Measurement of blood-retinal barrier permeability: a reproducibility study in normal eyes

Fluorescein penetration into the posterior vitreous depends on plasma-free fluorescein concentration and blood-retinal barrier (BRB) permeability. The reproducibility of two methods of deriving BRB permeability was studied in 19 normal eyes of 14 subjects using vitreous fluorophotometry on two separate occasions. Plasma-free fluorescence was measured at intervals over 1 hr and posterior vitreous fluorescence was measured before (background scan), within 6 min (bolus) and at 60 min (measurement) after intravenous fluorescein (14 mg X kg-1). A computer algorithm subtracted background fluorescence from the measurement scan which was then corrected for signal spread by using a "spread" function derived from the bolus scan. BRB permeability coefficient and vitreous diffusion coefficients were derived by fitting a mathematical model to the plasma and corrected vitreous fluorescence data. A permeability index was also calculated by dividing the area under the vitreous fluorescence by the area under the plasma fluorescence curve. There were no significant differences in the results between right and left eyes. Mean +/- SD values on first and second occasions for all eyes were permeability coefficient: (1.91 +/- 0.94) and (2.08 +/- 0.95) X 10(-7) cm X s-1; diffusion coefficient: (1.33 +/- 0.68) and (1.19 +/- 0.54) X 10(-5) cm2 X s-1; and permeability index: (2.05 +/- 1.03) and (2.11 +/- 1.02) X 10(-7) cm X s-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

In vitro human scleral permeability of fluorescein, dexamethasone-fluorescein, methotrexate-fluorescein and rhodamine 6G and the use of a coated coil as a new drug delivery system.

PURPOSE: To determine the in vitro human scleral permeability of several dyes and drugdye combinations with varying molecular weights (MW) and lipid solubilities (fluorescein, dexamethasone-fluorescein, methotrexate-fluorescein, and rhodamine). Coils coated with rhodamine were also evaluated for scleral permeability and sustained release. METHODS: Scleral sections excised from moist chamber stored human globes were mounted in a 2-compartment perfusion chamber. A small depot of drug/dye (100 microl of 10(-4) M fluorescein, dexamethasone-fluorescein, methotrexate-fluorescein or rhodamine) or a coated coil in 100 microl of BSS was added to the episcleral surface while perfusing BSS to the choroidal side. The perfusate was collected and measured for fluorescence. Permeability was calculated as Ktrans from the flux measurements. RESULTS: Ktrans values (cm/sec, mean +/- SE) for the studied dyes and drug-dye combinations were 5.21 +/- 0.71 x 10(-6) for fluorescein, 1.64 +/- 0.17 x 10(-6) for dexamethasone-fluorescein, 3.36 +/- 0.62 x 10(-6) for methotrexate-fluorescein, 1.86 +/- 0.39 x 10(-6) for rhodamine and 2.18 +/- 0.23 x 10(-6) for the rhodamine from the coils. We found a significant difference between the permeability of the sclera to fluorescein and dexamethasone-fluorescein (P < 0.001), methotrexate-fluorescein (P < 0.05) and rhodamine (P < 0.001). Steady state flux was observed from the rhodamine coil. CONCLUSION: The rank order of scleral permeability to the studied dyes is as follows: fluorescein > methotrexate-fluorescein > rhodamine coil > rhodamine 6G > dexamethasone-fluorescein. Differences in scleral permeability are related to MW and lipid solubility. Prolonged transscleral diffusion of rhodamine delivered by solution and by coil are similar.     

