AQUEOUS HUMOR FORMATION

Active and Passive Mechanisms of Formation

Aqueous humor is secreted into the posterior chamber by the nonpigmented ciliary epithelium. Aqueous humor production requires an active sodium-potassium adenosine triphosphatase similar to that found in other secretory epithelial tissues throughout the body.^1–4 In addition, the enzyme carbonic anhydrase is intimately involved in the secretory mechanism.^5–8 Passive flow of aqueous humor into the posterior chamber has also been postulated to account for a significant fraction of aqueous humor formation⁹; however, this seems unlikely. Passive flow would result from hydrostatic and osmotic pressures acting as driving forces. If passive formation were significant, an appreciable decrease in the rate of aqueous humor formation should occur when the intraocular pressure was raised. However, the baseline rate of aqueous humor formation in the primate, approximately 2.0 μL/min, decreases only 0.02 μL/min per millimeter of mercury (mm Hg) as intraocular pressure is raised.¹⁰ As pointed out by Bill,¹¹ the protein colloid osmotic pressure of the interstitial fluid is 14 mm Hg. This would draw fluid from the posterior chamber into the ciliary processes. For passive fluid flow to occur in the opposite direction, the ciliary process hydrostatic pressure would have to exceed the intraocular pressure by more than 14 mm Hg.

Measurement of the Rate of Aqueous Humor Formation

Goldmann¹² calculated the rate of aqueous humor flow through the anterior chamber by measuring the relationship between aqueous humor fluorescein concentration and plasma fluorescein concentration after intravenous administration of the dye. Subsequently, oral administration of fluorescein was used for this purpose.^13,14 The chief disadvantage of both methods is that knowledge of the fluorescein concentration in the posterior chamber is required. This is difficult to determine. In addition, frequent measurements of the unbound plasma fluorescein concentration are necessary.

Jones and Maurice¹⁵ showed that after application of fluorescein to the cornea by iontophoresis it was possible to measure aqueous humor flow by monitoring the changes of fluorescein concentration in the corneal stroma and anterior chamber. Topical administration using fluorescein eye drops has also been effective.¹⁶ The typical time course of the fluorescein concentration in the corneal stroma and anterior chamber after topical administration is shown in Figure 1. In method I of Jones and Maurice,¹⁵ aqueous humor flow was calculated from the magnitude of the peak anterior chamber concentration and the time required to reach this peak. Because the exact time of the peak fluorescein concentration in the anterior chamber was difficult to determine, owing to the relative flatness of the curve, this method was seldom used. Method II of Jones and Maurice¹⁵ used data from those portions of the cornea stroma and anterior chamber fluorescein decay curves that were parallel. The magnitude of the anterior chamber aqueous humor flow, F_a, is a function of the anterior chamber volume, V_a, the slope of the decay curve, A, and the ratio of the mass of fluorescein in the cornea to that in the anterior chamber, M_c/M_a, as shown in equation 1.

F_a = V_aA[1 + M_c/M_a]

M_c = V_c C_c , where V_c is the corneal stroma volume and C_c is the corneal stroma fluorescein concentration. Similarly, M_a = V_a C_a, where C_a is the anterior chamber concentration of fluorescein. Making these substitutions in equation 1 yields:

F_a = V_aA[1 + V_cC_c/V_aC_a]

If the volumes of the cornea and anterior chamber are assumed to remain constant in the steady state, then F_a is a function of A, the steepness of the decay curve, and C_c/C_a, which is the ratio of fluorescein concentrations in the corneal stroma and aqueous humor. The logarithm of C_c/C_a is represented by the distance between the parallel decay curves (see Fig. 1). Equation 2 shows that the more rapid the rate of aqueous humor flow, F_a, the steeper will be the decay curves and the larger the magnitude of the distance between the two decay curves (i.e., C_c/C_a will increase). At the other extreme, if F_a becomes equal to zero, the fluorescein concentrations in the anterior chamber and corneal stroma will equalize by diffusion and decay curves will be flat (A = 0).

Equation 2 rests on the assumptions that (i) all fluorescein in the corneal stroma leaves by diffusion into the anterior chamber, (ii) the anterior chamber is a well-mixed compartment, and (iii) all anterior chamber fluorescein is removed by convective flow.^15–17 These assumptions have been shown to be reasonably accurate.^15,18 In addition, there is close agreement between the empirically measured changes in fluorescein concentration and the theoretical values predicted by the model.^15–18

The chief advantage of the fluorophotometric method is that aqueous humor flow can be measured without invasive procedures. The normal physiology is altered in only two ways: (i) fluorescein is present in the eye and (ii) the eye is intermittently exposed to blue light. The blue light causes the dye in the corneal stroma and aqueous humor to fluoresce. This fluorescence is measured by the fluorophotometer and is directly proportional to the fluorescein concentration.

Invasive methods have been used to measure aqueous humor flow in experimental animals.^10,19–21 These methods suffered from the obvious disadvantage that they could not be carried out without an obligatory insult to the eye.²² They usually measured the rate of change in the concentration of a tracer substance injected into the aqueous humor or measured the rate of flow from a reservoir into the cannulated anterior chamber as the infusion pressure was changed.^11,20 Tracer molecules can also be used to determine the routes of flow from the anterior chamber; in this manner uveoscleral flow was first discovered.²³

Tonography has been used to measure anterior chamber aqueous humor flow. The method is noninvasive. However, at best, tonography can only reveal the rate of flow across the trabecular meshwork (i.e., the pressure-dependent flow). Because tonography measures the change in aqueous humor flow caused by an increase in intraocular pressure, it yields no information regarding the magnitude of the pressure-independent flow of aqueous humor, usually referred to as uveoscleral outflow, F_u. Therefore, it is not possible to determine the rate of aqueous humor formation on the basis of tonography alone. As shown in equation 3, in the steady state, the rate of anterior chamber aqueous flow, F_a, equals the sum of trabecular outflow, F_tr, and uveoscleral outflow, F_u.

F_a = F_tr + F_u

AQUEOUS HUMOR OUTFLOW

Trabecular Meshwork Outflow

The inner wall of the canal of Schlemm and the juxtacanalicular tissue seem to be the sites of the major resistance to flow through the trabecular meshwork.²⁴ Tonography is used to measure this resistance to the flow of aqueous humor. In its simplest form, tonography is accomplished by placing the Schiøtz tonometer on the cornea of the supine patient for 4 minutes and observing the change in intraocular pressure over the test period. Electronic tonography machines allow a continuous recording of the intraocular pressure.

The flow of aqueous humor into the canal of Schlemm seems to be a passive process and therefore obeys the general physical law relating passive flow, F, the hydrostatic pressure driving force, Δp, and the resistance to flow, R (equation 4).

ΔP = FR

When applied to the trabecular flow, it yields

IOP-P_CS = F_trR_tr

where the driving force is IOP − P_cs, the difference between the hydrostatic pressure of the anterior chamber, IOP, and that of the canal of Schlemm, P_cs. F_tr is the transtrabecular flow of aqueous humor, and R_tr is the resistance to flow between the anterior chamber and the canal of Schlemm. The inverse of R_tr is C_tr, the trabecular outflow facility.

C_tr = 1/R_tr

Combining equations 5 and 6 yields:

F_tr = C_tr(IOP–P_cs)

If in equation 7 IOP is changed by ΔIOP, while C_tr and P_cs remain constant, the trabecular meshwork flow will change by ΔF_tr, as shown in equation 8.

C_tr = ΔF_tr/ΔIOP

Thus, in order to measure the trabecular outflow facility, C_tr, one must only measure ΔF_tr and ΔIOP.

