Aqueous Humor Dynamics

Sanket parajuli

Aqueous humor• Clear and optically transparent fluid

Functions• Brings oxygen and nutrients to cells of lens, cornea, iris • Removes products of metabolism and toxic substances • Provides optically clear medium for vision • Inflates globe --by maintaining IOP• High ascorbate levels protect against ultraviolet-induced

oxidative products, e.g., free radicals • Facilitates cellular and humoral responses of eye to

inflammation and infection

Bodystructure of ciliary body • Parts: Pars plana and pars plicata

• Attached anteriorly to the scleral spur, creating a potential space (supraciliary space) between itself and the sclera

• Iris inserts into the short anterior side of the ciliary body, leaving a narrow width of ciliary face visible on gonioscopy between the peripheral iris and the scleral spur

• Iris in turn divides the aqueous space into the

posterior and anterior chambers

• The junction of the iris, sclera, and cornea is called the anterior chamber angle

• Ciliary body runs from the scleral spur to the ora serrata, a distance of approximately 6 mm

• The anterior portion : pars plicata ---has approximately 70 to 80 radial ridges (the ciliary processes) on its inner surface

• The ciliary processes are approximately 2 mm in length, 0.5 mm in width, and 0.9 mm in height

• Pars plicata has a large surface area (approx 5* the surface area of the corneal endothelium) for active fluid transport

• The ciliary body is composed of muscle, vascular tissue, and epithelium

• Ciliary muscle consists of three separate muscles fibers –

a) longitudinal muscle • anteriorly to the scleral spur and trabecular

meshwork • posteriorly to the suprachoroidal lamina, with

some fibers connecting to the choroid and sclera as far posteriorly as the equator of the globe

• When the longitudinal muscle contracts, it pulls open the trabecular meshwork and Schlemm’s canal

b)circular muscle fibers • run parallel to the limbus • When these fibers contract they relax the

zonules, allowing the lens to change shape

c) Radial muscle connects the longitudinal and circular muscles Function of the radial muscle is not entirely clear, but it is postulated that contraction of the radial fibers may widen the uveal trabecular spaces

Ultrastructure of the ciliary processes

• Each ciliary process is composed of ::

• a central core of stroma and capillaries covered by a double layer of epithelium

• Function of the pigmented epithelium is not entirely clear

• Non-pigmented epithelial layer is composed of columnar cells, which are separated from the aqueous humor by a basement membrane

• Non-pigmented ciliary epithelium numerous mitochondria, well-developed rough endoplasmic reticulum, and tight junctions connecting adjacent apical cell membranes

• The potential space between the two epithelial layers is called the ciliary channel.

• One group of investigators postulates that aqueous humor is secreted into this space, particularly after stimulation of the system with various agents, such as adrenergic agonists(requires further study)

• Adjacent cells are connected by gap junctions, & desmosomes

• The non-pigmented ciliary epithelial cells are also joined at their apical membranes by tight junctions (zonulae occludentae), which are thought to be an important component of the blood–aqueous barrier

• AH is produced in the anterior portion of the ciliary processes

• Non-pigmented ciliary epithelium have abundant Na-K ATPase

• Mechanism of aqueous Formation Formation of AH is a complex

Aqueous formation has several important processes that normally happen simultaneously: • Diffusion• Ultrafiltration and • Active secretion

Ultrafiltration• The process by which a fluid and its solutes cross

a semipermeable membrane under a pressure gradient (e.g., capillary blood pressure) is called ultrafiltration

• More than twice the weight of the ciliary processes themselves of blood flows through the ciliary processes each minute

• As blood passes through the capillaries of the ciliary processes, about 4% of the plasma filters through the fenestrations in the capillary wall into the interstitial spaces between the capillaries and the ciliary epithelium

• Rate of protein leakage through the vessel walls into the tissue space of the ciliary processes is relatively low

• However, the ciliary epithelial layers are even less permeable to the passage of colloids into the posterior chamber

• Thus the colloid concentration in the tissue space of the ciliary processes is approximately 75% of that in plasma

• The high concentration of colloids in the tissue space of the ciliary processes favors the movement of water from the plasma into the ciliary stroma but retards the movement of water from the stroma into the posterior chamber

• Although --postulated that ultrafiltration is responsible for the majority of aqueous humor formation -- it is unlikely that the hydrostatic pressure difference between the ciliary capillaries and the posterior chamber can overcome the large oncotic pressure differential

