CHAPTER 9

Effects of Circulatory Events on Aqueous Humor Inflow and Intraocular Pressure

Herbert A. Reitsamer* and JeVrey W. Kiel{

*Department of Ophthalmology, Paracelsus Medical University, Salzburg, Austria {Department of Ophthalmology, University of Texas Health Science Center at San Antonio, San Antonio, Texas 78229

I. Overview

II. IOP EVects on Ocular Blood Flow

III. Ocular Blood Flow EVects on IOP

IV. Ciliary Blood Flow and Aqueous Production

A.Ciliary Body Blood Supply

B.Measurement of Ciliary Blood Flow

C.Sampling Depth

D.Measurement Site

E.Aqueous Flow Measurement

F.Relationship Between Ciliary Blood Flow and Aqueous Flow V. Episcleral Venous Pressure and IOP

VI. Conclusion

References

I. OVERVIEW

This chapter will review the role of the ocular circulations in intraocular pressure (IOP) homeostasis. Historically, glaucoma was considered an ischemic disease caused by elevated IOP; however, it is now evident that ocular hypertension is not a prerequisite for glaucoma and that the progressive death of retinal ganglion cells can arise from multiple etiologies. Nonetheless, high IOP is a primary risk factor for glaucoma and lowering IOP is its primary treatment (Heijl et al., 2002). Since either situation has the potential to aVect the ocular circulations, we will begin with a brief description of IOP

FIGURE 1 Relationship between choroidal blood flow (choroidal flux), intraocular pressure (IOP) and perfusion pressure (MAP IOP). The left panel shows the response of choroidal blood flow to increasing intraocular IOP at diVerent fixed values of MAP (right panel). Choroidal blood flow declines as IOP approaches the MAP and stops when IOP exceeds the MAP. The right panel shows the same dataset with choroidal blood flow plotted against perfusion pressure; the curves superimpose demonstrating that MAP – IOP is the eVective ocular perfusion pressure. [Modified from Kiel and Van Heuven (1995)].

III. OCULAR BLOOD FLOW EFFECTS ON IOP

Historically, two conceptual models have served as frameworks for understanding IOP. One model views the IOP in terms of the ocular pressure– volume relationship, which is an exponential function of the total volume of the ocular contents and the elastic properties of the ocular coats. This approach provides the theoretical basis for indentation tonometry and tonography (Friedenwald, 1937; Grant, 1950). The other model considers the steady state IOP in hydraulic terms as an ohmic function of aqueous flow and outflow resistance (Barany, 1963). This approach provides a theoretical basis for understanding ocular hypertension and hypotony as well as the current pharmacological and surgical manipulations of IOP. Each model is useful for understanding diVerent aspects of the IOP (Figure 2).

If the elasticity of the eye is constant, any change in IOP must involve a change in ocular volume. Under normal conditions, the primary contributors to the total ocular volume are the vitreous, lens, aqueous, and blood. The vitreous and lens volumes are relatively stable over time and seldom have an acute influence on the IOP. By contrast, the volumes of aqueous and blood are more labile and account for most variations in the IOP. Changes in aqueous volume result from transient imbalances in aqueous production and outflow. Similarly, changes in ocular blood volume occur during transient imbalances in the flow of blood into and out of the eye. Most of the

FIGURE 3 (Top) Blood pressure and IOP recorded by direct cannulation of the central ear artery and vitreous compartment in an anesthetized rabbit. (Bottom) Computer simulation showing cardiac synchronous IOP pulsations due to fluctuations in blood volume caused by pulsatile inflow of blood and steady outflow (Kiel, 1998).

under control conditions, the IOP immediately before and after death are typically 15 and 7 mm Hg, respectively. This occurs in seconds, which is too fast for aqueous dynamics to play a role in the IOP decrease.

FIGURE 4 IOP falls immediately when the heart is stopped with an overdose of pentobarbital (100 mg/kg, iv) in a deeply anesthetized rabbit. Continued venous outflow without corresponding arterial inflow results in the net loss of blood volume responsible for the fall in IOP. (Time in seconds.)

