Week 9: Aqueous and Vitreous Humor and Vitreo-Retinal Attachments PDF

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Week 9: Aqueous and Vitreous Humor and VitreoRetinal Attachments List the structures and function of the filtration angle Anterior Segment Structure: 1. Anterior Chamber:  Cavity behind cornea and in front of the iris  Fluid filled (aqueous)  Consists of: o Glucose o Amino acids o Ascorbic acid o Dissolved gases 2. Posterior chamber: (parts of it)  Not to be confused with vitreous chamber  Narrow area behind iris, in front of zonules and ciliary processes Together, form the anterior chamber angle, interested in the flow of aqueous that is produced at ciliary processes through pupil and into angle. Eventually leaves. (arrows)

The Angle Region of anterior chamber where cornea and iris join. 1. Schwalbe’s line 2. Trabecular Meshwork (nonpigmented) 3. Trabecular meshwork (pigmented) 4. Scleral spur 5. Ciliary body band: peripheral iris extends anteriorly to attach to ciliary body, narrow width of ciliary face visible known as ciliary body band. 1. Schwalbe’s line: Marks the termination of Descemet’s membrane, most anterior in angle, appears as an opaque line. 

Marks separation between corneal stroma and endothelium, indicates we are observing the end of the cornea when we see Descemet’s membrane.

2. Trabecular Meshwork Forms the aqueous outflow pathway, together with scleral spur, ciliary muscle and Schlemm’s canal. Apex of outflow pathway is at Schwalbe’s line, the vase is at the scleral spur.  

Inner wall of TM faces anterior chamber Outer wall of TM is attached to inner wall of Schlemm’s canal

When aqueous leaves ciliary processes, travels in front of lens, out through pupil and leaves through TM, has collagen fibres and pores to capture debris  can get clogged up (glaucoma). Non-pigmented: Anterior portion is usually less pigmented and is considered the non-filtering portion of the meshwork Pigmented: As fluid tends to flow through here, the posterior trabecular meshwork tends to collect/accumulate pigment with age as it is shed from other structures e.g. the iris. Divided histologically into 3 components: 1.   

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Uveal Meshwork (inner) Large pores exist between tissue and lamellae, very open Contributes little resistance to aqueous outflow Cells of outer layers act primarily as pre-filters, aggressively phagocytic, removing cellular debris from aqueous humour before fluid moves deeper into less porous Juxtacanalicular meshwork and Schlemm’s canal. Biggest pores, largest stuff get caught here Highly variable between eyes

2. Corneo-scleral meshwork (central)  Makes up the bulk of the trabecular meshwork, pores smaller than uveal  Constructed from 10-15 perforated collagenous sheets suspended between Schwalbe’s line and ciliary muscle  Outer layer arise from scleral spur  Inner layers arise from ciliary muscle fibre insertions  The cells of the outer layers of the trabecular meshwork act primarily as pre-filters and are aggressively phagocytic, removing cellular debris from the aqueous humour before the fluid moves deeper into the less porous Juxtacanalicular meshwork (JCM) and Schlemm’s Canal. 3.    

Juxtacanalicular tissue (cribriform layer, outer) Outer portion of trabecular meshwork (closest to cornea) Lies between Schlemm’s canal and outer lamella of corneoscleral meshwork Outer aspect formed by endothelial cells which line inner wall of Schlemm’s canal Principle site of aqueous outflow resistance TM is involved in homeostasis: robust response to increased IOP by reducing outflow resistance.

3. Scleral Spur  Protrusion of sclera into anterior chamber  Continuous with ciliary body posteriorly trabecular meshwork anteriorly 4. Ciliary body band  Iris inserts into the short anterior side of the ciliary body, leaving a narrow width of ciliary face visible using gonioscopy  Ciliary body band is the ciliary face between the peripheral iris and scleral spur observed in gonioscopy.

Clinical: Sampaolesi’s line Pigment visible anterior to, or along, Schwalbe’s line 

Can be idiopathic, but presence of this line can be associated with both pigment dispersion syndrome, pseudoexfoliation syndrome and glaucoma.

Canal of Schlemm  Circular vessel considered to be a venous channel for aqueous drainage  Lies parallel and continuous with outer aspect of trabecular meshwork  Oval shape in crosssection  Outer wall is perforated by 30-35 collector channels draining to intra-scleral veins

Discuss aqueous production and drainage demonstrating your understanding of the structures and physiology involved. Aqueous Humor 

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Aqueous consists of clear liquid containing: o Electrolytes, ascorbic acid, sugars, sodium chloride, oxygen, amino acids, waste products (i.e. CO2, lacticacid)  no blood vessels bc must stay transparent Generally: Constantly in motion, may be no movement in pupil block causing angle closure glaucoma Rate of formation 2 µL/min Entire volume is replaced every 1-2 hrs Volume of Anterior Chamber = 0.2 ml (most of it) Volume of Posterior Chamber = 0.06ml Function: keep the shape of the globe constant, maintains the pressure of the eye and corneal nourishment

2 Mechanisms: 1. Active Secretion  80% is secreted by non-pigmented layer of ciliary epithelium by metabolic pump independent of IOP 2. Passive Secretion  20% is produced by diffusion and ultrafiltration dependent upon blood pressure plasma oncotic (osmotic pressure induced by proteins) pressure and IOP, pushed out by blood pressure.

Outflow Flows from ciliary processes through posterior chamber  

Pars plicata (folded anterior portion of CB) Pars plana (flat portion in uvea)

Flows through pupil, then to anterior chamber where it is drained from eye.

Humour Drainage Trabecular (conventional) pathway  Pressure dependent, runs along a pressure gradient o 90% drained through TM into Schlemm’s canal  Flows from Schlemm’s canal to collector channels and venous plexi, exit eyes through episcleral veins (veins = flow towards heart, arteries = flow away from heart)  Episcleral venous pressure level can affect the outflow with higher venous pressure reducing the outflow  Effect of high blood pressure is small (10mmHg raises IOP by 0.26mmHg) Uveoscleral (unconventional) pathway  Independent of pressure at IOP levels > 7-10mmHg  Aqueous passes through ciliary muscle into supraciliary (anteriorly) and suprachoroidal (posteriorly) spaces o Drained by venous circulation in the ciliary body, choroid and iris root (crypts allow aqueous to communicate with deep tissue)  provide nourishment and remove waste o Age related decline in uveoscleral outflow.

Describe the physiology of intra-ocular pressure and its measurement IOP is determined by the following factors: 1. Rate of aqueous secretion, higher rate, higher pressure 2. Resistance to aqueous outflow 3. Level of episcleral venous pressure o Difference in pressure between arteries entering tissue and veins leaving it - Higher venous pressure reduces outflow Normal IOP = 15.9 mmHg, 10-21mmHg  out of range, homeostasis may not be working   

Differences between patients (physiological IOP) who may be well normal Fluctuates with time of day, body position, heartbeat, blood pressure, respiration o Elevated in morning, variation in 50yo Can result in pathology (retinal detachment, retinal tears)...