L45 Aqueous and Vitreous Humour

Question 1

Where can aqueous humour be found?

Answer 1

It can be found in the anterior segment. This segment consists of both an anterior and posterior chamber.

Question 2

Where can vitreous humour be found?

Answer 2

Vitreous cavity which may be referred to as the posterior SEGMENT.

Question 3

What is the aqueous humour?

Where is it derived from?

Is it produced and drained constantly?

Is it in constant motion?

What does it do?

Answer 3

A clear, colourless fluid

Blood plasma

Yes

Yes

Provides nutrients to the lens, cornea and trabecular meshwork as well as remove waste products of metabolism.

Question 4

Why does the aqueous humour have virtually no proteins in it/

Answer 4

Generally, proteins are big molecules and so would scatter light.

Question 5

What is the aqueous humour produced by?

Answer 5

The ciliary body.

Question 6

What generates intra-ocular pressure (IOP)?

Answer 6

Aqueous Humour. The IOP is determined by the balance between aqueous production and drainage.

Question 7

Why is there a need for intraocular pressure?

Answer 7

Adequate IOP is necessary for maintaining structural integrity and normal optical function of the eye.

Question 8

What can an abnormal intraocular pressure lead to?

Answer 8

Glaucoma and a loss of vision.

Question 9

How can glaucoma be treated?

Answer 9

Pharmacological suppression of aqueous secretion OR enhanced drainage.

Question 10

Vitreous humour fills what volume of the interior eye?

Answer 10

80% (approximately 4 ml volume).

Question 11

What are the potential functions of the vitreous humour?

Answer 11

Helps maintain shape of the eye.

Supports retinal attachment

Storage area for metabolites of the retina and lens

Viscoelastic -i.e. shock absorber protecting retinal tissue during eye movement.

Question 12

What is vitreous humour in contact with (and so interacts with)?

Answer 12

The lens, Ciliary Body and Retina.

Question 13

Is the Vitreous humour always transparent?

Answer 13

No it is not always transparent.

Question 14

What are the three fundamental components of the Vitreous humour?

Answer 14

Collagen Fibres (mainly type II, but some IV & IX)

Proteoglycans ( Protein core + glysosaminoglycan (GAG) - GAG in vitreous humour is hyaluronic acid (HA))

Water (This is the primary component - the vitreous is mainly water)

Question 15

Describe the Vitreous Humour

Answer 15

Vitreous is widely spaced matrix of collagen fibrils and HA molecules

Spaces are filled with water molecules, mostly bound to HA

It is a gel-like structure.

Collagen-HA matrix makes up only 1% of vitreous by weight

99% water – constantly replaced by diffusion, mainly from aqueous

Question 16

How can floaters be produced in the vitreous humour?

Answer 16

Collagen fibrils can aggregate and if bundles are large enough produce “floaters”

Question 17

What produce Hyaluronic acid (HA) and collagen in the vitreous humour?

Answer 17

Hyalocytes ( these also have a phagocytic capcity - as in that they can also function as phagocytes).

Question 18

Where in the Vitroeus humour are the hyalocytes found?

Answer 18

In the vitrous periphery (known as vitreous cortex), adjacent (just next to) the retina.

Question 19

What is the canal of Cloquet?

Answer 19

In the early stages of embryology, there was an artery that went from the back of the eye (optic nerve head) to the lens- called the hyaloid artery. As the eye begins to form that artery regressed (basically it is gotten rid of). The canal of Cloquet is the remnants of that.

“Site of embryonic hyaloid artery. Site of first vitreous to form (primary vitreous)- outside canal is secondary vitreous. Primary and secondary vitreous are separated by remnants of artery wall

Question 20

Describe where in the vitreous is the collagen fibre density as its highest.

Answer 20

Collagen fibrils in vitreous cortex denser than in inner vitreous

Question 21

Why are regional subdivisions of the vitreous hard to see and how can they be imaged?

Answer 21

Due to the transparency of the vitreous humour they are hard to see.