It should be noted that ocular blood volume is relatively well regulated under normal conditions, at least in response to changes in arterial pressure (Kiel, 1994). As shown in Fig. 5, an acute increase in arterial pressure induced mechanically by occluding the descending aorta elicits only a small increase in IOP due to choroidal vasoconstriction under control conditions. Although both autoregulatory myogenic (Kiel, 1994) and autonomic neural mechanisms (Bill et al., 1977) have been proposed to explain this response, the regulatory mechanism responsible for this choroidal vasoconstriction is beyond the scope of this chapter. What is important to note, however, is that when this choroidal regulation is abolished with a systemic vasodilator, a similar acute elevation of arterial pressure can elicit a significantly larger increase in IOP. It is unclear whether choroidal regulation is impaired in glaucoma (Weigert et al., 2005), but if such large spikes in IOP occur during normal variations in arterial pressure they could contribute to optic neuropathy.

FIGURE 5 IOP responses to acute mechanically induced increases in arterial pressure in an anesthetized rabbit. Raising arterial pressure to 110 mm Hg elicits a modest increase in IOP under control conditions (left) and a much larger increase in IOP when choroidal regulation is impaired with a systemic vasodilator (right). The vasodilated IOP response is the largest we have recorded (Kiel, 1994).

As in tonography, a sustained pressure induced increase in ocular blood volume does not produce a sustained increase in IOP as shown in Fig. 6. Instead, the elevated IOP increases the pressure gradient for aqueous outflow, which causes a compensatory decrease of aqueous volume and IOP gradually returns to baseline (Kiel, 1994). If the increase in blood volume is small, the compensation is relatively quick, whereas compensation for a larger increase in blood volume, as occurs when choroidal regulation is impaired, takes longer. In either situation, IOP falls below baseline when the arterial pressure induced distention of the vasculature is abruptly ended, reflecting the compensatory loss of aqueous volume, which is gradually restored by continued aqueous production, which returns IOP to baseline. Such a compensatory volume shift was noted almost 100 years ago by Duke Elder who observed a marked shallowing of the anterior chamber and rise of IOP to 80–90 mm Hg upon ligation of the vortex veins in anesthetized dogs, the most extreme method to cause choroidal engorgement.

Tonometry and tonography are based on the ocular pressure–volume (P–V) relationship. Here too, the eVect of blood volume on IOP is evident. In the anesthetized rabbit, step increases in volume when arterial pressure is held at diVerent levels give diVerent P–V relationships, which are in turn diVerent from that obtained in postmortem eyes (Fig. 7; Kiel, 1995). The

FIGURE 6 IOP responses to sustained increases in arterial pressure under control and vasodilated conditions in anesthetized rabbits. Raising arterial pressure mechanically to 110 mm Hg under control conditions elicits a relatively small increase in IOP that returns to baseline relatively quickly; raising arterial pressure to the same level under vasodilated conditions results in a larger increase in IOP that takes longer to return to baseline. Restoration of baseline arterial pressure ends vascular engorgement and the undershoot of IOP reveals the compensatory lose of aqueous volume that returned IOP to baseline during the arterial pressure elevation (Kiel, 1994).

Mean Arterial Pressure (mm Hg)

FIGURE 7 EVect of arterial pressure on the ocular pressure–volume relationship and ocular rigidity in anesthetized rabbits (Kiel, 1995).

original Friedenwald tables used in tonometry and tonography were based on enucleated eyes refilled with saline to achieve a normal IOP. This procedure precluded any blood volume buVering of the IOP response to subsequent saline injections. However, as Fig. 7 shows, the P–V relationship and the ocular rigidity coeYcient are significantly dependent on the arterial pressure distending the ocular circulations.

IV. CILIARY BLOOD FLOW AND AQUEOUS PRODUCTION

A prevalent assumption in glaucoma pharmacology is that drugs that reduce aqueous production act directly on the ciliary epithelium, either by interfering with neural stimulation (e.g., prejunctional inhibition of norepinephrine release by a2 adrenergic agonists), antagonism of neurohumoral stimulation (e.g., adrenergic receptor binding by b adrenergic antagonists), or by altering key intracellular enzymes underlying the ionic transport

FIGURE 8 Ciliary blood flow (CilBF) responses to changes in perfusion pressure.