In conventional tonography, ΔIOP is equal to the mean increase above the baseline intraocular pressure produced by the tonometer. Measurement of ΔF_tr is based on the change in the volume of the globe during tonography. This change in volume of the globe is indirectly derived from the change in intraocular pressure that occurs during the 4-minute tonography period. The relationship between the volume of the globe and the intraocular pressure, although somewhat controversial, has been reasonably well worked out.^25,26 The intraocular volume decrease determined by tonography is assumed to be equal to the magnitude of the flow of aqueous humor across the trabecular meshwork in excess of the normal rate of flow.

Overestimations of the trabecular meshwork flow, F_tr, could result if (i) part of the intraocular volume decrease during tonography were caused by increased flow of fluid leaving the eye by nontrabecular meshwork routes (e.g., if blood and extracellular fluid were squeezed out of the choroid) and (ii) the rate of aqueous humor secretion were decreased during tonography. These and possibly other sources of overestimation seem to occur. Overestimation errors are collectively referred to as pseudofacility. As shown by equation 9, the difference between the total outflow facility measured by tonography, C_tot, and the trabecular outflow facility, C_tr, equals the pseudofacility, C_ps.²⁷

C_ps = C_tot – C_tr

Changes in outflow facility determined by tonography could be caused by either a change in C_tr, as is usually assumed, or a change in C_ps, or both.

Attempts have been made to measure C_ps in humans by measuring the relationship between an induced increase in episcleral venous pressure and the resultant increase in intraocular pressure.^28,29 However, this method is fraught with potential artifacts and therefore is of limited usefulness.

An attempt to determine human C_tr noninvasively has been made by Yablonski and co-workers.^30,31 They used fluorophotometry to measure ΔF_tr after administration of a β-blocker and/or a carbonic anhydrase inhibitor. With the measurement of the concomitant ΔIOP, assumed to be entirely caused by decreased aqueous humor formation, C_tr was determined using equation 8. This method avoids the confounding influence of C_ps and ocular rigidity; however, because of inherent variability, its main use is in studies of many subjects rather than of individuals.

Invasive methods of tonography have been used in experimental animals.^22,32,33 These methods involve cannulation of the anterior chamber and thereby permit direct measurement of ΔFtr caused by ΔIOP without having to deduce volume changes from intraocular pressure changes. These methods also permit the injection of tracers into the anterior chamber and thus increase the capabilities of these studies; however, the invasive techniques also create artifacts caused by anesthesia, a variable inflammatory response, and penetration of the globe, that are not present in noninvasive procedures.^22,34

The value of (IOP − Pcs) in equation 7 is the outflow pressure, the driving force for fluid flow into the canal of Schlemn. The value of P_cs is assumed to be nearly equal to the value of the episcleral venous pressure, which is approximately 10 mm Hg; therefore, a change in intraocular pressure from 16 mm Hg to 12 mm Hg represents a 25% decrease in intraocular pressure but a 67% decrease in outflow pressure. On the other hand, a decrease in intraocular pressure from 40 mm Hg to 30 mm Hg is also a 25% decrease in intraocular pressure but only a 33% decrease in outflow pressure. When studying drugs that affect the trabecular outflow, it is more meaningful to express changes in the steady-state condition as percent changes in outflow pressure rather than as percent changes in intraocular pressure.

Uveoscleral Outflow

In 1937, after the injection of indigo carmine dye into the anterior chambers of rabbits, von-Seidel noted staining in the sclera and uvea.³⁵ Thirty years later in an experimental primate model, Bill^36,37 identified the alternate (uveoscleral) outflow pathway with a description of the consecutive progression of radiolabeled aqueous humor from the anterior chamber through the interstitial spaces of the ciliary body muscle to the suprachoroidal space and though the sclera. Some fluid is removed by the ciliary body capillaries³⁸ and a strong case has been made by Yablonski³⁹ for the final route of uveoscleral flow to be largely into the uveal blood rather than entirely across the sclera. The basis for this hypothesis is the large protein colloid osmotic pressure difference between the uveal blood and aqueous humor which would act as a strong driving force for aqueous humor flow into the uveal capillaries. According to this hypothesis the term uveoscleral flow is a misnomer because most of the flow is not across the sclera. It has similarly been suggested that pressure-insensitive outflow is a better term.⁴⁰ Nevertheless, we will continue to use the term uveoscleral outflow in this chapter in order to be consistent with the rest of the literature. Direct measurement of uveoscleral outflow involves the use of radiolabeled tracers, is invasive and requires a disturbance to the eye. Uveoscleral flow of tracer molecules injected into primate eyes accounted for 30% to 50% of the total aqueous humor egress.⁴¹ Bill⁴² identified twelve human eyes that were to undergo medically necessary enucleation for suspected and/or confirmed malignant melanoma. Prior to enucleation, Bill injected ¹³¹I-albumin into the eyes and, at carefully measured intervals, quantified the amount of radioactive tracer identified in the ocular tissues. The average amount of uveoscleral outflow was approximately 10%.

Data obtained using noninvasive indirect measurements suggest that uveoscleral outflow can vary significantly with changes in the pathophysiologic condition of the eye. In young, healthy human subjects, Townsend and Brubaker⁴³ used tonography to determine that 36% of outflow occurred through the uveoscleral pathway. In a fluorophotometry study comparing the rates of uveoscleral outflow in young and elderly nonglaucomatous people, Toris et al⁴⁴ calculated that the uveoscleral outflow accounts for 54% of the total outflow in the young, and 46% of the total outflow in elderly. The 54% of outflow that is uveoscleral in young humans (n = 51) is consistent with earlier data from tracer experiments in young monkeys.⁴⁵ In contrast, fluorophotometry showed 23% of the aqueous passed through the uveoscleral pathway of ocular hypertensive patients.⁴⁶

Eyes with primary open-angle glaucoma (n = 15) treated with maximally tolerated medical therapy have fluorophotometry measured uveoscleral flow rates that are 73% of total outflow,³⁰ which suggests high resistance in the trabecular meshwork may result in the therapeutic redirection of aqueous to the uveoscleral pathway.

Episcleral Venous Pressure

In 1901 scientists postulated that the aqueous humor drains into the episcleral veins.⁴⁷ This theory was substantiated 20 years later by Uribe-Troncoso⁴⁸ and von Seidel⁴⁹ with their independent descriptions of the movement of aqueous humor from the anterior chamber to the episcleral veins in rabbits. Aqueous humor was identified in the episcleral veins by Ascher.^50–52

Although there are different methods to measure the episcleral venous pressure (P_ev), all are rooted in the principle introduced by von Seidel⁵³ that the intravascular pressure of the vein will be equal to the amount of external pressure necessary to elicit a change in the caliber of that vein. This theory assumes that vein walls have negligible rigidity and tension, and therefore, the vein will not become reduced in caliber until the extravascular pressure exceeds the intravascular pressure. Brubaker⁵⁴ corroborated this assumption with the observation that episcleral venous pressures measured by external compression of the vein correlated with the venous pressures measured by vessel cannulation. Episcleral venous pressure may vary up to 1 mm Hg with changes in posture.^55,56 Regardless, most studies conclude that P_ev is normally 8 to 12 mm Hg in humans.⁵⁷ In most forms of glaucoma, the episcleral venous pressure remains relatively stable.^55,58,59

