Ординатура / Офтальмология / Английские материалы / Retinal Vascular Disease_Joussen, Gardner, Kirchhof_2007

rate constants of each plasma component. Thus, k can be found from a single measurement of enhancement, provided R1, Tk, and the plasma parameters are known. Setting the vitreous volume in the slice Vv = Aregion-of-interest(slice thickness), PS then is:

This equation assumes that all tracer in the slice originated from the portion of retina in that slice. This assumption has been verified experimentally [7].

To determine the accuracy of this approach, the outer BRB was first destroyed via an intravenous injection of sodium iodate [9]. Sodium iodate causes extensive disruption of the tight cell-cell junctions of the retinal pigment epithelium (outer BRB) within 24 h post-treatment [20]. At this time, BRB PS was determined using DCE-MRI, and eyes were then enucleated and frozen prior to measuring vitreous levels of Gd-DTPA [9]. BRB PS was then calculated using standard physiologic modeling [9]. The plasma time course was separately determined and used in both MRI and physiologic assessments. As shown in Fig. 8.2.5, agreement was found between the DCEMRI and “gold standard” PS measures. Furthermore, following sodium iodate exposure, vitreous GdDTPA distribution in gas-compressed vitrectomized eyes was different from that of eyes with normal vitreous (Fig. 8.2.7). The BRB PS calculated from con-

trol eyes and those exposed to sodium iodate were

not different (P > 0.05) [10]. In other words, changes I 8 in vitreous fluidity are not a confounding factor for DCE-MRI BRB PS measurements. More recent work

has confirmed the linearity and accuracy of the technique (Fig. 8.2.8) [7]. A linear relationship between vitreous enhancement (E) and Gd-DTPA dose was demonstrated when the injected Gd-DTPA dose was compared to the detected signal in sodium iodate treated rats (Fig. 8.2.8) [7]. Further, DCE-MRI BRB PS measurements were in agreement with PS measured using a more conventional approach with radiolabeled sucrose [7]. The concurrence between DCE-MRI BRB PS and “gold standard” values, given the methodological differences between the studies [e.g., destructive versus non-destructive tissue analysis and different tracers (Gd-DTPA and sucrose)] provides strong support for the underlying assumptions, linearity, and accuracy of the DCE-MRI approach.

Quantifying BRB damage using DCE-MRI is relatively new to ophthalmology. To help understand its weaknesses and strengths, comparisons with known clinical methods have been performed. One disadvantage is that DCE-MRI BRB measurements cannot be performed in a doctor’s office. However, DCEMRI using Gd-DTPA is widely available in most medical centers and we envision its use, not as a pri-

mary screening tool, but as a useful method when quantitative measures of BRB damage need to be assessed over time, such as in a drug clinical trial, or when fluorescein methods are not applicable or produce equivocal results. A potential weakness of DCEMRI was postulated by Plehwe et al., who measured T1 in selected volumes of vitreous after administration of Gd-DTPA and suggested that, at the lower limit of detection, MRI has a 1,200-fold lower sensitivity for detection of Gd-DTPA by reduction of T1 than that for the detection of fluorescent material by VFP [48]. However, Plehwe et al. did not present data to support this theoretical hypothesis and subsequent studies have clearly demonstrated that their prediction was incorrect [42, 70, 71]. To further demonstrate this, direct comparison of the detection sensitivity of VFP and DCE-MRI was performed using higher doses of Gd-DTPA, acquisition of a thicker slice, and use of T1-weighted images emphasizing relative differences in T1 between pixels rather than measurement of absolute T1. These studies were performed under a variety of experimental conditions, including focal retinal photocoagulation and cryopexy, grid photocoagulation obscured by vitreous hemorrhage, albino fundus, and endophthalmitis

(Table 8.2.1, personal communication with Harsha Sen, Noburo Ando, Brian Conway, and Eugene de Juan). DCE-MRI was found to reliably detect GdDTPA leakage from three diode laser burns, the smallest photocoagulation lesion distinguishable from untreated eyes by VFP (Table 8.2.1). These results strongly suggest that the sensitivity of MRI is at least comparable to that of VFP when based on detectable lesion size (Table 8.2.1). Conditions such as vitreous hemorrhage, albino fundus, and endophthalmitis can confound VFP data but had no effect on the analysis of the DCE-MRI data (some of this data is presented in Table 8.2.1, personal communication with Harsha Sen, Noburo Ando, Brian Conway, and Eugene de Juan) [10]. In addition, we also confirmed that increased vitreous fluidity, which can occur with age and disease and confound the accuracy of BRB damage assessments, as measured by the VFP method, does not affect the BRB PS as assessed by DCEMRI (Fig. 8.2.7) [10].

