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

[Viscocanalostomy: preliminary clinical results]

PURPOSE

Filtration surgery has shifted in the past 20 years to a nonperforating surgery to reduce complications. The purpose of this study was to assess the short-term clinical results and complications of viscocanalostomy.

PATIENT AND METHODS

In a prospective and nonrandomized study, 107 consecutive eyes of 67 patients who underwent viscocanalostomy were analyzed. The surgeon conducted postoperative care. The minimal follow-up was 1 year, with a mean follow-up of 13.1 months (range, 12-18 months). The criteria for success were defined as intraocular pressure (IOP) less than 21 mmHg without treatment.

RESULTS

The mean preoperative intraocular pressure was 28.3 mmHg while the mean postoperative intraocular pressure was 5.4 mmHg on the first day and 10.2 mmHg at 13 months. The rate of patients who had intraocular pressure below 21 mmHg with or without treatment was 98% at 13 months. The complete success rate without treatment was 80% at 13 months. Seven cases of ocular hypotony lasting more than 1 month were noted.

CONCLUSION

Viscocanalostomy is a promising procedure because in the short term it provides good tonometric results in glaucomatous patients without the complications of trabeculectomy. However, it remains a technique with a learning curve.

Glaukos iStent Trabecular Micro-Bypass

The iStent trabecular micro-bypass system (Glaukos Corp. Laguna Hills, CA) was developed to address the limitations of current medical and surgical therapies for glaucoma treatment. The iStent^® is inserted ab interno through a small temporal clear corneal incision, bypassing the trabecular meshwork and placed in Schlemm's canal at the lower nasal quadrant. Implantation of this stent into Schlemm's canal allows aqueous humor to drain directly from the anterior chamber into Schlemm's canal bypassing the obstructed trabecular meshwork. For this review, a Medline search was performed using the terms “trabecular micro-bypass stent” and “trabecular bypass stent.” The online abstract database for the American Academy of Ophthalmology was also reviewed. Abstracts which duplicated published articles were excluded. All relevant papers (n is equal to three) and abstracts (n is equal to one) were included in this review. Multiple, prospective multi-country, clinical trials have demonstrated the safety and efficacy of iStent in reducing IOP, when compared to traditional treatment modalities, while reducing/ eliminating the need for ocular antihypertensive drugs when implanted in OAG patients during combined cataract surgery or in patients with glaucoma refractory to traditional treatment modalities.

The changing paradigm of outflow resistance generation: towards synergistic models of the JCT and inner wall endothelium

Aqueous humor outflow resistance is the primary determinant of intraocular pressure (IOP), and increased outflow resistance is the basis for elevated IOP associated with glaucoma. Experimental evidence suggests that the bulk of outflow resistance is generated in the vicinity of the inner wall endothelium of Schlemm's canal, its basement membrane and the juxtacanalicular connective tissue (JCT). However, attempts to sort out the contribution of each of these tissues to total outflow resistance have not been successful. Conventional understanding of outflow resistance assumes that the resistance of each tissue strata (i.e., the inner wall endothelium, its basement membrane and JCT) in the outflow pathway adds in series to contribute to total outflow resistance generation. However, this perspective leads to a paradox where the apparent resistances of all tissues in the outflow pathway are much lower than the measured total resistance. To resolve this paradox, we explore synergistic models of outflow resistance generation where hydrodynamic interactions between different tissue strata lead to a total resistance that is greater than the sum of the individual tissue resistances. We closely examine the "funneling" hypothesis that has emerged as a leading synergistic model, and we review the basis of funneling, mechanical and biological requirements for funneling and evidence in support of this hypothesis. We also propose refinements to the funneling model and describe how funneling may relate to segmental variability of aqueous humor outflow patterns observed within the trabecular meshwork. Pressure gradients across the JCT and inner wall endothelium will generate mechanical loads that influence the morphology of these tissues. Because tissue morphology may in turn affect outflow resistance, there exists the potential for a two-way coupling or a "fluid-solid interaction" between outflow hydrodynamics and the mechanical behavior of the inner wall and JCT. Furthermore, the adhesions and tethers between the inner wall and JCT must be physically capable of supporting such loads. We examine the structure and mechanical strength of these adhesions, and provide evidence that these adhesions and tethers are unable to support the full load imposed by the bulk of outflow resistance generation unless a substantial fraction of outflow resistance is generated within the JCT, consistent with the funneling model. This indicates that these attachments between the inner wall and JCT have considerable physiological importance for outflow resistance regulation, by maintaining the proximity between these two tissues to facilitate funneling. Further study is greatly needed to better characterize these important interactions.

