aqueous production and drainage Flashcards
1
Q
What is the anterior chamber ?
A
the space behind the cornea and infront of the iris
2
Q
Where is the vast majority of the aqeuous ?
A
in the anterior chamber with a
3
Q
What is the posterior chamber ?
A
the narrow space behind the iris surrounding the lens infornt of the vitreous humoiur
4
Q
What is the diameter of the anterior chamber?
A
11.3-12.4mm
5
Q
What is chamber depth?
A
the distance between the posterior surface of the cornea and the anterior surface of the lens
-measurement is taken from the corneal apex to the apex lens.
6
Q
Who has greater depth?
A
myopes
7
Q
Who has shallower depth ?
A
hyperopes
8
Q
Why does the anterior chamber depth reduce with age?
A
because the lens continues to grow and pushes iris forward and is effectively narrowing the anterior chamber
9
Q
Why is the drainage angle important?
A
the angle between the periphery of the cornea and periphery of the iris -is the iridocorneal angle
- that part of the angle of the aqueous; leaves the eye
- so a narrowing angle could impede aqueous drainage
10
Q
What is angle-closure glaucoma?
A
when angle becomes progressively narrow
which impedes the outflow of the aqueous from behind and that leads to a raised IOP - causing damage
11
Q
What is the appearance of the aqueous humour?
A
is a transparent colourless fluid
12
Q
Where is aqueous humour derived from?
A
blood plasma
13
Q
What is an important function of aqueous humour?
A
responsible for the nutrition of the avascular lens and cornea (as they don’t have a. blood supply)
14
Q
What is another important function of the aqueous humour?
A
removes metabolic waste products
15
Q
What generates IOP?
A
Aqueous
16
Q
What is IOP determined by?
A
by the balance between aqueous production and aqueous drainage
17
Q
What rate is the aqueous humour produced?
A
at a rate of 2-3µl per min
18
Q
When are there higher rates of aqueous production?
A
during the day
19
Q
Whats is the electrolyte composition of aqueous similar to?
A
plasma
20
Q
How does the aqueous differ?
A
Differs in the composition of certain organic solutes e.g. ascorbate (vitamin c- which is an important antioxidant in the aqueous to reduce the risk of oxidative damage in the lens) and lactate levels are higher
21
Q
What do the cornea and lens (aqeuous and plasma) do?
A
metabolise glucose primarily through anaerobic glycolysis - don’t get through an oxidative breakdown.
-They then generate high levels of lactate and that lactate is eliminated as a waste product from aqueous
22
Q
What is another important thing about the aqueous and plasma?
A
contains very low levels of protein and it is less than <1% plasma.
-It is maintained by the blood-aqueous barrier - as proteins are large and scatter light which is not good for an optical system.
23
Q
What are the low levels of protein for?
A
to reduce the intraocular scatter
24
Q
What happens if the inflammation of the anterior uvea (iris/ciliary body)? (the eye is inflamed)
A
causes a breakdown of the blood-aqueous barrier leading to high levels of proteins in plasma which is referred to as ‘flare’.
25
What are the 2 important things that happen in the inflammation of the iris?
1. picture in the top right- tiny little dots - inflammatory cells - showing the structure of cornea - inflammatory debris
2. picture in left- shows flare- the breakdown of breakdown of blood-aqueous barrier- light scatter due to high levels of protein
26
What are the functions of the blood-aqueous barrier?
blood vessels of iris have tight junctions between endothelial cells- impermeable - so don't leak proteins
- Tight junctions between ciliary epithilalel cells
27
In terms of dynamics, how is the aqueous produced ?
Produced by the epithelium of the ciliary body (pars placata)
28
Where does the aqueous travel ? (dynamics)
- From the posterior chamber aqueous outflow passes through the pupil into the anterior chamber
- Aqueous drains at the irido-corneal angle
- Conventional and uveoscleral outflow pathways
29
What are the details of the aqueous dynamics when travelling ?
produced by epithelium of the ciliary body
-aqueous outflow from the cite of production (ciliary epithelium) - which then flows in the space behind the iris- posterior chamber and filters through the narrow gap between the posterior surface of the iris of lens, just towards the tip of iris then enters anterior chamber and then it drains at iridcorneal angle
30
What is the ciliary body?
the middle part of the uveal tract
31
What is the uveal tract ?
used to describe the iris, ciliary body and choroid
32
What are the 2 parts the uveal tract can be divided into ?
Anterior uvea - has iris and ciliary body
| -posterios uvea - has choroid
33
What is the ciliary body didved into ?
