Ординатура / Офтальмология / Учебные материалы / Atlas of Glaucoma Second Edition Choplin Lundy 2007

Normal visual function requires the shape of the globe to remain fixed and the optical pathway from the cornea to the retina to remain clear. This requires that the nutrition of the intraocular tissues in the optical pathway must occur with a minimum number of blood vessels. All of this is accomplished very efficiently by the production of the clear aqueous humor, its circulation into the anterior chamber and its drainage from the anterior chamber angle through tissues with high resistance (trabecular meshwork and uvea). The intraocular pressure thus is maintained, the shape of the eye is preserved, the refracting surfaces are kept in place and the avascular cornea and lens are provided with nourishment and waste removal.

FLOW OF AQUEOUS HUMOR

Aqueous humor is produced by the ciliary body and is secreted into the posterior chamber. Most of this fluid then flows into the anterior chamber and drains out of the chamber angle via one of two outflow pathways, through the trabecular meshwork or through the ciliary muscle. Trabecular outflow is pressure dependent and flow through the ciliary muscle (often called uveoscleral outflow) is relatively pressure independent (Figure 3.1).

AQUEOUS PRODUCTION MECHANISMS

The primary function of the ciliary processes is to produce ocular aqueous humor. These processes constitute the internal aspect of the pars plicata of the ciliary body. The anterior portions of the larger processes have many capillary fenestrations and epithelial mitochondria, indicating specialization for aqueous humor production. Ciliary processes have a highly vascularized core (Figures 3.2 and 3.3) and their perfusion directly regulates the volume

Figure 3.1 Circulation and drainage of aqueous humor.

Ocular aqueous humor is secreted by the ciliary processes of the ciliary body into the posterior chamber. Some fluid circulates between the lens and the iris through the pupil into the anterior chamber (large green arrow). It drains from the anterior chamber by passive bulk flow via two pathways. Trabecular outflow is the drainage of aqueous humor sequentially through the trabecular meshwork, Schlemm’s canal, collector channels, episcleral veins, anterior ciliary veins and into the systemic circulation (small green arrow). Uveoscleral outflow is the drainage of aqueous humor from the chamber angle into the tissue spaces of the ciliary muscle and on into the suprachoroidal space. From there some fluid percolates through the scleral substance and emissarial canals and some fluid is reabsorbed by uveal blood vessels (blue arrows). Although the bulk of the posterior chamber aqueous humor exits the eye through the pupil, some drains across the vitreous into the retina and retinal pigment epithelium (red arrow). The active transport of fluid out of the eye by the retinal pigment epithelium utilizes a mechanism similar to that of the non-pigmented ciliary epithelium.

of capillary ultrafiltration and indirectly influences active secretion by controlling the supply of blood-borne nutrients to the ciliary epithelium.

The first step in the formation of aqueous humor is the development of a stromal pool of

Anterior arterioles

MAC

posterior arteriole

Marginal capillaries

Figure 3.2 Ciliary process vasculature. One ciliary process of a monkey is shown in lateral view. The vasculature consists of a complex anastomotic system supplied by anterior and posterior arterioles radiating from the major arterial circle (MAC). The anterior arterioles supply the anterior aspect of the process and drain posteriorly into the choroidal veins. The posterior arterioles provide posteriorly draining capillaries generally confined to the base of the process. The choroidal veins drain blood from the iris veins, ciliary processes and the ciliary muscle. (Redrawn from Morrison JC, Van Buskirk EM. Ciliary process microvasculature of the primate eye. Am J Ophthalmol 1984; 97: 372–83.)

plasma filtrate by ultrafiltration through the ciliary process capillary wall (Figure 3.3). Active transport of ions out of the cell establishes an osmotic gradient in the intercellular spaces of the ciliary epithelium (Figure 3.4). Finally, water moves into the posterior chamber along the osmotic gradient (Figures 3.4 and 3.5). The posterior chamber aqueous humor is modified by diffusion of molecules into or out of the surrounding tissue. A small percentage of the modified aqueous humor flows posteriorly through the vitreous and across the retina and retinal pigment epithelium, but most flows through the pupil into the anterior chamber (Figure 3.1). The flow of aqueous humor into the anterior chamber is easily measured giving a reasonable estimation of the aqueous production rate.