Movement of fluorescein and fluorescein glucuronide across the isolated rabbit iris-ciliary body

Movement of fluorescein and fluorescein glucuronide, a fluorescent metabolite of fluorescein, across the isolated iris-ciliary body of the albino rabbit was determined under short-circuit conditions using a modified Ussing's chamber. The permeabilities of this tissue to these dyes were calculated. The outward permeability (from the aqueous to the stromal side) of the iris-ciliary body preparation averaged 6.63 +/- 0.86 for fluorescein and 1.51 +/- 0.47 X 10(-6) cm/sec for fluorescein glucuronide, and the inward permeability (from the stromal to the aqueous side) was 1.68 +/- 0.41 for fluorescein and 1.37 +/- 0.77 X 10(-6) cm/sec for fluorescein glucuronide, respectively. Application of probenecid or ouabain decreased the outward permeability of fluorescein, but it had no significant effect on the fluorescein glucuronide movement. Application of 10(-5) M 2,4-dinitrophenol showed no significant effect on the fluorescein or fluorescein glucuronide movement, but application of 5 X 10(-4) M 2,4-dinitrophenol decreased the outward fluorescein transfer, which was also markedly suppressed by incubation at 0 degrees C. It is possible that an active transport mechanism is involved in the outward fluorescein movement across the iris-ciliary body, while the inward movement of fluorescein and also the fluorescein glucuronide movement across this tissue is mainly by passive diffusion.     

The blood-retinal barrier permeability to fluorescein in normal subjects and in juvenile diabetics without retinopathy

The blood-retinal barrier permeability to fluorescein was determined in 20 eyes from 17 normal volunteers (mean age 31 years) and in 20 eyes from 19 juvenile diabetics without apparent retinopathy (mean age 35 years - mean duration of diabetes 6 years). The permeability was in normal subjects (1.1 +/- 0.4) X 10(-7) cm/sec (mean +/- 2 X SD) and in juvenile diabetics (1.1 +/- 0.7) X 10(-7) cm/sec (mean +/- 2 X SD). Thus a break-down of the blood-retinal barrier cannot be demonstrated as a very early and general phenomenon in the early course of the diabetic disease. The fluorescein diffusion coefficient in the vitreous body was determined and juvenile diabetics without apparent retinopathy showed a diffusion coefficient of (0.80 +/- 0.25) X 10(-5) cm2/sec (mean +/- 2 X SD), which was the same as in normals where the diffusion coefficient was (0.69 +/- 0.46) X 10(-5) cm2/sec (mean +/- 2 X SD).     

Estimation of the permeability of the blood-retinal barrier in normal individuals

The fluorescein kinetics in the vitreous was simulated with a computer to consider several factors such as permeability of the blood-retinal barrier, outward active transport, plasma fluorescein dynamics, diffusion of fluorescein in the vitreous, and fluorescein leakage from the blood-aqueous barrier. Kinetic vitreous fluorophotometry was performed in normal individuals to estimate the inward and outward permeability of the blood-retinal barrier based on the theory of the simulation model. The results of the simulation studies suggest that the fluorescein concentration in the posterior vitreous after intravenous administration is dependent mainly on the inward permeability and on the plasma concentration and that the outward permeability has little influence on the fluorescein kinetics at the early phase. In the pharmacokinetic analysis of the results of kinetic vitreous fluorophotometry, we obtained average values of 1.8 X 10(-5) cm/min and 5.6 X 10(-4) cm/min for the inward permeability and outward permeability coefficients, respectively. The diffusion coefficient of fluorescein in the vitreous was estimated at 7.9 X 10(-4) cm2/min on the average. The outward permeability of the blood-retinal barrier is approximately 31 times the inward permeability. This suggests that a facilitated process that transports fluorescein outward from the vitreous cavity exists in the blood-retinal barrier of human eyes.     

Human corneal endothelial permeability to fluorescein and fluorescein glucuronide

The corneal endothelial permeability coefficient (Pac) for fluorescein and fluorescein glucuronide was determined in ten normal young volunteers. After oral administration of fluorescein, the apparent concentrations of both dyes in the corneal stroma and the anterior chamber were measured by differential fluorometry. The apparent dye levels calculated directly from the in vivo fluorometric measurements were converted to the true ones, based on the result of a normalization experiment performed in rabbit eyes. The value of Pac averaged 5.44 +/- 1.77 X 10(-4) cm/min for fluorescein and 3.77 +/- 1.10 X 10(-4) cm/min for fluorescein glucuronide (mean +/- SD, N = 20); the former was significantly greater than the latter (paired t-test, P less than 0.001). The aqueous-cornea distribution ratio was 0.50 +/- 0.14 for fluorescein and 0.66 +/- 0.16 for fluorescein glucuronide; the latter was significantly greater than the former (paired t-test, P less than 0.001). It was suggested that the previously reported values of Pac for fluorescein in the human eye were underestimates.     