The spatial sensitivity of DCE-MRI BRB, which is considered a strength of the method, was evaluated by taking advantage of the retinal anatomy of the rabbit. The rabbit retina is largely avascular except for a band of blood vessels that extends temporally

Table 8.2.1. Comparison of VFP and DCE-MRI BRB assessment (mean ± SEM)

VFP was measured from vitreous fluorescence 2 mm anterior to retina 1 h after intravenous fluorescein injection.

N = number of eyes, P = p value for difference from control using Student’s unpaired t-test, PS’ = permeability surface area product normalized per unit area determined from the MR images

and nasally. This anatomy greatly simplifies the separate identification and quantitation of inner and outer BRB damage on an MRI slice perpendicular to the band of blood vessels. To demonstrate this, pigmented rabbits were treated with intravenous sodium iodate 30 mg/kg (a specific RPE cell poison), intravitreal N-ethylcarboxamidoadenosine (NECA, which specifically disrupts the vascular BRB), or retinal diode laser photocoagulation [60]. The pattern of enhancement observed in eyes treated with NECA clearly showed only inner BRB damage in contrast to the panretinal leakage that appeared following sodium iodate exposure. Simultaneous leakage from the outer and inner BRB in eyes treated with dense retinal laser photocoagulation could be localized and measured independently. Taken together, these data demonstrate that the detection and spatial sensitivity of DCE-MRI is at least as good as fluoresceinbased methods for monitoring BRB breakdown.

Nearly 15 years of research has also shown that DCE-MRI has wide applicability with regard to the study of BRB breakdown in various conditions and retinopathy models. It has been established that DCE-MRI BRB PS measurements are capable of detecting subtle changes in vascular permeability following ischemia/reperfusion, proliferative vitreoretinopathy, endophthalmitis, argon vs. diode photocoagulation, and ocular surgery, in addition to the diabetic applications discussed above [4, 7, 23, 24, 42, 67, 69 – 71]. Further, quantifiable changes in BRB permeability have proven useful in the assessment of various therapeutic interventions, including the effects of corticosteroid (triamcinolone acetonide) treatment [4, 24, 58, 70]. DCE-MRI using Gd-DTPA is routinely available in most medical centers and has many advantages over FA and VFP including the absence of optical limitations, the ability to produce images in any of three dimensions, and use of a tracer that is non-metabolized, non-toxic, non-actively transported, and has a well defined plasma concentration curve. Collecting DCE-MRI is thus relatively robust (i.e., can be done under a variety of adverse conditions) and uses a tracer that allows consider-

able simplification of, and confidence in, the data analysis. Importantly, these results lay the foundation for future translational studies in which the exact same biomarker and technique, BRB integrity measured by DCE-MRI, can be used to evaluate drug treatments in animal models of retinopathy and in human clinical studies.

To further explore the usefulness of DCE-MRI, additional comparisons have been made with nonclinical methods for assessing BRB damage as applied to the streptozocin-induced hyperglycemic rat, a model of diabetic retinopathy. The diabetic rat develops retinal pathological damage similar to the early stages that are characteristic of the non-proliferative stage of the human disease. Many investigators have attempted to examine early BRB in diabetic rat models as indirect evidence for increased risk of extracellular (or vasogenic) edema formation. The results of these studies, however, have been quite controversial [35]. Using a variety of non-MRI detection methods and tracers in diabetic or galactose-fed rats, some investigators have reported little or no change in BRB permeability in early diabetes [11, 19, 20, 25, 26, 29 – 31, 36, 63, 68]. In contrast, using a histologic detection method with an endogenous tracer (extravasation of albumin) only a subset of diabetic rats are reported to develop some leakage [44, 66, 67]. In further contrast, there are many reports, using either fluorescein, albumin, or Evans blue dye as the tracer, of BRB damage occurring as early as 2 weeks after inducing diabetes in rats [18, 27, 43, 66, 73].

