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(Investigative Ophthalmology and Visual Science. 2001;42:433-438.)
© 2001 by The Association for Research in Vision and Ophthalmology, Inc.

Diabetic Macular Edema: Passive and Active Transport of Fluorescein through the Blood–Retina Barrier

Birgit Sander, Michael Larsen, Birgitte Moldow and Henrik Lund-Andersen

From the Department of Ophthalmology, Herlev Hospital, University of Copenhagen, Denmark.


   Abstract
Top
 Abstract
Introduction
 Subjects and Methods
 Results
 Discussion
 References
 
PURPOSE. To investigate the passive bidirectional and active outward transport of fluorescein through the blood–retina barrier (BRB) in diabetic patients with clinically significant macular edema and in healthy controls.

METHODS. The passive and active transport of fluorescein through the BRB was quantitated by vitreous fluorometry. A previously developed method was used to model passive transport. A new simulation model was developed and evaluated for estimation of active transport. The study included 10 eyes of 5 healthy controls and 31 eyes of 20 diabetic patients with clinically significant diabetic macular edema (CSME) in at least one eye, totalling 25 eyes with CSME.

RESULTS. Passive permeability of fluorescein was increased by a factor of 12 in eyes with edema compared to healthy controls (edema, 23.7 nm/sec; healthy subjects, 1.9 nm/sec, P < 0.01), whereas the active transport was doubled (edema, 84.1 nm/sec; healthy subjects, 43.5 nm/sec, P < 0.01). Unlike active transport, passive permeability was related to the degree of retinopathy, in that eyes with severe non-proliferative diabetic retinopathy had a passive permeability that was significantly increased compared to moderate retinopathy (32.1 nm/sec and 14.6 nm/sec, respectively, P < 0.05). The passive movement quantitated with vitreous fluorometry was larger for diffuse and mixed leakage compared to focal (P = 0.07).

CONCLUSIONS. Insofar as the movement of fluorescein can be taken as a probe for the movement of electrolytes and water, the pathogenesis of diabetic macular edema seems to involve a disruption of the BRB, presumably its inner component. The active resorptive functions of the blood–retina barrier appear to be compensatorily increased to counteract edema formation, although the increase is too small to prevent edema in the face of severe leakage through the blood–retina barrier.


   Introduction
Top
 Abstract
 Introduction
Subjects and Methods
 Results
 Discussion
 References
 
The blood–retina barrier separates the neuroretina from the blood. The barrier function is located at the retinal pigment epithelium and the endothelium of the retinal vessels. The movement of water through the blood–retina barrier appears to have two dominant components: A passive (bidirectional) transport and an active transport directed from the retina to the blood. Theoretically, macular edema develops when the inflow of fluid into the retina exceeds the outflow. Since the movement of water across the blood–retina barrier cannot be traced noninvasively, clinical research has been confined to the use of fluorescein as a surrogate marker of fluid and electrolyte movement. Passive transport (permeability) of fluorescein has been shown to increase in relation to the development of retinopathy in the presence of macular edema.1 2

In vitro studies of isolated retinal pigment–epithelium–choroid preparations have shown that the outward active transport of fluorescein is substantially larger than the passive transport and that this transport is inhibited by metabolic (oubain) and competitive inhibitors (probenecid).3 4 5 In humans, the active transport of fluorescein measured with vitreous fluorometry has also been shown to be inhibited by probenecid.6 A study of patients with retinitis pigmentosa complicated with macular edema7 has shown an increase in active transport, whereas the role of active transport in diabetic macular edema is unknown.

In the clinical study presented here, we have examined the passive and the active transport of fluorescein through the blood–retina barrier to evaluate the relative importance of these components in the pathogenesis of diabetic macular edema. In addition, a simulation method has been developed for the calculation of active transport.


