounds. All CDI examinations were performed in a specially designated room with the individuals in an upright sitting position. The right eye was selected as the study eye. The left eye was chosen only if the right eye did not meet the inclusion criteria. After resting for several minutes, a baseline Doppler examination was performed. Subsequently, the retina was stimulated by a white 10-Hz flicker light through the closed eye lids (flicker light source: Digital Stroboscope DT-2269; Voltcraft, Taipai, Taiwan). The source of flicker light was placed close under the Doppler probe and was directed onto the closed eye lids. This placement led to a bright illumination of the entire visual field. The blood flow velocities were reassessed 30 seconds after onset of the flicker light. Measurements were repeated immediately after, and 60 and 120 seconds after termination of the flicker stimulation. In some subjects this flicker stimulation cycle was repeated five times to get information on the reproducibility of the measurements. Measurements were performed either in the central retinal artery (CRA) or in the short posterior ciliary artery (SPCA). 
Measuring Blood Flow Velocities by Color Doppler Imaging
CDI was performed with an ultrasound system (Sonoline Elegra Advanced System; Siemens, Erlangen, Germany) equipped with a phased array transducer (model 7.5L40; Siemens). Ultrasound frequency was 6.5 MHz in the pulsed Doppler mode. The transducer was carefully set on the closed eye lid, minimizing pressure on the bulb. Acoustic coupling between transducer and skin was optimized by a carbomeric gel (Vidisic; Dr. Mann Pharma, Berlin, Germany). The examination technique is described in detail elsewhere. 11,12  
The changes of blood flow velocities over the course of a cardiac cycle were recorded continuously. PSV and EDV were recorded directly in the Doppler mode. 
Statistical Analysis
The effect of flicker stimulation on retrobulbar blood flow velocities was analyzed by applying a repeated-measures 2 × 2 ANOVA model. PSV and EDV in the SPCA and the CRA were the dependent variables. The time point of measure (before, during, shortly after, and 60 and 120 seconds after flicker light stimulation) and the group (glaucoma control) were the fixed factors. P < 0.05 was considered to be statistically significant. Reproducibility was determined by calculating the intraclass correlation coefficient (ICC). ICC > 0.75 indicates good reproducibility. All data are shown as the mean ± SEM. 
Results
Seventy-nine subjects were enrolled in the trial. Forty-four were healthy control subjects and 35 were patients with primary open-angle glaucoma. Of the patients with primary open-angle glaucoma, nine had not shown an IOP > 21 mm Hg in their histories and thus had been categorized as having normal-tension glaucoma. All patients with glaucoma received topical antiglaucoma therapy with an average of 2.0 ± 0.2 antiglaucoma agents per patient. Twenty patients received prostaglandins, 14 β-adrenoceptor antagonists, 13 dorzolamide, 12 brimonidine, 6 brinzolamide, and 4 pilocarpine. Demographic and ophthalmic baseline data are given in Table 1
Table 1.
 
Summary of the Demographic and Ophthalmic Baseline Data of the Control and Glaucoma Groups
Table 1.
 
Summary of the Demographic and Ophthalmic Baseline Data of the Control and Glaucoma Groups
Age (y) M:F BCVA (dec) IOP(mm Hg) Antigl. Agents CD MD (−dB) PSD (dB)
Controls 39.9 ± 3.1 26:18 0.87 ± 0.47 13.1 ± 1.3 0 0.15 ± 0.09 ND ND
Glaucoma 64.1 ± 2.4 11:24 0.89 ± 0.02 14.1 ± 0.5 2.0 ± 0.2 0.81 ± 0.03 4.54 ± 0.92 4.32 ± 0.59
Short Posterior Ciliary Artery
In the SPCA of healthy control subjects flicker stimulation led to an increase in PSV from 9.7 ± 0.8 to 12.5 ± 0.8 cm/s (P < 0.001; n = 24; Fig. 1A) and of EDV from 2.4 ± 0.3 to 3.6 ± 0.3 cm/s (P < 0.001; n = 24; Fig. 1A). This effect was not detectable in patients with glaucoma (PSV: 9.4 ± 0.8 vs. 11.1 ± 1.0 cm/s; P = 0.235; EDV: 2.5 ± 0.3 vs. 2.7 ± 0.3 cm/s; P = 0.279; n = 20 for PSV and EDV; Fig. 1C). In the healthy control subjects the flicker-induced hemodynamic changes were completely reversible after 60 and 120 seconds. 
Figure 1.
 