(A) Perfusion pressure was changed by step increases (arrows) in IOP, while mean arterial pressure (MAP) was left unchanged. (B) Pressureflow relationship calculated from the tracings shown in Fig. 7A. At each point MAP and IOP are shown (as MAP/IOP). (C) Manipulation of arterial pressure by occlusions of the descending thoracic aorta (aortic) and the inferior vena cava (caval). (D) Pressureflow relationship for the ascending and descending ramp of the aortic and caval occlusions shown in Fig. 7C. Figure 7B and D show evidence of ciliary blood flow autoregulation at perfusion pressures above 30–35 mm Hg.

A. Ciliary Body Blood Supply

The ciliary body is supplied by branches oV the major arterial circle of the iris which is fed by the long posterior ciliary arteries and, in some species, from the anterior ciliary arteries. The vessels of the ciliary processes divide into zones (Morrison et al., 1987a; Funk and Rohen, 1990; Lutjen Drecoll and Rohen, 1994). One zone is at the anterior base of the processes and consists of arterioles and capillaries that drain into a venular system separate from the other zones. This zone is the boundary between the nonfenestrated capillaries of the iris and the fenestrated capillaries of the ciliary processes. A second zone originates at the anterior base but extends more anteriorly into the processes and then drains into marginal venules running along the inner edge of the processes. A third zone supplies the posterior portion of the major

FIGURE 9 LDF measurement depth is suYcient for transscleral ciliary blood flow measurement. (Left) rabbit ciliary blood flow (Flux) response to caval occlusion at the same site overlying the ciliary body with and without a layer of unperfused sclera harvested from the fellow eye ( 300 mm thick) interposed between the probe and the sclera. Flow response detected through the interposed tissue shows LDF sampling depth is suYcient to measure through sclera. ( P¼MAP IOP; up and down arrows mark beginning and end of caval occlusion; adapted from Kiel et al., 2001.) (Right) Transscleral LDF measurement as IOP is increased by saline infusion (on, oV) and fluid withdrawal ( V ) to restore normal IOP while mean arterial pressure (MAP) remained constant. Detection of blood flow changes in response to IOP indicates LDF detects intraocular blood flow (authors’ unpublished observation).

D. Measurement Site

Transscleral LDF measurements have a distinct anterior to posterior profile corresponding to the underlying tissues as shown in Fig. 10. The blood flow (flux) recorded between the visible limbal vessels and the signal nadir over the pars plana originates primarily from the ciliary body. The ciliary muscle in the rabbit is poorly developed, so most of the flow signal is from the ciliary processes (Prince and Eglitis, 1964). However, the vascular casts by Morrison et al. (1987a,b) suggest the anterior processes are more specialized for secretion. Whether the LDF measurement includes both the anterior and posterior processes is unclear, but Fig. 11 shows that LDF probe placement over the ciliary body does not detect changes in iris blood flow.

E. Aqueous Flow Measurement

Measurements of aqueous flow are based on the clearance of fluorescein from the cornea and anterior chamber after topical application. The commercially available FM2 fluorophotometer (Ocumetrics, Mountain View,

FIGURE 10 LDF transscleral ciliary blood flow measurement based on distinct blood flows corresponding to underlying tissues. Typical probe placement is 1 mm posterior to the ring of superficial limbal vessels. (Top) Blood flow profile as laser Doppler probe is moved posteriorly from the limbus in 1 mm steps. (Bottom) Corresponding rabbit anatomy. co, cornea; cj, conjunctiva; cb, ciliary body; os, ora serata; rt, retina; ch, choroid; sc, sclera. (Adapted from Kiel et al., 2001.)

