Photoablative laser-grid trabeculectomy in glaucoma filtering surgery: histology and outflow facility measurements in porcine cadaver eyes

BACKGROUND AND OBJECTIVE: Mid-infrared laser technology opens new perspectives in experimental glaucoma surgery. The aim of the study was to determine the efficacy of grid-laser trabeculectomy to reduce aqueous outflow resistance sufficiently in perfused porcine cadaver eyes. MATERIALS AND METHODS: Twenty-five freshly enucleated porcine cadaver eyes were randomly assigned to either laser-grid trabeculectomy (LGT), conventional trabeculectomy (CT), or control. In all surgical cases a scleral flap was prepared in a standardized manner. In order to penetrate into the anterior chamber a 1 x 2 mm sclerectomy was performed in CT, and 10 transscleral ablation craters were created in LGT. An erbium:YAG laser (2.96 microm) was used for transscleral photoablation. Radiation was delivered in a single 6 mJ-pulse of 200 micros duration by means of an articulated zirconium fluoride optical fiber and a 200 microm quartz fiber-tip. Outflow facility was measured at a constant perfusion pressure. RESULTS: Laser-grid trabeculectomy (LGT) yielded reproducible ablation areas varying between 210-300 microm in diameter, with 90% being full-thickness penetrations. Collateral thermal damage was less than 30 microm. Outflow facility measurements revealed a statistically significant increase (mean +/- SD) from 0.255 +/- 0.096 microL/min/mm Hg in control eyes (n = 9) to 0.772 +/- 0.157 microL/min/mm Hg in the LGT group (n = 9), and 2.957 +/- 0.602 microL/min/mm in the CT group (n = 7). Flattening of the anterior chamber, folding of Descemet's membrane, and iris prolapse were only observed in the CT eyes but not in the GLT group. CONCLUSION: Application of the laser-grid trabeculectomy produces patent transscleral perforation with minimal collateral tissue damage and effects a significant reduction (67%) in outflow resistance without any postoperative hypotony-related complications.     

Influence of non-penetrating glaucoma surgery on aqueous outflow facility in isolated porcine eyes

PURPOSE: To investigate, in vitro, the influence of non-penetrating glaucoma surgery (NPGS) and the influence of tightly suturing the superficial scleral flap on the aqueous outflow facility of isolated porcine eyes.Materials and methods: The anterior chambers of 12 enucleated porcine cadaver eyes were cannulated and perfused. NPGS was performed by the same surgeon. The overall ocular aqueous outflow facilities were assessed before and after the surgical interventions of NPGS, as well as after scleral flap closure. RESULTS: The mean (SD) aqueous outflow facility, which was 0.164 (0.014) microl/min/mm Hg before surgery, increased significantly after NPGS to 1.584 (0.217) microl/min/mm Hg, p<0.001. When the superficial flap was closed, the aqueous outflow facility significantly decreased (0.754 (0.107) microl/min/mm Hg, p<0.001) but remained significantly higher than preoperatively (p<0.01). After suturing the superficial flap, the overall resistance increased to 1.625 (0.210) microl/min/mm Hg. The difference in the resistance to outflow before and after flap closure was 0.848 (0.169) microl/min/mm Hg. CONCLUSION: After NPGS suturing the scleral flap can modulate aqueous outflow resistance. The experimental set up described might provide an efficient model for the technical training of glaucoma surgeries.     

Effect of intraocular pressure on uveoscleral outflow following cyclodialysis in the monkey eye

Cyclodialysis was performed in both eyes of five cynomolgus monkeys. Two days later the intraocular pressure (IOP) had fallen from 17.7 +/- 0.8 to 7.1 +/- 1.4 mm Hg (P less than 0.001). At that time, both eyes were perfused for 30 min with fluorescein-isothiocyanate (FITC) dextran (MW 70,000), one at 35 mm Hg and the other at 4 mm Hg. Four pairs of control eyes (without cyclodialysis) were perfused in the same manner. At 4 mm Hg, uveoscleral outflow was 0.02 +/- 0.02 microliter/min in control eyes and 0.05 +/- 0.04 microliter/min in eyes following cyclodialysis. However, at 35 mm Hg, uveoscleral outflow in eyes with cyclodialysis increased to 2.13 +/- 0.47 microliters/min compared to 0.32 +/- 0.10 microliter/min in control eyes. Thus the "facility" of uveoscleral outflow in control eyes is 0.01 microliter/min/mm Hg and in eyes following cyclodialysis is 0.07 microliter/min/mm Hg. It is concluded that cyclodialysis results in a pressure-dependent increase in uveoscleral outflow.     