To the best of our knowledge, these fundamental discrepancies regarding if and when BRB opening occurs in diabetic rats have not been resolved. It is noteworthy that a number of confounding variables have been shown to substantially affect detection of barrier integrity using fluorescein, albumin, and Evans blue dye, such as the size and properties of the probe, interpretation of increased albumin-based tracers, rapid binding of Evans blue dye to tissue protein, possible contamination from the aqueous humor, as well as the issues listed above that unambiguous interpretation of increased passive BRB

yet another potential factor that might help explain the contradictory conclusions of studies of diabetesinduced alteration in retinal permeability. Most of the experimental methods used to assess BRB integrity require animal death and/or enucleation before determining the extent of BRB damage. Using DCEMRI, we have shown that BRB becomes leaky immediately before and after death, possibly causing an artificial increase in retinal permeability in methods that require enucleation or retinal isolation to assess permeability (Fig. 8.2.9) [7]. In control rats, signal intensities change were minimal while the animal was alive (P > 0.05), indicating an intact BRB. However, clear evidence for BRB damage (P < 0.05) was found before loss of respiratory movement (i.e., death) (Fig. 8.2.9). In separate studies, BRB opening was also evident within minutes after sacrifice in animals with intravenous KCl injection (data not shown). Thus, near death conditions were associated with a rapid breakdown in BRB permeability. We speculate that control and diabetic rats might demonstrate this near death artifact to varying degrees producing different extents of BRB opening. In addition, extensive manipulation of the rats prior to BRB assessment may exacerbate the contribution of the death artifact. For example, Xu et al. allowed Evans blue dye to circulate for 90 min in ketamine/xylazine anesthetized rats before opening the chest cavity and perfusing via the left ventricle [72]. Note that the pneumothorax produced by opening of the chest

Fig. 8.2.9. Plots of vitreous humor % signal enhancement (E, filled symbols), normalized to the 1 min time point value, as a function of time postcontrast injection in control rats (n = 6, left panel), and control rats receiving additional urethane boluses (2×0.2 ml) i.v. (arrowheads; n = 3, right panel). In the right panel, respiratory movement (open circle), evident as low-intensity image smearing in the phase-encode direction, is also plotted and used to assess time of animal death

cavity will significantly compromise the animals’ physiology and soon lead to the animals’ death. In addition, the perfusion fluid maintained at pH 3.5 was used because it allowed optimal binding of Evans blue dye to albumin. It is possible that a pH 3.5 solution could produce a change in cell shape and BRB disruption. Nagy, Szabo, and Huttner found that cerebral perfusion with acidic pH buffer induced substantial blood-brain barrier leakage [45]. We have preliminary DCE-MRI data (not shown) suggesting that progressively lower arterial pHs will increase vitreous signal intensity enhancements (E) by Gd-DTPA in control rats. Although more work is needed, it appears possible that the use of low pH wash solutions alone can induce BRB damage. In addition, the rat was then perfused for 2 min at a physiological pressure of 120 mm Hg. However, ketamine/xylazine anesthetic will lower mean arterial blood pressure [73]. The act of perfusing the animal at a physiological pressure of 120 mm Hg may produce a relative acute hypertensive event. Such events are associated with damage to the bloodbrain barrier [40]. It is also possible that some dilution of extravascular dye by the perfusate also occurred. Taken together, these considerations raise the possibility that some combination of death, pneumothorax manipulation, acidic flush solution, and in situ perfusion could artificially induce BRB damage so that some of the Evans blue dye leaked into the retina.

To avoid these potentially confounding factors, including the death artifact, DCE-MRI was applied to rats that were diabetic for either 2, 4, 6, or 8 months [7]. No evidence for passive BRB leakage in rats was found until after 6 months of diabetes (Fig. 8.2.10) [7]. In addition, we did not see increased vitreous enhancement (E) following injection of gadolinium bound to human serum albumin (Fig. 8.2.11). It is possible that in vivo and ex vivo methods measure different aspects of BRB integrity. The MRI approach only measures tracer levels in the vitreous, while ex vivo approaches only monitor tracer concentrations intraretinally. Nonetheless, it is noteworthy that, given the extensive validation of the DCE-MRI technique discussed above, there does not appear to be evidence for early BRB damage in diabetic rats.