Reduction of the available area for aqueous humor outflow and increase in meshwork herniations into collector channels following acute IOP elevation in bovine eyes

PURPOSE

To understand how hydrodynamic and morphologic changes in the aqueous humor outflow pathway contribute to decreased aqueous humor outflow facility after acute elevation of intraocular pressure (IOP) in bovine eyes.

METHODS

Enucleated bovine eyes were perfused at 1 of 4 different pressures (7, 15, 30, 45 mm Hg) while outflow facility was continuously recorded. Dulbecco PBS + 5.5 mM glucose containing fluorescent microspheres (0.5 mum, 0.002% vol/vol) was perfused to outline aqueous outflow patterns, followed by perfusion-fixation. Confocal images were taken along the inner wall (IW) of the aqueous plexus (AP) in radial and frontal sections. Percentage effective filtration length (PEFL; IW length exhibiting tracer labeling/total length of IW) was measured. Herniations of IW into collector channel (CC) ostia were examined and graded for each eye by light microscopy.

RESULTS

Increasing IOP from 7 to 45 mm Hg coincided with a twofold decrease in outflow facility (P < 0.0001), a 33% to 57% decrease in PEFL with tracer confined more to the vicinity of CC ostia, progressive collapse of the AP, and increasing percentage of CC ostia exhibiting herniations (from 15.6% +/- 6.5% at 7 mm Hg to 95% +/- 2.3% at 30 mm Hg [P < 10(-4)], reaching 100% at 45 mm Hg).

CONCLUSIONS

Decreasing outflow facility during acute IOP elevation coincides with a reduction in available area for aqueous humor outflow and the confinement of outflow to the vicinity of CC ostia. These hydrodynamic changes are likely driven by morphologic changes associated with AP collapse and herniation of IW of AP into CC ostia.

Extracellular matrix in the trabecular meshwork

The extracellular matrix (ECM) of the trabecular meshwork (TM) is thought to be important in regulating intraocular pressure (IOP) in both normal and glaucomatous eyes. IOP is regulated primarily by a fluid resistance to aqueous humor outflow. However, neither the exact site nor the identity of the normal resistance to aqueous humor outflow has been established. Whether the site and nature of the increased outflow resistance, which is associated with open-angle glaucoma, is the same or different from the normal resistance is also unclear. The ECMs of the TM beams, juxtacanalicular region (JCT) and Schlemm's canal (SC) inner wall are comprised of fibrillar and non-fibrillar collagens, elastin-containing microfibrils, matricellular and structural organizing proteins, glycosaminoglycans (GAGs) and proteoglycans. Both basement membranes and stromal ECM are present in the TM beams and JCT region. Cell adhesion proteins, cell surface ECM receptors and associated binding proteins are also present in the beams, JCT and SC inner wall region. The outflow pathway ECM is relatively dynamic, undergoing constant turnover and remodeling. Regulated changes in enzymes responsible for ECM degradation and biosynthetic replacement are observed. IOP homeostasis, triggered by pressure changes or mechanical stretching of the TM, appears to involve ECM turnover. Several cytokines, growth factors and drugs, which affect the outflow resistance, change ECM component expression, mRNA alternative splicing, cellular cytoskeletal organization or all of these. Changes in ECM associated with open-angle glaucoma have been identified.