   Subjects and Methods
Top
 Abstract
 Introduction
 Subjects and Methods
Results
 Discussion
 References
 
Subjects
Twenty-four diabetic patients with clinically significant diabetic macular edema (CSME) in at least one eye were examined consecutively. Both patients with IDDM and NIDDM were included. Patients with proliferative retinopathy, cataract, pseudophakia, or aphakia were excluded. Clinically significant macular edema was defined according to the ETDRS criteria as retinal thickening within 500 µm of the umbo, as hard exudates within the same 500 µm if associated with retinal thickening, and as a >1 optic disc area of retinal thickening if any part of the edematous area is within 1-disc diameter from the umbo.8

In eyes with posterior vitreous detachment or vitreous liquefaction near the optical axis, vitreous fluorometry is unreliable as a tool to assess the blood–retina barrier, because convection replaces the otherwise steep preretinal diffusion gradient by a flat curve in front of the retina.7 Consequently, we used fluorometry scans obtained 30 minutes after fluorescein injection to assess the qualitative properties of the vitreous. If the vitreous curve was flat immediately in front of the retina or throughout the posterior vitreous, the eye was excluded. Fifteen eyes were excluded because of such signs of vitreous liquefaction. Two additional eyes were excluded because of a lid defect and vitreous hemorrhage. Of the remaining 31 eyes, 25 had CSME, whereas no CSME was found in 6 eyes (Table 1) . The mean age was 56 years (range, 28–74), and the mean diabetes duration was 10 years (range, 1–34). Metabolic control and blood pressure had been determined every 3 months in the year before examination. The mean arterial blood pressure value was 102 mm Hg (range, 83–117) and the mean HbA1c was 8.9 (range, 5.3–12.1).


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  		Table 1. Clinical Characteristics and Visual Acuity (VA)

 
Five healthy subjects (mean age, 24 years; range, 22–27) without eye diseases, vitreous liquefaction, or known systemic disease were also included.

The study was approved by the local medical ethics committee. All participants gave their written informed consent after full information according to the Helsinki declaration.

Methods
Stereoscopic color fundus photography and fluorescein angiography were recorded using a Canon fundus camera (CF-60UV). Retinopathy was graded on 60° fundus photographs using a procedure adapted to the modified Arlie House description.9 Presence of CSME was evaluated on 40° fundus photographs by one ophthalmologist and one technician. In case of disagreement, another ophthalmologist made the final decision as to the grading level.

The fraction of leakage originating from microaneurisms versus more diffuse leakage was evaluated on fluorescein angiography based on the ETDRS system as focal, mixed or diffuse leakage (more than 67%, between 33 and 67% and below 33% leakage from microaneurisms).10

Fluorescein and its metabolite fluorescein glucuronide were measured using an ocular fluorometer (Fluorotron; OcuMetrics, San Jose, CA) adapted to differential spectrofluorometry. With this method, the light source is an argon laser changing rapidly between two different excitation wavelengths (458 and 488 nm) allowing separate determination of fluorescein and fluorescein glucuronide.11 12 13 Pupillary dilation was induced by topical phenylephrine 10% and cyclopentolate 0.5%. After a bolus injection of 14 mg/kg disodium fluorescein, postinjection scans were performed at 30 and 60 minutes for the calculation of the passive permeability of the blood–retina barrier, and at 7, 8, 9, and 10 hours for the active transport (4 scans at each session). Blood samples were obtained at 5, 15, 30, and 60 minutes and at the time of the late postinjection scans. Visual acuity was measured with standard, retroilluminated ETDRS charts.

After intravenous fluorescein injection, the dye diffuses from the retina into the vitreous. The concentration close to the retina is high shortly after injection, decreasing drastically toward the center of the eye (Fig. 1) . In the anterior part of the eye, a high concentration of fluorescein is seen in the anterior chamber, and a lower gradient builds up behind the lens because fluorescein diffuses into the anterior vitreous from the posterior chamber.



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  		Figure 1. Vitreous fluorescein concentration curve 60 minutes after fluorescein injection in a diabetic patient with macular edema. The fluorescein concentration is high near the retina (0 mm), decreasing rapidly toward the center of the eye. In the anterior chamber (appr. 25 mm, open arrow) a high fluorescein concentration is seen due to the leakage through the blood-aqueous barrier. The autofluorescense signal from the lens has been removed (18 to 23 mm). Note the peak at 17 mm (black arrow) due to fluorescein behind the lens that is derived from the anterior part of the eye.