We next asked the question: what if DCE-MRI is not sensitive enough to detect a subtle diabetes induced lesion that might occur early in the time course of the disease process? If previous studies are correct and BRB damage occurred as early as 2 weeks of hyperglycemia, then, we reasoned, retinal edema might have developed by 3 months of diabetes. Interestingly, Park et al. claimed to have detected an increase in inner nuclear layer thickness at 1 week after inducing diabetes in rats [46]. However, no sta-

Fig. 8.2.10. Summary of BRB PS from control rats (n = 10), and

(*P < 0.05) was only found between the control and 8 month diabetic groups. These results are similar to other studies that find no evidence for early BRB damage [11, 19, 20, 25, 26, 29 – 31, 36, 63, 68]. We reasoned that perhaps an albumin-based contrast agent, as used in other studies of BRB damage, might reveal BRB damage. However, as shown in Fig. 11, no leakage was detected with a larger contrast agent

Fig. 8.2.11. Summary of vitreous humor % signal enhancement (E) in control and 3 month diabetic rats measured using a higher MW contrast agent [human serum albumin-(Gd-DTPA)30 (MW 93 kDa)] than Gd-DTPA (MW 590 Da). No betweengroup differences (P > 0.05) in vitreous signal intensity were found with albumin-(Gd-DTPA)30. These data provide additional evidence of lack of early BRB damage (Fig. 8.2.10)

tistical analysis of the data was presented to confirm that retina thickness actually increased over control values [46]. If edema was present in the study of Park et al., then it was brief since retinal thickness at later time points was not increased. Consequently, the relevance of the findings of Park et al. to more chronic edema typically found clinically is unclear [46]. In contrast, we, and others, have reported a decrease in retinal thickness relative to controls in regions about

diabetes [6, 38, 46]. This lack of evidence for chronic edema is not consistent with the reported BRB damage starting at 2 weeks of diabetes and continuing, nor is it consistent with the formation of intracellular edema. In other words, if there is BRB damage starting at 2 weeks of diabetes, and edema does not develop, how important is that BRB damage? Our data, and that of others, suggests the alternative: that edema does not form in diabetic rat retinas by 3 months (which is consistent with an intact BRB) [11, 19, 20, 25, 26, 29 – 31, 36, 63, 68].

Although diabetic rats do not appear to develop an early passive leakage through damaged BRB, it remains of value to measure how well treatments, such as receptor antagonists, correct increases in BRB PS induced by exogenous administered soluble mediators, such as vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VFP) [28]. This approach could help optimize treatment dosing, route, and scheduling. For example, a single intravitreous injection of VEGF/VPF is reported to induce a threeto fourfold increase in BRB permeability without disruption of retinal architecture [2, 3, 16, 39, 52]. Derevjanik et al. reported evidence of a roughly fourfold increased BRB damage to mannitol (182 Da) 6 h after intravitreous injection of 10–6 M VEGF in mice [16]. These findings are supported by DCE-MRI studies that found a threefold increase in BRB PS in the rat under similar conditions [7]. Since the dose of VEGF used in this latter study was higher than that used in other studies, it may appear that the sensitivity of DCEMRI is lower. For example, Xu et al. utilized a tenfold lower dose (50 ng) of VEGF and reported a fourfold increase in BRB leakage [72]. However, based on our calculations (shown below), the VEGF dose used in the present study (and by Derevjanik et al. [16]) and in the work of Xu et al. are both higher than needed to achieve substantial equilibrium binding. Assuming a vitreous volume is 50 μl in the rat [34], our VEGF dose of 0.5 μg corresponds to a vitreous concentration of 0.24 × 10–6 M and the dose used by Xu et al. corresponds to a vitreous level of 0.24 × 10–7 M. Since the Kd of VEGF for its two receptors is ~10–10– 10–11 M [13, 14], both VEGF doses are clearly higher than needed to achieve substantial equilibrium binding. DCE-MRI BRB PS measurements appear to be a sensitive and quantitative approach for monitoring treatment efficacy on VEGF/VPF-induced BRB damage in rats.

Finally, we have demonstrated another advantage of DCE-MRI: providing a useful surrogate marker of BRB breakdown in patients with diabetic macular

edema (DME) [64]. We found that the mean slope of 8 I the signal enhancement function was significantly greater in diabetic patients than in controls, indicating a greater time-dependent accumulation of contrast in the pre-macular vitreous of diabetic patients [64]. These data are consistent with results from previous VFP studies that demonstrate significantly greater contrast influx in diabetic patients with edema than in control groups [55, 56, 57]. Previously reported differences between patients with non-clin- ically significant edema and clinically significant edema subgroups [55, 56, 57] were not found in this study and may be due to the different sizes of the subgroups studied by VFP and DCE-MRI. We think that DCE-MRI will be most beneficially applied together with other quantitative methods for evaluating edema, such as OCT, since DCE-MRI provides a snapshot in time of BRB dysfunction that may not be obtained from potentially more slowly changing structural measures but may be independent of retinal thickening. Future studies comparing DCE-MRI

with OCT are now warranted.