Targeted gene transfer to Schlemm's canal by retroperfusion

The goal of the present study was to specifically modify protein expression in the resistance-generating region of the conventional outflow pathway, namely the inner wall of Schlemm's canal (SC) and the juxtacanalicular region of the trabecular meshwork, in perfused human anterior segments. Anterior segments from human cadaveric eyes were prepared for organ culture using standard techniques and were perfused at constant flow while recording pressure. After reaching a stable outflow facility within physiological limits, forward perfusion was stopped and a fluid-tight fence encircling the limbus was installed and filled with media containing an adenovirus encoding the lacZ reporter gene (either 2 x 10(6) or 6 x 10(6)PFU/ml). With the limbus submerged, pressure inside the chamber was lowered to -1 mmHg to facilitate reverse perfusion of virus into SC ("retroperfusion"). After 30-60 min at zero pressure (with some mixing), forward perfusion was restarted and continued for 5-7 days, after which anterior segments were fixed and processed for visualization of lacZ activity. Retroperfusion of nine anterior segments with adenovirus encoding a reporter gene did not appreciably alter baseline outflow facility (0.27+/-0.05 versus 0.29+/-0.08 microl/min per mmHg post-retroperfusion). Gross examination of outflow tissues showed focal distribution of lacZ activity around the circumference of SC, presumably near collector channels. In segments that were sequentially tilted during retroperfusion, the distribution of lacZ activity appeared more uniform. Sagittal histological sections showed lacZ activity in all portions of the conventional drainage tract, particularly cells in the resistance-generating region. Taken together, the results demonstrate that candidate protein expression by cells in the resistance-generating region of the conventional drainage pathway can be specifically modified by retroperfusion of adenovirus and examined for effects on outflow facility.

Trabecular bypass: effect of schlemm canal and collector channel dilation

PURPOSE

To theorize the effect of Schlemm canal (SC) and/or collector channel (CC) dilation combined with a trabecular bypass on intraocular pressure (IOP) in eyes with primary open angle glaucoma.

METHODS

The elliptic shaped SC is dilated in conjunction with a trabecular bypass and its expanded height is largest at the bypass and linearly deceases to the nondilated height over the dilated circumferential length. The CC dilation is modeled with a reduced outflow resistance of second order polynomial over the same dilated length. Equations governing the pressure and circumferential flow in SC are solved numerically for both the unidirectional and bidirectional bypasses. The reduced IOP is deduced from the solution.

RESULTS

IOP is reduced substantially with moderate SC dilation from the normal height of 20 microm to 40 to 50 microm at the bypass; additional IOP reduction diminishes with further dilation. SC dilation is more effective for eyes with smaller SC. CC dilation also lower IOP significantly. With the trabecular bypass alone, the elevated IOP in primary open angle glaucoma is expected to drop to the mid-to-high teens. IOP can be further reduced by another 3 to 6 mm Hg with moderate SC and CC dilation. The circumferential length of dilated SC affects the efficacy of IOP reduction. In theory, the dilation of SC with a trabecular bypass is analogous to a partial trabeculotomy in terms of IOP reduction.

CONCLUSIONS

In theory, a moderate dilation of SC and CC in conjunction with a trabecular bypass reduces the IOP to the low-to-mid teens.

Efficacy and safety of ab interno excimer laser trabeculotomy in primary open-angle glaucoma: two years of follow-up

Ab interno trabeculotomy was performed using the recently developed XeCl excimer laser in 21 eyes of 21 patients with primary open-angle glaucoma refractory to medical therapy. The patients were followed at the Eye Department of S. Antonio Hospital in Padova, for an average of 25.3 +/- 1.3 months. Intraocular pressure (IOP), visual acuity and ocular complications were evaluated. The laser procedure was quick and relatively easy, with minimal manipulation of tissues; complications were clinically insignificant. At the last follow-up, a marked IOP-lowering effect compared to baseline was observed (from 24.8 +/- 2.0 to 16.9 +/- 2.1 mm Hg; -31.8%, p < 0.0001). Nineteen patients (90.5%) had an IOP lowering of 20% or more; however, 8 of these eyes (38.1%) required additional IOP-lowering medical therapy. The procedure failed in 2 cases (9.5%) despite additional therapy. In conclusion ab interno excimer laser trabeculotomy seems effective to decrease IOP, serves to reduce the number of antiglaucoma medications and is relatively safe, proving to be a promising therapeutic option in glaucoma surgery.

Ab Interno Trabeculectomy: Development of a Novel Device (Trabectome???) and Surgery for Open-Angle Glaucoma

PURPOSE

To design an instrument to selectively remove trabecular meshwork and Schlemm's canal inner wall (SCIW), and demonstrate its effectiveness by histologic analysis of treated cadaveric human tissue.