Time course of the increase of blood flow velocities in the flicker experiment. Results for the SPCA (A) and the CRA (B) in healthy control subjects. Data recorded for the SPCA (C) and the CRA (D) in patients with glaucoma. *P < 0.05.
Figure 1.
 
Time course of the increase of blood flow velocities in the flicker experiment. Results for the SPCA (A) and the CRA (B) in healthy control subjects. Data recorded for the SPCA (C) and the CRA (D) in patients with glaucoma. *P < 0.05.
Central Retinal Artery
In the CRA flicker light led to an increase of EDV from 2.1 ± 0.2 to 3.0 ± 0.3 cm/s (P = 0.002; n = 20; Fig. 1B) in healthy volunteers and from 1.3 ± 0.2 to 2.0 ± 0.2 cm/s in patients with glaucoma (P = 0.004; n = 15; Fig. 1D). PSV was not affected by flicker stimulation in the healthy volunteers and patients with glaucoma (controls: 10.0 ± 0.9 vs. 11.9 ± 1.0 cm/s; P = 0.239; n = 20; Fig. 1B; patients with glaucoma: 9.2 ± 0.7 vs. 9.4 ± 0.8 cm/s; P = 0.619; n = 15; Fig. 1D). Flicker-induced hemodynamic changes in EDV were completely reversible after 60 and 120 seconds. 
Comparison of the Glaucoma and Control
The only statistically significant difference between the glaucoma and control groups was the response to flicker stimulation in the short posterior ciliary artery. PSV and EDV was significantly different during flicker stimulation (P = 0.048), but not before, shortly after, or 60 or 120 seconds after the flicker stimulus. The PSV and EDV measures of both groups are shown in Table 2
Table 2.
 
Measurements at the Five Time Points of the Experiment
Table 2.
 
Measurements at the Five Time Points of the Experiment
t1 t2 t3 t4 t5
Peak systolic velocity
    CRA
        Controls (n = 20) 10.0 ± 0.9 11.9 ± 1.0 11.0 ± 0.8 9.8 ± 0.8 9.3 ± 0.8
        Glaucoma (n = 15) 9.2 ± 0.7 9.4 ± 0.8 8.4 ± 0.7 8.4 ± 0.7 8.9 ± 0.7
    SPCA
        Controls (n = 24) 9.7 ± 0.6 12.5 ± 0.8* 9.6 ± 0.7 9.0 ± 0.5 8.0 ± 0.4
        Glaucoma (n = 20) 9.4 ± 0.8 11.1 ± 1.1 10.5 ± 1.7 8.5 ± 0.5 8.4 ± 0.5
End diastolic velocity
    CRA
        Controls (n = 20) 2.1 ± 0.2 3.0 ± 0.3* 2.3 ± 0.2 1.9 ± 0.2 1.7 ± 0.2
        Glaucoma (n = 15) 1.3 ± 0.2 2.0 ± 0.2* 1.3 ± 0.1 1.3 ± 0.1 1.5 ± 0.1
    SPCA
        Controls (n = 24) 2.4 ± 0.3 3.6 ± 0.3* 2.4 ± 0.2 2.3 ± 0.2 2.3 ± 0.2
        Glaucoma (n = 20) 2.5 ± 0.3 2.7 ± 0.3 2.7 ± 0.5 2.1 ± 0.1 2.0 ± 0.2
Reproducibility of the Measurements
In some healthy subjects, the flicker stimulation was repeated five times in the CRA (n = 6) or SPCA (n = 8). The flicker response did not differ significantly between the five test cycles at each time point (Table 3). 
Table 3.
 
ICCs for PSV and EDV in the SPCA and CRA
Table 3.
 