```
2 parts
pars plana (posterior 2/3) and pars plicata (anterior 1/3)
```
34
What is the pars plicata characterised by ?
by 70-80 radially orientated ciliary processes
35
What is the pars plicata involved in?
aqueous production
36
What are the 2 functions of ciliary body?
- Aqueous production
| - Accommodation
37
What are the 4 layers of the ciliary body ?
- Ciliary epithelium- closest to the inside of eye
- Ciliary stroma- connective tissue
- Ciliary muscle
- Supraciliaris- a thin layer that lies between the ciliary body and overlying sclera
38
What does the ciliary epithelium consist of?
double epithelial layer - 2 epithelial cells attached to each other
39
What is the ciliary processes?
ciliary epithelium with double epithelila layer
- 1 layer is pigmented = PE
- 1 layer is not pigmented = NPE
40
What is the stroma of ciliary processes?
highly vascular
| -contains numerous fenestrated capillaries- highly permeable
41
What are the characteristics of the ciliary epithelium?
- Pigmented and non-pigmented cells joined apex to apex- have basal lamina on both sides
- Both cells show features of a secretory epithelium: and those include the presence of numerous mitochondria marked infolding of the plasma membrane (increase surface area)
- Can see a variety of cell junctions are present: and are linked by desmosomes, gap junctions and tight junctions-
- gap junctions-show these cells are working as a functional unit.
42
What are more characteristics of the ciliary epithelium?
- tight junction in NPE- maintains blood-aqueous barrier hence a low level of protein within the aqueous.
- Gap junctions allow electrical and metabolic coupling between cells
- Basal folding of the plasma membrane of both cells (PE AND NPE cells) considerably increases the cell surface area for the transport of water, ions and small molecules
43
What do the PE and NPE act as?
a functional unit in aqueous production - linked via gap junctions
44
What is the firsrt event of the mechanism of aqueous production?
- As aqueous formation involves metabolically driven ion transport systems
- Firstly, ions are transported actively from the stroma into the pigmented epithelium and then via gap junctions into the NPE. (which is adjacent to the posterior chamber)
45
What is the final stage of the mechanism of aqueous production?
- those electrolytes are actively transported from the NPE into the stroma. (active transport )
46
What are the different types of ion transport proteins involved in. the mechanism of aqueous production. ?
- These ions that come in through the PE get transported via gap junctions into NPE.
- Sodium, chloride, and bicarbonate ions are then transported across the basal surface of the non-pigmented cells (Active process) - into the posterior chamber
- An osmotic gradient is created which causes osmotic water flux
47
What do the ions establish/create?
-An osmotic gradient which causes osmotic water flux- high proportion of water and el;ectrolytes/molecules
48
What forms the aqeuous humour ?
The water with its ions and dissolved molecules
49
How is aqueous production regulated?
- by the autonomic nervous system- (Adrenergic nervous system)
50
What are the 2 receptors in the ANS?
- Alpha and beta receptor
- They are present on the ciliary epithelium and is coupled to an enzyme called adenylate cyclase
- This enzyme resides within the plasma membrane
51
How can the receptor regulate rate of production?
e. g something binds to alpha 2receptor - will activate the adenylate cyclase molecule- lead to an increase of cyclin AMP molecule
- cAMP is a second messenger- transport protein - and this would effectively reduce IOP- so switch of aqueous production
52
What happens when we want to increase aqueous production?
-can stimulate the beta 2 receptor- which will inhibit the adenylate cyclase and will reduce the levels of cAMP in the cytoplasm- which will lead to an increase in IOP
53
What are most of the drugs which reduce IOP do?
- interact with adrenergic receptors e.g beta-blockers (blocks beta 2 receptor- stop simulating a particular receptor), alpha agonists.
- Carbonic anhydrase inhibitors reduce the amount of bicarbonate available for transport and thereby reduce IOP.
54
What are the 2 ways you can look into the iridocorneal angle?
- use a new lens called a gonioscope - a contrast lens which is attached tot he surface of the eye and contains an angled mirror which allows you to look into the drainage angle
- can look directly into the trabecular meshwork located within the angle
- can look at the external features of the aqueous drainage and pathway
55
Why is gonioscopy essential?
by which conditions determine whether the angle is open or close
-evaluate patients with suspected glaucoma, slight increased IOP.
56
What are the 2 aqueous pathways?
