Aqueous Humour Dynamics

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Aqueous Humour Dynamics : 

Aqueous Humour Dynamics Dr. Padmaja Phade

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Aqueous humour is clear colourless watery solution continuously circulated from posterior chamber of the eye through out the anterior chamber Maintenance of IOP and pathophysiology of glaucoma revolves around aq. Humour dynamics


ANATOMY Ciliary body Forward continuation of choroid at Ora serreta Triangular in cut section Inner side of triangle is divided into Pars plicata- 2-2.5mm Pars plana- 5mm temporally, 3mm nasaly

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Continue…. Microscopy Supraciliary lamina- outer most condensed part of the stroma Stroma- consist of collagen tissue and fibroblast with ciliary muscle, vasculature and nervs Layer of pigmented epithelium Layer of non pigmented epithelium Internal limiting membrane

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Continue… Ciliary processes 70-80 Whitish finger like projections 2 X 0.5mm Composed of central capillary network with fenestrated thin endothelium and pericytes surrounded by stroma and two layers of epithelium and ILM Inner nonpigmented and outer pigmented epithelium with juxta pposed apical surfaces Inner nonpigmented epi. Characterised by mitochondria, zonula occludentes (ZO)and lateral surface interdigitations The tight junctions contribute to the blood aqueous barrier

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Posterior chamber Triangular space 0.06ml of aqueous Divided into prezonular, zonular and retro zonular space

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Anterior chamber 2.5mm deep in centre, Contains 0.25ml aqueous Bounded ant-post surface of cornea, Post- anterior surface of ciliary body and iris Comunicates through the pupil with post. Chamber Chamber volume decreases by 0.11μl/year of life Chamber depth decreases by 0.01mm/year of life Chamber depth is shallower in hypermtropic than myopic Chamber depth is slightly decrease during accommodation partly by lens curvature and partly by forward translocation of lens.

Angle of Anterior chamber : 

Angle of Anterior chamber Peripheral recess of ant. Chamber Formed mainly by TM Formed post. To ant. By Ciliary band Scleral spur Trabecular meshwork Schwalbe’s line

Gonioscopic grading of angle width : 

Gonioscopic grading of angle width

Aqueous outflow system : 

Aqueous outflow system Consist of two pathways Trabecular / conventional outflow Uveoscleral / unconventional outflow Trabecular meshwork It is sieve like structure bridging the scleral sulcus cosist of 3 parts Uveal meshwork- inner most, extend from iris root and ciliary body to the schwalbes line. The trabeculea are chord like and 2-3 layer thick. Arrangement creates 25-75μ Each trabeculae has 3 concentric layers with central collagenous core enclosed by abasement membrane and trabecular cells.

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Continue…. 2. Corneoscleral meshwork From scleral spur to lateral wall of scleral sulcus Cosist of flat sheet of trabeculae with elliptical opening ranging from 5-50 μ become progressively smaller towards the schlemms canal 3. Juxtacanalicular meshwork Outermost layer connects corneoscleral meshwork to schlemms canal Consists of 2-5 layers of loosely arranged cells embedded in ECM (hyluronic acid and other GAG) lined on either side by endothelial cells Offers main resistance to aqueous flow Outer endothelial layer of juxta canalicular meshwork comprises inner wall of schlemms canal Inner endothelial layer continue with corneo scleral meshwork

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Continue…. Schlemms canal Endothelium lined oval channel present in scleral sulcus Endothelial cells of inner wall are irregular and contain giant vacuoles Collect or channel 25-35 Intrascleral aq. Vessels Leaving schlemms canal at oblique angles to terminate into episcleral veins. Valveless, wide at their origin and taper towards the anastomosis with venous channel. Direct system- Indirect system- Episcleral veins Drain ultimately in cavernous sinus via ant. Ciliary and sup. Ophthalmic vein

Formation of Aqueous Humour : 

Formation of Aqueous Humour Ciliary processes are the site of aqueous formation which is primarily derived from the plasma within capillaries of cilliary processes. Mechanisms involved 1. Diffusion-mol of gas/solution distribute themselves uniformly throughout the space in which they are contained by net flux of particles from area of higher conc. to area of lower conc.

continuation : 

continuation Fick’s law of diffusion Rate of movt.=k( c1-c2) K is constant which depends on nature and permeability of memb, nature solute and solvent and temp. C1- conc of substance on side with higher conc. C2-conc of substance on side with lower conc. 2. Ultrafiltration- depends on hydrostatic pressure and solute conc of plasma in capillaries of cilliary processes 3. Secretion-active process against conc gradient water sol substances of large mol size and greater charge are actively secreted


STEPS OF AQUEOUS FORMATION Secretion basically depends on transfer of solute from stromal surface to post chamber This establishes an osmotic gradient driving water passively into aqueous . Composition of aqueous is similar to that of protein free plasma except for higher ascorbic acid and bicarbonate content.