Fluorescein transport through the blood-aqueous and blood-retinal barriers in diabetic macular edema

PURPOSE: To investigate the integrity of the blood-aqueous barrier (BAB) and the blood-retinal barrier (BRB) in diabetic patients with clinically significant macular edema (CSME). METHODS: The rate constant (Kd(F)) of the BAB and the permeability of the blood-retinal barrier for non-protein bound fluorescein were determined simultaneously by fluorometry. Results were analysed pairwise in diabetic patients (n = 25) with CSME in one eye and without CSME in the other eye. RESULTS: Kd(F) for the eyes with CSME was significantly increased compared to eyes without CSME (444. 10(-6) min( -1) and 387. 10(-6) min(-1) respectively, p = 0.01). The passive permeability of the BRB was also significantly increased in CSME (5.7 nm/sec and 3.5 nm/sec respectively, p = 0.009, n = 19). CONCLUSIONS: Both the rate constant of the BAB and the BRB permeability were significantly increased in CSME indicating that eye-specific factors are common for both barriers in diabetic patients with CSME.     

Quantitative vitreous fluorophotometry applying a mathematical model of the eye

A slit-lamp fluorophotometric method is presented that permits calculation of a blood-retinal barrier permeability to fluorescein (P) and a diffusion coefficient for fluorescein in the vitreous body (D). The calculations are performed by relating the time course of the free--not protein bound--fluorescein concentration in the bloodstream with the fluorescein concentration profile in the vitreous body. The combination is performed automatically on a computer by applying a simplified mathematical model of the eye. P refers to the area of the barrier of the model eye. In a group of six normal persons, the mean P was (1.1 +/- 0.4) X 10(-7) cm/sec (mean +/- SD), while in six diabetic patients with background retinopathy and macular edema the mean P was (7.1 +/- 3.8 ) X 10(-7) cm/sec. The mean D was (7.4 +/- 3.4) X 10(-6) cm2/sec in the normal group and (9.6 +/- 2.0) X 10(-6) cm2/sec in diabetic patients, corresponding as a first approximation to free diffusion in water. Model calculations show that knowing the fluorescein concentration in the bloodstream is considerably significant for the calculation of the permeability, contributing factors up to 50%. For the low-permeation situation, subtraction of the preinjection scan contributes a factor of 50% for both permeability and diffusion coefficient. The exact placement in the vitreous body of the concentration profile, by applying a formalism that transforms slit-lamp movement to intraocular distance, contributes a factor of 20% on the diffusion coefficient. The permeability obtained with the model can be calculated as the ratio between area of vitreous and plasma fluorescein concentration curves within 20%. Active transport of fluorescein across the blood-retinal barrier in the direction of vitreous to blood does not seem to be significant within the first 2 hr after fluorescein injection.     

Movement of fluorescein monoglucuronide in the rabbit cornea. Diffusion in the stroma and endothelial permeability

The movement of fluorescein monoglucuronide, a fluorescent metabolite of fluorescein, was studied in the rabbit cornea in vitro and in vivo. A stromal strip was exposed to fluorescein monoglucuronide, and the diffusion rate and the distribution in the stroma were measured every hr for 24 hr. The diffusion coefficient was 0.94 +/- 0.11 (+/- S.D.) X 10(-6) cm2/sec, and the saline/stroma distribution ratio was in a range of 0.67 to 0.69. The concentration of fluorescein monoglucuronide in the anterior chamber and the cornea was measured every hr for 8 hr following intravenous administration. The endothelial permeability was 4.7 +/- 1.0 X 10(-4) cm/min, and the aqueous/cornea distribution ratio was 0.56 +/- 0.05. It appears that the corneal endothelial permeability in the living eye determined hitherto from systemic administration of fluorescein is most likely the permeability to fluorescein monoglucuronide.     