CA) is typically used to measure the changes in fluorescein concentration over time. Detailed descriptions of the technique can be found elsewhere (Brubaker, 1991a,b). However, in anesthetized animals held in a stereotaxic head holder, the standard procedure can be modified to take advantage of the fact that the fluorophotometer can measure the same optical path throughout the experiment, which minimizes scan variability and permits flow calculations over shorter time intervals. This is important when ciliary blood flow is varied by mechanically manipulating MAP, which limits the

FIGURE 11 Evidence that the LDF sampling volume in the rabbit is deep enough to reach the ciliary body, yet small enough to avoid the iris. Trace shows responses to topical brimonidine (0.15%, 40 ml) given at 0.5 hours. Ocular mean arterial pressure (OMAP), orbital venous pressure (OVP), and heart rate (HR) were unaVected. LDF measurement through the sclera over the ciliary body indicates a rapid decrease in blood flow that was not detected by a second LDF probe directed through the cornea at the iris ( 3 mm from the iris root). The diVerent flow responses at neighboring sites indicate the LDF sampling volume is adequately selective for the ciliary body (Reitsamer et al., 2006).

duration of the experiments. Figure 12 shows representative triplicate scans at 15 min intervals illustrating the minimal scan variability and an example of the rapid changes in fluorescein clearance that can be detected with more frequent scans.

It should be noted that fluorophotometry measures the flow of aqueous through the anterior chamber, which is assumed to be 90% of total aqueous production; the remaining 10% is thought to flow posteriorly through the vitreous and retina to the choroid where it is reabsorbed (Brubaker, 1991a).

A

500Tripple scan 12:00 Tripple scan 13:15

FIGURE 12 Measurement of aqueous flow. Three triplicate scans taken over 150 min show a steady decrease of fluorescein concentration (A). During experiments, scans can be made for 60–90 min before and after drug administration or perturbations of ocular perfusion pressure

(B) to obtain the fluorescein concentration decay curves (C) used to calculate the aqueous flow rate. Panels B and C are from the same animal and show a step decrease of blood pressure causing the decrease of fluorescein removal from the anterior chamber, consistent with the calculated 50% reduction in aqueous flow. (Panels B and C from Reitsamer and Kiel, 2003.)

F. Relationship Between Ciliary Blood Flow and Aqueous Flow

In order to determine the relationship between ciliary blood flow and aqueous production, it is necessary to vary ciliary blood flow and measure the resulting eVect on aqueous production. Ideally, only ciliary blood flow should be varied (i.e., neurohumoral input to the eye, the ocular PP, and blood flow through the other ocular circulations should all be constant), but this is not technically possible. What has been done instead is to manipulate ciliary blood flow by mechanically holding the arterial pressure at diVerent levels (80, 70, 55, and 40 mm Hg) in anesthetized rabbits. The results of the initial study are shown in Fig. 13 (left); the relationship between ciliary blood flow and aqueous production based on additional experiments are also shown (Fig. 13, right).

The graphs in Fig. 13 show that when ciliary blood flow is increased above its normal level, aqueous production is relatively unchanged. This is also true for reductions in ciliary blood flow of 25%, but further reductions result in proportional decreases in aqueous production. In order words, there is a critical level of ciliary blood flow below which aqueous production is blood flow dependent. Above that critical level, aqueous production is independent of ciliary blood flow. A similar relationship is found in the gastric mucosa, where acid secretion is blood flow independent until mucosal blood flow is

FIGURE 13 Relationship between ciliary blood flow and aqueous humor production in anesthetized rabbits. (Left) Original relationship based on 43 experiments. (Right) Revised relationship based on additional experiments. The open circle is the average baseline value at spontaneous ciliary blood flow and aqueous flow. The curve shows a plateau, where aqueous flow is relatively independent of ciliary blood flow; however, if ciliary blood flow decreases below roughly 75% of baseline, aqueous flow becomes dependent on ciliary blood flow and decreases with further reductions in ciliary blood flow (Reitsamer and Kiel, 2003).

reduced below a critical level, whereupon it becomes blood flow dependent. In addition, work with secretory stimulants in the stomach shift the relationship upward, whereas secretory inhibitors shift the relationship downward (Perry et al., 1983; Holm and Perry, 1988). If ciliary secretion behaves similarly, the following hypotheses seem reasonable: (1) aqueous production requires an adequate supply of oxygen and nutrients delivered by the ciliary circulation; (2) the neurohumoral milieu of the ciliary processes determines the level of secretory stimulation so that the rate of aqueous production at a given PP and ciliary blood flow can be stimulated or inhibited pharmacologically; and (3) the autoregulatory mechanisms governing ciliary blood flow are modulated by endogenous neurohumoral factors and that ciliary autoregulation can be overridden by the administration of exogenous vasoactive compounds or large changes in PP can exceed the autoregulatory ability of the ciliary circulation.