The rate at which aqueous humor enters the anterior chamber in the normal human eye averages about 2.5 l/min. There is a circadian fluctuation in aqueous flow such that the rate is lowest during sleep at night and highest around noon to 1 p.m. (Figure 3.6). The circadian rhythm is not affected by eyelid closure, supine posture, sleep deprivation, sleep in a lighted room or short naps. The factors mediating the reduction in aqueous flow are unclear, but corticosteroids and catecholamines may play a role.

Intraocular pressure can be reduced by administering drugs that reduce aqueous production.

Posterior chamber

P = 20 mmHg

π = 0 mmHg

Figure 3.3 Production of aqueous humor. The ciliary process consists of a core containing capillaries and interstitial tissue surrounded by two epithelial layers oriented apex to apex. The innermost layer consists of non-pigmented ciliary epithelial cells which are connected to each other by tight junctions and desmosomes. The outer layer consists of pigmented ciliary epithelial cells connected to each other by gap junctions. The production of aqueous humor by the ciliary processes involves both ultrafiltration and active secretion. Plasma filtrate enters the interstitial space through capillary fenestrations (ultrafiltration). The net filtration of fluid from the capillaries (3.2 l/min) has to correspond to the formation of the aqueous humor (3 l/min) plus any loss of tissue fluid from the processes (0.2 l/min). The net filtration of filtrate from the capillaries corresponds to about 4% of plasma flow. The capillary wall is a major barrier for the plasma proteins but the most significant barrier is in the non-pigmented ciliary epithelium where tight junctions occlude the apical region of the intercellular spaces. The outcome of this design is a high protein concentration in the tissue fluid. This causes a high oncotic pressure in the tissue fluid and a reduction in the transcapillary difference in the oncotic pressure. The hydrostatic pressure (P) and colloid osmotic pressure ( ) of the blood, interstitial tissue and posterior chamber are listed for the rabbit. The effect of hydrostatic and oncotic pressure differences across the ciliary epithelium is a pressure of about 13 mmHg tending to move water into the processes from the posterior chamber. Therefore, under normal conditions the movement of fluid into the posterior chamber requires secretion. Because the is zero in the posterior chamber, the only way to secrete fluid into this chamber is via active transport across the ciliary epithelial layers. (Modified from Bill A. Blood circulation and fluid dynamics in the eye. Physiol Rev 1975; 55: 383–417.)

Clinically available drugs with this ocular hypotensive mechanism of action are listed in Table 3.1.

AQUEOUS HUMOR OUTFLOW PATHWAYS

In the anterior chamber, some of the aqueous humor may be lost via the cornea and iris or gained

Figure 3.5 Cole’s hypothesis of fluid production. A standing gradient osmotic flow system is represented by this diagram. The system consists of a long, narrow channel with a restriction at the apical end (tight junction). Solute pumps line the walls of the channel and solute is continuously and actively transported from the cells into the intercellular channel (blue arrows). This makes the channel fluid hyperosmotic. As water flows along the path of least resistance towards the open end of the channel (black arrow), more water enters across the walls because of the osmotic differential (red arrows). In the steady state a standing gradient is maintained. The relative osmolarity is depicted by the blue color. Volume flow is directed towards the open end of the channel. (Modified from Cole DF. Secretion of the aqueous humour. Exp Eye Res 1977; 25 (Suppl): 161–76.)

from iridial blood vessels, but these gains or losses are very small so that, in a steady state, the rate of drainage of aqueous humor through the chamber angle is nearly equal to the rate of inflow of aqueous humor through the pupil. Thus, aqueous flow (Fa) can be defined as the sum of aqueous humor