8.2.3 Conclusions

In summary, it would appear that clinical management of retinal edema based only on measurement of either retinal thickness or BRB integrity is not adequate for evaluating medical treatment of edema because the ratio of the contribution of intraand extracellular mechanisms to edema or how this ratio changes between patients, between different retinopathies, and over time is not well defined [32]. In principle, quantitatively measuring both retinal thickness and BRB integrity can provide enough information to substantially improve diagnosis and prognosis of retinal edema. For example, finding that an increase in retinal thickness occurs with an intact BRB would imply the presence of intracellular edema. As another example, if treatments corrected a BRB defect but have no effect on elevated retinal thickness, it would imply a strong intracellular component. Currently, such distinctions cannot be made. One reason is that present methods for detecting (e.g., FA) and/or measuring (e.g., VFP) BRB damage clinically are suboptimal, as described above. Simpler, more available, robust, and accurate measures of BRB PS are needed, especially to quantify the impact of medical therapy on edema. In this chapter, we highlight DCE-MRI as addressing this need, since clinical MRI machines are widely available, in vivo measures of BRB PS are obtained without any manipulation of the eye, and the method is sensitive and well validated against a range of other methods. While DCE-MRI is not expected to be screening tool, we anticipate its increasing use in the clinic when fluorescein-based

methods are not applicable or produce equivocal results. As a research tool for use in clinical trials or in experimental settings, DCE-MRI BRB PS studies hold even more promise. Further, DCE-MRI BRB PS studies are applicable experimentally and have helped define the conditions under which other techniques reported to measure the extent of BRB breakdown should be validated (e.g., they should be linear and provide a physiologic measure of PS). Finally, DCEMRI BRB PS studies have identified animal models that are useful (e.g., evaluating drug treatments for VEGF-induced BRB opening) or not (e.g., early changes in the diabetic rat).

References

1.Aiello LP (2002) The potential role of PKC beta in diabetic retinopathy and macular edema. Surv Ophthalmol 47 Suppl 2:S263–S269

2.Aiello LP, Bursell SE, Clermont A, Duh E, Ishii H, Takagi C, et al. (1997) Vascular endothelial growth factor-induced retinal permeability is mediated by protein kinase C in vivo and suppressed by an orally effective beta-isoform-selec- tive inhibitor. Diabetes 46(9):1473 – 1480

3.Alikacem N, Yoshizawa T, Nelson KD, Wilson CA (2000) Quantitative MR imaging study of intravitreal sustained release of VEGF in rabbits. Invest Ophthalmol Vis Sci 41(6):1561 – 1569

4.Ando N, Sen HA, Berkowitz BA, Wilson CA, de Juan E, Jr (1994) Localization and quantitation of blood-retinal barrier breakdown in experimental proliferative vitreoretinopathy. Arch Ophthalmol 112(1):117 – 122

5.Antonetti DA, Barber AJ, Khin S, Lieth E, Tarbell JM, Gardner TW (1998) Vascular permeability in experimental diabetes is associated with reduced endothelial occludin content: vascular endothelial growth factor decreases occludin in retinal endothelial cells. Penn State Retina Research Group. Diabetes 47(12):1953 – 1959

6.Barber AJ, Lieth E, Khin SA, Antonetti DA, Buchanan AG, Gardner TW (1998) Neural apoptosis in the retina during experimental and human diabetes. Early onset and effect of insulin. J Clin Invest 102(4):783 – 791

7.Berkowitz BA, Roberts R, Luan H, Peysakhov J, Mao X, Thomas KA (2004) Dynamic contrast-enhanced MRI measurements of passive permeability through blood retinal barrier in diabetic rats. Invest Ophthalmol Vis Sci 45(7): 2391 – 2398

8.Berkowitz BA, Sato Y, Wilson CA, de Juan E (1991) Bloodretinal barrier breakdown investigated by real-time magnetic resonance imaging after gadolinium-diethylenetria- minepentaacetic acid injection. Invest Ophthalmol Vis Sci 32(11):2854 – 2860