The factors influence obtaining accurate measurements of episcleral venous pressure in humans: (i) identifying the correct vessel, (ii) applying the measurement device at an appropriate position over the vessel, and (ii) the determination of the end point. To obtain the most accurate representation of the pressure of the canal of Schlemm, the episcleral venous pressure should be measured in an aqueous- containing vein. Aqueous veins originate at the limbus and are paler than the blood filled episcleral veins. Zeimer⁶⁰ suggests placing the measurement device slightly distal to the junction of the aqueous and episcleral vein. If these vessels are difficult to visualize, the device can be placed 3 mm distal to the limbus. Proposed end points^60–63 range from the first visible change in the vessel,^55,64,65 either in color or shape, to the complete occlusion of the vessel.⁵⁴ The controversy centers on how much of a change in vein appearance best represents just exceeding the episcleral venous pressure. In the same eye, different endpoints produce different results. The excess increase in external pressure required to reach the first detectible change in caliber of the vein has been reported to be 0.25 to 2 mm Hg.^55,62 As much as 7 mm Hg⁶⁴ more may be required to fully collapse the vein. The end point measured as either a 50% reduction in the color or width of the vessel is generally accepted as an appropriate choice.⁵⁷

The pressure chamber technique, first described by von-Seidel⁵³in 1923, is generally accepted as the most reliable and accurate method to measure episcleral venous pressure.⁶⁶ Although the instrument has been modified over the years to improve accuracy and ease of use, the principle underlying this technique is unchanged. A transparent distensible membrane covers one end of the chamber and the other end enables the observer to view the vessel. The observer applies external pressure by placing the membrane end of the chamber on the conjunctiva overlying the vein. Pressure is applied until a change in the caliber of the vessel, the end point, is noted. The pressure of the chamber at the end point is measured by a manometer connected to the apparatus. Zeimer et al⁶⁰ made a significant contribution to the pressure chamber method in 1983 with the development of the episcleral venomanometer. This device has many advantages: it can be operated by a single observer, it is compact, and it can easily be attached to a slit-lamp biomicroscope to provide a stereoscopic view. The episcleral venomanometer has been demonstrated to have both intraobserver and interobserver reproducibility.⁶⁰ Calibration of this instrument is not necessary, and an end point of half-balancing of the selected vein is recommended.

Additional methods have been proposed to measure episcleral venous pressure but none have been able to achieve the accuracy and reproducibility of the episcleral venomanometer and/or pressure chamber technique. The force method, developed by Goldmann,⁶⁷ involves the application of a variable force to a constant area over the episcleral vein. Pressure is calculated, as the force required, to reach the endpoint divided by the area of the applanation tip. The reproducibility of this method is controversial: Brubaker⁵⁴ demonstrated that the end point is difficult to reproduce with the force method. However, Podos et al⁵⁹ reported a reproducibility of ±1.2 mm Hg. The size and shape of the applanation tip are factors that influence the calculation of pressure; if the flat tip does not completely conform to the curved globe, the force will not be evenly distributed.^54,59,68 The air jet method, like the pressure chamber method, is based on the theory that the pressure of the vein will be equal to the pressure from a jet of air that elicits a change in the caliber of the vein. This device must be calibrated to account for the loss of pressure due to resistance as the air travels from the manometer toward the eye. The accuracy of this method is decreased if the jet of air does not hit the vein directly.⁶¹ An indirect tonographic method⁶⁹ has also been proposed, but it was found to provide reliable information only in normal eyes.⁷⁰

Alternative Model for Aqueous Humor Dynamics

The preceding discussion and equations are derived from the traditional two-compartment model of aqueous humor dynamics. The two-compartment model describes the passage of fluorescein between the systemic circulation, the anterior chamber, and the cornea. In recent years, McLaren et al⁷¹ have proposed that the mathematical calculations of aqueous humor dynamics should be routed in a three-compartment model. This theory arose as a result of the observation that the existing (two-compartment) equations do not accurately predict the time in which fluorescein enters the anterior chamber from the systemic circulation. The three-compartment model adds an additional transfer coefficient that takes into account the diffusion of the fluorescein through a tissue component, the iris, prior to its appearance in the anterior chamber. Although the three-compartment model has yet to gain wide acceptance, there are convincing arguments on its behalf.

The eye may be thought of as an elastic sphere of sclera and cornea, inflated by a liquid at a pressure equal to the intraocular pressure. Alterations in intraocular volume will transiently change intraocular pressure. Equation 10 gives the change in the value of the log of the intraocular pressure as the product of ΔV, the change in intraocular volume, and E, the ocular rigidity coefficient.²⁵

Δlog (IOP)= EΔV

Equation 11 expresses this relationship using the natural logarithm and assumes that the normal value for E is 0.0215/μL

Δln (IOP)=2.303 Δlog(IOP)=(2.303)(0.0215) (ΔV)

For small changes in V, dV, equation 11 becomes

dln (IOP)=d(IOP)/IOP =(2.303)(0.0215) (dV)=0.05 (dV)

There is an approximate 5% change in intraocular pressure for every microliter change in volume.^25,26 Equations 10 to 12 apply only when intraocular volume is changing. Changes in intraocular volume cannot be sustained unless there is a coincident change in the rate of aqueous humor secretion, the outflow facility, the uveoscleral flow, or the episcleral venous pressure. For example, if 1 μL of fluid were somehow removed from the globe, the intraocular pressure would decrease by 5%, however, the globe's volume and intraocular pressure would soon return to their prealtered level if the factors that determine the steady-state level of intraocular pressure were unchanged.

A sphere is the geometric configuration with the smallest surface area per unit volume or, stated differently, it has the largest volume per unit surface area. Any alteration in its shape results in smaller volume. Deformation of a sphere without a decrease in volume results in its being overinflated. Hence, deformation of the eye results in an increase in intraocular pressure.

Tonography and ophthalmodynamometry deform the eye. During tonography, because none of the steady-state determinants of intraocular pressure has been altered, the pressure decreases exponentially toward its pretonography level. When the extraocular muscles rotate the globe, the eye is deformed by their pull. Usually, the resultant change in intraocular pressure is minimal,⁷² however, when a muscle is mechanically restricted, the pull of the antagonist muscle may result in considerable deforming force. Thus, in a case of extraocular muscle entrapment after orbital floor fracture, or of an extraocular muscle adhesion due to endocrine ophthalmopathy,⁷³ attempts to rotate the globe away from the direction of action of the restricted muscle will result in an elevation of the intraocular pressure. With each heart beat a slight increase in the intraocular vascular volume occurs; hence, there is a transient increase in intraocular pressure.