• Ultrafiltration --does not explain why active ion transport inhibitors such as ouabain are capable of reducing aqueous humor formation by 70–80%

• Thus--ultrafiltration helps to move fluid out of the capillaries into the stroma but alone is insufficient to account for the volume of fluid moved into the posterior chamber

• The latter step requires an active metabolic process. Ultrafiltration and active secretion occur in tandem

Active transport

• energy-dependent process-moves substance against its electrochemical gradient

• The selective active transport system that participates in the secretion of aqueous humor by ciliary epithelium are:• Na+,K+-2Cl- symport ,• Cl- –HCO3

- and NA+,H+ antiports ,• Cation channels • Water channels• Na+, K+ –ATPase ,• K+ channels • H+ –ATPase • Cl- channels

• It is not clear which ion or ions are actively transported across the non-pigmented ciliary epithelium, though most theories include sodium, chloride, and/or bicarbonate

• Most suggest--the active transport of the sodium ion is the key process

• Enough Na-K ATPase activity is present in the ciliary non-pigmented epithelium to drive aqueous humor formation mainly by the sodium gradient

• Candia et al suggested that while active transport of Na is important, it cannot account for the entire rate of aqueous formation in vivo

• Instead Cl-is actively trans-ported across the non-pigmented ciliary epithelium and may be the driving force of aqueous formation

Diffusion

• Diffusion is the movement of a substance across a membrane along its concentration gradient

• As the aqueous humor passes from the posterior chamber to Schlemm’s canal, it is in contact with the ciliary body, iris, lens, vitreous, cornea, and trabecular meshwork

• There is sufficient diffusional exchange with the surrounding tissues, so that the anterior chamber aqueous humor resembles plasma more closely than does the posterior chamber aqueous humor

• Aqueous humor provides glucose, amino acids, oxygen, and potassium to surrounding tissues and removes carbon dioxide, lactate, and pyruvate

Control of aqueous formation

• Adrenergic innervation: No adrenergic innervation of ciliary epitheliumVessels of ciliary body----adrenergic fibers

• Vasopressin:Supports active transport of Na ions across ciliary epithelium and increases aqueous formation

• adenylcyclase

• α2 stimulation----inhibits adenylcyclase---decreases aqueous formation

• ß2 stimulation—stimulates adenylcyclase—increases aqueous formation

• Reduce IOP--- α2 agonists/ ß2 antagonists

Blood–aqueous Barrier

• consists of all of the barriers to the movement of substances from the plasma to the aqueous humor

Main::• the tight junctions (zonae occludentes) connecting

the apical portions of adjacent non-pigmented epithelial cells in the ciliary processes and the tight junctions in endothelium of ciliary processes

• Not as ‘tight’ as their name would imply – in that they do allow passage of some small ions and water

• The blood–aqueous barrier is responsible for maintaining the differences in chemical composition between the plasma and the aqueous humor.

• Many endogenous and exogenous stimuli increase the permeability of the epithelia and vascular endothelium (i.e., break the blood–aqueous barrier), producing an increase in aqueous humor protein concentration

• The breach of the barrier often is accompanied by a transient rise in IOP, which is then followed by a period of prolonged hypotonia

• Sometimes (e.g., intraocular infection), a breakdown of the blood–aqueous barrier is clearly therapeutic because it brings mediators of cellular and humoral immunity to the interior of the eye

• In other situations (e.g., some forms of uveitis and following trauma), the breakdown of the barrier is inappropriate and favors the development of complications, such as cataract and synechia formation

Mechanisms for compromise of the blood–aqueous barrier

• One important mechanism is mediated by prostaglandins

• This system is activated by a variety of stimuli, including paracentesis, and is blocked by NSAIDS agents such as aspirin and indomethacin

• Another important mechanism is mediated by sensory innervation and neural peptides, including substance P

• This system is activated by a variety of stimuli (e.g., topical nitrogen mustard) and is blocked by retrobulbar lidocaine (lignocaine)

• When the blood–aqueous barrier is broken, protein-rich fluid collects in cysts beneath and between the epithelial cells of the ciliary body

• The contents of these cysts eventually burst into the posterior chamber

• Protein-rich fluid also enters the eye via reflux from Schlemm’s canal in some situations

Clinical application:

• Hyperosmotic agents such as mannitol penetrate the eye poorly because of the barrier and is widely distributed in extracellular spaces of the body and water is drawn out from the cells and ocular fluid