Aqueous dynamics in experimental ab externo trabeculectomy

PURPOSE: This study assessed the decrease in intraocular pressure (IOP), the residual outflow resistance of the trabeculum and the filtration site by histology in enucleated pig and human eyes after ab externo trabeculectomy. METHOD: Measurement of the resistance to aqueous outflow was performed using the constant pressure method before and after ab externo trabeculectomy. RESULTS: The outflow facility was 0.31 +/- 0.13 microl/min/mm Hg in pig eyes and 0.24 +/- 0.08 microl/min/mm Hg in human eyes before surgery, and 79.0 +/- 47. 6 microl/min/mm Hg in pig eyes and 6.33 +/- 6.67 microl/min/mm Hg in human eyes after ab externo trabeculectomy. On histological examinations the aqueous outflow pathway was seen to be at the level of the residual trabecular meshwork. CONCLUSION: This study showed that ab externo trabeculectomy significantly lowers IOP and improves outflow facility in enucleated pig and human eyes.     

Diurnal variations in outflow facility

Tonography was carried out at 8 AM, 2 PM, and 8 PM on 43 normal eyes, 58 open-angle glaucomatous eyes and 10 ocular hypertensive eyes. Diurnal variations in outflow facility and their relation to those in intraocular pressure (IOP) were studied. Diurnal variations in outflow facility were present in almost all eyes, with approximately a 10% exception. The average diurnal variation in true outflow facility (deltaCtrue) was 0.10 microliter/min/mm Hg in normal eyes and 0.07 microliter/min/mm Hg in glaucomatous eyes. The average rate of diurnal variation in true outflow facility: formula: (see text) was 43.6% in normal eyes and 69.1% in glaucomatous eyes. The value was greater in glaucomatous eyes than in normal eyes. The curve of diurnal variations in true outflow facility was divided into 5 types, and in glaucomatous eyes the value of outflow facility was apt to increase in the morning and decrease in the evening. In 42% of the normal and 45% of the glaucomatous eyes, inverse relation was seen between diurnal variations in outflow facility and diurnal rhythm in IOP. It is thought that diurnal variations in outflow facility, along with diurnal fluctuations in the rate of aqueous formation, are one of the factors responsible for the diurnal rhythm found in IOP.     

Change in outflow pathway of porcine eyes in vitro by nonpenetrating filtering surgery

Objective: To study the change in different outflow pathways of porcine eyes in vitro by nonpenetrating filtering surgery.Design: Experimental study.Participants: Sixty-four enucleated porcine eyes were studied.Methods: Deep sclerectomy was performed on isolated porcine eyes (Group 1A), then the superficial scleral flap was watertight sealed (Group 1B), and finally the oracles and the exterior wall of the Schlemm canal were watertight sealed (Group 1C). In another series of experiments, deep sclerectomy was performed with the scleral lake volume of 4 mm × 4 mm × 0.5 mm (Group 2A), 4 mm × 2 mm × 0.5 mm (Group 2B), and 4 mm × 1mm × 0.5 mm (Group 2C), respectively; then the superficial scleral flap was watertight sealed. The control eye (Groups 1D and 2D) underwent creation and watertight sealing of a superficial scleral flap. Intraocular pressure (IOP) and outflow facility were determined preoperatively and postoperatively.Results: Compared with the preoperative value, IOP was decreased and outflow facility was increased in Groups 1A, 1B, and 1C (p < 0.05). In deep sclerectomy, the outflow facility was decreased by more than 0.18 μL/min/mm Hg after the conjunctival pathway was blocked. After the oracles and external wall of the Schlemm canal were blocked, the outflow facility decreased further by more than 0.09 μL/min/mm Hg but was still 0.06 μL/min/mm Hg higher than before surgery. There was a positive linear correlation between the deep sclerectomy volume and the ratio of IOP decrease or outflow facility increase postoperatively.Conclusions: Deep sclerectomy can increase the outflow of porcine eyes in vitro. The major factors maintaining the postoperative outflow increase include the subconjunctival pathway, the functional deep scleral lake, and the opening of the Schlemm canal.     