- Majority of aqueous (70-90%) drains through the trabecular meshwork/ canal of Schlemm route (conventional pathway)
- An alternative uveo-scleral pathway accounts for 10-30% of aqueous outflow (USP)
57
What is the conventional outflow pathway? (CP)
Majority of aqueous drains through the trabecular meshwork via canal of Schlemm route
58
What is the Coventional aqueous outflow pathway?
1. Trabecular meshwork
2. Canal of Schlemm
3. Intra-scleral venous plexus (veins which take the aqueous through the sclera )
4. Episcleral veins
59
What are the 3 paths the trabecular meshwork consists of?
1. Uveal trabeculae- chord like structures-looking at the extreme periphery of the cornea-back surface of the cornea - corneal endothelium- these cells enlarge
2. Corneo-scleral trabeculae- flattened sheets of collagen surrounded by cells
3. Juxta-canalicular layer- adjacent to canal of shcelmn- collector channels take the aqueous from canal of schelmn
60
What does the trabecular meshwork act as?
Trabecular meshwork acts as a filter and provides the resistance (which allows pressure to build up) for the generation of an IOP
61
What is the canal of schlemn?
- Circular venous channel- runs 360 degrees around eye
- Lined by a single layer of endothelial cells
- Endothelial cells of the inner wall of the canal of Schlemm linked by tight junctions (which means it cant go between cells)
62
How does the aqueous pass through the barrier?
through two types of pore:
- Paracellular- pores occur at cell junctions of cells- allow aqueous to pass through a controlled way
- Transcellular- pores which allow aqueous to pass through
63
How are transcellular pores formed?
they occur preferentially at sites of giant vacuole formation
- the aqueous is indenting the basal surface of the endothelial cell which lines the inner wall of the canal of schlemn
- that vacuole then starts to enlarge and breaks through to apical surface
64
Whys is the transcellular process an effective way of regulating aqueous outflow?
the process of giant vacuole formation can be regulated e.g if IOP is high, and want to reduce, will lead to increase the number of giant vacuoles and increase the number of transcellular pores which will allow more aqueous through and reduce IOP
65
What happens once the aqueous is gone through the canal of schlemn?
- taken by the collector channels
| - Collector channels drain from the canal of Schlemm into the intrascleral venous plexus
66
What are the 2 ways the aqueous joins the vascular system via collector channels?
The intrascleral venous plexus ( collection of blood vessels) drains into episcleral veins(large veins on surface of eye)
- An alternative is the presence of these aqueous veins which is a direct communication from the canal of chelmn directly up to the ocular surface and you can see the mixing of the aqueous and blood will happen on the surface of the eye- the majority of mixing will happen in the sclera
- ultimately the aqueous leaves the ey and then drains into the episcleral veins on the surface of the eye
67
What are the aqueous veins?
direct communication for the cancal of shclemn to ocular surface
68
What is the reason why the aqueous is flowing in this direction?
-A pressure gradient exists from the anterior chamber to the ocular surface (from high pressure to low pressure)
69
What is the alternative pathway for aqueous outflow?
uveo-scleral pathway
70
What happens in the uveo-scleral pathway?
- Aqueous enters the ciliary body and passes between ciliary muscle fibres into supra-ciliary and suprachoroidal spaces
- which is why it ultimately rejoining this vascular system
71
What about the conventional pathway?
- Pressure independent pathway- if IOP is high , the rate of aqueous outflow will be high.
- Uveo-scleral pathway is independent of IOP-
- Pressure lowering action of prostaglandin analogues e.g. Latanaprost is thought to be due to enhanced uveo-scleral outflow
72
How can aqueous outflow be regulated?
by active contraction of the ciliary muscle (which will open spaces in a meshwork) and also trabecular cells through modulation of their actomyosin system
- Contraction of the ciliary muscle expands the trabecular meshwork and increases outflow and decrease IOP
- Contraction of trabecular cells decreases outflow and increases IOP
73
What is glaucoma associated with?
- an increased resistance to outflow
- In primary open-angle glaucoma outflow resistance of the JCT/ inner wall region is typically higher than normal.
- May involve changes to the cells of the JCT or extracellular matrix
74
What are the downfalls in glaucoma?
Changes in outflow pathway in POAG difficult to differentiate from normal ageing changes with similar outflow facility
- “Plaque-like” deposits have been described in the JCT
- Secondary glaucomas can arise from intertrabecular spaces being blocked by cellular or non-cellular material
75
What surgery can be done to improve drainage in glaucoma ?
- Selective laser trabeculoplasty
- Trabeculectomy
- Minimally invasive glaucoma surgery (MIGS)