METHODS

The design parameters of the instrument were the ability to permanently remove a segment of trabecular meshwork and Schlemm's canal inner wall without causing damage to surrounding tissue, and to allow use with standard anterior segment surgical techniques and equipment via an ab interno approach. Treatment was applied to 20 segments of human corneoscleral rims. The treated areas were examined using a confocal microscope and compared with matching areas in untreated controls and simulated goniotomy.

RESULTS

The resultant instrument system surgically removes the trabecular meshwork and Schlemm's canal inner wall from an anterior chamber approach. It consists of a disposable surgical handpiece with irrigation, aspiration, and electrocautery to focally ablate the target tissues. The attached console includes a high-frequency (550 KHz) electrosurgical generator and irrigation/aspiration controlled by a foot pedal. Histologic examination of specimens treated with the Trabectome displayed disruption of the trabecular meshwork and Schlemm's canal inner wall without damage to surrounding structures. The specimens treated by simulated goniotomy displayed significant damage to the outer wall of Schlemm's canal and the surrounding sclera. The controls showed no disruption or damage to any tissues.

CONCLUSIONS

The Trabectome system is designed for performing trabeculectomy via an ab interno approach. It successfully removed sections of trabecular meshwork and Schlemm's canal inner wall with less injury to the adjacent tissue compared with goniotomy knife in vitro. Theoretically, this procedure should provide direct access of aqueous humor to Schlemm's canal.

'What controls aqueous humour outflow resistance?'

The bulk of aqueous humour outflow resistance is generated in or near the inner wall endothelium of Schlemm's canal in normal eyes, and probably also in glaucomatous eyes. Fluid flow through this region is controlled by the location of the giant vacuoles and pores found in cells of the endothelium of Schlemm's canal, but the flow resistance itself is more likely generated either in the extracellular matrix of the juxtacanalicular connective tissue or the basement membrane of Schlemm's canal. Future studies utilizing in vitro perfusion studies of inner wall endothelial cells may give insights into the process by which vacuoles and pores form in this unique endothelium and why inner wall pore density is greatly reduced in glaucoma.

Alterations in human trabecular meshwork cell homeostasis by selenium

Epidemiological evidence indicates that selenium supplementation may increase risk for glaucoma and ocular hypertension. The purpose of this study was to determine the effects of selenium on trabecular meshwork cells, a likely site of pathology for glaucoma. Human trabecular meshwork (HTM) cells and human umbilical vein endothelial cells (HUVECs) were treated with selenium (MSeA) at or near physiologically relevant concentrations. Selenium uptake by cells was monitored using mass spectrometry. Alterations in protein secretion, intracellular signaling, and cell morphology were monitored; and the role of integrin signaling in MSeA-induced morphological alterations was investigated using divalent cation treatments. Radiolabeling was used to assess protein synthesis and secretion, while luciferase and MTT assays monitored total cellular ATP and cell viability, respectively. Whereas detectible changes in intracellular selenium were observed after exposure to 1-10 microM MSeA for 24hr, the majority remained in the conditioned medium. Selenium-induced morphological changes (< or =3 hr) occurred before alterations in protein secretion and intracellular signaling (3-6 hr). Zinc treatment prevented selenium-mediated alterations in protein secretion and changes in cell-matrix adhesion. MSeA treatment (5 microM) led to a 60% decrease in protein synthesis after 3 hr and a 30% reduction in secretion, although significant alterations in cell viability and total ATP were not observed after MSeA treatment. Selenium altered several indicators of HTM cell homeostasis, but did not affect viability at physiologically relevant doses. Similar results with HUVECs have implications for understanding selenium's mechanisms of action as an anti-angiogenic agent.

A Trabecular Bypass Flow Hypothesis

PURPOSE

To introduce a hypothesis that theorizes the effect of a trabecular bypass, a channel created through the trabecular meshwork, on the facility of outflow and the intraocular pressure (IOP).

METHODS

Equations that govern the pressure and circumferential flow in Schlemm's canal are established, based on the linear relationships between pressure drop and flow, the balance of flows in the canal, and a uniform leaking structure of collector channels. Two types of bypasses permitting either unidirectional or bidirectional flow are incorporated through boundary conditions to solve the equations and to derive the facility of outflow and the reduced IOP.

