Week 10: Aqueous, Vitreous Humour & Vitreo-Retinal Attachments Flashcards
Describe the anterior segment structure
- Anterior Chamber
- Filled with aqueous humour (0.2 mL)
- Measures 3 mm at greatest depth centrally
- Anterior limit: cornea & sclera
- Posterior limit: iris & lens - Posterior Chamber
- Filled with aqueous humour (0.06 mL)
- Narrow area
- Anterior limit: iris
- Posterior limit: lens & zonules
- Peripheral limit: ciliary processes
What are the angle structures of the anterior chamber from anterior to posterior.
- Schwalbe’s Line
- Trabecular Meshwork (non-pigmented)
- Trabecular Meshwork (pigmented)
- Scleral Spur
- Ciliary Body Band
Describe Schwalbe’s Line
- Most anterior structure in the angle
- Looks like an ‘opaque’ line
- Termination of Descemet’s membrane
- Pigment visible anterior to Schwalbe’s line is called a sampaolesis line
Describe Trabecular Meshwork in general
- Forms aqueous outflow pathway with the scleral spur, ciliary muscle and Schlemm’s canal
- Apex of the outflow pathway is at Schwalbe’s line and base is at the scleral spur.
- Inner wall of the TM faces the anterior chamber
- Outer wall of the TM is attached to the inner wall of Schlemm’s canal
- Involved with homeostasis
Describe Trabecular Meshwork non-pigmented
- Anterior portion is usually less pigmented
- The non-filtering portion of the meshwork
Describe Trabecular Meshwork pigmented
- As fluid flows through here, posterior TM tends to collect pigment with age as it is shed from other structures e.g. the iris
What are the Trabecular meshwork 3 components histologically from posterior to anterior
- Uveal Meshwork (inner)
- Corneo-scleral Meshwork (central)
- Juxtacanalicular tissue/Cribriform layer (outer)
Describe the uveal meshwork
- Large pores exist between tissue and lamellae
- Contributes little resistance to aqueous outflow
- Thickness highly variable between eyes
- Cells of the outer layers of TM acts as pre-filters and are aggressively phagocytic
Describe the corneo-scleral meshwork
- Makes up bulk of TM
- Inner layers arise from ciliary muscle fibre insertions and outer layer arise from scleral spur
- Cells of outer layers of the TM act as pre-filters and are aggressively phagocytic
Describe the juxtacanalicular tissue/cribriform layer
- Outer portion of trabecular meshwork
- Outer aspect formed by endothelial cells, lining inner wall of Schlemm’s canal
- Principle site of aqueous outflow resistance
Describe scleral spur
- Protrusion of sclera into anterior chamber
- Connects with TM anteriorly and ciliary body band posteriorly
Describe ciliary body band
- Ciliary face between the peripheral iris and scleral spur
- Can be observed through gonioscopy
Describe canal of schlemm
- Circular vessel and oval shape in cross-section
- Lies parallel and continuous with outer aspect of trabecular meshwork
- Function: venous channel for aqueous drainage
What structures do you observe with wide open grading scale
- Ciliary body band
What structures do you observe with Grade I
- Schwalbe’s line
What structures do you observe with Grade II
- Trabecular meshwork
What structures do you observe with Grade III
- Scleral spur
What structures do you observe when it’s closed
- No structures
- Schwalbe’s line not visible
What does the aqueous humour consist of
- Consists of clear liquid
- Contains: electrolytes, glucose, amino acids, ascorbic acid, dissolved gases & waste products
Describe what the aqueous humour and its dimensions
- Constantly in motion & entire volume is replaced every 1-2 hours
- Rate of formation 2 µL/min
- Volume of Anterior Chamber = 0.2 mL
- Volume of Posterior Chamber = 0.06 mL
What is the function of the aqueous humour?
- Maintains the shape of the globe constant
- Maintains the pressure of the eye and corneal nourishment
Describe the aqueous humour production
- Active Secretion
- 80% is secreted by non-pigmented layer of ciliary epithelium by metabolic pump independent of IOP - Passive Secretion
- 20% is produced by diffusion & ultrafiltration dependent upon blood pressure, plasma oncotic pressure and IOP
Describe the aqueous humour outflow
- Flows from ciliary processes through posterior chamber
• Pars plicata
• Pars plana - Through the pupil
- Flows to anterior chamber where it is drained from the eye
Explain the aqueous drainage of: trabecular (conventional pathway)
- 90% drained through TM into Schlemm’s canal
- Aqueous flows from Schlemm’s canal to collecting channels and venous plexi
- Exits the eye through the episcleral veins
Explain the aqueous drainage of uveoscleral (unconventional pathway)
- Aqueous passes through ciliary muscle into supraciliary anteriorly and suprachoroidal posteriorly spaces
- Drained by venous circulation in the ciliary body, choroid and iris roots
Describe the physiology of intraocular pressure and its measurement
- Normal IOP = 15.9 mmHg
- Physiological IOP differs in patients
- Fluctuates and elevates in morning and diurnal variation
- Goldman equation determines IOP in mmHg
IOP is determined by:
- Rate of aqueous secretion
- Resistance to aqueous outflow
- Level of episcleral venous pressure
Discuss the blood aqueous barrier
- Restriction of solutes travelling from ocular vasculature to aqueous humour
- Consists of:
• Tight junctions of the ciliary processes non-pigmented epithelium
• Inner wall of Schlemm’s canal
What is the break down of the blood aqueous barrier?
- Disease (i.e. intraocular infections)
- Trauma (i.e. blunt trauma, alkali burns, corneal abrasions)
- Drug induced
- Inflammation (ie. anterior uveitis)
Describe the vitreous chamber
- Filled with gel-like vitreous body
- Makes up 80% volume of the eye
What are the three zones of the vitreous chamber and describe them briefly?
- Vitreous Cortex
- Outer zone
- Composed of tightly packed collagen fibrils - Intermediate Zone
- Fine fibres that are unbranched
- Run anterior to posterior - Cloquet’s canal
- Located in centre of vitreous body
- Has an S shape
List the vitreous points of attachment
- Ora serrata
- Vitreous base
- Anterior hyaloid
- Posterior hyaloid
- Retrolental ligament
- Retrolental space
What are the functions of the vitreous body?
- Support function for the retina
- Filling function to maintain eye shape
- Diffusion barrier between anterior and posterior segments of the eye
- Metabolic buffer
What is glaucoma and what are some types?
- Pathological loss of retinal ganglion cell axons
Types:
- Primary Open Angle Glaucoma: no identifiable cause
- Primary Closed Angle Glaucoma
- Secondary glaucoma
What is posterior vitreous detachment?
- Gel volume decreases and liquid volume increases
- Traction may detach the posterior vitreous from the retina