A fluorophotometric study of the effect of argon laser trabeculoplasty on aqueous humor dynamics

Fluorophotometry and tonography were used before and after argon laser trabeculoplasty to measure aqueous humor dynamics in 15 eyes of 14 patients. A new fluorophotometric method used to measure true outflow facility showed that it increased from a mean pretreatment value of 0.016 microliter/min/mm Hg to a mean posttreatment value of 0.075 microliter/min/mm Hg. Similarly, total outflow facility, measured tonographically, increased from 0.112 microliter/min/mm Hg to 0.151 microliter/min/mm Hg. Uveoscleral flow decreased after laser therapy; however, this was secondary to the decrease in intraocular pressure. No effect was found on the rate of aqueous humor production. We concluded that the primary effect of argon laser trabeculoplasty was to increase true outflow facility, thereby decreasing intraocular pressure.     

Characterization of uveoscleral outflow in enucleated porcine eyes perfused under constant pressure

PURPOSE: A new technique was developed to measure the flow of aqueous humor through the uveoscleral pathway in porcine eyes and to examine whether there is any outflow through the choroid into the vortex veins. METHODS: Enucleated porcine eyes were perfused in vitro under a constant pressure of 10 mm Hg. After total outflow was measured, the episcleral vessels were blocked with cyanoacrylate to eliminate outflow through the conventional pathway. The vortex veins were then blocked, to assess the amount of choroidal drainage. RESULTS: The average outflow in control eyes was found to be 2.8 +/- 0.9 microL/min. After the exit sites of the conventional pathway were blocked, the average outflow decreased to 1.1 +/- 0.5 microL/min. Blocking the vortex veins did not appear to alter uveoscleral outflow further (1.2 +/- 0.8 microL/min). CONCLUSIONS: The results suggest that choroidal drainage into the vortex veins is insignificant in the absence of blood perfusion. No significant washout effects in porcine eyes were observed.     

Diode laser transscleral cyclophotocoagulation for refractory glaucoma

PURPOSE: To evaluate the efficacy of contact diode laser transscleral cyclophotocoagulation using different treatment parameters. METHODS: All eyes undergoing contact diode laser transscleral cyclophotocoagulation between April 1991 and September 1997 at our two institutions were enrolled. Success was defined as an intraocular pressure less than or equal to 22 mm Hg or absence of pain for poorly sighted and blind eyes. RESULTS: Two hundred six eyes of 204 patients (Salzburg, 117 eyes; New York, 89 eyes) were enrolled. Mean patient age was 65.8 +/- 18.7 years (range, 4-96 years). Mean follow-up was 9.2 +/- 11.2 months (range, 3-56 months). Mean preoperative and postoperative IOPs were 42.1 +/- 11.0 mm Hg (range, 24-76 mm Hg) and 17.3 +/- 10.9 mm Hg (P < 0.001) and 20.3 +/- 13.2 mm Hg (P < 0.001) at 12 and 24 months, respectively. The number of laser applications (mean, 18.6 +/- 4.2; range, 10-40) and maximal laser power (mean, 2,352 +/- 408.5 mW; range, 1,500-3,000) were not associated with a lower postoperative IOP. An IOP less than or equal to 22 mm Hg was achieved in 72.7% of eyes at the mean follow-up of 9 months. Thirty-three (16.0%) eyes required at least one retreatment. Phthisis occurred in four (1.9%) eyes. CONCLUSION: Contact diode laser transscleral cyclophotocoagulation is useful in eyes with refractory glaucoma in which the risks of outflow surgery are deemed unacceptable.     

Uveoscleral aqueous outflow in the rhesus monkey: importance of uveal reabsorption.

The uveal absorption of aqueous humor at two different intraocular pressures was measured in rhesus monkeys by sampling vortex vein blood during anterior chamber perfusion of Ringer's solution containing both fluorescein and 125I albumin. At 20 mm. Hg, an excess of fluorescein equivalent in amount to 0.45 microliter/min. of anterior chamber perfusate and an excess of 125I albumin equivalent to 0.18 microliter/min. of perfusate was found in the vortex vein blood compared to systemic blood. At 32 mm. Hg, an excess of 0.86 microliter/min. of fluorescein and 0.26 microliter/min. of 125I albumin was measured. The increase in fluorescein absorption at the higher intraocular pressure was significant (0.025 less than p less than 0.05), with a uveal outflow facility of 0.034 microliter/min./mm. Hg. At an intraocular pressure of 20 mm. Hg, uveal aqueous outflow is less than 10% of total aqueous outflow. The pressure dependence of the uveal uptake of fluorescein implies an aqueous reabsorption into the uveal vessels by ultrafiltration. Uveoscleral aqueous "outflow" appears to consist of intraocular uveal reabsorption of water and small molecules from the aqueous, with only larger molecules like albumin actually leaving the eye through posterior scleral vessel perforations. This reabsorption plays a minor role in intraocular pressure regulation under normal conditions.     