RESULTS

In normal healthy eyes, the facility of outflow increases by 13% and 26% in the presence of a unidirectional and bidirectional bypass, respectively. The circumferential flow is significant only in the immediate quadrant to the bypass. The elevated IOP due to the abnormally high resistance in trabecular meshwork in open angle glaucoma is substantially reduced with a single bypass. Mean physiological level of IOP is attained if the bidirectional bypass is placed; slightly higher IOP is attained if a unidirectional bypass is placed. In either case, the higher the baseline IOP, the greater the reduction. The effectiveness of the bypass on IOP reduction is related to the resistances in the canal and the collector channels. Multiple bypasses can be used to further reduce the IOP.

CONCLUSION

It is theoretically demonstrated that a single patent trabecular bypass can enhance the facility of outflow and reduce the IOP to physiological levels.

Trabecular bypass stents decrease intraocular pressure in cultured human anterior segments

PURPOSE

To determine the effect on intraocular pressure (IOP) of bypassing the trabecular meshwork in cultured human anterior segments.

DESIGN

Prospective laboratory investigation using normal human eyes obtained at autopsy.

METHODS

Anterior segments from 21 eyes were placed in perfusion culture, and trabecular bypass stents were inserted through the trabecular meshwork, with the lumen of the tube opening into Schlemm's canal. Eyes received from one to four stents, placed equidistant apart. In eyes receiving one or two stents, additional stents were later added to a maximum of four per eye.

RESULTS

Intraocular pressure was lowered after placement of a single stent, from 21.4 +/- 3.8 mm Hg to 12.4 +/- 4.2 (P < .001). This corresponded to an 84% increase in facility of outflow. Eyes receiving more than one stent had final IOP of 11.9 +/- 3.7 mm Hg. Nine eyes had sequential addition of stents, and seven of these had a further decrease of IOP (13.6 +/- 4.1 to 10.0 +/- 4.3; P = .02). Excision of the entire meshwork, between stents, dropped IOP to 6.3 +/- 3.2 mm Hg, indicating some residual meshwork or canal resistance remained even after placement of three stents.

CONCLUSIONS

Bypass of the trabecular meshwork lowers IOP in cultured human anterior segments. One stent produced the greatest change in pressure. The sequential addition of more stents further lowered pressure in seven of nine eyes. This technique holds promise as a new clinical surgery for glaucoma.

Deep sclerectomy in patients with capsular glaucoma

PURPOSE

To compare the results of deep sclerectomy in capsular glaucoma (CG) with those in primary open-angle glaucoma (POAG).

METHODS

This consecutive, prospective study comprised 24 CG patients (28 eyes) and 25 POAG patients (29 eyes) who underwent deep sclerectomy. Two different implants were used: either an absorbable collagen implant (Aqua-Flow) or a non-absorbable hydrophilic acrylic implant (T-Flux). The number of glaucoma medications, intraocular pressure (IOP) and complications were compared postoperatively. The definition of complete success was IOP below 19 mmHg without therapy.

RESULTS

After a mean follow-up of 19.9 +/- 10.9 months (range 6-36 months) in the CG group and 16.2 +/- 10.0 months (range 6-36 months) in the POAG group, complete success was seen in 60.7% and in 37.9% of eyes, respectively (p=0.085). After adjustments for disparities in baseline characteristics, survival analysis demonstrated that success rates were better over time in CG eyes than in POAG eyes (p=0.038). At all time-points, except at 24 months, the IOP was lower in the CG group than in the POAG group. This difference was statistically significant at 1 week (p=0.050) and 3 months (p=0.006). At 18 months, the mean decrease in number of medications was 77.3% in the CG group and 65.9% in the POAG group (not statistically significant). Levelled hyphema occurred more frequently in CG eyes (35.7%) than in POAG eyes (13.8%).

CONCLUSION

Capsular glaucoma patients had significantly higher success rates over time than POAG patients following deep sclerectomy with implant.

Nonpenetrating filtering surgery

Trabeculectomy is currently the standard filtration procedure for surgical treatment of glaucoma. This technique has advantages over full thickness procedures but can cause early postoperative complications such as hyphema, shallow or flat anterior chamber, hypotony with the risk of choroidal detachment, and maculopathy. Nonpenetrating procedures are alternatives to trabeculectomy with the advantage of minimizing the risk of postoperative complications related to hypotony. The two major variations of nonpenetrating glaucoma surgery are nonpenetrating deep sclerectomy and viscocanalostomy. Techniques and results are discussed in this paper.