Accurate and precise measurement of blood-retinal barrier breakdown using dynamic Gd-DTPA MRI.

Dynamic T1-weighted magnetic resonance imaging (MRI) after the injection of Gd-DTPA is a promising method for investigating breakdown of the blood-retinal barrier (BRB). Previously, the authors demonstrated that in a T1-weighted image, the initial rate of change in the vitreous water MRI signal as gadolinium diethylenetriaminepentaacetic acid (Gd-DTPA) enters the vitreous space strongly correlated with the extent of BRB breakdown. Here, a practical approach to measuring a more relevant physiologic parameter is presented: the permeability surface area product (PS). The theory is a development of earlier work used in investigating the breakdown of the blood-brain barrier. The accuracy and precision of this approach was investigated in rabbits pretreated with sodium iodate (30 mg/kg intravenously). The MRI-derived PS normalized to the area of leaky retina (5.65 +/- 0.25 x 10(-4) cm/min, mean +/- standard error of the mean; n = 6) was compared to a similarly normalized PS calculated using a classical physiologic method (4.12 +/- 0.73 x 10(-4) cm/min; n = 6). Good agreement between the two methods was found (P = 0.09). This result demonstrates that the MRI-derived PS is an accurate and precise measure of BRB breakdown under these conditions. The mathematical model of Gd-DTPA distribution in vivo also is validated. Based on these results, several potential sources of error are discussed, including the effect of back-flow of Gd-DTPA from the vitreous space to the plasma, the underlying vascular patency, and MRI slice selection.     

Carboxyfluorescein. A dye for evaluating the corneal endothelial barrier function in vivo

The relationship of the cornea-aqueous distribution ratio (r ca) and concentration in vitro was established for fluorescein and carboxyfluorescein. The value of rca for fluorescein was found to fall from 3.20 +/- 0.25 (mean +/- S.D., n V 6) to 1.78 as the concentration of the free fluorescein in the bathing medium rose from 5.8 X 10(-8) to 5.9 X 10(-5) g ml-1. For carboxyfluorescein, it remained unchanged over the same concentration range, and the average for total determinations was 1.29 +/- 0.16 (n = 20). The value of rca for carboxyfluorescein determined in vivo was 1.62 +/- 0.23 (mean +/- S.D., n = 6) and the corneal endothelial permeability to carboxyfluorescein in normal rabbits was 3.31 +/- 0.66 X 10(-4) cm min-1 (n = 11), which was 35% lower than that for fluorescein. Because of its lower endothelial permeability and a value of rca which is unchanged over a wide range of concentration, carboxyfluorescein may be better suited for the in vivo evaluation of the barrier function of the corneal endothelium than fluorescein.     

Blood-retina barrier permeability in diabetes during acute ACE-inhibition.

We assessed the acute effect of ACE-inhibition (captopril) on blood-retina barrier (BRB) permeability in 10 hypertensive insulin-dependent diabetic patients with background retinopathy in a double-masked placebo controlled cross-over study. All patients underwent ophthalmological examination, fundus photography, fluorescein angiography, vitreous fluorometry, and continuous blood pressure recording within 3 h of the drug/placebo administration. The decrease in mean arterial blood pressure, from placebo treatment 149/92 +/- 17/7 to captopril treatment 132/83 +/- 14/7 mmHg (mean +/- SD), P less than 0.01 was not accompanied by a significant decrease in BRB permeability, which was 2.51 (1.24-9.15) with placebo and 3.02 (1.25-13.93).10(-7) cm/s during captopril treatment (geometric mean and-range), NS. Our study suggests that abnormal leakage through the BRB in hypertensive insulin-dependent diabetic patients with background retinopathy is caused predominantly by structural changes in the retinal vessels whereas hydrostatic forces play a minor role.     

Blood-retinal barrier dysfunction at the pigment epithelium induced by blue light.