 
The passive transport or permeability (Ppassive) of the blood–retina barrier and the diffusion coefficient of fluorescein in the vitreous (D) is calculated from the vitreous curve 1 to 5 mm in front of the retina.14 15 The model assumes a homogeneous blood–retina barrier without active transport being involved in the movement of fluorescein and an intact vitreous gel. The simplified modeling of the barrier is justified, as the flux of fluorescein is nearly unidirectional during the early phase, driven by the steep concentration gradient from the plasma toward the vitreous. The model corrects for variations in plasma concentration and absorption of the lens.

In time, the flux of fluorescein from the plasma to the eye diminishes and the net movement changes at the outward direction from the vitreous to the blood. The preretinal gradient reverses so that the concentration at the retina is lower than in the center of the vitreous (Fig. 2) . Assuming equal electrical charges on both sides of the retina and absence of bulk flow, the outward flux through the blood–retina barrier equals the equidirectional flux in the nearby vitreous.



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  		Figure 2. Vitreous fluorescein concentration curves 8 hours after fluorescein injection. The preretinal fluorescein gradient is reversed compared to Figure 1 and the net, outward transport from the vitreous to the blood is indicated by the preretinal gradient.

 
Assuming that the outwardly directed transport is mainly due to active transport, a quantitative estimate can be derived from the equation: Outward transport = (D * dC/dx) * Cr-1, where dC/dx is the preretinal fluorescein gradient, and Cr-1 is the fluorescein concentration at the retina.16 With this method, the passive component of the outward transport is ignored and the active transport is calculated from measurements at fixed time intervals, 7 to 10 hours after injection. A previous study attempted to account for the non-negligible concentrations of fluorescein in the plasma at the time of the late vitreous scans and their contribution to the net outward flux at this time.7 17 The effect of this correction is small and does not solve the additional problem that there is a time lag between the disappearance of fluorescein from the vitreous and the plasma. In other words, it does not account for the non-steady–state nature of the kinetics of fluorescein. This is of particular concern in studies of macular edema, where considerable inter-eye variation is seen in both the speed of diffusion in the vitreous (D) and in passive and active transport. Consequently, a model is needed, where active transport is calculated by taking the non-steady–state nature of the system into consideration.

We made no attempt at determining the active transport of the metabolite fluorescein glucuronide, because animal studies have shown that fluorescein glucoronide movement across the blood–retina barrier is predominantly passive and because the late-phase concentrations of fluorescein glucuronide are close to the lower limit of the effective range of the fluorometer.

The preretinal fluorescein curve was simulated using a compartment model with two different types of geometry. In healthy subjects, a spherical model of the eye was initially constructed as previously reported.3 18

The space between the retina and the center of the eye was divided into approximately 300 conical cells (Fig. 3) , the length of each cell being coupled to the diffusion coefficient of fluorescein in the vitreous and defined from the Einstein equation for diffusion in one direction, that is, along the optical axis of the eye (cell length = {surd}D * 2 * t), where D represents the diffusion coefficient and t is time. The vitreous fluorescein concentration calculations were reiterated once for each second elapsed between injection and vitreous concentration measurement. Passive transport from the blood into the first cell of the model was calculated from the concentration difference between the observed plasma concentration curve and the outermost cell multiplied by the passive permeability. An initial estimate of outward transport was obtained using a previously described simpler method (outward transport = D * dC/dx * Cr-1). The diffusion between cells was calculated using Fick’s law. The innermost cell in the center of the eye was regarded as the point of symmetry. The method for estimation of the active transport (Tactive) employs successive iterative model calculations comparing model and experimental data using a least-quadrant method to obtain the best possible fit between model and observations.



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  		Figure 3. Simplified spherical model of the blood–retina barrier. The distance from the retina to the center of the eye is divided into a large number of conical cells. The fluorescein concentration in the cell next to the retina is estimated from the plasma fluorescein concentration curve and passive and active transport estimates.