9.Berkowitz BA, Tofts PS, Sen HA, Ando N, de Juan E, Jr (1992) Accurate and precise measurement of blood-retinal barrier breakdown using dynamic Gd-DTPA MRI. Invest Ophthalmol Vis Sci 33(13):3500 – 3506

10.Berkowitz BA, Wilson CA, Tofts PS, Peshock RM (1994) Effect of vitreous fluidity on the measurement of blood-ret- inal barrier permeability using contrast-enhanced MRI. Magn Reson Med 31(1):61 – 66

11.Caspers-Velu LE, Wadhwani KC, Rapoport SI, Kador PF

(1995) Permeability of the blood-retinal and blood-aque- ous barriers in galactose-fed rats. J Ocul Pharmacol Ther 11(3):469 – 487

12.Cunha-Vaz JG (2004) The blood-retinal barriers system. Basic concepts and clinical evaluation. Exp Eye Res 78(3): 715 – 721

13.Cunningham SA, Stephan CC, Arrate MP, Ayer KG, Brock TA (1997) Identification of the extracellular domains of Flt- 1 that mediate ligand interactions. Biochem Biophys Res Commun 231(3):596 – 599

14.Cunningham SA, Tran TM, Arrate MP, Brock TA (1999) Characterization of vascular endothelial cell growth factor interactions with the kinase insert domain-containing receptor tyrosine kinase. A real time kinetic study. J Biol Chem 274(26):18421 – 18427

15.Dallal MM, Chang SW (1994) Evans blue dye in the assessment of permeability-surface area product in perfused rat lungs. J Appl Physiol 77(2):1030 – 1035

16.Derevjanik NL, Vinores SA, Xiao WH, Mori K, Turon T, Hudish T, et al. (2002) Quantitative assessment of the integrity of the blood-retinal barrier in mice. Invest Ophthalmol Vis Sci 43(7):2462 – 2467

17.DiMattio J (1991) In vivo use of neutral radiolabelled molecular probes to evaluate blood-ocular barrier integrity in normal and streptozotocin-diabetic rats. Diabetologia 34(12):862 – 867

18.Enea NA, Hollis TM, Kern JA, Gardner TW (1989) Histamine H1 receptors mediate increased blood-retinal barrier permeability in experimental diabetes. Arch Ophthalmol 107(2):270 – 274

19.Ennis SR (1990) Permeability of the blood-ocular barrier to mannitol and PAH during experimental diabetes. Curr Eye Res 9(9):827 – 838

20.Ennis SR, Betz AL (1986) Sucrose permeability of the blood-retinal and blood-brain barriers. Effects of diabetes, hypertonicity, and iodate. Invest Ophthalmol Vis Sci 27(7): 1095 – 1102

21.Ferris FL, III, Patz A (1984) Macular edema. A complication of diabetic retinopathy. Surv Ophthalmol 28 Suppl: 452 – 461

22.Frank JA, Dwyer AJ, Girton M, Knop RH, Sank VJ, Gansow OA, et al. (1986) Opening of blood-ocular barrier demonstrated by contrast-enhanced MR imaging. J Comput Assist Tomogr 10(6):912 – 916

23.Funatsu H, Wilson CA, Berkowitz BA, Sonkin PL (1997) A comparative study of the effects of argon and diode laser photocoagulation on retinal oxygenation. Graefes Arch Clin Exp Ophthalmol 235(3):168 – 175

24.Garner WH, Scheib S, Berkowitz BA, Suzuki M, Wilson CA, Graff G (2001) The effect of partial vitrectomy on blood-ocu- lar barrier function in the rabbit. Curr Eye Res 23(5):372 – 381

25.Grimes PA (1985) Fluorescein distribution in retinas of normal and diabetic rats. Exp Eye Res 41(2):227 – 238

26.Grimes PA (1988) Carboxyfluorescein distribution in ocular tissues of normal and diabetic rats. Curr Eye Res 7(10):981 – 988

27.Jones CW, Cunha-Vaz JG, Rusin MM (1982) Vitreous fluorophotometry in the alloxanand streptozocin-treated rat. Arch Ophthalmol 100(7):1141 – 1145