Exposure to low-intensity white light can induce dysfunction of the blood-retinal barrier (BRB) at the retinal pigment epithelium (RPE). To determine whether the shorter wavelengths white light are responsible for this dysfunction, rabbit retinas were exposed to blue light (400-520 nm) or yellow light (510-740 nm). The permeability of the BRB, a parameter for the integrity of the barrier, was quantified with vitreous fluorophotometry. Morphologically, the barrier at the RPE was visualized on light and electron microscopy using horseradish peroxidase (HRP) as a tracer. Seventeen pigmented rabbits were exposed to blue light and 11 were exposed to yellow light. Vitreous fluorescein leakage increased with the exposure energy according to a power function (correlation coefficient > 0.79). The threshold energy for an increase in BRB permeability was 50 J/cm2 (0.014 W/cm2 for 1 hr) after blue and 1600 J/cm2 after yellow light. HRP tracing demonstrated that after blue light exposure, a significant fluorescein leakage on fluorophotometry corresponded to the presence of HRP in the RPE cells and in the subretinal space. After yellow light exposures of < 3700 J/cm2 and in rabbits with no significant fluorescein leakage, the HRP was limited to the choroidal capillaries and Bruch's membrane. These results demonstrate that the blue component of white light causes dysfunction of the BRB at the RPE 30 times more effectively than the longer wavelength fraction of white light. As a result, a blue light blocking filter should be used in ocular surgery on humans when an operating microscope is being used (light power 0.1-0.9 W/cm2).     

Quantitative MR imaging study of intravitreal sustained release of VEGF in rabbits

PURPOSE: To determine whether sustained elevation of vascular endothelial growth factor (VEGF) in the vitreous cavity causes retinal hyperpermeability [blood-retinal barrier (BRB) breakdown] before the development of retinal neovascularization (NV) and to document the kinetics of the integrity of BRB breakdown versus time. METHODS: Poly(L-lactide-co-glycolide)based devices loaded with VEGF were implanted intravitreally in rabbit eyes. Contrast-enhanced magnetic resonance imaging (MRI) methods were used to identify and quantitate the retinal permeability at various time points after implantation. This was done with the newly developed MR tracer AngioMARK (Epix Medical, Boston, MA). After the MRI measurements, fundus photography and fluorescein angiography (FA) also were performed on the same set of animals. RESULTS: At 3 days after implantation, the MR images showed a significant retinal leakage into the vitreous cavity (BRB breakdown) of the VEGF-implanted eyes. To quantitate this leakage, the permeability surface area product (PS) was measured. At 3 days, the mean PS product was 1.25 +/-0.25 x 10(-5) cm3/min. Based on the VEGF in vitro release study, this 3-day BRB breakdown corresponded to a total sustained release of 7.42 +/- 0.54 microg/ml of VEGF. The fundus and FA photographs of these VEGF-implanted eyes taken at 4 days after implantation also showed a considerable level of retinal vascular dilation and tortuosity. By 12 days after implantation, the mean PS product decreased to 5.83 +/- 1.38 x 10(-6) cm3/min. However, the retinal NV was observed only after the second week after implantation. By this time, a total of 10.70 +/- 0.92 microg/ml of VEGF was released in a sustained fashion. Also, after the retinal NV development, retinal detachment also was observed. The control eyes, however, which were implanted with blank devices, remained unchanged and normal during the entire course of this study (PS = 5.57 +/- 0.66 x 10(-7) cm3/min). CONCLUSIONS. The findings indicate that sustained delivery of elevated amounts of VEGF in the vitreous cavity induces a BRB breakdown even earlier than 3 days after implantation. This was achieved after a total sustained release of 7.42 +/- 0.54 microg/ml of VEGF. This retinal leakage regressed by more than half by the time the retinal NV developed. Furthermore, a retinal detachment occurred after this retinal NV. These results are similar to proliferative diabetic retinopathy (PDR). The sustained elevation of VEGF in the vitreous cavity of rabbit eyes is potentially a good model to test VEGF antagonists to treat or prevent PDR in humans. The quantifiable change of BRB breakdown by the contrast-enhanced MRI method is ideal to assess the therapeutic intervention in vivo without killing the animal and may prove to be clinically useful in humans.     