 
Simulated vitreous curves are shown in Figure 4 , from 30 minutes to 10 hours. The time-related changes in the curves are as expected from experimental data for healthy subjects.



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  		Figure 4. Vitreous curves calculated with the spherical model, from 30 minutes to 8 hours after fluorescein injection. Input data (Ppassive, 2.5 nm/sec; Tactive, 25 nm/sec) corresponds to a healthy subject.

 
The late-phase preretinal fluorescein concentration curves observed in patients with marked diabetic macular edema generally did not fit the curves predicted by the mathematical model (Fig. 5) . To correct this discrepancy, the model was changed using a cubic cell shape (Fig. 6) with the same cell length as the conical cell that was used previously. After this change of geometry, the model fitted well in cases of marked edema.



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  		Figure 5. Vitreous curves (8 hours) calculated with the spherical model and experimental data from a healthy subject (Ppassive, 2.5 nm/sec; Tactive, 25 nm/sec) and a patient with a large CSME (Ppassive, 53 nm/sec; Tactive, 86 nm/sec). The model fits closely to experimental data for the healthy subject but a large discrepancy is seen in macular edema, which cannot be corrected by changes in the active transport.

 


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  		Figure 6. Simplified cylindrical model of the blood–retina barrier. Assuming diffusion from the anterior part of the eye into the vitreous and low leakage in the peripheral part of the retina compared to the posterior pole, the concentration gradients will form elipsoid curves in the vitreous (thin lines). The major direction of fluorescein flux is orthogonal to the concentration.

 
The theoretical justification for the use of the cylindrical model is the high degree of macular leakage that is not matched by a similar increase in peripheral results in a preretinal concentration profile similar to that obtained from a planar surface with a uniform leakage. Since the 60° fluorescein angiograms used in the study did not show marked peripheral leakage in any patient and because Dalgaard et al. has shown that leakage from outside the macula does not contribute the concentrations measured on the optical axis of the eye, we assume that the sources of variation are to be found within the macula.19

Tests of the cubic model on healthy subjects and patients with minor degrees of diabetic macular edema demonstrated that the cubic model failed to fit the scans from these subjects. Our presumptive explanation is that at 6 to 8 hours after the injection of fluorescein, the concentration of fluorescein in the vitreous is determined by diffusion from both the retina and from the anterior segment of the eye. The relative leakage varies markedly between the groups of subjects in the present study. In healthy subjects and diabetic patients with minor degrees of leakage through the blood–retina barrier, the fluorescein concentration in the anterior chamber is approximately 5 times the concentration 1 mm in front of the retina at one hour after injection. It is apparent at this point that fluorescein from the posterior chamber and the ciliary body has reached the anterior vitrous behind the lens (Fig. 1) . Given 5 to 7 hours more time to diffuse from here to the rest of the vitreous, it is obvious that retrolental fluorescein will have moved into the posterior vitreous. Consequently, the conventional assumption of spherical symmetry used in the modeling of posterior vitreous fluorescein kinetics had to be abandoned. Instead, the center of symmetry of the model was moved from the geometrical center of the posterior vitreous hemisphere to a position closer to the retina and the magnitude of this movement was adjusted according to the observed difference in concentration between the anterior chamber and the preretinal vitreous. Using such a model, acceptable agreement with the observed vitreous curves was found both in healthy subjects and in diabetic patients with various degrees of macular edema (Fig. 7) . The results confirm earlier studies,4 18 which have shown that diffusion in both directions between the anterior and the posterior part of the eye is important when vitreous fluorometry is performed many hours after injection.



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  		Figure 7. Experimental and model curves (cylindrical) of vitreous fluorescein for a patient with a large CSME (Ppassive, 53 nm/sec; Tactive, 116 nm/sec) and a patient with moderate CSME (Ppassive, 9 nm/sec; Tactive, 97 nm/sec), both at 8 hours after fluorescein injection. Large differences are seen in the vitreous concentrations corresponding to large differences in passive permeability. The model fits closely to the experimental data in both patients and also in healthy subjects (not shown). Minor deviations are seen very close to the retina, where the measurements are unreliable because of optical limitations of the ocular fluorometer and near the center of the eye.