ALTERATION OF STEADY STATE DETERMINANTS OF INTRAOCULAR PRESSURE BY PHARMACOLOGIC AGENTS

The goal of pharmacologic glaucoma therapy is to prevent visual field loss. The only proven method is to lower the intraocular pressure.^74–78 In order for a drug to be effective, it must alter one or more of the factors that maintain the steady-state intraocular pressure: the rate of aqueous humor formation, the rate of aqueous humor outflow (trabecular meshwork and uveoscleral pathways), and the episcleral venous pressure. None of the commonly used eye medications seem to significantly affect the episcleral venous pressure.⁷⁹

α-ADRENERGIC AGONISTS

α-Agonists were first noted to have an ocular hypotensive effect in 1966 with the observation that oral clonidine, an α₂-agonist, decreased intraocular pressure in humans.⁸⁰ Topical clonidine was found to have potent systemic side effects which included systolic and diastolic hypotension, bradycardia, and sedation.^81–83 The dose-dependent systemic hypotension resulted in a decrease in ophthalmic artery pressure, which may put the optic nerve at increase risk for ischemic damage.⁸⁴ In 1978, apraclonidine, a clonidine analogue, was the first α₂-agonist approved in the United States for treating ocular hypertension. Apraclonidine, compared to clonidine, had decreased blood-aqueous and blood–brain permeabilities, and therefore had a safer systemic side effect profile.⁸⁵ Brimonidine, introduced subsequently, was 23-fold to 32-fold more α₂-adrenoceptor selective than apraclonidine, so much so that it is considered a reference compound.^86–88

In vivo fluorophotometry data in rabbits, monkeys, and humans indicate suppression of aqueous humor formation following administration of clonidine, apraclonidine, and brimomidine.^89–91 In humans it has been suggested that, with long-term therapy, brimomidine might lower intraocular pressure by another mechanism (i.e., not by a reduction in aqueous formation). Toris et al⁸⁹ observed with fluorophotometry that, acutely, both brimomidine and apraclonidine decrease aqueous humor formation.^92,93 By day 8 of treatment, the magnitude of the inhibition of aqueous flow decreased for both drugs and the outflow facility in apraclonidine treated eyes increased, thereby sustaining the decrease in intraocular pressure.⁹³ After 8 days of treatment with brimomidine, fluorophotometry measured aqueous humor flow was reduced by 0.44 μL/min, compared to that on day 1, 0.70 μL/min. By day 29 the rate of aqueous flow had returned to pretreatment levels,⁹² yet the pressure-lowering effect remained stable. Toris and co-workers⁹² explanation was that uveoscleral outflow was presumed to have increased by 60% above baseline by day 8, and remained at this elevated level through day 29. Trabecular outflow facility and episcleral venous pressure were unchanged by brimomidine. The mechanism by which brimomidine may cause such an increase in uveoscleral outflow is unknown. Some suggest that stimulation of α₂-adrenergic receptors by brimomidine may result in the release of endogenous prostaglandins.⁹² It is widely believed that prostaglandins enhance uveoscleral outflow.^94,95

β-ADRENERGIC AGONISTS

Epinephrine is an α₁-, α₂-, β₁-, and β₂-adrenergic agonist. It is believed to act as an ocular hypotensive agent primarily through its β₂-agonist activity, because selective β₂-agonists such as salbutamol can mimic epinephrine's hypotensive action.⁹⁶ α₁-Agonists, such as phenylephrine,⁹⁷ have no significant hypotensive effect.

Based on earlier tonographic studies,^98,99 and even some early fluorometric studies,^13,100 it was erroneously concluded that topical epinephrine lowered intraocular pressure primarily by decreasing the rate of aqueous humor formation. This was based on the observation that the increase in C_tot produced by epinephrine was not sufficient to account for the decrease in intraocular pressure. Because epinephrine did not significantly affect the episcleral venous pressure⁶⁶ and the concept of uveoscleral flow had not been fully appreciated, it was assumed that the decrease in intraocular pressure was primarily because of a decrease in aqueous humor secretion. Subsequent fluorometric studies have found that epinephrine actually increases, by approximately 20%, aqueous humor formation.^17,38,43

Topical epinephrine, and other β₂-agonists, seem to act primarily by increasing uveoscleral flow,^17,43 with the increase in trabecular outflow facility being of secondary importance. However, the literature is not consistent. Of those parameters (intraocular pressure, episcleral venous pressure, rate of aqueous humor secretion, outflow facility, and uveoscleral flow) evaluated, human uveoscleral flow is the most indirectly and possibly the least accurately, calculated. Conflicting findings exist. For example, Wang et al¹⁰¹ investigated the effects in 26 human volunteers of twice-daily doses of topical epinephrine hydrochloride 2% on aqueous humor dynamics after 1 week of use. Their results contradicted almost all of the preceding statements. Fluorophotometry was used to measure both aqueous humor formation and trabecular outflow facility. The trabecular outflow facility was calculated as the ratio of the change in aqueous humor flow to the change in IOP (equation 8). Aqueous formation decreased by 12% after epinephrine treatment, and trabecular outflow facility increased approximately 44%. Episcleral venous pressure was determined with an episcleral venomanometer and was not significantly affected by treatment with epinephrine. Measurement of uveoscleral outflow with indirect mathematical methods suggested that epinephrine did not alter F_u.

β-ADRENERGIC ANTAGONISTS

Aqueous humor flow measurements, determined by fluorophotometry, show that after either a single dose of 0.5% timolol,^16,102 or 1 week of twice-daily topical administration,¹⁷ a 50% reduction in the rate of aqueous humor formation results. One drop of timolol maleate reduced flow from 2.24 μL/min to 1.17 μL/min. The intraocular pressure fell from 26.8 mm Hg to 18.7 mm Hg. Based on the assumption that the episcleral venous pressure was 10 mm Hg, this decrease in intraocular pressure represented a 50% decrease in outflow pressure (IOP − P_ev) from 16.8 mm Hg to 8.7 mm Hg. The 50% decreases in aqueous humor flow and outflow pressure suggested that the entire intraocular pressure effect of timolol maleate was caused by the reduction in aqueous humor production. These assumptions were substantiated by the absence of significant alteration of the episcleral venous pressure,¹⁰³ outflow facility,^17,103 and uveoscleral flow.¹⁷

CARBONIC ANHYDRASE INHIBITORS

For over 40 years, carbonic anhydrase inhibitors have been important agents used in the treatment of glaucoma. The intraocular pressure-lowering effects of carbonic anhydrase inhibitors in humans were first noted by Becker,¹⁰⁴ shortly after acetazolamide was introduced as a diuretic. Oral carbonic anhydrase inhibitors have a high frequency of adverse effects, and often are not tolerated by patients. Attempts to produce a topical carbonic anhydrase inhibitor were initially unsuccessful because of difficulty achieving the 99% inhibition of carbonic anhydrase needed to elicit a significant psysiologic response.^105,106 In 1983, Maren et al¹⁰⁷ demonstrated that topical trifluoromethazolamide lowered the intraocular pressure of rabbits. Eventually, a safe and effective topical carbonic anhydrase inhibitors for use in humans was developed.

Acetazolamide, methazolamide, and dichlorphenamide are oral carbonic anhydrase inhibitors approved for use in the United States. These drugs lower intraocular pressure by decreasing the rate of aqueous humor secretion as evidenced by fluorophotometry^108,109 and tonography data.^{6,103,110,111} The initial decrease in acetazolamide treatment has been found to be as much as 75% in dogs, determined by radioactive tracer anterior chamber dilation.²⁰ Tonographic data based on equation 7 has been shown to reduce aqueous humor formation by approximately 50% in most studies of animals¹¹² and humans.^110,113 Fluorophotometry in humans has also shown a 50% decrease in the steady-state rate of aqueous humor formation.^108,109

Using fluorophotometry, 2% topical dorzolamide was shown to induce a 38% decrease in aqueous humor secretion in monkeys.¹¹⁴ In humans, a 20% fluorophotometric reduction of aqueous secretion was measured within 2 to 5 hours of topical dorzolamide administration.¹¹⁵ The effect of carbonic anhydrase inhibitors on uveoscleral outflow is not known; however, it seems unlikely that a change occurs in this parameter since the reduction in aqueous humor formation is sufficient to explain the entire effect on intraocular pressure.