ICCs for PSV and EDV in the SPCA and CRA
PSV EDV
CRA (n = 6) 0.933 0.898
SPCA (n = 8) 0.962 0.956
Discussion
In the present study, the healthy volunteers showed a significant increase in retrobulbar blood flow velocities in response to flicker light stimulation of the retina in the paraoptic SPCA supplying the peripapillary vascular bed. In glaucoma subjects, the flicker response in the SPCA was reduced. 
Flicker light stimulation of the retina has been used experimentally since the 1990s to investigate neurovascular coupling. 13,14 Most of the published scientific work on ocular neurovascular coupling was done with laser Doppler flowmetry (LDF) until now. The retina and the inner choroidal layers are the preferred measuring sites for this technique. Using LDF, most publications have reported increased blood flow measurements in response to flicker light stimulation. 15,16 Regarding glaucoma, LDF represents a compromise, since it assesses hemodynamics only in the superficial layers of the optic disc. The retrolaminar parts of the blood supply are also relevant in this regard—in particular, the paraoptic SPCAs. This blood supply can only be investigated using CDI. We therefore decided to try to combine CDI of those vessels with the flicker procedure to obtain a tool for investigating neurovascular coupling in the retrobulbar parts of the circuit of Zinn-Haller. 
To our knowledge, until now the only other studies in which a comparable ultrasound Doppler methodology combined with retinal flicker light stimulation was applied were Delles et al. 17 and Michaelson et al. 9 They limited their study to the CRA. Comparing the present results with the outcome of Michelson et al., 9 the effect of flicker light on blood flow velocities in the CRA was more pronounced in their work than in the present data. A direct comparison of both studies is difficult because of the different experimental conditions used; the CDI measurements in the study by Michelson et al. were undertaken in supine patients, whereas in our study the measurements were taken in sitting subjects. It was recently shown that the CDI results may be posture dependent. 3,18  
Since the present study applies a novel methodology and assesses the neurovascular coupling effects in a previously invisible vascular bed, a direct comparison with literature is difficult. However, other reports are in general support of our observed enhancing effect of flicker light on the peripapillary blood flow (e.g., in the small vessels near the rim of the optic nerve head measured by LDF). 15,19 The results from these more superficial vessels are most likely to be influenced by the regulation in the circuit of Zinn-Haller for which the SPCA forms an important blood supply source. Neurovascular coupling has also been investigated in the subfoveal choroid, but results are contradictory. Lovasik et al. 20 reported an enhancing effect of flicker light on choroidal circulation, whereas the pioneering study in that field from Garhofer et al. 15 did not find a significant effect. 
What does the increase of PSV and EDV mean for blood flow in the peripapillary region? Unfortunately, none of the current methodologies for measuring ocular perfusion is capable of directly measuring blood flow in volume per unit of time. 12 A concomitant increase in both PSV and EDV, as seen in the SPCA of the healthy volunteers, is regarded by many users of the CDI methodology as a good indicator of an actual increase in blood flow volume. 21 This might be, as in the retina, the physiological response to an increased metabolic demand in the peripapillary region, which is due to increased neuronal activity in the axons of the ganglion cells. Such a probably metabolically mediated phenomenon is present not only in the ocular segments of the visual system, but also in the posterior cerebral artery during flicker stimulation of the retina. 22 Therefore, perfusion may depend on the neuronal activity along the entire optic tract, and one could postulate that this is a physiological feature of the entire visual system. 
This physiology seems to be disturbed in patients with glaucoma since the present study revealed an altered regulatory response in the glaucoma subjects to flicker light in the paraoptic SPCA. This observation may be limited by the fact that the glaucoma and control groups were not age matched in the present study, although previous data obtained with the retinal vessel analyzer indicate only very weak dependence or even independence of the magnitude of flicker response and age. 23 In addition, it cannot be completely ruled out that this observation is a secondary effect as the consequence of an altered perception of the flicker stimulus because of glaucomatous ganglion cell death. On the other hand, vascular dysregulation is known to play an important role in the pathogenesis of glaucoma, and numerous reports indicate a disturbed regulatory capacity of small vessels in glaucoma (reviewed in Refs. 24, 25). Our previous work set a specific emphasis on the role of blood flow in the paraoptic SPCA of patients with glaucoma. It has been shown that the dysregulation in this vessel is particularly pronounced when comparing the adaptation of hemodynamics in healthy control subjects and patients with glaucoma after posture change. 3 In addition, it has been demonstrated that diminished blood flow velocities in the SPCA are a predictor of glaucoma progression. 26 Thus, the present results fit well with these previous postulates and appear to be congruent with the most current literature on vascular dysregulation in patients with glaucoma. 
The present study has given insight into the predominantly physiological aspects of the regulation of perfusion of the optic disc and has shown a first application of using CDI after flicker stimulation in patients with glaucoma. Regarding the further use of this method in the clinic and in research, the experiments indicating good reproducibility represent a first step. On the other hand, like all hemodynamic measurement methodologies, the combination of flicker and CDI also has the problem that its discriminatory power is too weak to clearly separate healthy subjects from patients, since the overlap between both groups is too large. So, from today's viewpoint, combining flicker with CDI measurements will be helpful for intraindividual comparisons or for statistic comparisons of cohorts, as in the present study. Despite these limitations, our results could represent another step for future interventional research aimed at investigating the mechanisms behind glaucoma-associated disturbance of ocular hemodynamics in more detail. 
Footnotes
 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.
References
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Figure 1.
 