 
To evaluate the final model, the calculated values for fluorescein concentration at the retina (Cr) were compared with the observed curves for all subjects. The correlation between calculated and simulated values was very high (r = 0.90), indicating that the model fitted well over the range of healthy controls and diabetic patients.

Active transport as calculated by the final simulation model was linearly correlated with the previous method6 16 (r = 0.91) with an overall increase in Tactive of 65%. The reason for the increase compared to the previously used method is probably that the model calculates the active transport right at the retina and not by backward extrapolation of the preretinal curve observed from 1 to 5 mm into the vitreous. The latter leads to underestimation of the gradient at the retina, because the gradient theoretically must be steeper the closer it is to the retina.


   Results
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
Discussion
 References
 
Passive and Active Transport
In CSME, the passive permeability was significantly increased by one order of magnitude compared to both healthy subjects and eyes with diabetic retinopathy without edema (log-transformed t-test, P < 0.01, Table 2 ). In patients with CSME, the variation in passive transport was considerable (1.9 to 72.2 nm/sec), corresponding to the wide range of severity found by clinical evaluation and fluorescein angiography.


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  		Table 2. Passive Permeability (Ppassive) and Active Transport (Tactive) in Diabetic Patients

 
No significant difference was found between the passive permeability of fluorescein (Ppassive 24.7 ± 19.7 nm/sec (mean ± SD)) and fluorescein glucuronide (Ppassive 27.5 ± 25.5 nm/sec).

The outward, active transport in macular edema was doubled (log-transformed t-test, P < 0.01, Table 2 ) compared to healthy controls.

The ratio between active and passive transport was significantly reduced in patients with macular edema compared to healthy controls (P < 0.02, Mann–Whitney U test, Table 2 and Fig. 8 ). In diabetic eyes without edema, the ratio was higher than in healthy controls, but not significantly (P = 0.2).



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  		Figure 8. The ratio of active to passive transport in healthy subjects, diabetic patients without edema, and patients with CSME. The ratio is significantly reduced when edema is present. In diabetic eyes without edema, the ratio is significantly increased compared to CSME and higher than in healthy subjects.

 
Retinopathy
Eyes with CSME had either mild to moderate (ETDRS levels 35 to 47, n = 13) or severe (ETDRS level 53, n = 12) diabetic background retinopathy, whereas all diabetic eyes without CSME had only mild or moderate retinopathy. The passive permeability was significantly increased in eyes with severe retinopathy (32.1 ± 21.8 nm/sec) compared to eyes with mild or moderate retinopathy (14.6 ± 12.4 nm/sec; P < 0.05, log transformed t-test).

Focal and Diffuse Leakage
Passive fluorescein permeability was lowest for edema primarily due to microaneurisms (focal edema), intermediate for mixed, and largest for diffuse edema, where less than 33% of the leakage is estimated to originate from microaneurisms. Ppassive for focal, mixed, and diffuse edema was found to 14.7 ± 18.5 (n = 8), 26.3 ± 17.5 (n = 9) and 29.9 ± 22.2 nm/sec (n = 8), respectively. The difference was not statistically significant (ANOVA P = 0.07). Tactive was found to 80.9 ± 51.7, 87.6 ± 52.2, and 83.6 ± 40.2 nm/sec, respectively.


   Discussion
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
References
 
Using fluorescein as a marker, we have examined the passive permeability and active outward transport through the blood–retina barrier with the aim of elucidating the pathogenesis of diabetic macular edema.

Passive permeability was significantly increased in macular edema by a factor of 12 compared to healthy subjects, with a considerable variation that was associated with the variation in extent of the edema. The more severe levels of passive transport were found in eyes with advanced retinopathy and large areas of diffuse leakage on fluorescein angiograms. Moderately increased passive transport was seen in cases with leakage originating from microaneurisms and type I patients were typically in this group.