Fluorophotometry has been utilized in studies investigating the effects of topical dorzolamide and oral acetazolamide used in combination. While both treatments result in a statistically significant reduction in aqueous humor flow,^115,116 oral acetazolamide produces a greater effect: acetazolamide, 30%;dorzolamide, 17%.^116,117 Most data suggest that, used concurrently, topical and oral carbonic anhydrase inhibitors are not additive.^116,118,119 However, Maus et al¹¹⁷ found that the addition of oral acetazolamide to patients already receiving topical dorzolamide reduced aqueous humor formation by an additional 16%. Adding dorzolamide to patient's already receiving acetazolamide did not result in further aqueous flow reduction.

Topical carbonic anhydrase inhibitors are less potent than β-blockers when used as monotherapies.¹¹⁸ In combination, timolol and dorzolamide produce additive lowering effects on both aqueous humor formation and intraocular pressure. Their additive effects have been shown to reduce aqueous flow by approximately 55%.¹¹⁹

CHOLINERGIC AGENTS

Direct-acting cholinergic agents such as pilocarpine and carbachol, and indirect-acting cholinergic agents, such as physostigmine (Eserine) and echothiophate iodide appear to lower the intraocular pressure entirely on the basis of their effects on C_tr.³³ Fluorophotometry has shown no effect of echothiophate iodide on the rate of human aqueous humor formation.¹²⁰ These agents also have no significant effect on P_ev.⁷⁹ In vivo studies in humans¹²¹ and primates¹²² indicate that pilocarpine decreases the magnitude of F_u. Presumably, the contraction of the ciliary body musculature decreases the ease with which aqueous humor can penetrate the ciliary body. The increase in C_tot, determined tonographically, is often larger than appears appropriate for the decrease in intraocular pressure. This may be due to the decrease in F_u, which would tend by itself to raise the intraocular pressure.

CORTICOSTEROIDS

Topical corticosteroids raise intraocular pressure without affecting aqueous humor flow.^123–125 There is a reasonably consistent tonographic finding of reduced outflow facility,^126,127 suggesting that the increase in intraocular pressure is due to a negative but reversible effect on the trabecular structure.

PROSTAGLANDIN ANALOGUES

The ocular hypotensive effect of intracameral prostaglandins was noted in 1971.¹²⁸ Camras et al¹²⁹ showed a similar hypotensive effect from topically applied prostaglandins in 1977. Prostaglandin F_α2 analogues, latanoprost, bimatoprost, unoprostone isopropyl, and travoprost have emerged as effective therapies for the treatment of glaucoma.

Prostaglandins lower the intraocular pressure in large part by increasing the uveoscleral outflow. The exact mechanism in which this occurs is not known. One theory is that prostaglandins initiate a remodeling of the extracellular matrix of the ciliary body that results in improved drainage of aqueous humor through the ciliary body face. Experimental evidene exists that, in primates, prostaglandin F_α2 analogues decrease the extracellular matrix surrounding the ciliary body smooth muscle cells after 4 to 8 days of treatment.^130,131 This finding corresponds to the observation that a larger intraocular pressure reduction was observed in monkeys after 4 to 5 days of daily latanoprost dosing.132,133 Bimatoprost was also found to increase tonographic outflow facility, C_tot.⁴⁰ Similar, but not quite statistically significant increases in outflow facility were found, by fluorophotometry, for latanoprost.⁹³

Initially, based on data from primate experimental models, there was concern that pilocarpine might inhibit prostaglandin induced enhancement of uveoscleral outflow.^134,135 However, in humans, fluorophotometry data indicate that pilocarpine does not have this effect^136–138; prostaglandin derivatives and pilocarpine are effective in lowering intraocular pressure when used together.

Hyperosmotic agents (e.g., urea, mannitol, and glycerol) decrease the intraocular pressure by reducing the intraocular volume. They increase the osmotic pressure exerted by the blood and drive intraocular fluid into the capillaries. As a result, fluid is drawn out of the vitreous and the extracellular spaces.^142,143 The exact route of the fluid escape is not clear. Perhaps uveoscleral flow is increased, since there is some evidence that this flow is a function of the magnitude of the blood osmotic pressure.^36,38,39

Vasoactive agents that reduce the intraocular blood volume can cause a transient decrease in intraocular pressure. For example, oxygen administration has been shown to lower the intraocular pressure¹⁴⁴ and to cause a decrease in choroidal blood volume.¹⁴⁵ However, the basis of the more sustained lowering of intraocular pressure during oxygen administration is a reduced episcleral venous pressure. On the other hand, intravenous administration of vasodilators such as amyl nitrite causes an increase in intraocular pressure that is assumed to be caused by increased uveal blood volume.¹⁴⁶

1. Riley MV: The sodium-potassium-stimulated adenosine triphosphatase of rabbit ciliary epithelium. Exp Eye Res 3:76, 1964

2. Härkönen M, Palkama A, Uusitalo R: Functional dependence of the ciliary epithelium ATPase activity and intraocular pressure on the autonomic nervous system. Acta Physiol Scand 86:327, 1972

3. Bill A: Blood circulation and fluid dynamics in the eye. Physiol Rev 55:383, 1975

4. Cole DF: Secretion of the aqueous humor. Exp Eye Res 25(suppl):161, 1977

5. Becker B: The effects of carbonic anhydrase inhibitor, acetazolamide, on the composition of the aqueous humor. Am J Ophthalmol 40:129, 1955

6. Becker B: Carbonic anhydrase and the formation of aqueous humor. Am J Ophthalmol 47:342, 1959

7. Berggren L: Direct observation of secretory pumping in vitro of the rabbit eye ciliary process. Invest Ophthalmol 3:266, 1964

8. Maren TH: HCO₃ formation in aqueous humor: Mechanism and relation to the treatment of glaucoma. Invest Ophthalmol 13:479, 1974

9. Green K, Pederson JE: Contribution of secretion and filtration to aqueous humor formation. Am J Physiol 222:1218, 1972

10. Bill A: Effects of longstanding stepwise increments in eye pressure on the rate of aqueous formation in a primate (Ceropithecus ethiops). Exp Eye Res 12:184, 1971

11. Bill A: The role of ciliary blood flow and ultrafiltration in aqueous humor formation. Exp Eye Res 16:287, 1973

12. Goldmann H: Über Fluorescein in der menschlichen Vorderkammer. Ophthalmologica 119:65, 1950

13. Nagataki S: Effects of adrenergic drugs on aqueous humor dynamics in man. Acta Soc Ophthalmol Jpn 81:1795, 1977

14. Araie M, Sawa M, Takase M: Physiological study of the eye as studied by oral fluorescein: I. Oral administration of fluorescein and its pharmacokinetics. Acta Soc Ophthalmol Jpn 84:1003, 1980

15. Jones RF, Maurice DM: New methods of measuring the rate of aqueous flow in man with fluorescein. Exp Eye Res 5:208, 1966

16. Yablonski ME, Zimmerman TJ, Waltman SR, et al: A fluorophotometric study of the effect of topical timolol on aqueous humor dynamics. Exp Eye Res 27:135, 1978

17. Schenker HW, Yablonski ME, Podos SM, et al: Fluorophotometric study of epinephrine and timolol in human subjects. Arch Ophthalmol 99:1212, 1981

18. Pederson JE, Gaasterland DE, MacLellan HM: Accuracy of aqueous humor flow determination by fluorophotometry. Invest Ophthalmol Vis Sci 17:190, 1978

19. Becker B: The measurement of rate of aqueous flow with iodide. Invest Ophthalmol 1:52, 1962

20. O'Rourke J, Macri FJ, Berghoffer B: Studies in uveal physiology: I. Adaptation of isotope clearance procedures for external monitoring of anterior uveal blood flow and aqueous humor turnover in the dog. Arch Ophthalmol 81:526, 1969

