IOP recovery subsequent to paracentesis reflects the refilling of the AC with “secondary” aqueous. The rate of such IOP recovery, with concomitant AH refilling, was stimulated by sildenafil in two different animal models, implying that the vasodilator not only provides more fluid for secondary aqueous formation after paracentesis, but may also increase AH turnover in the normal eye. Although we have not measured AH turnover directly, both the accelerated rates of IOP restoration after paracentesis in animals administered sildenafil, as well as the fact that sildenafil increases vascular flow in the eye due to dilations of intraocular arteries,
9 suggest that the vasodilator should increase the turnover of AH in the AC.
The measurement of AH turnover in the presence of systemic sildenafil administration is an important parameter that should be characterized in future studies to corroborate our findings and interpretations. This could be done in animal models, as well as in human subjects, noninvasively using the fluorescein depot method.
21 –23 The rate of AH turnover in the AC could be monitored under control conditions, followed by sildenafil ingestion to determine whether turnover is indeed increased, as reflected by an enhanced rate of fluorescein clearance from the AC. We posit that such result is very likely attainable, and would occur independently of paracentesis. At steady state with IOP constant, AH inflow = AH outflow. This flow is the AH turnover. On an increase in inflow, which we predict sildenafil will elicit, the IOP would immediately rise, and this increased pressure would accelerate AH outflow, resulting in a new steady state with a higher IOP and a larger AH turnover, without any changes in AH outflow facility. The magnitude of the IOP increase would directly correlate with the extent of the AH inflow. Should the baseline value of the outflow facility be sufficiently large, a large increase in AH inflow could result in a relatively small change in IOP, which is a consequence of the relative rates of inflow and outflow. The resulting increase in both inflow and outflow would be detected as an increase in AH turnover with the noninvasive fluorescein technique.
Conversely, it is also hypothetically possible that sildenafil inhibited the AH outflow facility by an unknown mechanism. In such a case, and presuming that the PDE5 inhibitor had no stimulatory effect on inflow, the refilling of the AC with “secondary” aqueous after paracentesis plus sildenafil ingestion would still occur faster than the rate of refilling after paracentesis in the absence of systemic PDE5 administration. Our results cannot directly exclude this possibility. If such mechanism comes into play, a reduction in outflow facility would be quantifiable. Putative changes in outflow facility evoked by sildenafil could be determined in live animal models administered the drug,
15,22,24 as well as in human subjects noninvasively.
23 However, we consider this prospect unlikely based on contemporary knowledge of the physiological effects of sildenafil.
In our present experiments with paracentesis, the calculated AH refilling rates represent an average for the entry of “secondary” aqueous after the removal of AH from the AC. At the moment of paracentesis (i.e., t = 0), the hydrostatic pressure difference between the capillaries of the ciliary body stroma and the AC would be maximal, resulting in a higher rate of ultrafiltration from the vasculature that should gradually decline as the IOP recovers. Moreover, simultaneous with AC refilling there is an increased outflow of AH consequent to the buildup in pressure in the AC, which would also contribute to minimizing our estimate of the AH refilling rate. As such, our results represent an average that may be the result of a very complicated refilling curve.
Interestingly, it was observed with rabbits that paracentesis, in itself, evoked a large IOP overshoot above the control level as the pressure was restored (
Fig. 3), a phenomenon in rabbits attributed to the base of the iris bowing forward, thereby closing the angle and blocking the outflow.
25 We merely recorded this excess in pressure and calculated the time point at which the IOP was approximately equal to that of the control level for determining the recovery time. Since the protocol was designed so that AH was withdrawn from each eye only once, the IOP overshoot evoked by paracentesis was a unilateral effect that did not influence the IOP of the fellow eye. In our data, elevated IOPs evoked by paracentesis gradually declined to control levels within 90 minutes (
n = 24 eyes).
The postparacentesis overshoot in IOP to a level above that of the control value was observed only with rabbits, a species with which paracentesis also induces miosis, which allows for the above-noted pupillary block.
25 In contrast, we never observed miosis with sheep, and observed that the relatively large, rectangular-shaped pupils of this animal exhibit a mostly fixed shape, with minimal responses to ambient light.