16 Atlas of glaucoma

Time of day

Figure 3.6 Aqueous flow versus time of day. In humans, aqueous humor flow into the anterior chamber is highest between 1200 and 1300 hours peaking at a rate of about 3.2 l/min. It decreases through the afternoon and into the night to a rate of about 1.6 l/min at 0400 hours. This represents a 45% reduction in aqueous flow. The magnitude of reduction is greater than that produced by pharmacological aqueous humor suppressants. (Data from Reiss GR, Lee DA, Topper JE, Brubaker RF. Aqueous humor flow during sleep. Invest Ophthalmol Vis Sci 1984; 25: 776–8.)

drainage through two outflow pathways, the trabecular meshwork (Ftrab) and the uvea (Fu):

Trabecular outflow

In the primate trabecular outflow pathway, the aqueous humor drains sequentially through the trabecular meshwork, Schlemm’s canal, collector channels, episcleral veins, anterior ciliary veins and into the systemic circulation (Figure 3.1). It is generally agreed that the tissues between the anterior chamber and Schlemm’s canal provide the major portion of normal resistance to aqueous drainage. Which tissue constitutes the greatest resistance is somewhat controversial. To exit the eye via Schlemm’s canal, aqueous humor must traverse three layers of trabecular meshwork and the inner wall of Schlemm’s canal (Figure 3.7). The resistance to fluid flow increases along this pathway. The outer layer contains a delicate meshwork of elastic-like fibers which are connected at one end with the endothelium of Schlemm’s canal, and at the other end with tendons of the ciliary muscle. Contraction of the ciliary muscle may increase spaces between the plates of the meshwork and reduce resistance to flow. Pilocarpine may reduce

Figure 3.7 Three layers of the trabecular meshwork. Aqueous humor first passes through the inner layer or the uveal meshwork. This layer is a forward extension of the ciliary muscle. It consists of flattened sheets which branch and interconnect in multiple planes. The uveal meshwork does not offer any significant resistance to aqueous drainage because of the presence of large overlapping holes in these sheets. The middle layer, the corneoscleral meshwork, includes several perforated sheets of connective tissue extending between the scleral spur and Schwalbe’s line. Relative to the openings in the uveal meshwork, the openings in these sheets are small and do not overlap. The sheets are connected to each other by tissue strands and endothelial cells. The result of this architectural design is a circuitous path of high resistance to the flow of aqueous humor. The outer layer, the juxtacanalicular meshwork, lies adjacent to the inner wall of Schlemm’s canal. It contains collagen, a ground substance of glycosaminoglycans and glycoproteins, fibroblasts and endothelium-like juxtacanalicular cells. It also contains elastic fibers that may provide support for the inner wall of Schlemm’s canal. This meshwork contains very narrow, irregular openings providing high resistance to fluid drainage.

trabecular outflow resistance (increase trabecular outflow facility) by this mechanism. The inner wall of Schlemm’s canal consists of a monolayer of endothelial cells interconnected by tight junctions. It has been hypothesized that this wall serves as the major barrier to aqueous humor drainage. One possible means for fluid to traverse this barrier is via transcellular channels within the endothelial cells (Figure 3.8).

In humans, trabecular outflow accounts for 5 to 95% of aqueous drainage depending on the health of the eye, while in healthy monkeys the rate is about 50%. This flow is pressure dependent, meaning that the flow is proportional to the difference between IOP and the hydrostatic pressure in the canal.

Uveoscleral outflow

Anterior chamber aqueous humor that does not drain through the trabecular meshwork, flows from

Schlemm's canal

Cribriform pathway

Trabecular meshwork

Figure 3.8 Inner wall of Schelmm’s canal and the juxtacanalicular meshwork (also called cribriform meshwork). This wall contains a monolayer of spindle-shaped endothelial cells interconnected by tight junctions. One theory is that fluid may move through this area in transcellular channels. These channels may form and recede in a cyclic fashion, beginning as invaginations on the meshwork side of the endothelial cell and progressing to a transcellular channel into Schlemm’s canal. Only a few channels appear to open at a time, providing the major resistance to trabecular outflow. One channel is drawn as a vacuole and one is shown opened to Schlemm’s canal. The arrows indicate the direction of fluid flow. (Modified from Rohen JW. Why is intraocular pressure elevated in chronic simple glaucoma? Anatomical considerations. Ophthalmology 1983; 90: 758–65.)