Continuation : 

Continuation Formation of stromal pool: formed by ultra filtration of plasma in capillaries of ciliary processes due to fenestrations in endothelium proteins are also present in the stromal pool this ultrafiltrate accumulates behind the tight junctions of the NPE.

Conti…. : 

Conti…. 2. Uptake of fluid from stromal pool PE takes up solute(NaCl) by 2 major electroneutral processes Na+ / H+ counter exchanger NHE1 antiport in parallel with the AE2 antiport anion counter exchanger causing Cl- influx and HCO3- eflux carbonic anhydrase II stimulates NaCl uptake by increasing the delivery of H+ and HCO3- to the 2 antiports.

Conti….. : 

Conti….. Na +-K+ 2Cl- SYMPORT situated on basolateral membrane of of PE cells rate of transport of the 3 ions depends on their conc gradient especially on ratio of extracellular to intracellular Cl- net solute transfer through symport is zero at intracellular Cl- conc of 50mM

Conti… : 

Conti… PASSIVE DIFFUSION mostly water diffuses along osmotic gradient established by Nacl transport. cell memb have low content of sphyngomyelin and cholesterol hence relative high water permeability POTENTIAL SOLUTE RECYCLING In order to minimize fluctuations in the cell vol due to mismatch in rates of uptake of solutes and water by PE at stromal surf and their release at aqueous surf there is autocrine regulation of these processes at the PE level

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Conti… Excess fluid and solute uptake at stromal surf than release at the aqeous surf of PE , NPE Cellular swelling ATP release by PE and NPE cAMP formation Activation of Cl- channels on stromal surf Efflux of excess Cl- and water from PE, NPE

CONTI……… : 

CONTI……… 3. FLUID TRANSFER THROUGH GAP JUNCTIONS Gap junctions between PE and NPE formed by connexins Cx43 and Cx40 Gap junctions are also present within PE and NPE but are functionally less significant Aqueous is thus formed by parallel couplets of PE NPE cell gap junctions.

Conti… : 

Conti… 4. FLUID TRANSFER INTO AQUEOUS HUMOUR final step in aqueous secretion. Solutes and water are transported across the basolateral membrane of NPE. Na+, K+ ATPase releases(70%) Na+ against electrochemical gradient into aqueous, remaining (30%/) transported passively or by ultra filtration. Cl- is released along its electrochemical gradient through Cl- channels. Water released along osmotic gradient established by solute transfer into aqueous through AQP1 and AQP4. Bicarbonate exits through HCO3-/Cl- exchangers as well as Cl- channels. K+ transported by secretion and diffusion Ascorbic acid secreted against a conc gradient Amino acids are secreted by 3 diff carrier proteins each for acidic , basic and neutral molecules.


RATE OF AQUEOUS HUMOUR FORMATION 2.5micro liters/min in an undisturbed human eye Rate of aqueous inflow is 2 ½ times higher during waking than in nocturnal hours REGULATION OF AQUEOUS FORMATION 1.Adrenergic receptors- α2 receptor stimulation lowers aqueous secretion via adeylate cyclase inhibition. epinephrine stimulates PGF2α production which lowers IOP. β2 receptor stimulation leads to increased aqueous secretion via activation of adenylate cyclase.

Conti….. : 

Conti….. Catecholamines Stimulation of adenylate cyclase Icreased c-AMP formation activation of PK specific protien phosphorylation increased permeability of PE,NPE cells to solutes and water. increased aqueous formation

Conti…….. : 

Conti…….. 2. Ultrafiltration and diffusion these passive mechanisms depend on blood pressure in cilliary capillaries , plasma oncotic pressure,and IOP. 3. Vasopressin vasopressin stimulates NaCl transport through PE ,NPE and thus aqueous formation. vasopressin levels in turn are indirectly proportional to plasma osmolarity.


MEASUREMENT OF RATE OF AQUEOUS PRODUCTION Class 1 methods measure rate of appearance and dissapearance of a substance from aqueous Fluorescein techniques Radioactive labeled isotopes Intravenous PAH technique Class 2 methods flow= C(Po-Pv) C= facility of aqueous outflow Po= IOP Pv=episcleral venous pressure

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Conti……. Perfusion of eyes at a constant pressure Tonography Perilimbal suction cup method


BIOCHEMICAL COMPOSITION OF AQUEOUS HUMOUR Water: constitutes about 99.9% of aqueous Proteins: is about 5-16 mg/dl ie1/500 of plasma protein content (6-7g/dl) A:G ratio is same as that of plasma IgG and IgM arepresent plasminogen and its proactivators are present FGF,TGFβ,IGF1 Amino acids:- conc varies with aqueous/plasma conc (0.08-3.14) Non colloidal constituents:- similar to that of plasma ascorbate, lactate, pyruvate is higher than that in plasma conc of glucose and urea is higher than that of plasma bicarbonate, ascorbate levels in post chamber is higher than in ant chamber chloride conc in post chamber is lower than in ant chamber