An additional comparative difference between the two animal models was that sildenafil ingestion did not directly increase the IOP of rabbits. Neither the elevated IOP produced secondarily by paracentesis, nor the baseline IOP of the contralateral eye not yet subjected to paracentesis, was affected by sildenafil (
Fig. 3). With three other rabbits that were not subjected to paracentesis and solely fed sildenafil, no significant effects on IOP were observed (data not shown). As such, sildenafil accelerated the AH refilling rate independently of whether it also increased baseline IOP, as saliently occurs with sheep (
Figs. 1 and
2).
12
Thus, an explanation is necessary for the absence in rabbits of an IOP-elevating effect by the drug, which nevertheless stimulated the AH refilling rates after paracentesis. We presume that if the drug indeed increased AH turnover in the normal rabbit eye, it concomitantly increased the pressure-dependent outflow via the trabecular meshwork in this species, thereby not producing a detectable change in IOP. Our present emphasis is the finding that sildenafil increased the AH refilling rate in a species in which such refilling after paracentesis occurs rapidly. After the removal of 50 and 100 μL AH, IOP recovered, on average, within 14 minutes (
Table 4), resulting in estimated AH-refilling rates of 3.9 and 8.3 μL/min, respectively (
Table 4). These rates are markedly higher than the 2 to 3 μL/min rate of AH formation attributed to secretion by the CE in rabbit.
26 –28 After sildenafil treatment, the time necessary for IOP restoration was approximately halved, and the estimated AH refilling rates therefore doubled.
As classically accepted, the formation of AH first entails the leak of fluid from the fenestrated capillaries of the ciliary processes into the surrounding ciliary body (CB) stroma. Because of the fenestrations, the leaked fluid contains proteins that subtract from the oncotic pressure within the capillaries, thereby reducing fluid reabsorption into the capillaries. The leaked fluid (not reabsorbed by the ciliary capillaries) is estimated to be approximately 4% of the rate of 75 to 100 μL/min blood flow in the CB capillaries.
10 AH formation across the ciliary epithelium (CE) then results from two driving forces: hydrostatic pressure gradients between the CB stroma and the PC, and osmotic forces produced by active electrolyte transport across the CE.
10,29 The first mechanism has been referred to as ultrafiltration and the second as secretion.
30 Ultrafiltration across the CE is currently thought to represent only a minor component of AH production when the blood–aqueous barrier (BAB) is intact,
29 so that most AH results from secretion across the CE into the PC and subsequent entry into the AC via the pupil.
10,29
As originally considered by Bill,
10,30 we proposed that fluid could also directly enter the AC across the anterior face of the iris,
12 given the absence of an anatomic barrier between the CB stroma and the AC via the iris root. This flow could occur when there is a pressure difference between the CB stroma and the AC, and putatively, sildenafil could induce, or increase, such a pressure difference by increasing the rate of leakage of protein-rich fluid from the capillaries. Consistent with this possibility, sheep given sildenafil exhibit an increased protein concentration in the AC.
12 Sildenafil could also increase ultrafiltration across the CE, particularly when the permeability of the CE tight junctions is increased, as occurs secondary to paracentesis in rabbits, cats, and monkeys.
17,31,32
To the best of our knowledge, there are no indications that sildenafil disrupts the BAB. We are unaware of patients, who take sildenafil for various vascular diseases, reporting incidents of proteinaceous “flare” in their visual axis. The most commonly recognized adverse effect of sildenafil in the eye is a transient blue tinge to vision and increased sensitivity to bright lights that has been attributed to an inhibition of PDE6, a critical enzyme in the regulation of the phototransduction cascade. As such, we suggest that the increase in AC protein content elicited by sildenafil in the sheep animal model probably resulted from an increase in flow of plasma-like fluid directly from the CB stroma to the AC via the iris in accord with Freddo's model.