Time course of the increase of blood flow velocities in the flicker experiment. Results for the SPCA (A) and the CRA (B) in healthy control subjects. Data recorded for the SPCA (C) and the CRA (D) in patients with glaucoma. *P < 0.05.
Figure 1.
 
Time course of the increase of blood flow velocities in the flicker experiment. Results for the SPCA (A) and the CRA (B) in healthy control subjects. Data recorded for the SPCA (C) and the CRA (D) in patients with glaucoma. *P < 0.05.
Table 1.
 
Summary of the Demographic and Ophthalmic Baseline Data of the Control and Glaucoma Groups
Table 1.
 
Summary of the Demographic and Ophthalmic Baseline Data of the Control and Glaucoma Groups
Age (y) M:F BCVA (dec) IOP(mm Hg) Antigl. Agents CD MD (−dB) PSD (dB)
Controls 39.9 ± 3.1 26:18 0.87 ± 0.47 13.1 ± 1.3 0 0.15 ± 0.09 ND ND
Glaucoma 64.1 ± 2.4 11:24 0.89 ± 0.02 14.1 ± 0.5 2.0 ± 0.2 0.81 ± 0.03 4.54 ± 0.92 4.32 ± 0.59
Table 2.
 
Measurements at the Five Time Points of the Experiment
Table 2.
 
Measurements at the Five Time Points of the Experiment
t1 t2 t3 t4 t5
Peak systolic velocity
    CRA
        Controls (n = 20) 10.0 ± 0.9 11.9 ± 1.0 11.0 ± 0.8 9.8 ± 0.8 9.3 ± 0.8
        Glaucoma (n = 15) 9.2 ± 0.7 9.4 ± 0.8 8.4 ± 0.7 8.4 ± 0.7 8.9 ± 0.7
    SPCA
        Controls (n = 24) 9.7 ± 0.6 12.5 ± 0.8* 9.6 ± 0.7 9.0 ± 0.5 8.0 ± 0.4
        Glaucoma (n = 20) 9.4 ± 0.8 11.1 ± 1.1 10.5 ± 1.7 8.5 ± 0.5 8.4 ± 0.5
End diastolic velocity
    CRA
        Controls (n = 20) 2.1 ± 0.2 3.0 ± 0.3* 2.3 ± 0.2 1.9 ± 0.2 1.7 ± 0.2
        Glaucoma (n = 15) 1.3 ± 0.2 2.0 ± 0.2* 1.3 ± 0.1 1.3 ± 0.1 1.5 ± 0.1
    SPCA
        Controls (n = 24) 2.4 ± 0.3 3.6 ± 0.3* 2.4 ± 0.2 2.3 ± 0.2 2.3 ± 0.2
        Glaucoma (n = 20) 2.5 ± 0.3 2.7 ± 0.3 2.7 ± 0.5 2.1 ± 0.1 2.0 ± 0.2
Table 3.
 
ICCs for PSV and EDV in the SPCA and CRA
Table 3.
 
ICCs for PSV and EDV in the SPCA and CRA
PSV EDV
CRA (n = 6) 0.933 0.898
SPCA (n = 8) 0.962 0.956
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