• Systemically adminiseterd drugs like chloramphenicol and ampicillin (low mol wt) can penetrate barrier but penicillin erythromycin (high mol wt) can’t

Aqueous outflow::

• Aqueous returns to the venous system primarily by means of the • conventional or canalicular pathway (83–96% of flow)

• the uveoscleral or unconventional route 5 to 15% of flow

Anatomy of the conventional outflow system • limbus is the transitional region between the

cornea and sclera

• At the inner surface, the limbus contains a scleral indentation called the scleral sulcus

• At the anterior margin of the scleral sulcus is Schwalbe’s line

• scleral sulcus is defined posteriorly at its internal margin by the scleral spur, an anterior extension of sclera that partially encloses the posterior portion of the sulcus

• Trabecular meshwork is interposed between the anterior chamber and Schlemm’s canal by means of its attachment anteriorly to Schwalbe’s line and posteriorly to the scleral spur

• The trabecular meshwork thus composes the inner wall of Schlemm’s canal

schwalbe’s line • Schwalbe’s line (composed of collagen and elastic

tissue) is an irregular elevation that runs circumferentially around the globe

• This line or zone marks the transition from trabecular to corneal endothelium, the termination of Descemet’s membrane, and the insertion of the trabecular meshwork into the corneal stroma.

• Secretory cells, called Schwalbe’s line cells, are present in this area that produce a phospholipid material thought to facilitate aqueous flow

Scleral spur

• attached anteriorly to the trabecular meshwork and posteriorly to the sclera and the longitudinal portion of the ciliary muscle

• consists of collagen types I and III 5% elastic tissue

• When ciliary muscle contracts-- pulls scleral spur posteriorly

• The largest trabecular lamellae near the anterior chamber are attached to the scleral spur and accordingly are rotated inward and posteriorly by ciliary muscle contraction

• Inward movement of the trabecular meshwork results in an

enlargement of intertrabecular spaces and an increase in the size of Schlemm’s canal, reducing the tendency of the canal lumen to narrow or collapse

Trabecular meshwork tissues

• apex at Schwalbe’s line and its base at the scleral spur

• Composed of multiple layers each of which consists of collagenous connective tissue core covered by continuous endothelial lining

• Functions as one way valve

• It’s cells are phagocytic

1.Uveal part: innermost, adjacent to AC , cord like mesh

extending from root of iris & ciliary body to peripheral cornea (schwalbe line)

intertrabecular space is larger

2.Corneoscleral part:sheet like, larger middle portion extending from

scleral spur to schwalbe line, intertrabecular space is smaller

Endothelial(juxtacanalicular) part: • narrow outer part of the trabeculum

• links Corneoscleral part with endothelium of the inner wall of schelmm canal

• Major proportion of normal resistance to aqueous outflow is in this part of TM

• Partly because the pathway is narrow, tortuos and partly because of the resistance offered by the extracellular proteoglycans and glycoproteins

SCHLEMM’s CANAL

• Circumferential single channel bridged by septa

• Surrounded by sclera trabecular meshwork and scleral spur

• Inner wall is lined by irregular spindle shaped endothelial cells containing giant vacuoles and have direct communication with trabecular spaces

• The outer wall is lined by smooth flat cells ,has opening for the collector channels—lacks vacuoles

• Complex system of veins connects the canal to episcleral veins

Schlemm’s canal is a modified vessel

• In other vessels, pressure gradients are higher in the vessel lumen & Fluid moves from the higher pressure in the lumen of vessels across vessel walls to the lower pressure in adjacent tissues as a response to the hydrostatic pressure gradient

• In contrast, pressures are higher external to the lumen of Schlemm’s canal & aqueous flows from the anterior chamber, across the modified vascular wall represented by the trabecular meshwork, into the lower pressure vascular lumen of Schlemm’s canal

Collector channel’s

• Schlemm’s canal is drained by 20-30 collector channels which drain into complex system of intrascleral episcleral and subconjunctival venous plexus

Trabecular /conventional/pressure dependent outflow• Most of the aqueous exits the eye by the way of

trabeculum into schlemm’s canal and drained to venous system, about 90%

• Schlemm’s canal----episcleral veins----ant ciliary & sup ophthalmic vein----cavernous sinus

• It acts as a one way valve-it permits the aqueous to leave the eye but limits the flow in other direction independently of energy

• It is increased by miotics, sympathomimetics, laser trabecoloplasty and trabeculotomy

Uveoscleral/unconventional/ pressure independent outflow

• About 10% of aqueous outflow is by this route

• AC----ciliary muscle----supra ciliary & suprachoroidal space----fluid exit through the intact sclera or along the nerves & vessels that penetrate the sclera.