the chamber angle into the supraciliary space and ciliary muscle, and then seeps through the scleral substance or flows through emissarial canals or is absorbed into uveal blood vessels (Figure 3.1). This is called ‘uveoscleral’ outflow. Other names for this drainage include ‘unconventional’, ‘pressure independent’ and ‘uveovortex’ outflow. These names arose from attempts to describe a relatively undefined seepage route. It would be more accurate to refer to this drainage as simply ‘uveal’ outflow, to include all anterior chamber aqueous humor egress from the uvea. The term appearing most often in the literature is ‘uveoscleral outflow’ hence its use in this chapter.

In contrast to trabecular outflow, uveoscleral outflow is effectively pressure independent. At IOPs greater than about 5 mmHg this outflow remains relatively constant and the facility of uveoscleral outflow is very low (approximately 0.02 l/min per mmHg). Uveoscleral outflow reportedly accounts for anywhere from 4 to 45% of total aqueous drainage in humans. It can be increased by drugs that relax the ciliary muscle and be reduced by drugs that contract it (Figure 3.9). In general, topical prostaglandin (PG)-F2 analogs reduce IOP predominantly by increasing uveoscleral outflow. The mechanism of the flow reduction appears to be a relaxation of the ciliary muscle causing the acute effect and the biochemical modification of its intercellular matrix, causing a more chronic effect. The mechanism for the pressure reduction in clinically

available topical ocular drugs is summarized in Table 3.1.

TECHNIQUES FOR MEASURING

AQUEOUS FLOW

Aqueous flow was first measured by injecting a tracer such as para-aminohippuric acid or fluorescein into the blood stream, allowing it to enter the anterior chamber, and then sampling the aqueous humor to determine the rate of its disappearance. The relationship between aqueous humor tracer concentration and plasma tracer concentration was monitored to determine aqueous flow. Subsequently, oral administration of fluorescein was used for the same purpose. Injecting a large-molecular-weight tracer such as albumin or fluorescein isothiocynate dextran directly into the anterior chamber and collecting aqueous samples over time provided a more direct measure of aqueous flow. The rate of disappearance of the tracer from the aqueous humor is a function of aqueous flow.

Another method to determine aqueous flow in humans was a non-invasive photogrammetric technique. With the anterior chamber filled with fluorescein, freshly secreted aqueous humor appeared as a clear bubble in the pupil. The increase in volume of this bubble was determined by geometric optics and was an indication of the rate of aqueous flow.

Currently the method of choice to assess aqueous flow is fluorophotometry. Fluorescein is administered to the eye either topically (Figure 3.10) or by corneal iontophoresis. Fluorescein traverses the cornea, enters the anterior chamber and drains through the chamber angle (Figure 3.11). When an

equilibrium has been established, the concentrations of fluorescein in the cornea (Cc) and the anterior chamber (Ca) decrease over time. A fluorophotometer (Figure 3.12) is used to monitor these changes. The instrument can focus separately on the cornea and then on the anterior chamber, thus allowing a discrete measurement of each region.

The rate of aqueous flow determines the cornea and anterior chamber fluorescein decay curves. The typical time course of the tracer in the corneal stroma and anterior chamber is illustrated in Figure 3.13. The magnitude of the anterior chamber aqueous humor flow (Fa) is a function of the anterior chamber volume (Va), the absolute value of the slope of the decay curve (A) and the ratio of the mass of fluorescein in the cornea to that in the anterior chamber (Mc /Ma) (Equation 2):

Mc can be rewritten as Vc /Cc where Vc is the corneal stroma volume and Cc is the corneal stroma fluorescein concentration. Similarly, Ma can be rewritten as Va/Ca where Ca is the anterior chamber concentration of fluorescein. Equation 2 then becomes

Assuming that Vc and Va remain constant in the steady state, then Fa is a function of the slope of the decay curve (A) and Cc/Ca. The logarithm of Cc/Ca is represented by the distance between the parallel decay curves (Figure 3.13). Equation 3 shows that the more rapid the rate of aqueous humor flow (Fa) the steeper will be the decay curves and the larger the magnitude of the distance between the two

decay curves. At the other extreme, if Fa becomes zero, the fluorescein concentrations in the anterior chamber and corneal stroma will equalize by diffusion and the decay curves will be flat (A 0).