The passive permeability of fluorescein and the larger metabolite fluorescein glucuronide did not differ in spite of a large difference in octanol/water partition coefficient between these compounds. Studies of postmortem retinas from patients with diabetic retinopathy have demonstrated transendothelial channels and changes in the tight junctions20 which could lead to equal transport of fluorescein and fluorescein-glucuronide and the present study is in agreement with previous studies on diabetic patients and macular edema in retinitis pigmentosa3 4 7 indicating similarities in the breakdown of the blood–retina barrier despite a different stage of disease or a different pathology.

Compared to healthy eyes, active transport was increased significantly by a factor of approximately 2, indicating a stimulation of outward, active transport concomitant with the increase in passive permeability. Theoretically, a decrease in active transport may be involved in the development of macular edema, but this theory cannot be confirmed by this study. The significance of the increase compared to the changes in passive permeability is not known and has to be investigated with interventional studies.

The active, reabsorptive transport was larger than the passive permeability in absolute values. Our knowledge of the transport systems in the blood–retina barrier is limited and neither the stoichiometric relation between passive and active transport nor the natural substrate of the system that actively transports fluorescein are known. Our results do not indicate whether the increase is caused by an increased number of transport carrier sites or changes in affinity.

A small fraction of patients with macular edema demonstrated low values for both passive and active transport. In four such eyes, the passive permeability averaged 3.0 nm/sec, equal to the mean value in healthy subjects + 2 SD. The active transport in these eyes averaged 31.8 nm/sec, which is below the mean value in healthy subjects. By comparison, in the four eyes with the largest values for passive permeability (51.2 nm/sec), the active permeability averaged 114.5 nm/sec. It remains to be investigated whether the spontaneous prognosis and the response to photocoagulation treatment differs from that seen in the other patients.

In conclusion, the present study indicates that the major change in transport through the blood–retina barrier in diabetic macular edema is due to an increase in passive leakage through a damaged barrier, whereas the active transport, located at the retinal pigmentary epithelium, is intact and probably responsible for the increased resorptive activity that seems to arise in response to the disruption of the inner blood–retina barrier. By a combination of angiographic, clinical, and experimental evidence, we conclude that the defect is located in the inner blood–retina barrier and pharmacological agents should focus on preventing or reversing the defect. Whether a medical stimulation of the active transport is feasible remains to be investigated.


   Acknowledgements
 
The authors thank clinical photographer Hans-Henrik Petersen for skillful assistance and nurse Anni Nielsen for photographic readings. Claus Engler, Peter Dalgaard, and Peter Koch Jensen contributed with insightful remarks.


   Footnotes
 
Supported by grants from The Danish Diabetes Association and Danish Eye Health Society.

Submitted for publication November 2, 1999; revised April 14 and September 14, 2000; accepted November 8, 2000.

Commercial relationships policy: N.

Corresponding author: Birgit Sander, Department of Ophthalmology, Herlev Hospital, University of Copenhagen, DK 2730 Herlev, Denmark. bsander{at}idea.dk


   References
Top
 Abstract
 Introduction
 Subjects and Methods
 Results
 Discussion
 References
 