BLOOD OCULAR BARRIER BLOOD AQUEOUS BARRIER : formed by tight junctions (zonula occludens and zonula adherans) between cells of inner NPE of ciliary processes and non fenestrated epithelium of iris capillaries. BLOOD RETINAL BARRIER : INNER : tight junctions of retinal capillaries and endothelial cells OUTER: tight junctions between adjacent RPE Blood ocular barrier prevents proteins and large mol wt substances from entering the ocular cavities Lipid solubility facilitates ocular penetration Medium mol wt substances penetrate at a slower rate than their transit through capillary walls. With breakdown of blood aqueous barrier protein and antibody conc of aqueous equilibrates with that of plasma to form SECONDARY OR PLASMOID AQUEOUS. Fibrinogen may cause clotting .


CAUSES OF INTERRUPTION OF BLOOD OCULAR BARRIER OCULAR TRAUMA mechanical : paracentesis corneal abrasion intraocular surgeries stroking iris Physical: X rays atomic radiation Chemical : alkali irritants

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PATHOPHYSIOLOGICAL 1. inflamations 2. intraocular and corneal infections 3.anterior segment ischemia PHARMACOLOGICAL 1.MSH 2.Cholinergic drugs 3. Cholinesterase inhibitors 4. Nitrogen mustard


FUNCTIONS OF AQUEOUS HUMOUR Maintenance of IOP Metabolism of avascular stuctures of eye Optical function Clearing function


PHYSIOCHEMICAL PROPERTIES OF AQUEOUS VOLUME: 0.31 ml ( 0.25ml in AC ; 0.06 ml in post chamber) Refractive index : 1.336 pH : 7.2 ( acidic) Density: slightly denser than water ( 1.040: 1.025) Osmotic pressure: hyperosmotic to plasma by 3 to 5 m osmo/l


AQUEOUS HUMOUR DYNAMICS Includes aqueous formation and drainage Aqueous flows from post to ant chamber through pupil and in AC flows along conventional current set up due to temp difference in ant part and post part of AC. From AC aqueous is drained by Trabecular(conventional) outflow Uveoscleral( unconventional) outflow

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Ciliary process Aq. In post. Chamber Anterior chamber Trabecular meshwork ciliary body Schlemms canal suprachoroidal space Collector channel venous circn. Cil. Body, sclera and orbit Episcleral veins Trabecular outflow 90% Uveoscleral outflow 10%


TRABECULAR OUTFLOW Drains 75 to 90% aqueous Free flow occurs through TM till the juxtacanalicular tissue which offer some resistance to the outflow. SPECIAL CHARACTERISTICS OF TM CELLS: High levels of cytoskeletal actin and lower levels of microtubules Presence of cellular vimentin and desmin AQP1 PROTIENS High levels of surface tPA GAG degrading enzymes and acid phosphatases Β2 adrenergic receptors and TIGR Specialized endocytic / phagocytic properties

MECHANISM OF AQUEOUS TRANSPORT THROUGH TM1. VACUOLATION THEORY:- vesicles and vacuoles in endothelium open and close intermittently to transport aqueous from TM cells to Schlemm’s canal : 

MECHANISM OF AQUEOUS TRANSPORT THROUGH TM1. VACUOLATION THEORY:- vesicles and vacuoles in endothelium open and close intermittently to transport aqueous from TM cells to Schlemm’s canal

Conti…… : 

Conti…… 2. LEAKY ENDOTHELIAL CELLS SONDERMAN’S CHANNELS : microtubules in TM cells help aqueous flow from corneoscleral trabecular meshwork into lumen of Schlemm’s canal . CONTRACTILE MICROFILAMENTS : PORES IN ENDOTHELIAL CELLS : (3μm ) about 20,000


UVEOSCLERAL OUTFLOW Drains 0.3μl/min Drains 10 to 25% of aqueous Independent of IOP PG increase uveoscleral flow to lower the IOP


FACILITY OF AQUEOUS OUTFLOW Pressure gradient of 10mm of Hg between IOP and episcleral venous plexus helps in drainage C- value expressed as aq. Outflow in μl/min/mm of Hg It represents quantitative aproximation of state of aq. Drainage system

Measurements of C value : 

Measurements of C value Perfusion method C= flow rate / Pi – Po Independent of ocular rigidity and corneal curvature C=0.28 μl/min/mm of Hg 2. Tonography Most commonly used non invasive method 3. Suction cup method

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97.5% population has C value >0.18 Most glaucoma pt. has C value <0.17 Significance of C value As adjuncting diagnosis of glaucoma C value < 0.10 or less in angle closure glaucoma after an acute attack suggest that peripheral iridectomy may not be sufficient Evaluation of drug mechanism and experimentally to study abnormality of various influences on aq. dynamics

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Thank You