They can be imaged using dark field optics.

Question 22

What is the vitreous cortex?

Answer 22

The vitreous membrane (or hyaloid membrane or vitreous cortex) is a layer of collagen separating the vitreous humour from the rest of the eye. At least two parts have been identified anatomically. The posterior hyaloid membrane separates the rear of the vitreous from the retina.

Question 23

What is the anterior hyaloid membrane (AHM)?

Answer 23

The anterior surface of vitreous is a distinct layer of dense collagen called the anterior hyaloid membrane (AHM)

Question 24

What is the patella fossa?

Answer 24

The cup shaped depression in the anterior hyaloid membrane (AHM) which is occupied by the posterior surface of the lens.

Question 25

What makes up the vitreous base?

Answer 25

Strong attachment of anterior vitreous at pars plana epithelium and with inner limiting membrane of retina at ora Serrata.

Question 26

Is collagen very dense at vitreous base?

Answer 26

Yes and therefore forms a distinct layer!

Question 27

How does Vitreous humour change with age?

Answer 27

The change relates to the relative amounts of gel and liquid compartments of the vitreous liquid compartment.

Where:

Gel refers to parts of vitreous with collagen and HA matrix

Water refers to parts with no collagen

At birth, vitreous is all gel – liquid compartment increases throughout life

Furthermore,

Collagen is present as fibrils in young vitreous however in older vitreous, it is present in bundles (floaters).

Question 28

How do floaters affect the retina?

Answer 28

They form a shadow upon the retina.

Question 29

What is Posterior Vitreous detachment?

How does it occur?

How does it occur to?

What symptoms may the patient notice and why are these caused?

Answer 29

A pathology of the vitreous humour.

As we get older, the water level gets to a particular point and so the gel cannot maintain its shape. Thus it collapses in upon itself, pulling away from the back.

Common in older eyes (65% of people over 65) however, can occur in young patients if they are particularly myopic.

(Important to note there is no change in vitreal volume- just rearrangement of gel and water components).

Some patients can be asymptomatic whilst others may notice flashes of light particularly in the periphery. These flashes come about because there can sometimes be abnormal areas of attachment between the vitreous and underlying retina, such that when it collapses it tugs on the retina slightly. This tug on the retina generates that flash of light.

They might also notice an increased number of floaters.

Question 30

What is photopsia?

Answer 30

Photopsia is the presence of perceived flashes of light in the field of vision. It is most commonly associated with: posterior vitreous detachment

Question 31

Is Posterior Vitreous Detachment dangerous?

Answer 31

Not really, it can happen and not lead to any other underlying issues however sometimes the pulling away from the retina can lead to a tear in the retina.

Question 32

How can Posterior Vitreal Detachment be problematic?

Answer 32

Patient experiences a pronounced circular floater (Weiss ring) essentially in the location of the blind spot - this floater consists of avulsed (the word means pulled or torn away) glial tissue from around the optic nerve head. (Basically as the vitreous is detaching it pulls this tissue from around the optic nerve head off and this tissue becomes a massive floater).

PVD can tear the retina – liquid vitreous can then leak behind photoreceptors leading to retinal detachment

Also, detached vitreous can “tug” on retinal blood vessels and produce haemorrhage.

Vitro-retinal traction could in worse case scenario also lead to a hole in the macular (known as Macular hole) which obviously reduced vision.

Question 33

What is OCT and what does it stand for?

Answer 33

Optical coherence tomography (OCT) is a non-invasive imaging test. OCT uses light waves to take cross-section pictures of your retina. With OCT, your ophthalmologist can see each of the retina’s distinctive layers.

Question 34

What are some developmental abnormalities that may be seen?

Answer 34

Persistent hyaloid artery - in normal people it regresses completely so you see no sign of it but in people with developmental abnormalities, remnants of it may be seen:

Mittendorf dot - is when a remnant of the hyaloid artery is seen on the posterior surface of the lens.

Bergmeister’s papilla- is when a remnant of the hyaloid artery is seen on optic nerve head.