Increases in intraocular pressure during hemodialysis in eyes during early postvitrectomy period

BACKGROUND AND OBJECTIVES: To identify risk factors for intraocular pressure (IOP) increases during hemodialysis (HD) in the early postoperative period following pars plana vitrectomy (PPV). PATIENTS AND METHODS: Comparisons of 36 vitrectomized eyes of 22 diabetics undergoing maintenance HD with those of 138 eyes of 69 nonoperated patients. Serial IOPs were measured before PPV, after PPV/before dialysis, and during dialysis. Serum osmolarity and blood pressure were also recorded. RESULTS: In 20 of 36 operated eyes (55.6%) and 18 of 138 nonoperated eyes (13.0%) IOP was increased (> or =4 mm Hg) during HD. In the operated group, marked increases (> or =7 mm Hg) were noted in 9 eyes and ocular pain in 5 eyes. Eyes with pre-existing outflow obstruction and/or acute postoperative outflow compromise carried high risks. No correlation with changes in serum osmolarity or blood pressure was found. CONCLUSION: Monitoring of IOP during HD seems warranted when diabetics with pre-existing outflow obstruction and/or postoperative outflow compromise, undergo HD following PPV.     

Aqueous dynamics in exfoliation syndrome.

Eighteen untreated patients with unilateral exfoliation syndrome without glaucoma were studied with regard to differences in aqueous dynamics between the affected and unaffected fellow eyes. Additionally, both eyes of the patients with exfoliation were compared to the eyes of 18 age-matched and gender-matched control subjects. Intraocular pressure was significantly higher (13.9 +/- 3.0 mm Hg [mean +/- standard deviation]) in the affected compared with the unaffected eyes (12.2 +/- 2.5 mm Hg), but no difference in aqueous humor flow was detected when eyes in the three groups were compared (affected eyes, 2.40 +/- 0.60 microliter/min; unaffected eyes, 2.40 +/- 0.65 microliter/min; control eyes, 2.61 +/- 0.60 microliter/min). Anterior chamber volumes were significantly smaller in affected (122 +/- 37 microliters) and unaffected eyes (121 +/- 37 microliters) of the patients with exfoliation compared with the eyes of the control subjects (145 +/- 30 microliters). Apparent resistance was higher in the eyes with exfoliation (6.5 +/- 1.9 min mm Hg/microliters) compared with both unaffected (5.4 +/- 1.2 min mm Hg/microliters) and control groups (5.2 +/- 1.8 min mm Hg/microliters), although these differences were not statistically significant.     

Size of donor corneal button and outflow facility in aphakic eyes.

Outflow facility was measured in 6 human eye bank eyes following penetrating keratoplasty and cataract extraction. A 7.5 mm corneal button was removed and replaced by an 8 mm button using conventional mid-to-deep stromal sutures. Baseline outflow facility in the phakic eyes (0.28 +/- 0.12 microliter/min/mm Hg, mean +/- SD) was unchanged (0.29 +/- 0.12) following the cataract extraction and penetrating keratoplasty using the larger corneal button.     