21. Obenberger J, Babický A: Alkali and acid burns of the rabbit eye: Measurement of aqueous flow by means of intravenously injected Na ¹²⁵I and Na ¹³¹I. Graefes Arch Klin Exp Ophthalmol 193:253, 1975

22. Sears ML: Outflow resistance of the rabbit eye: Technique and effects of acetazolamide. Arch Ophthalmol 64:823, 1960

23. Bill A: Conventional and uveo-scleral drainage of aqueous humor in the cynomolgus monkey (Macaca irus) at normal and high intraocular pressures. Exp Eye Res 5:45, 1966

24. Rohen JW, Futa R, Lütjen-Drecoll E: The fine structure of the cribriform meshwork in normal and glaucomatous eyes as seen in tangential sections. Invest Ophthalmol Vis Sci 21:574, 1981

25. Friedenwald JS: Contribution to the theory and practice of tonometry. Am J Ophthalmol 20:985, 1937

26. Perkins ES: Ocular volume and ocular rigidity. Exp Eye Res 33:141, 1981

27. Bárány EH: A mathematical formulation of intraocular pressure dependent on secretion, ultrafiltration, bulk outflow, and osmotic reabsorption fluid. Invest Ophthalmol 2:584, 1963

28. Kupfer C, Sanderson P: Determination of pseudofacility in the eye of man. Arch Ophthalmol 80:194, 1968

29. Kupfer C, Ross K: Studies of aqueous humor dynamics in man: I. Measurements in young normal subjects. Invest Ophthalmol 10:518, 1971

30. Yablonski ME, Cook DJ, Gray J: A fluorophotometric study of the effect of argon laser trabeculoplasty on aqueous humor dynamics. Am J Ophthalmol 99:579, 1985

31. Hayashi M, Yablonski ME, Novack GD: Trabecular outflow facility determined by fluorophotometry in human subjects. Exp Eye Res 48:621, 1989

32. Becker B, Constant MS: The facility of aqueous outflow: A comparison of tonography and perfusion measurements in vivo and in vitro. Arch Ophthalmol 55:305, 1956

33. Bárány EH: The immediate effect on outflow resistance of intravenous pilocarpine in the vervet monkey Cercopithecus ethiops. Invest Ophthalmol 6:373, 1967

34. Bárány EH, Becker B: Resistance to aqueous outflow in glaucoma. In Newell FW, ed. Transactions of the First Conference, December 5–7, 1955. New York: Josiah Macy, Jr. Foundation, 1956:209–212

35. Bárány EH: Pseudofacility and uveo-scleral outflow routes. Glaucoma Symposium, Tutzing Castle, 1966. New York: Karger, 1967:27–51

36. Bill A: The aqueous humor drainage mechanism in the cynomolgus monkey (Macaca irus) with evidence for unconventional routes. Invest Ophthalmol 4:920, 1965

37. Bill A, Hellsing K: Production and drainage of aqueous humor in cynomolgus monkey (Macaca irus). Invest Ophthalmol 4:920, 1965

38. Pederson JE, Gaasterland DE, MacLellan HM: Uveoscleral aqueous outflow in the rhesus monkey: Importance of uveal reabsorption. Invest Ophthalmol Vis Sci 16:1008, 1977

39. Yablonski ME: Some thoughts on the pressure dependence of uveoscleral flow [Letter]. J Glaucoma 12) :90,2003; author reply93-94.

40. Brubaker RF: Mechanism of action of bimatoprost (Lumigan™) Surv Ophthalmol 45:5347, 2001

41. Bill A, Hellsing K: Production and drainage of aqueous humor in the cynomolgus monkey (Macaca irus). Invest Opthalmol 4:920, 1965

42. Bill A: Uveoscleraal drainage of aqueous humor in human eyes. Exp Eye Res 12:275, 1971

43. Townsend DJ, Brubaker RF: Immediate effect of epinephrine on aqueous formation in the normal human eye as measured by fluorophotometry. Invest Ophthalmol Vis Sci 19:256, 1980

44. Toris CB, Yablonski ME, Wang Y-L, et al: Aqueous humor dynamics in the aging human eye. Am J Ophthalmol 127:407, 1999

45. Bill A: Aqueous humor dynamics in monkeys (Macaca irus and Cercopithecus ethiops). Exp Eye Res 11:195, 1971

46. Toris CB, Yablonski ME, Camras CB: Uveoscleral outflow and the outflow facility are reduced in ocular hypertensive patients. Invest Ophthalmol Vis Sci 39(Suppl 4):8, 1998

47. Lauber H: Beitrage zur anatomile des vorderen augenabschnittes der wirbeltiere. Anat Heft 18:369, 1998

48. Uribe-Troncoso M: The physiologic nature of the Sclemm canal. Am J Ophthalmol 4:321, 1921

49. Von Seidel E: Weitere experimentelle Untersuchungen uber die Quelle und den Verlauf der intraokularen Saftstromung. IX. Mitteilung. Uber den Abflu? des vorderen Augenkammer. Graefes Arch Clin Exp Ophthalmol 104:357, 1921

50. Ascher KW: Aqueous veins: preliminary note. Am J Ophthalmol 25:31, 1942

51. Ascher KW: Aqueous veins; physiologici importance of visible elimination of intraocular fluid. Am J Ophthalmol 25:1174, 1942

52. Ascher KW: Aqueous veins: II local pharmacologic effects on aqueous veins: Glaucoma and aqueous veins. III Am J Ophthalmol 25:1301, 1942

53. Von Seidel E: Weitere experementelle Untersuchungen uber die Quelle und den Verlauf der intraokularen Saftstromung. XX. Mitteilung. Uber die Messung des Blutdrukesin dem episcleralen Venengeflect, den vorderen Cliar und den Wirlvenen normaler Augen. Graefes Arch Clin Exp Ophthalmol 112:252, 1923

54. Brubker RF: Determination of episcleral venous pressure in the eye: A comparison of three methods. Arch Ophthalmol 77:110, 1967

55. Leith AB: Episcleral venous pressure in tonography. Br J Ophthalmol 47:271, 1963

56. Linner E, Rickenbach C, Werner H: Comparative measurements of the pressure in the aqueous veins and the conjunctival veins using different methods. Acta Ophthalmol (Copenh) 28:469, 1950

57. Asamoto A, Yablonski M: Physiology and measurement of episcleral venous pressure. In: Ritch R, Shields MB, Krupin T, eds. The Glaucomas, 2nd ed. St. Louis: Mosby, 1966:352

58. Linner E: The outflow pressure in normal and glaucomatous eyes. Acta Ophthalmol (Copenh) 33:101, 1955

59. Podos SM, Minas TF, Macri FJ: A new instrument to measure episcleral venous pressure: comparison of normal eyes and eyes with open-angle glaucoma. Arch Ophthalmol 80:209, 1968

60. Zeimer RC, Gieser DK, Wilensky JT, et al: A practical venomanometer. Measurement pf episcleral venous pressure and assessment of normal range. Arch Ophthalmol 101:1447, 1983

61. Krakau CET, Widakowich J, Wilke K: Measurements of episcleral venous pressure by means of an air jet. Acta Ophthalmol (Copenh) 51:185, 1973

62. Phelps CD, Armaly MF: Measurement of episcleral venous pressure. Am J Ophthalmol 85:35, 1978

63. Mins JL, Holland MG: Applanation and Schiotz tonometer standardizations for the owl monkey eye with a new technique for measuring episcleral venous pressure. Invest Ophthalmol 10:190, 1971