Quantitative control of the function of the corneal endothelium by fluorophotometry in the anterior eye segment

Whereas specular microscopy yields mostly qualitative information, fluorophotometry furnishes quantitative data of endothelial cell function. Determination of fluorescein permeability of the endothelial cell layer reflects the function of the endothelial barrier. Following topical application of fluorescein, the time-dependent change in the fluorescein concentration in corneal stroma and aqueous yields the transfer coefficient (Kc) of the corneal endothelium. With the Fluorotron Master, Kc was 3.77 +/- 0.57 X 10(-3)/min. for normal eyes; patients with cornea guttata or Fuchs's dystrophy had a significantly higher transfer coefficient (Kc = 7.9 +/- 2.88 X 10(-3)/min.). Normal Kc values were found 6-63 months (average 33.5 months) after phacoemulsification with implantation of a posterior chamber lens.     

Carboxyfluorescein transfer across the blood-retinal barrier evaluated by quantitative fluorescence microscopy: comparison with fluorescein

Carboxyfluorescein levels in ocular tissues of normal rats were measured using quantitative fluorescence microscopy and compared with fluorescein levels to determine the extent to which blood-retinal barrier permeability is affected by the difference in lipid solubility of these two dyes. Retinal fluorescence intensity measurements at 2 min after i.v. dye injection were very much lower for carboxyfluorescein than for fluorescein despite similar plasma free dye concentrations. Marked leakage of dye from the optic disc into peripapillary retina was identified. At 1- and 2 hr, retinal levels of the two dyes became more similar, because fluorescein was removed from retina faster than carboxyfluorescein. After sodium-iodate-induced damage of the pigment epithelium, high levels of both dyes were evident in retina, but carboxyfluorescein was localized chiefly within extracellular space whereas fluorescein also densely stained cell somata. The fluorescence intensity levels recorded, which are proportional to the total mass of dye in the tissue, were correspondingly lower for carboxyfluorescein than for fluorescein, indicating that they were markedly affected by the different distribution of the two dyes. To relate tissue fluorescence intensity directly to dye concentration in the extracellular fluid, measurements were obtained from isolated retinas incubated in dye solutions of known concentration. Log-log plots demonstrated a linear relation between fluorescence intensity and medium concentration for both dyes, but retinal fluorescence of carboxyfluorescein, in correspondence with its limited distribution in the tissue space, was consistently less than that of fluorescein. The ratio of carboxyfluorescein to fluorescein fluorescence varied with the retinal layers but was constant for each layer over the concentration range tested. These fluorescence intensity ratios then were used to adjust the in vivo data so that comparison between the two dyes more closely reflected their extracellular dye concentration. With this correction the amount of carboxyfluorescein present in outer retina shortly after dye injection was approx. 1/10 that of fluorescein, indicating that carboxyfluorescein penetrates the pigment epithelium less readily than fluorescein, as expected from the difference in lipid solubility of the two dyes. However, fluorescence of both dyes in retina and presumably in vitreous humor eventually reached similar levels. This is attributed to entry of the dyes at sites of barrier discontinuity, as at the optic disc, and by a difference in their rates of removal from the intraocular compartment.     

The permeability of the corneal endothelium to fluorescein in the normal human eye

Endothelial permeability to fluorescein was measured as part of fluorophotometric studies of aqueous humor flow in 112 normal subjects whose ages span six decades. The permeability was 2.4 +/- 0.2 X 10(-4) cm/min (mean +/- SD). Females had a slightly higher permeability than males. No correlation between age and permeability was noted. The precision of this method of measurement was estimated to be +/- 22%.     