How does nonpenetrating glaucoma surgery work? Aqueous outflow resistance and glaucoma surgery

Histologic, experimental, and theoretical studies of the aqueous outflow pathways point toward the juxtacanalicular region and inner wall of Schlemm's canal as the likely site of aqueous outflow resistance in the normal eye. At least 50% of the aqueous outflow resistance in the normal eye and the bulk of the pathologically increased resistance in the glaucomatous eye resides in the trabecular meshwork and the inner wall of Schlemm's canal. The uveoscleral, or uveovortex, pathway, which accounts for perhaps 10% of the aqueous drainage in the healthy aged human eye, can become a major accessory route for aqueous drainage after pharmacologic treatment. Surgeries designed to incise or remove the abnormal trabecular meshwork of glaucoma address the pathologic problem of the disease. Surgeries that unroof Schlemm's canal or expand the canal, such as viscocanalostomy, probably cause inadvertent ruptures of the inner wall and juxtacanalicular tissue, thus relieving the abnormal outflow resistance of glaucoma. This review is a summary of current thought on the pathophysiology of aqueous outflow resistance in glaucoma and, in light of this, provides an interpretation of the mechanism of pressure reduction created by these new surgeries.

Ab-interno goniotrabeculotomy versus mitomycin C trabeculectomy for adult open-angle glaucoma: a 2-year randomized clinical trial

OBJECTIVE

To evaluate the effect of ab-interno goniotrabeculotomy (AIGT) on the intraocular pressure (IOP) in adult patients with primary open-angle glaucoma (POAG), compared with the effects of mitomycin C trabeculectomy (MT).

DESIGN

Prospective, randomized, clinical trial.

PARTICIPANTS

Thirty-two eyes of 32 patients with medically uncontrolled POAG.

INTERVENTION

Standard limbus-based trabeculectomy with adjunct mitomycin C (0.3 mg/mL for 3 minutes) in 16 eyes of 16 patients; AIGT was performed in 16 eyes of 16 patients. The groups were matched for age, preoperative IOP, duration of preoperative antiglaucoma treatment, use of preoperative beta-blockers and parasympathomimetics, and use of beta-blockers in the fellow eye. The IOP (average of the two highest values measured in the diurnal curve, from 8 AM to 6 PM, every 2 hours) and complications were recorded 1, 3, 6, 12, 18, and 24 months after surgery.

MAIN OUTCOME MEASURES

Identification of complications and IOP.

RESULTS

All patients were followed up for 24 months. More postoperative complications occurred in the MT group during the 2-year follow-up. One month after surgery, IOP was 10 +/- 1.46 mmHg (range, 8-13) in the MT group and 12.12 +/- 1.63 mmHg (range, 8-14) in the AIGT group (Student's t test, P = 0.001). Three months after surgery, IOP was 11.5 +/- 1.59 mmHg (range, 8-14) and 12.75 +/- 1.57 mmHg (range, 10-16) in the MT and AIGT groups, respectively (Student's t test, P = 0.033). From the 6th to the 24th postoperative month, no statistically significant difference in IOP was found between the two groups. At the end of follow-up, 14 of 16 eyes (87.5%) of the AIGT group and 13 of the 16 eyes (81.25%) of the MT group showed an IOP < or = 14 mmHg.

CONCLUSION

Ab-interno goniotrabeculotomy appears to be a viable and safe surgical treatment for adult POAG. More extended follow-up, however, and a larger series of patients are needed to ascertain the actual effectiveness of this procedure in adult POAG.