  1. Krogsaa, B, Lund-Andersen, H, Mehlsen, J, et al (1981) The blood–retina barrier permeability in diabetic patients Acta Ophthalmol 59,689-694[Medline][Order article via Infotrieve]
  2. Bursell, S, Delori, F, Yoshida, A, et al (1984) Vitreous fluorometric evaluation of diabetics Invest Ophthalmol Vis Sci 25,703-710[Abstract/Free Full Text]
  3. Yoshida, A, Ishiko, S, Kojima, M. (1992) Outward permeability of the blood–retina barrier Graefe’s Arch Clin Exp Ophthalmol 230,84-87[Medline][Order article via Infotrieve]
  4. Cunha-Vaz, J, Maurice, D. (1967) The active transport of fluorescein by the retinal vessels and the retina J Physiol 191,467-486[Abstract/Free Full Text]
  5. Koyano, S, Araie, M, Eguchi, S. (1993) Movement of fluorescein and fluorescein glucuronide across retinal pigment epithelium-choroid Invest Ophthalmol Vis Sci 34,531-538[Abstract/Free Full Text]
  6. Engler, C, Sander, B, Koefoed, P, et al (1994) Probenecid inhibition of the outward transport of fluorescein across the human blood–retina barrier Acta Ophthalmol (Copenh) 72,663-667[Medline][Order article via Infotrieve]
  7. Moldow, B, Sander, B, Larsen, M, et al (1998) The effect of acetazolamide on passive and active transport of fluorescein across the blood-retina barrier in retinitis pigmentosa complicated by macular edema Graefe’s Arch Clin Exp Ophthalmol 236,881-889[Medline][Order article via Infotrieve]
  8. . Early Treatment Diabetic Retinopathy Study Research Group (1985) Photocoagulation for diabetic macular edema. Early treatment diabetic retinopathy study report number 1 Arch Ophthal 103,1796-1806[Abstract]
  9. . Early Treatment Diabetic Retinopathy Study Research Group (1991) ETDRS report number 10. Grading diabetic retinopathy from stereoscopic color fundus photographs—an extension of the modified Arlie House classification Ophthalmology 98,786-807[Medline][Order article via Infotrieve]
  10. . Early Treatment Diabetic Retinopathy Study Research Group (1991) ETDRS report number 11. Classification of diabetic retinopathy from fluorescein angiograms Ophthalmology 98,807-822[Medline][Order article via Infotrieve]
  11. Larsen, M, Dalgaard, P, Lund-Andersen, H. (1991) Differential spectrofluorometry in the human vitreous: blood–retina barrier permeability to fluorescein and fluorescein glucuronide Graefe’s Arch Clin Exp Ophthalmol 229,350-357[Medline][Order article via Infotrieve]
  12. Larsen, M, Dalgaard, P, Lund-Andersen, H. (1991) Determination of spatial coordinates in ocular fluorometry Graefe’s Arch Clin Exp Ophthalmol 229,358-362[Medline][Order article via Infotrieve]
  13. Larsen, M, Lund-Andersen, H. (1991) Lens fluorometry: light attenuation effects and estimation of total lens transmittance Graefe’s Arch Clin Exp Ophthalmol 229,363-370[Medline][Order article via Infotrieve]
  14. Lund-Andersen, H, Krogsaa, B, La Cour, M, Larsen, J. (1985) Quantitative vitreous fluorophotometry applying a mathematical model of the eye Invest Ophthalmol Vis Sci 26,698-710[Abstract/Free Full Text]
  15. Dalgaard, P, Larsen, M. (1990) Fitting numerical solutions of differential equations to experimental data: a case study and some general remarks Biometrics 46,1097-1109[Medline][Order article via Infotrieve]
  16. Engler, C, Sander, B, Larsen, M, et al (1994) Fluorescein transport across the human blood-retina barrier in the direction vitreous to blood Acta Ophthalmol 72,655-662[Medline][Order article via Infotrieve]
  17. Moldow, B, Larsen, M, Sander, B, Lund-Andersen, H. (1999) The effect of acetazolamide on passive and active transport of fluorescein transport across the normal blood–retina barrier Invest Ophthalmol Vis Sci 40(8),1770-1775[Abstract/Free Full Text]
  18. Ogura, Y, Tsukahara, Y, Saito, I, Kondo, T. (1985) Estimation of the permeability of the blood–retina barrier in normal individuals Invest Ophthalmol Vis Sci 26,969-976[Abstract/Free Full Text]
  19. Dalgaard, P, Barker, VA, Lund-Andersen, H. (1989) Vitreous fluorometry: mathematical analysis of the effect of peripheral leakage on axial scans Invest Ophthalmol Vis Sci 30,1522-1526[Abstract/Free Full Text]
  20. Ishibashi, T, Inomata, H. (1993) Ultrastructure of retinal vessels in diabetic patients Br J Ophthalmol 77,574-578[Abstract/Free Full Text]



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