Effect of Healon and Viscoat on outflow facility in human cadaver eyes

PURPOSE: To compare the acute effects of Healon (sodium hyaluronate) and Viscoat (sodium chondroitin sulfate-sodium hyaluronate) on outflow facility in human cadaver eyes and determine which viscoelastic agent is least likely to cause an intraocular pressure (IOP) spike after cataract surgery. SETTING: The Glaucoma Research Lab, University of Toronto, Ontario, Canada. METHODS: In this prospective paired study, 15 pairs of human cadaver eyes were used. Following the construction of a 3.0 mm scleral tunnel, 0.25 cc of Healon was injected into the anterior chamber of 1 eye and 0.25 cc of Viscoat was injected into the contralateral eye. The viscoelastic agents were removed from both eyes in a standardized fashion and the scleral tunnels closed. The eyes were then perfused at a constant IOP of 8.0 mm Hg, corresponding to 16.0 mm Hg in vivo. Outflow facility (microL/minute [min]/mm Hg) was recorded every 15 minutes for 24 hours using standard methods. RESULTS: Outflow facility in the Viscoat-treated eyes decreased appreciably for the first 3 hours, then recovered somewhat after 12 hours; facility in the Healon-treated eyes showed less of an overall decrease. Over the 24 hour perfusion period, mean outflow facility was 0.037 microL/min/mm Hg +/- 0.015 (SD) in the Viscoat-treated eyes and 0.060 +/- 0.012 microL/min/mm Hg in the Healon-treated eyes. Healon reduced outflow facility significantly less than Viscoat between 3.25 and 10.50 hours postoperatively (P < .05, 2-tailed t test). CONCLUSIONS: Healon reduced outflow facility less than Viscoat between 3.25 and 10.50 hours postoperatively.     

Uveoscleral outflow following cyclodialysis in the monkey eye using a fluorescent tracer

Cyclodialysis was performed in one eye of each of eight cynomolgus monkeys. Two days later, the intraocular pressure was 1.6 +/- 0.7 mmHg in eyes with cyclodialysis and 12.0 +/- 0.7 mmHg in fellow control eyes. 10(-4) M fluorescein-isothiocyanate dextran (70,000 molecular weight) was perfused into the anterior chamber of each eye for 30 min. The eyes were enucleated and dissected into sclera, choroid, retina, iris, and ocular fluid. Samples were homogenized and centrifuged, and the fluorescence of the supernatant was measured. Expressed as equivalent volumes of aqueous, the rate of anterior chamber movement of tracer via uveoscleral pathways was 1.40 +/- 0.17 microliter/min in cyclodialysis eyes and 0.34 +/- 0.10 microliter/min in control eyes. Cyclodialysis results in a fourfold increase in uveoscleral outflow, contributing to the observed hypotony.     

A modified in vitro method for aqueous humor outflow studies in enucleated porcine eyes.

The aim of this study was to develop and test a short-term in vitro method for aqueous humour outflow studies using enucleated porcine eyes. The method used was a modification of two methods that have previously been used: whole eyes and anterior segment cultures. The advantage of the model used in this study was that the anterior part of the eye, including the anterior and posterior chambers, remained intact as in whole enucleated eyes, but neither iridotomia nor trephination through the cornea was needed. The deepening of the anterior chamber during perfusion was avoided by regulating the "vitreal" pressure. Test compounds were administered topically or intracamerally to an anatomically normal anterior chamber. Fresh porcine eyes (n = 48) were sectioned at the equator, and the vitreous mass was carefully removed. This anterior bisection was bound around a specific plastic chamber, thus creating a closed eye. The anterior chamber was perfused at a pressure of 15 mmHg. The mean outflow rate in the nonmedicated eye group was 3.7 +/- 0.20 microL/min (mean +/- standard error of the mean), and it increased by 18% during 9 h owing to a wash-out effect. Compounds known to enhance the aqueous outflow were used for testing the validity of the preparation.     

Transscleral visible/near‐infrared spectroscopy for quantitative assessment of haemoglobin in experimental choroidal tumours

Purpose: To study the feasibility of using transscleral visible/near‐infrared spectroscopy (Vis/NIRS) to estimate the content of haemoglobin in choroidal tumour phantoms of ex vivo porcine eyes. Methods: Thirty enucleated porcine eyes were prepared with a tumour phantom made by injecting a suspension of gelatine, titanium dioxide and human blood into the suprachoroidal space. The blood concentrations used were 2.5%, 25% and 50%, with 10 eyes in each group. Alternating Vis/NIRS measurements were taken over the phantom inclusion and on the opposite (normal) side of each eye. For statistical analysis, a genetic algorithm was utilized to suppress insignificant wavelengths in the spectra. The processed spectra were then used to build a regression model based on partial least squares regression and evaluated by twofold cross‐validation. Results: Ultrasonography revealed that all phantoms were localized within the suprachoroidal space with no penetration through the retina. The largest mean diameters of the phantoms with 2.5%, 25% and 50% blood were 15.5, 15.2 and 15.7 mm, respectively (p > 0.05). The largest mean thicknesses were 4.5, 4.5 and 4.8 mm, respectively (p > 0.05). Statistical analysis of the spectral data showed that it was possible to correctly discriminate between the normal side and the tumour phantom side of the eyes in 99.88% of cases. The phantoms could be correctly classified according to their blood concentrations in 99.42% of cases. Conclusions: This study demonstrates that transscleral Vis/NIRS is a feasible and accurate method for the detection of choroidal tumours and to assess the haemoglobin content in such lesions.     