The effect of pH on the transfer of fluorescein across the blood-retinal barrier

The blood-retinal barrier (BRB) might be governed by the same permeability principles as the blood-brain barrier (BBB). For a weak acid like fluorescein, BRB permeability would be proportional to its pH-dependent lipid solubility, according to the pH partition hypothesis. A range of metabolic acidosis was produced in 20 rats by the oral administration of NH4Cl; six additional rats received normal saline. Four hours later, vitreous fluorophotometry, venous fluorescein values, and arterial pH were measured. Significant linear relationships were found between vitreous fluorophotometry readings and blood hydrogen ion concentrations (p less than 0.025) and plasma fluorescein concentrations (p less than 0.05). According to the linear relationship, changing the pH from 7.4 to 7.3 or 6.9 would result in an increase in vitreous fluorophotometry reading of 8.5 or 72%, respectively. Since the pH partition hypothesis predicts values of 52 or 640% our results suggest that the BRB conforms less to the hypothesis than does the BBB. Furthermore, although pH changes of a magnitude able to influence vitreous fluorophotometry readings substantially may occur under experimental conditions in animals, they are unlikely to occur in ambulatory human patients.     

Dysfunction and repair of the blood-retina barrier following white light exposure: a fluorophotometric and histologic study.

The purpose of this study was to pinpoint the site of blood-retina barrier disruption after white light exposure and determine the course of barrier repair. The retinas of 25 anaesthetized pigmented rabbits were exposed for 1 hr to the light of a xenon arc lamp filtered to eliminate ultraviolet and infrared light. The light intensities selected were near the threshold intensity causing visible retinal lesions in order to evaluate the function of the blood-retina barrier (BRB) in this range. Functional assessment of the BRB was made with vitreous fluorophotometry (VF), and electron microscopy (EM) after intra-arterial administration of horseradish peroxidase (HRP) as tracer. In 11 of the 14 rabbits exposed to threshold intensity (90-110 mW cm-2; retinal field of illumination, 0.64 cm2), a breakdown of the BRB was demonstrated by a 2-40-fold increase in the permeability of the BRB for fluorescein and by transcellular passage of HRP through the retina pigment epithelium (RPE). All 11 rabbits developed oedematous fundus lesions. Within a week, pigmentary alterations of the fundus were seen on ophthalmoscopy, while the BRB permeability for fluorescein and HRP had returned to normal. EM of the retina showed slight swelling of RPE during the period of increased permeability but no alterations of the neuroretina. After functional barrier repair, the RPE cells demonstrated irregularity of the melanin pigment alignment and some loss of the monocellular arrangement. In six rabbits exposed to subthreshold light intensity (65-89 mW cm-2) no fundus lesion developed and EM evaluation of the BRB was normal.2+ remains altered.     

Vitreous fluorophotometry in aphakic or pseudophakic eyes with persistent cystoid macular edema

By vitreous fluorophotometry, the degree of disruption of the blood-vitreous barrier was studied in 16 aphakic or pseudophakic eyes with persistent cystoid macular edema (CME) and 11 aphakic or pseudophakic eyes without CME; postoperative periods ranged from 7 to 22 months in both groups. The rate of fluorescein penetration into the vitreous was determined within 30 minutes after intravenous injection. It represented the permeability of the blood-vitreous barrier and averaged 10.28 +/- 4.52 (SD) X 10(-6) min-1 in eyes with CME and 3.05 +/- 1.21 X 10(-6) min-1 in eyes without CME; the difference between the two groups was statistically significant (P less than 0.002, Student t-test). The peak fluorescein concentration in the mid-vitreous (Cv) was determined and the concentration of free fluorescein in the blood serum (Cs) at the corresponding time was also estimated: the Cs/Cv ratio represented the balance between the inward and outward transport of fluorescein across the blood-vitreous barrier. The Cs/Cv ratio was 7.91 +/- 2.94 in eyes with CME and 12.91 +/- 3.68 in eyes without CME: the difference was statistically significant (P less than 0.001). In 6 eyes with the Cs/Cv ration of 5.0-9.2 the condition of CME deteriorated or remained unchanged during the follow-up of 4-8 months, but in 4 eyes with the ratio of 9-14.2 CME showed an improvement during the same period. In 3 eyes with vitreous tug syndrome, anterior vitrectomy improved the ratio from an average of 4.8 to 17.4. It was concluded that a functional disturbance of the blood-vitreous barrier underlies the development of persistent CME.