• Some aqueous also drain via iris

• It is decreased by miotics

• It is increased by cycloplegics, prostaglandins , sympathomimetics, surgery like cyclodialysis

COMPOSITION OF NORMAL AQUEOUS HUMOR

• It consists of :• Water ---98.92%• Inorganic and organic anions • Carbohydrates• Glutathione and urea• Proteins • Growth modulatory factors• Oxygen and Carbondioxide• Lipids

INORGANIC IONS• Concentration of sodium, potassium & magnesium in aqueous are similar to that of plasma

• Calcium is half the plasma

• Important anions are Chloride & bicarbonate

• There is apparent excess of Cl- and deficit of HCO3-

• Iron ,copper & zinc are found in aqueous at a concentration of 1mg/ml, same as that of plasma

• Phosphate is also present with very low concentration

ORGANIC ANIONS

Lactate:• most abundant,concentration is higher than that of plasma• Plasma & aqueous level is directly related• From metabolism of glucose by ciliary epithelium, retina,lens and cornea

Ascorbate:• Ascorbic acid or vitamin C• Unique constituent of aqueous humor• Its concentration ranges from 0.6-1.5 mM, ie, 10-50 times higher than in plasma

• No relation between plasma aqueous ascorbate level

• Serves as an Antioxidant

• Secreted actively by a Na-dependent L-ascorbic acid transporter

• Partially absorbs ultraviolet radiation

• Diurnal mammals have high ascorbic level about 20 time more than nocturnal mammals

CARBOHYDRATES

Glucose:• Aqueous concentration is about 70-80% of that of

plasma

• Enters by simple diffusion

• Aqueous glucose level is increased in diabetics due to higher

• concentration in lens

• Glucose from aqueous diffuses into cornea

PROTEINS

• Blood aqueous barrier normally limits the protein of aqueous humor to less than 1% of its plasma concentration

• One possible route to enter the AC is through the root of iris

• Aqueous contains 0.02 gm of protein/100ml whereas plasma contains 7gm/100ml

• Lower molecular weight proteins ---albumin and beta globulins are more prominent in the aqueous

• The heavy molecular weight proteins like beta lipoproteins and heavy immunoglobulins are present in trace amount

• The albumin : globulin is much higher in aqueous than in plasma

• Increased globulin content, decreased albumin: globulin-----retinoblastoma

Immunoglobulin:

• IgG is normally found in aqueous humor in a concentration of 3mg/100ml

• IgA, IgD and IgM are not normally present in detectable amount

• IgG increase , IgM &IgA becomes detectable ----uveitis

Complement:

• Found in minute quantity

Components of Coagulation & fibrinolytic system:

• Trace in amount

• Secondary aqueous contains major components of coagulation and fibrinolysis

Plasminogen & plasminogen proactivator:• Are present in significant amount

• Trace amount of inhibitor of plasminogen is present

• This keeps the aqueous outflow pathway free of fibrin

• Recent evidence suggest that human aq promotes coagulation by shortenning the PT and PTT,may act like a procoagulant rather than a thromboplastin-like substance

Enzymes:

• Hyaluronidase--important in the normal regulation of the resistance to outflow through TM

• Carbonic anhydrase—trace

• Lysozyme—provides significant antibacterial protection.

• In intraocular inflammation its level increases by 10 fold or more

• Growth modulatory factors

• Transforming growth factor1 & 2(TGF-1 and 2)• Acid and basic fibroblast growth factor(afgf&bfgf)• Insulin like growth factor 1 (IGF-1)• Insulin like growth factor binding protein(igfbp)• Vascular endothelial growth factor(vegf)• Transferin

• IGF-1 are increased in patients of DM• VGEF increased in ocular neovascularizationi. After occlusion of the central retinal veinii. Iris neovascularisation

OXYGEN AND CARBONDIOXIDE• PO2= 55mmHg

• Oxygen tension in Aq decreases with topical epinephrine and in contact lens users