Outflow resistance of enucleated human eyes at two different perfusion pressures and different extents of trabeculotomy

Aqueous outflow resistance of enucleated human eyes was measured at 7 and 25 mm Hg before and after partial and complete (12 clock hours) internal trabeculotomy. Following complete trabeculotomy, 71% of the resistance was eliminated at 25 mm Hg while only 49% was eliminated at 7 mm Hg. In contrast to published findings in eyes with intact trabecular meshwork where the resistance increased with increasing IOP (1), following complete trabeculotomy, the resistance decreased 2% per mm Hg with increasing IOP. Experiments with trabeculotomy limited to part of the circumference showed that a one hour trabeculotomy produced 41% (25 mm Hg) to 60% (7 mm Hg) of the effect of a twelve hour trabeculotomy. The results indicate that a surprisingly high fraction of aqueous outflow resistance resides in the distal aspects of the outflow system at normal IOP, and that this distal resistance drops as IOP is increased.     

The comparison of topical ibopamine 2% with tonography to identify the outflow resistance in eyes with ocular hypertension

BACKGROUND: To evaluate the ability of topical ibopamine 2% to detect outflow resistance by comparing it with the tonography test in eyes with ocular hypertension (OHT). METHODS: 62 eyes with OHT and 33 control eyes were included in this prospective study. Tonography was done manually as a standard outflow facility measurement. We used a C value of 0.18 mul/min/mm Hg or less and a P(o)/C value of 100 and above as a positive tonographic test. The ibopamine test performed on the following day was considered positive if there was an intraocular pressure (IOP) change of at least 3 mm Hg. RESULTS: The sensitivity of the tonography and ibopamine tests was 69 and 53%, respectively, in eyes with OHT. The specificity of both tests was 97%. Although the sensitivity of the tonography test is higher than that of the ibopamine test, the difference between both was not statistically significant in these eyes (p = 0.409). Positive results in tonography were associated with higher IOP, while the results were not dependent on the IOP in the ibopamine test. Both tests together were positive in 33.87% (21 eyes) and negative in 11.29% (7 eyes) in 62 eyes with OHT. CONCLUSION: This study revealed that the ibopamine provocation test can detect outflow system resistance in eyes with OHT comparable with tonography which is a traditional outflow facility measurement. Ibopamine, however, can detect the eye with outflow impairment by a different and IOP-independent way, while tonography depends on IOP.     

Human scleral hydraulic conductivity: age-related changes, topographical variation, and potential scleral outflow facility

PURPOSE: To measure the specific hydraulic conductivity (K) of human sclera over a range of ages, to assess topographical variation, and to provide a theoretical estimate of potential scleral outflow facility. METHODS: Human donor sclera (n = 18; mean age 56.7 +/- 25.9 years; range 4-89) was clamped in a modified Ussing chamber connected to a water column set at 15.7 mm Hg. Column descent was measured over 24 hours at 20 degrees C with a digital micrometer. Scleral thickness of glutaraldehyde-fixed specimens was measured by light microscopy, taking the mean of 15 measurements per donor. Topographical variation in hydraulic conductivity (HC) was determined in an additional 10 donor eyes (mean age, 54.1 +/- 26.4 years; range 12-89), comparing anterior, equatorial, and posterior sclera. The potential transscleral outflow facility was calculated by multiplying HC by total scleral surface area and adjusting water viscosity to core body temperature. RESULTS: Mean K +/- 1SD in adults (>18 years) was 5.85 +/- 3.89 x 10(-18) m(2). K tended to be higher in pediatric donors, but there was no statistically significant age-related change. However, when all data sets were combined (n = 28), HC showed a significant decline with age. There was no significant topographical variation in HC. The potential transscleral outflow facility was 0.33 microL.min(-1).mm Hg(-1). CONCLUSIONS: Quantifying HC may help refine ocular pharmacotherapy, as transscleral water movement increases intraocular drug elimination and impedes transscleral drug delivery. The potential scleral outflow is two to three times higher than that which occurs in vivo; hence, medical or surgical interventions that fully exploit this pathway have considerable capacity to lower intraocular pressure.