28.Kent D, Vinores SA, Campochiaro PA (2000) Macular oedema: the role of soluble mediators. Br J Ophthalmol 84(5): 542 – 545

29.Kirber WM, Nichols CW, Grimes PA, Winegrad AI, Laties AM (1980) A permeability defect of the retinal pigment epi-

ability changes at blood-retinal barrier in diabetes and effect of aldose reductase inhibition. Am J Physiol 259: R601–R605

31.Lightman S, Rechthand E, Terubayashi H, Palestine A, Rapaport S, Kador P (1987) Permeability changes in blood-reti- nal barrier of galactosemic rats are prevented by aldose reductase inhibitors. Diabetes 36:1271 – 1275

32.Lobo C, Bernardes R, Faria dA, Jr, Cunha-Vaz JG (1999) Novel imaging techniques for diabetic macular edema. Doc Ophthalmol 97(3 – 4):341 – 347

33.Luan H, Roberts R, Sniegowski M, Goebel DJ, Berkowitz BA (2006) Retinal thickness and subnormal retinal oxygenation response in experimental diabetic retinopathy. Invest Ophthalmol Vis Sci 47(1):320 – 8

34.Lukaszew RA, Mullins CM, Penn JS, Berkowitz BA (1997) Noninvasive and quantitative staging of hyaloidopathy in experimental retinopathy of prematurity. Invest Ophthalmol Vis Sci 38(4):S747

35.Lund-Andersen H (2002) Mechanisms for monitoring changes in retinal status following therapeutic intervention in diabetic retinopathy. Surv Ophthalmol 47 Suppl 2:S270– S277

36.Maepea O, Karlsson C, Alm A (1984) Blood-ocular and blood-brain barrier function in streptozocin-induced diabetes in rats. Arch Ophthalmol 102(9):1366 – 1369

37.Marmor MF (1999) Mechanisms of fluid accumulation in retinal edema. Doc Ophthalmol 97(3 – 4):239 – 249

38.Martin PM, Roon P, Van Ells TK, Ganapathy V, Smith SB (2004) Death of retinal neurons in streptozotocin-induced diabetic mice. Invest Ophthalmol Vis Sci 45(9):3330 – 3336

39.Mathews MK, Merges C, McLeod DS, Lutty GA (1997) Vascular endothelial growth factor and vascular permeability changes in human diabetic retinopathy. Invest Ophthalmol Vis Sci 38(13):2729 – 2741

40.Mayhan WG (2001) Regulation of blood-brain barrier permeability. Microcirculation 8(2):89 – 104

41.Menzies SA, Hoff JT, Betz AL (1990) Extravasation of albumin in ischaemic brain oedema. Acta Neurochir Suppl (Wien) 51:220 – 222

42.Metrikin DC, Wilson CA, Berkowitz BA, Lam MK, Wood GK, Peshock RM (1995) Measurement of blood-retinal barrier breakdown in endotoxin-induced endophthalmitis. Invest Ophthalmol Vis Sci 36(7):1361 – 1370

43.Murata T, Ishibashi T, Khalil A, Hata Y, Yoshikawa H, Inomata H (1995) Vascular endothelial growth factor plays a role in hyperpermeability of diabetic retinal vessels. Ophthalmic Res 27(1):48 – 52

44.Murata T, Nakagawa K, Khalil A, Ishibashi T, Inomata H, Sueishi K (1996) The relation between expression of vascular endothelial growth factor and breakdown of the blood-reti- nal barrier in diabetic rat retinas. Lab Invest 74(4):819 – 825

45.Nagy Z, Szabo M, Huttner I (1985) Blood-brain barrier impairment by low pH buffer perfusion via the internal carotid artery in rat. Acta Neuropathol (Berl) 68(2): 160 – 163

46.Park SH, Park JW, Park SJ, Kim KY, Chung JW, Chun MH, et al. (2003) Apoptotic death of photoreceptors in the strepto- zotocin-induced diabetic rat retina. Diabetologia 46(9): 1260 – 1268

47.Pelc NJ (1993) Optimization of flip angle for T1 dependent contrast in MRI. Magn Reson Med 29(5):695 – 699

DTPA MRI enhancement curve. J Magn Reson Series B 102:129 – 136

62.Tofts PS, Berkowitz BA (1994) Measurement of capillary permeability from the Gd enhancement curve: a comparison of bolus and constant infusion injection methods. Magn Reson Imaging 12(1):81 – 91