64. Gaasterland DE, Pederson JE: Episcleral venous pressure: A comparison of invasive and noninvasive measurements. Invest Ophthalmol Vis Sci 324:1417, 1983

65. Linner E: Measurement of the pressures in Schlemm's canal and in the anterior chamber of the human eye. Experientia 5:451, 1949

66. Yablonski ME, Podos SM: Glaucoma secondary to elevated episcleral venous pressure. In Ritch R, Shields MB, eds. The Secondary Glaucomas. St. Louis: Mosby, 1982:207–221

67. Goldmann H: An analysis of primary glaucoma. Trans Ophthalmol Soc UK 69:455, 1949

68. Goldman H: Abflussdruck, Minutenvolumen und Widerstand der Kammerwasserstrung des Menschen. Doc Ophthalmol 5/6:278, 1951

69. Stepanik J: Neues Verfahren zure Betimmung des extraokuralen Venendruckes. GraefesArch clin Exp Ophthalmol 177:116, 1969

70. Azuma I: Graphical expression of the intraocular pressure dynamics. Jpn J Ophthalmol 17:310, 1973

71. McLaren JW, Ziai N, Brubaker RF: A simplethree compartment model of anterior segment kinetics. Exp Eye Res 56:335, 1993

72. Jampel RS, Mindel J: The nucleus for accommodation in the midbrain of the macaque. Invest Ophthalmol 6:40, 1967

73. Hoskins HD Jr, Kass MA: Becker-Shaffer's Diagnosis and Therapy of the Glaucomas, 6th ed. St. Louis: Mosby, 1989:339

74. The AGIS Investigators: The Advanced Glaucoma Intervention Study (AGIS): 7. The relationship between control of intraocular pressure and visual field deterioration. Am J Ophthalmol 130:429, 2000

75. Bergea B, Boidn L, Svedbergh B: Impact of intraocular pressure regulation on visual fields in open-angle-glaucoma. Ophthalmology 106:997, 1999

76. Collaboration Normal-Tension Glaucoma Study Group: Comparison of glaucomatous therapeutically reduced intraocular pressures. Am J Ophthalmol 126:487, 1998

77. Mao LK, Steward WC, Shield MB: Correlation between intraocular pressure control and progressive glaucomatous damage in primary open-angle glaucoma. Am J Ophthalmol 111:51, 1991

78. Heijl A, Leske MC, Bengtsson B, et al: Reduction of intraocular pressure and glaucoma progression. Arch Ophthalmol 120:1268, 2002

79. Yablonski ME, Podos SM: Glaucoma secondary to elevated episcleral venous pressure. In Ritch R, Shields MB, eds. The Secondary Glaucomas. St. Louis: Mosby, 1982:207–221

80. Makabe R: Ophthalmological studies with dichlorophenylaminoimidazoline. Disch Med Wochenschr 91:1686, 1966

81. Hoffman BB, Lefkowitz RJ: Catecholamines, sympathomimetric drugs, adrenergic receptor antagonists. In: Hardman JG, Gilman AG, Limbird LE, eds. Goodman and Gilman's The Pharmacological Basis of Therapeutics, 9th ed. New York: McGraw-Hill, 1996:192–248

82. Hodapp E, Kolker AE, Kass MA, et al: The effect of topical clonidine on intraocular pressure. Arch Ophthalmol 99:1208, 1981

83. Kaufman PL, Barany EH: Loss of acute pilocarpine effect on outflow facility following surgical disinsertion and retrodisplacement of the ciliary muscle from the scleral spur in the cynomolgus monkey. Invest Ophthalmol 15:793, 1976

84. Krieglstein GK, Gramer E: The response of ophthalmic arterial pressure to topically applied clonidine. Graefes. Arch Clin Exp Ophthalmol 207:1, 1978

85. Gharagozloo NZ, Relf SJ, Brubaker RF: Aqueous outflow is reduced by the alpha-adrenergic agonist, apraclonidine hydrochloride (ALO 2145). Ophthalmology 95:1217, 1988

86. Cambridge D: UK-14, 304, a potent and selective α2-agonist for the characterization of α-adrenoceptor subtypes. Eur J Pharmacol 72:413, 1981

87. Konno F, Takayanagi I: Characterization of potsynaptic alpha-1 adrenoceptors in the rabbit dilator smooth muscle. Arch Pharmacol 333:271, 1986

88. Lattimer N, Rhodes KF: A difference in the affinity of some selective α2-drenoreceptor antagonists when compared on isolated vasa deferentia of rat and rabbit. Naunyn-Schmiedeberg's Arch Pharmacol 329:278-281, 1985

89. Toris CB, Gleason ML, Camras CB, et al: :Effects of brimomidine on aqueous humor dynamics in human eyes. Arch Ophthalmol 113:1514, 1995

90. Gabelt BT, Robinson JC, Hubbard WC, et al: Apraclonidine and brimomidine effects on anterior ocular and cardiovascular physiology in normal and sympathectomized monkeys. Exp Eye Res 59:633, 1994

91. Maus TL, Nau C, Brubaker RF: Comparison of the early effects of brimomidine and apraclonidine as topical ocular hypotensive agents. Arch Ophthalmol 117:586, 1999

92. Toris CB, Camras CB, Yablonski ME: Acute versus chronic effects of Brimonidine on aqueous humor dynamics in ocular hypertensive patients. Am J Ophthalmol 128:8, 1999

93. Toris CB, Tafoya ME, Camras CB, et al: Effects of apraclonidine on aqueous humor dynamics in human eyes. Ophthalmology 102:456, 1995

94. Toris CB, Camras CB, Yablonski ME: Effects of PhXA41, a new prostaglandin F2? analog, on humor dynamics in human eyes. Ophthalmology 100:1297, 1993

95. Poyer JF, Gabelt BT, Kaufman PL: The effect of topical PGF2? on uveoscleral outflow and facility in the rabbit eye. Exp Eye Res 54:277, 1992

96. Langham ME and Diggs E: α-Adrenergic responses in the eyes of rabbits, primates and man. Exp Eye Res 19:281, 1974

97. Lee DA, Brubaker RF: Effect of phenylephrine on aqueous humor flow. Curr Eye Res 2:80, 1982

98. Weekers R, Prijot E, Gustin J: Recent advances and future prospects in the medical treatment of ocular hypertension. Br J Ophthalmol 38:742, 1954

99. Becker B, Ley AP: Epinephrine and acetazolamide in the treatment of chronic glaucoma. Am J Ophthalmol 45:639, 1958

100. Weekers R, Delmarcelle Y, Gustin J: Treatment of ocular hypertension by adrenlin and diverse sympathomimetic amines. A J Ophthalmol 40:666, 1955

101. Wang Y-L, Hayashi M, Yablonski ME, et al: Effects of multiple doses of epinephrine on aqueous humor dynamics in human eyes. J Ocul Pharm Ther 18:53, 2002

102. Coakes RL, Brubaker RF: The mechanism of timolol in lowering intraocular pressure in the normal eye. Arch Ophthalmol 96:2045, 1978

103. Berson FG, Epstein DL: Separate and combined effects of timolol maleate and acetazolamide in open-angle glaucoma. Am J Ophthalmol 92:278, 1981

104. Becker B: Decrease in intraocular pressure in man by a carbonic anhydrase inhibitor, Diamox. Am J Ophthalmol 37:13, 1954

105. Maren TH: The relation between enzyme inhibition and physiologic in the carbonic anhydrase system. J Pharmacol Exp Ther 139:140, 1963