The dynamics of these hypotheses are more readily apparent in Fig. 14. The figure has three curves generated by a mathematical model (Kiel, 2000) of aqueous production plotted as a function of ciliary blood flow: a control curve for a normal level of secretory stimulation (Ctrl), a curve for a state of

FIGURE 14 Hypothetical curves for aqueous production versus ciliary blood flow under stimulated, control and inhibited conditions. Points show: (1) control production and blood flow; (2) inhibited production without change in blood flow; (3) stimulated production without change in blood flow; (4) ciliary vasoconstriction with flow dependent decrease in production;

(5) ciliary vasodilation with small flow dependent increase in production; (6) stimulated production with metabolic dependent vasodilation; and (7) inhibited production with metabolic dependent vasoconstriction.

FIGURE 15 (Left) Topical brimonidine reduces ciliary blood flow and the attendant fall in aqueous production is similar to that found when ciliary blood flow is reduced to the same extent by decreasing MAP so that the datum falls on the control curve (Reitsamer et al., 2006). (Right) Topical dorzolamide shifts the relationship downward consistent with a nonvascular, direct inhibition of aqueous production (Kiel and Reitsamer, 2006). (Note that the dorzolamide curve was obtained by varying ciliary blood flow with mechanical MAP manipulation after topical dorzolamide application.)

direct mechanism of action, topical dorzolamide in the rabbit causes a downward shift in the ciliary blood flow—aqueous flow relationship as shown in Fig. 15.

Figure 16 shows an example of a seemingly paradoxical drug response. In the rabbit, intravenous infusions of low and high doses of dopamine have opposite eVects on aqueous production—the low dose causes an increase in production, and the high dose causes a decrease (Reitsamer and Kiel, 2002a). One explanation for these disparate responses is the binding characteristics of the diVerent dopamine receptor subtypes in the epithelium. However, a simpler explanation might be that the high dose causes a significant decrease in ciliary blood flow, and as Fig. 16 shows, the decrease in ciliary blood flow places the high dose datum on the control curve, suggesting that the vascular mechanism is involved. The low dose, on the other hand, has little eVect on ciliary blood flow so that the datum is above the control curve, consistent with the low dose having a direct stimulatory eVect.

A key question for the role of ciliary blood flow in aqueous production is what is provided by the ciliary blood flow. Ciliary blood flow provides oxygen and nutrients to the ciliary epithelia and removes metabolic waste. It is unclear which of these is critical; however, oxygen is a reasonable candidate because the ionic transport systems responsible for the osmotic

FIGURE 16 Paradoxical response to dopamine. Intravenous infusion of a low dose of dopamine increases aqueous flow while an infusion of a high dose decreases aqueous production. Both doses likely stimulate epithelial ionic transport, but the high dose also causes ciliary vasoconstriction (presumably by activating vascular alpha adrenergic receptors), which deprives the epithelium of the blood flow needed to sustain aqueous production. (Adapted from Reitsamer and Kiel, 2002a.)

gradient that produces aqueous utilize oxidative metabolism for energy and they largely stop functioning when insuYcient oxygen is provided, despite the availability of other substrates (Burstein et al., 1984; Krupin et al., 1984). If ciliary blood flow is raised above the critical level, oxygen extraction decreases and the excess simply passes through in the venous eZuent. However, if blood flow is reduced below the critical level, even maximum oxygen extraction cannot provide suYcient high energy substrates (e.g., adenosine triphosphate) to drive ionic transport, and so aqueous production decreases. In support of oxygen as the critical factor, the change in the partial pressure of oxygen (PO2) adjacent to the ciliary body in the rabbit falls in response to topical brimonidine (Reitsamer et al., 2006). As Fig. 17 shows, the brimonidine induced decrease in ciliary blood flow is associated with a decrease in ciliary PO2. The decrease in PO2 indicates increased oxygen extraction rather than the decreased oxygen consumption that would be consistent with secretory inhibition, but whether the PO2 in the ciliary epithelia falls below the level needed to sustain ciliary metabolism and accounts completely for the brimonidine suppression of aqueous production requires further study.