63.Tornquist P, Alm A, Bill A (1990) Permeability of ocular vessels and transport across the blood-retinal-barrier. Eye 4(2):303 – 309

64.Trick GL, Liggett J, Levy J, Adamsons I, Edwards P, Desai U, et al. (2005) Dynamic contrast enhanced MRI in patients with diabetic macular edema: initial results. Exp Eye Res 81(1):97 – 102

65.Vinores SA, Derevjanik NL, Mahlow J, Berkowitz BA, Wilson CA (1998) Electron microscopic evidence for the mechanism of blood-retinal barrier breakdown in diabetic rabbits: comparison with magnetic resonance imaging. Pathol Res Pract 194(7):497 – 505

66.Vinores SA, McGehee R, Lee A, Gadegbeku C, Campochiaro PA (1990) Ultrastructural localization of blood-retinal barrier breakdown in diabetic and galactosemic rats. J Histochem Cytochem 38(9):1341 – 1352

67.Vinores SA, Van Niel E, Swerdloff JL, Campochiaro PA (1993) Electron microscopic immunocytochemical evidence for the mechanism of blood-retinal barrier breakdown in galactosemic rats and its association with aldose reductase expression and inhibition. Exp Eye Res 57(6): 723 – 735

68.Wallow IH (1983) Posterior and anterior permeability defects? Morphologic observations on streptozotocintreated rats. Invest Ophthalmol Vis Sci 24(9):1259 – 1268

69.Wilson CA, Berkowitz BA, Funatsu H, Metrikin DC, Harrison DW, Lam MK, et al. (1995) Blood-retinal barrier breakdown following experimental retinal ischemia and reperfusion. Exp Eye Res 61(5):547 – 557

70.Wilson CA, Berkowitz BA, Sato Y, Ando N, Handa JT, de Juan E, Jr (1992) Treatment with intravitreal steroid reduces blood-retinal barrier breakdown due to retinal photocoagulation. Arch Ophthalmol 110(8):1155 – 1159

71.Wilson CA, Fleckenstein JL, Berkowitz BA, Green ME (1992) Preretinal neovascularization in diabetic retinopathy: a preliminary investigation using contrast-enhanced magnetic resonance imaging. J Diabetes Complications 6(4):223 – 229

72.Xu Q, Qaum T, Adamis AP (2001) Sensitive blood-retinal barrier breakdown quantitation using Evans blue. Invest Ophthalmol Vis Sci 42(3):789 – 794

73.Yi-Ming W, Shu H, Miao CY, Shen FM, Jiang YY, Su DF (2004) Asynchronism of the recovery of baroreflex sensitivity, blood pressure, and consciousness from anesthesia in rats. J Cardiovasc Pharmacol 43(1):1 – 7

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I 9

9.1 Anatomy

The human eye is supplied by two vascular systems: the retinal and the uveal vessels. The uveal vascular system consists of the iris, the ciliary body and the choroid. The outer layers of the retina including the photoreceptors are nourished by the choroid, whereas the inner layers of the retina including the retinal ganglion cells are supplied by the retina. Approximately 65 % of the oxygen consumed by the retina is delivered from the choroid.

In humans both the choroidal and the retinal vessels are supplied from branches of the ophthalmic artery (Fig. 9.1). The central retinal artery provides the blood to the retina; the central retinal vein drains the retina. The anterior choroid is supplied from the long posterior ciliary arteries, whereas the posterior choroid is supplied from the short posterior ciliary arteries. The choroid drains into the vortex veins.

The retinal blood flow is characterized by a low perfusion rate (approximately 40 – 80 μl/min; Fig. 9.2), a high vascular resistance, a high oxygen extraction and accordingly a high arteriovenous oxygen extraction (35 – 40 %). By contrast, the choroid shows the highest perfusion rate per gram tissue of all vascular beds within the human body (approximately 1,200 – 2,000 μl/min), a low vascular resistance, a low oxygen extraction and a low arteriovenous oxygen

Fig. 9.1. Vascular supply of the eye (ONH optic nerve head, LPCA long posterior ciliary artery, MPCA middle posterior ciliary artery, CRA central retinal artery)

extraction (3 – 5 %). The driving force of blood flow in the eye is the ocular perfusion pressure (OPP). As in other vascular beds the perfusion pressure is the difference between the arterial and the venous pressures. The situation in the eye is, howev-