106. Kalina PH, Shetlar DJ, Lewis RA, et al: 6-amino-2-benzothiazole-sulfonamide. The effect of a topical carbonic anhydrase inhibitor on aqueous humor formation in the normal human eye. Ophthalmology 95:772, 1988

107. Maren TH, Jankowska L, Sanyal G, et al: The transcorneal permeability of sulfonamide carbonic anhydrase inhibitors and their effect on aqueous humor secretion. Exp Eye Res 36:457, 1983

108. Starr PAJ: Changes in aqueous flow determined by fluorophotometry. Trans Ophthalmol Soc UK 86:639, 1966

109. Bloom JN, Levine RZ, Thomas G, et al: Fluorophotometry and the rate of aqueous flow in man. Arch Ophthalmol 94:435, 1976

110. Friedenwald JS: Carbonic anhydrase inhibition and aqueous flow. Am J Ophthalmol 40:159, 1955

111. Becker B: The mechanism of the fall in intraocular pressure induced by the carbonic anhydrase inhibitor Diamox. Am J Ophthalmol 39:177, 1955

112. Yablonski ME, Sirota M, Charap P, et al. Effect of acetazolamide on aqueous flow in rabbits. Invest Ophthalmol Vis Sci 19 (suppl):125, 1980

113. de Carvalho CA, Cartaret L, Stone HH: Acetazolamide (Diamox) therapy in chronic glaucoma. Arch Ophthalmol 59:840, 1958

114. Wang RF, Serle JB, Podos SM, et al: MK-59=07 (L-671,152), a topically active carbonic anhydrase inhibitor reduces aqueous humor production in monkeys. Arch Ophthalmol 109:1297, 1991

115. Yamazaki Y, Miyamoto S, Sawa M: Effect of MK-507 on aqueous humor dynamics in normal human eyes. Jpn J Ophthalmol 38:92, 1994

116. Larsson LI, Alm A: Aqueous humor flow in human eyes treated with dorzolamide and different doses of acetazolamide. Arch Ophthalmol 116:19, 1998

117. Maus TL, Larsson LI, McLaren JW, et al: Comparison of dorzolamide and acetazolamide as suppressors of aqueous humor flow in humans. Arch Ophthalmol 115:45, 1997

118. Wayman L, Larsson LI, Maus T, et al: Comparison of dorzolamide and timolol as suppressors of aqueous humor flow in humans. Arch Ophthalmol 115:1368, 1997

119. Rosenberg LF, Krupin T, Tang LQ, et al: Combination of systemic acetazolamide and topical dorzolamide in reducing intraocular pressure and aqueous humor formation. Ophthalmology 105:88, 1998

120. Mindel JS, Yablonski M, Tavitian HO, et al: Dipivefrin and echothiophate efficacy of combined use in human beings. Arch Ophthalmol 99:1583, 1981

121. Bill A, Phillips CI: Uveoscleral drainage of aqueous humor in human eyes. Exp Eye Res 12:275, 1971

122. Bárány EH: Relative importance of autonomic nervous tone and structure as determinants of outflow resistance in normal monkey eyes (Cercopithecus ethiops and Macaca irus). In Rohen JW, (ed.): The Structure of the Eye: II. Symposium. Stuttgart. 1965:223–236

123. Anselmi P, Bron AJ, Maurice DM: Action of drugs on the aqueous flow in man measured by fluorophotometry. Exp Eye Res 7:487, 1968.

124. Godel V, Feiler OV, Stein R: Systemic steroids and ocular fluid dynamics II. Systemic versus topical steroids. Acta Ophthalmol (Copenh) 50:664, 1972

125. Rice SW, Bourne WM, Brubaker RF: Absence of an effect of topical dexamethasone on endothelial permeability and flow of aqueous humor. Invest Ophthalmol Vis Sci 24:1307, 1983

126. Miller D, Peczon JD, Whitworth CG: Corticosteroids and functions in the anterior segment of the eye. Am J Ophthalmol 59:31, 1965

127. Bernstein HN, Schwartz B: Effects of long-term systemic steroids on ocular pressure and tonographic values. Arch Ophthalmol 68:742, 1962

128. Starr MS: Further studies on the effect of prostaglandin on intraocular pressure in the rabbit. Exp Eye Res 11:170, 1971

129. Camras CB, Bito LZ, Eakins KE: Reduction of intraocular pressure by prostaglandins applied topically to the eyes of conscious rabbits. Invest Ophthalmol Vis Sci 16:1125, 1977

130. Lütjen-Drecoll E, Tamm E: Morphologic study of the anterior segment of cynomolgus monkey eyes following treatment with prostaglandin F_2α. Exp Eye Res 47:761, 1988

131. Lutjen-Drescoll E, Tamm E: The effects of ocular hypotensive doses of PGF F2?-1-isopropylester on anterior segment morphology. In Bito LZ, Stjernschantz J, eds. The Ocular Effects of Prostaglandins and Other Eicosanoids. Alan R. Liss: New York, 1989:737–776

132. Camras CB, Podos SM, Rosenthal JS, et al: Multiple dosing of prostaglandin F2α or epinephrine on cynomolgus monkey eyes. I. Aqueous humor dynamics. Inves Ophthalmol Vis Sci 28:463, 1987

133. Crawford K, Kaufman PL, Gabelt TB: Effects of topical PGF2α on aqueous humor dynamics in cynomolgus monkeys. Curr Eye Res 6:1035, 1987

134. Crawford K, Kaufman PL: Pilocarpine antagonizes prostaglandin F2-alpha induced ocular hypertension in monkeys. Evidence for enhancement of uveoscleral outflow by prostaglandin F2 alpha. Arch Ophthalmol 105:1112, 1987

135. Bill A: Effects of atropine and pilocarpine on aqueous humor dynamics in cynomologus monkey (Macaca irus). Exp Eye Res 6:120, 1967

136. Kent AR, Vroman DT, Thomas , et al: Interaction of pilocarpine with latanoprost in patients with glaucoma and ocular hypertension. J. Glaucoma 8:257, 1999

137. Toris CB, Alm A, Camras CB: Latanoprost and cholinergic agonists in combination. Surv Ophthalmol 47(Suppl 1):S141, 2002

138. Toris CB, Zhan G-L, Zhao J, et al: Potential mechanism for the additivity of pilocarpine and latanoprost. Am J Ophthalmol 131:722, 2001

139. Lincoff HA, Ellis CH, DeVoe AG, et al: The effect of succinylcholine on intraocular pressure. Am J Ophthalmol 40:501, 1955

140. Lincoff HA, Breinin GM, DeVoe AG: The effect of succinylcholine on the extraocular muscles. Am J Ophthalmol 43:440, 1957

141. Duke-EIder WS: The metabolism of the eye: II. Clinical applications. Arch Ophthalmol 6:158, 1931

142. Davis M, Duehr P, Javid M: The clinical use of urea for reduction of intraocular pressure. Arch Ophthalmol 65:526, 1961

143. Galin MA, Aizawa F, McLean JM: Urea as an osmotic hypotensive agent in glaucoma. Arch Ophthalmol 62:347, 1959

144. Gallin-Cohen P, Podos SM, Yablonski ME: Oxygen lowers intraocular pressure. Invest Ophthalmol Vis Sci 19:43, 1980

145. Gallin PF, Yablonski ME, Shapiro D, et al: Oxygen effect on aqueous flow and episcleral venous pressure. Invest Ophthalmol Vis Sci 19(suppl):82, 1980

146. Duke-EIder WS: System of Ophthalmology, Vol IV, The Physiology of the Eye and Vision. St. Louis, Mosby, 1968:321