FIGURE 17 EVect of brimonidine induced decrease in ciliary blood flow on ciliary PO2. Representative trace showing ciliary PO2 response to topical brimonidine and rapid decline in ciliary PO2 upon cessation of ciliary perfusion at death. Decrease in ciliary PO2 indicates increased oxygen extraction rather than decreased oxygen consumption.

Additional evidence that oxygen is the critical factor delivered in ciliary blood flow is shown in Fig. 18. In this experiment, anesthetized rabbits (n¼12) were respired with a mixture of room air and nitrogen suYcient to lower their percent hemoglobin saturation with oxygen to 75%. Ciliary blood flow increased slightly but not significantly, yet aqueous production decreased by 22%. In this situation, the delivery of other nutrients and removal of metabolic waste remained constant and only oxygen delivery was reduced. Thus, it appears that oxygen is indeed a critical factor delivered in ciliary blood flow.

FIGURE 18 EVect of hypoxia on ciliary blood flow and aqueous production. Lowering the percent saturation of hemoglobin with oxygen from 95% to 75% decreases aqueous production without changing ciliary blood flow or the delivery of other nutrients and washout of metabolites (authors’ unpublished results).

V. EPISCLERAL VENOUS PRESSURE AND IOP

The final circulatory parameter to be mentioned in this chapter is episcleral venous pressure (EVP). Of all the variables involved in IOP homeostasis, EVP is the least understood, and yet it accounts for more than 50% of the normal IOP (15mm Hg) in the Goldman equation (IOP¼aqueous flow divided by outflow facility plus EVP) given current EVP estimates (8–10mm Hg) (Zeimer, 1989). Whether EVP plays a role in glaucoma or can be lowered with drugs or surgery as a treatment for ocular hypertension is unclear, but these are research questions clearly worthy of pursuit (Funk et al., 1996; Toris et al., 2002; Selbach et al., 2005). A potential problem regarding EVP is that it is often assumed to be constant in calculations of tonographic outflow facility and uveoscleral outflow. This assumption is unlikely to be correct in many circumstances, but unfortunately it is diYcult to prove one way or the other because EVP is diYcult to measure.

For example, Fig. 19 shows the IOP, EVP, and orbital venous pressure (OVP) responses to changes in MAP (Reitsamer and Kiel, 2002b). The EVP response to MAP demonstrates that EVP is not constant. Another noteworthy aspect of these results is the pressure gradient from the episcleral veins to the orbital venous sinus. The source of the resistance responsible for

FIGURE 19 Anesthetized rabbit responses to step changes in MAP for IOP. MAP and OVP were measured by direct cannulation. EVP was measured by inserting a fine tipped glass pipette (2–3 mm tip) connected to a servo null instrument into episcleral veins with a visible plume of aqueous joining the stream of red blood cells. IOP varied exponentially with MAP, whereas EVP and OVP varied linearly with MAP (A). EVP varied linearly with OVP (B). The regression lines and correlation coeYcients (r) are shown for the pooled data in A and for individual experiment in B.

this pressure gradient is unknown, but the relatively long length of the small caliber episcleral veins and their course through the extraocular muscles en route to the orbital sinus are likely contributors.

VI. CONCLUSION

As noted at the beginning of this chapter, the ocular circulations play diverse roles in IOP homeostasis, and IOP in turn can have profound eVects on the ocular circulations. How and when the ocular circulations contribute to the etiology and treatment of ocular hypertension and glaucoma are subjects worthy of the intense study they receive, which continues to provide new insights into this devastating disease.

Acknowledgments

This work was supported by NIH grant EY09702 (J.W.K.), a Research to Prevent Blindness Lew R Wasserman Merit Award (J.W.K.), Austrian FWF J1866 MED (H.A.R.), San Antonio Lions, Lions International, and an unrestricted grant from Research to Prevent Blindness Inc.