11 Moreover, such putative increased inflow of fluid directly into the AC of sheep treated with sildenafil, and other longer-lasting PDE5 inhibitors,
12 likely accounts for the IOP elevation observed with this species (
Figs. 1 and
2). Presently, the only protocol that we are aware of that could be used to directly test this interpretation is to examine the integrity of the CE tight junctions morphologically in animals given sildenafil or tadalafil. If such junctions remain unperturbed in the presence of the PDE5 inhibitors, ultrafiltration of plasma-like fluid between the CE cells is unlikely.
Separately, we do not have evidence indicating whether paracentesis, in itself, disrupts the BAB in sheep; nor are we aware of published reports on the rate of AH formation in this species. Indirectly, we can estimate the sheep AH turnover to be approximately 3 μL/min from a control IOP of 10 mm Hg and an outflow facility of approximately 0.3 μL/min per mm Hg.
15 As such, we can only speculate that the spontaneous rate of AH refilling observed with sheep after the paracentesis of 60 and 120 μL (i.e., 1.2 and 2.2 μL/min;
Table 1) may possibly reflect secretion by the CE. However, after the paracentesis of 300 μL, as well as after the sheep ingested sildenafil, the postparacentesis AH refilling rates ranged between 3.2 and 8.1 μL/min (
Table 1), or rates generally larger than those reported for AH formation attributed to secretion by the CE, i.e., usually 2 to 3 μL/min.
26 Hypothetically, in these latter conditions, either an increased inflow into the AC via the anterior aspect of the iris occurred, and/or ultrafiltration across the CE increased, if the resistance of the tight-junction barrier declined. Nevertheless, it was clear that the postparacentesis rates of AH refilling after the ingestion of sildenafil were markedly higher than published estimates for AH secretion by the CE.
Although experimental paracentesis in rabbits has been extensively done to characterize the mechanisms underlying paracentesis-induced disruption of the BAB, which is thought to be mediated primarily by prostaglandins,
17,19,20,25,31 we are unaware of reports quantifying the time necessary for IOP restoration and/or the AH refilling rate. We presume that the rapid refilling rates that we estimated for secondary aqueous, which is plasmoid in nature in rabbits, cats, and monkeys,
17,31 –33 resulted from a substantial increase in ultrafiltration across the CE. The technique of fluorescein angiography has been used to qualitatively visualize the entry into the eye of systemically administered fluorescein in rabbits and monkeys subsequent to experimental paracentesis.
17,32 Fluorescein initially entered the eye from the capillaries of the ciliary body stroma into the PC, followed by entry into the AC via the pupil, such that most fluorescein entered the AC via transpupillary flow.
17,32 Interestingly, in pigmented rabbits, a diffusion of fluorescein from the anterior surface of the iris into the AC, which occurred sequentially to the fluorescein entry via the pupil, was also observed.
17,33 We posit that sildenafil may have increased the flow via both of these pathways in the experimental paracentesis of the rabbit eye due to its intraocular vascular dilating effects.
Given the above indications that sildenafil should increase the AH turnover of the normal eye (presumably due to increased flow in the CB stroma-to-iris pathway), as well as the rate of AH refilling after paracentesis (due primarily to an increased ultrafiltration), we suggest that the drug be considered as a potential prophylactic agent to augment the rate of AH refilling after eye surgeries of the anterior segment. In some cases, the angle collapses, leading to a potential complication of damage to the corneal endothelial cells due to mechanical interaction with the iris. Presently, many surgeons inject saline into the AC and/or with viscoelastic material to maintain normal ocular dimensions until CE secretory function and AH volume are fully restored. However, it can take 4 to 8 hours for freshly secreted AH to completely replace the saline in the AC.
34 One surgeon recommended withdrawing the aqueous at the beginning of surgery and returning it at the end of the procedure.
35 Sildenafil could be tested as an important adjunct in these types of procedures. It may also have utility in treating cases of ocular hypotony that result from insufficient AH formation.
36,37
Supported by National Institutes of Health/National Eye Institute Grants EY00160 and EY01867 (OAC) and an unrestricted grant from Research to Prevent Blindness, Inc., New York, NY.
The authors thank Aldo C. Zamudio for expert technical assistance with the paracentesis and Tono-Pen readings in rabbits.