Several assumptions are included in the fluorophotometric measurement of aqueous flow. The fluorescein is evenly distributed in the anterior chamber, there is minimal diffusional loss of fluorescein into limbal or iridial vessels or into the

vitreous cavity, and there are no obstructions causing light scatter. The method does not work in eyes that are aphakic, are pseudophakic, have undergone iridectomy or have any condition in which fluorescein can exit the eye by means other than through the physiological chamber angle outflow pathways. The method is also invalid in eyes with cornea opacity, or severe uveitis because of the light scatter.

Photomultiplier

tube

Recorder

Figure 3.12 The fluorophotometer. Several hours after fluorescein is applied topically to the cornea, a vertical beam of blue light is projected into the eye. The blue light is produced by placing an excitation filter in the light path. The blue light excites the fluorescein, causing it to fluoresce. Simultaneously, a detector filtered for fluoresced light is focused on the same point in the eye. The fluorescence signal at the intersection of the two light paths is detected by the fluorophotometer and is transformed to an electrical impulse by a photomultiplier tube. The intersection of the two light paths can be moved, providing discrete measurements of the fluorescence from the cornea through the anterior chamber. The signal is sent to a recorder and converted into units of ng/ml by referencing a standard curve.

TECHNIQUES FOR MEASURING

AQUEOUS DRAINAGE

Po is the IOP before the weighted tonometer is applied and Pt is the average IOP at the end of the test. Tonography rests on a number of assumptions that have been debated for years. Two include pseudofacility and scleral rigidity. Tonography is visually depicted in Figure 3.15.

In experimental work, the method of choice to determine outflow facility has been the constantpressure perfusion technique (Figure 3.16). The eye is cannulated and the pressure is set by the height of an external reservoir. The inflow of fluid from the reservoir into the anterior chamber (F1 and F2) is measured at each of two different pressures (P1 and P2). The assumption is made that the aqueous production rate and the venous pressure do not change during the measurement. Also assumed is that the tissue fluid volume and the blood volume do not change except within a few seconds after the change in the IOP. Under these conditions, outflow facility is calculated from the formula:

Trabecular outflow

The trabecular outflow pathway is IOP dependent because flow into the canal of Schlemm is passive in response to the hydrostatic pressure driving force, which is equal to IOP minus the hydrostatic pressure in the canal. This pressure difference, for practical purposes, can be assumed to be equal to the episcleral venous pressure, Pev, although the actual pressure in the canal is slightly greater than Pev.

There are three ways to measure facility of trabecular outflow: tonography, perfusion, and fluorophotometry. Tonography has been the most widely used method and, although it is no longer used as a routine clinical test, its use has provided a large body of information about the pathophysiology of glaucoma and the mechanism of action of

In a variation of the constant-pressure perfusion method, the flow of fluid from the reservoir into the anterior chamber is set with the constant rate of infusion by a syringe pump. At each of two different flows (F1 and F2), a resulting pressure is recorded (P1 and P2) and formula 6 is used again. This is the constant-flow perfusion method.

Two major problems exist with the perfusion method. One is the need to insert needles in the eye which can cause trauma and breakdown of the blood–aqueous barrier in some animals. Also, this precludes its use in humans. The other is the ‘washout’ phenomenon. When an eye is perfused experimentally, the flow resistance of the eye decreases progressively, probably from the depletion of proteins in the trabecular meshwork.

Loge Fluorescein concentration

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