Adenoviral reporter gene transfer to the human trabecular meshwork does not alter aqueous humor outflow. Relevance for potential gene therapy of glaucoma

Obstruction of the aqueous humor outflow from the anterior chamber of the eye leads to an elevation of intraocular pressure in glaucoma, the second major cause of blindness worldwide. Our goal is to be able to modulate aqueous humor outflow resistance by gene transfer to the cells of the trabecular meshwork (TM). We have previously shown that adenoviral vectors are able to transfer a reporter gene to the TM of postmortem human donors. However, assessing gene therapy for glaucoma requires models that can monitor changes in aqueous humor outflow facility (C = flow/pressure). In this study we used four replication-deficient adenoviruses in two such perfusion models. In the first model, whole porcine eyes were infected, perfused at constant pressure and flow changes recorded for 5 h. In the second one, anterior segments from human eyes were infected, perfused at constant flow and pressure changes recorded for 3 days. A single dose of 10(8) adenovirus plaque forming units (pfu) causes a reduction in C while single doses of 10(7), 10(6) and 10(5) p.f.u. do not affect outflow facility and retain positive gene transfer. These findings indicate that adenovirus, at effective doses, could become useful vectors for gene therapy of glaucoma.

Regulation of outflow rate and resistance in the perfused anterior segment of the bovine eye

Contractile properties of isolated trabecular meshwork strips have recently been described. In the present paper we characterize the regulation of the outflow pathway in the isolated perfused anterior segment of the bovine eye. Anterior segments of bovine eyes with detached iris, ciliary body and ciliary muscle were perfused at constant pressure of 8.8 mmHg. A constant outflow of approximately 6-8 microliters min-1 could be obtained for at least 3 hr. The calculated outflow resistance was in the range 1.1-1.4 mmHg min microliter-1. The relative outflow was significantly reduced after application of carbachol, reaching a maximal inhibition of 30%. EC50 for carbachol was 3 x 10(-8) mol l-1. Atropin completely blocked the effect of carbachol on outflow. Morphological examination of perfused anterior segments which were perfused with carbachol revealed an intact fine structure of the meshwork cells. Pilocarpine at 10(-5) mol l-1 reduced outflow by 15%. Epinephrine at 10(-5) mol l-1 reduced outflow, while epinephrine at 10(-6) mol l-1 slightly increased the outflow rate. This effect could be blocked by metipranolol. Endothelin-1 in concentrations of 2 x 10(-9) and 2 x 10(-8) mol l-1 inhibited relative outflow by > 30%. Carbachol, pilocarpine, endothelin and a high dose of epinephrine, which have been shown to induce contractions in isolated bovine trabecular meshwork and ciliary muscle strips, induced a reduction of outflow rate and an increase of outflow resistance of the anterior segment. Thus, at least in the bovine eye, the trabecular meshwork per se is directly involved in the regulation of aqueous humor outflow.

Pressures in the juxtacanalicular tissue and Schlemm's canal in monkeys

A micropuncture technique involving the use of microcannulas with tip diameters less than 5 microns was used to measure the pressure in Schlemm's canal and in the meshwork at distances approximately 7 and 14 microns from the inner wall of Schlemm's canal. In one set of experiments where the spontaneous intraocular pressure (IOP) was 12.2 +/- 0.5 cmH2O and the Schlemm's canal pressure (PSc) was 7.6 +/- 0.7 cmH2O, the pressure at 7 microns from the inner wall of Schlemm's canal was found to be 8.9 +/- 0.7 cmH2O and at a distance of 14 microns, 11.0 +/- 0.5 cmH2O--that is, 1.3 +/- 0.2 and 3.4 +/- 0.3 cmH2O respectively, higher than the PSc. In another set of experiments, the spontaneous IOP and PSc were also measured and then the IOP was increased by means of an external reservoir and measured once again. Spontaneous IOP was 16.0 +/- 1.3 cmH2O and the PSc was 11.5 +/- 1.4 cmH2O before the IOP was increased. After the IOP was increased to 20.2 +/- 1.2 cmH2O, the PSc was 11.7 +/- 1.6 cmH2O. When the microcannula was introduced into the juxtacanalicular tissue to locations at about 7 and 14 microns from the inner wall of Schlemm's canal the pressure measured at 7 microns was 16.9 +/- 1.3 and at 14 microns it was 18.9 +/- 1.4 cmH2O--that is, 5.2 +/- 0.8 and 7.2 +/- 1.0 cmH2O respectively, higher than the PSc. The results indicate that at the spontaneous IOP about 75% of the resistance between the anterior chamber and Schlemm's canal is located within 14 microns from the canal with some 50% being located within the region 7 and 14 microns from the canal. After a small increase in IOP, the tissue causing most of the outflow resistance became relocated to a region within 7 microns from the canal.