• PCO2= 40-60mmHg contributing 3% of bicarbonate

• CO2 is continuously lost from aqueous by diffusion across the cornea into the tear film

pH: 7.5-7.6

• Intraocular malignancy:

plasma : aqueous LDH ratio > 1.5 (retinoblastoma)<0.6 in rubeosis iridis, persistent hyperplastic primary vitreous, ROP and

malignant melanoma

• Uveitis:

IgG increase…IgM and IgA detectable

Factors influencing aqueous formation:

• Hydrostatic pressure alteration in capillaries: if venous obstruction—Raised IOP

• Permeability of capillaries

• Osmotic pressure of blood: Hypotonicity induces a rise in IOP

Decreased production:

• Hypothermia

• Acidosis

• Injection of hyperosmotic substance

• Drugs - Halothane, barbiturates, ketamine, vasopressin

• Retinal detachment

• Ocular inflammation

Factors increasing aqueous:• Hyperthermia• Alkalosis• Injection of PGs, Arachidonic acid and calcium

IOP

• Refers to pressure exerted by intraocular contents on coats of eyeball

• Normal IOP maintained by AH production-outflow and episcleral venous pressure

• IOP=10-21 mm Hg

• IOP raises 0.8 mm Hg for every 1 mm Hg rise in episcleral venous pressure

Factor influencing IOP:

• Age: after 40 IOP increases

• Heriditary: family h/o glaucoma—high IOP

• Sex: higher in women

• Axial length: Myopes tend to have high IOP

• Diurnal fluctuations: serum level of adrenocortical steroids

• Seasonal : High IOP in winter

• Cardiovascular: BP must rise by 100 mm Hg to raise IOP by 2 mm Hg

• Exercise: strenuous exercise reduces IOP as a result of acidosis

• Postural changes: IOP rises by 3-6 mm Hg from sitting to supine

• Inflammation: IOP is reduced as AH formation decreases

• IOP decreases after ocular surgery

Episcleral venous Pressure

• Aqueous humor leaving the eye by trabeculo canalicular outflow eventually passes into the venous system

• The pressure in the veins that receive the aqueous humor is referred to as episcleral venous pressure

• Episcleral venous pressure:: 8–11.5 mmHg

• The venous pressure levels are affected by body position

• When episcleral venous pressure is raised by some disease process, IOP is usually increased as well

• Many conditions (e.g., carotid cavernous fistula) that increase episcleral venous

pressure also cause ocular ischemia, which reduces aqueous humor formation and IOP

• Furthermore, acute elevations of episcleral venous pressure increase the facility of outflow, whereas chronic elevations may produce secondary changes in the angle structures and decrease the outflow facility

Drugs lowering IOP:

• Increasing outflow: PGs

• Decreasing episcleral venous pressure: PGs

• Increasing uveoscleral outflow: brimonidine

• Decreasing aqueous production: Beta blockers, CAI, adrenergic agonist

Diurnal variation of IOP:

• 2-6mm Hg variation over 24 hours

• Peak pressure in the morning

• Higher IOP is associated with higher fluctuation and a diurnal fluctuation of > 10 mm Hg suggestive of glaucoma

Angle of AC

1. Ciliary band

2. scleral spur

3. Trabecular meshwork

4. Schwalbe’s line

Shaffer’s system of grading the angle width

Facility of aqueous flow:

• Aqueous outflow--Calculated as C-value• Normal= 0.28 microliter/min/mm Hg

Measured by:

A) perfusion method : in vitro

B) Tonography: by use of schiotz tonometer baseline IOP calculated—then with help of tonographic tables C-values calculatedPreviously-- C value was important for diagnosis of glaucoma—now not much role

c) Suction cap method—similar to tonographic methods—not used

• Aqueous formation (F), facility of outflow (C), and episcleral venous pressure (Pv) are the major intraocular determinants of IOP (P0)

• Goldmann equation: P0=F/C +Pv

MEASUREMENT OF IOP

• Digitally

• Schiotz tonometer—indentation of the cornea by a known weight

• Applanation tonometer—based on Imbert-Fick principle which states that pressure inside an ideal dry ,thin walled sphere equals the force necessary to flatten its surface divided by the area of the flattening=== P=F/A

• Goldmann applanation tonometerArea of the cornea ---3.06mm diameter

• Noncontact (air puff) tonometer

• Potable electronic applanation device (tonopen)

• Pneumatic tonometer or pneumatonometer

References:

• Wolff’s anatomy of eye and orbit• Shield’s textbook of glaucoma• Becker’s and schaffer’s• Snell’s anatomy