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Aqueous humour and IOP Flashcards

1
Q

what is aqeuous humour

A

transparent watery fluid

total voloume = 0.2ml

it is in the anterior and posterior chambers

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2
Q

what is aqeuous humour secreated by

A

the non pigmented epithelium of the pars plicata cillary body

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3
Q

what is aqeuous humour made from

A
  • it is made from 98% water
  • amnio acids (1% plasma)
  • electrolytes , sodium , pottasium , chloride , hydrogencarbonate
  • ascorbic acid
  • gluecose

midly alkalia (ph 7.4)

contains immunoglobulins (has a role in immune defence)

amino acids and gluecose are important for nourishing tissues

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4
Q

what are the functions of aqueous humour

A

maintains iop- maintains the structures of the anterior and posterior chambers

protects against uv light - by absorbing some of the light from sunlight

contains immunoglobins - has a role in immune defence

the eye is an immune privledged site- their is no communication between the blood in the body and the contents of the eye - it has a blood retinal barrier and a blood aqueous barrier - i.e. if their is an infection in the blood the eye is protected

transparent - allows the passage of light through the lense to reach the retina

nurition to avascular ocular tissues - posterior cornea
trabecular meshwork
lens
anterior virteous

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5
Q

where is the aqueous humour produced

A
  • non- pigmented glandular epithelium of pars plicata of the cillary body
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6
Q

where is aqeuous humour secreated into

A

actively secreated into the posterior chamber

then flows freely around the iris into the anterior chamber

produced at 2ul/min

entire volume replaced in 100mins

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7
Q

describe the cillary bodies anatomy

A
  • goes all the way around the edge of the eye
  • it has 3 parts
    cillary body stroma - tissue within the cillary body

cillary muscle - important for accomodation because it allows us to change the shape of the lens

cillary epithelium - lines the outside of structures

  • pigmented
  • non pigmented
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8
Q

what epithelium secreates aqeuous humour

A

the non pigmented epithelium activley secreates aqeuous humour

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9
Q

what is the cillary body connected

A

connected to zonules - the zonules of zinn which attach to the lens

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10
Q

describe the site of aqeuous humour secreation

A

the non pigmented epithelium is the most superfical epitheium

it has a flat part called the pars plana and then a bumpy part called the pars plicata

comes through the posterior chmaber along the anterior surface of the lens and then circulates in the anterior chamber comes down towards the angle and then drains

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11
Q

what are the three main functions of the cillary body

A

3 main functions

accomodation

aqueous humour production

zonule maintence

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12
Q

when the cillary msucle contracts what happens to the shape of the lens

A

lens becomes more convex which allows you to accomodate on near objects

the cillary muscle comtracts and the zonular fibres relax

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13
Q

what is the blood aqueous barrier

A

important to stop mixing of the blood into the aqueous - loss of transparency- immune protection

selectively permeable membrane formed by the non- pigmented cillary body epithelium (same structure that secreates aqueous humour ) and endothelium of iris vasculature

tight junctions between adjacent cells keep barriers continous

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14
Q

what are the three processes involved in the production of aqueous humour

A
  • active secretion - 80- 90%

ultrafiltration

simple diffusion

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15
Q

how is active secretion involved in the production of atp

A

uses atp to move fluid from the plasma in blood vessels

accounts for the majority of aqueous production

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16
Q

how is ultrafiltration involved in aqueous humour production

A
  • hydrostatic pressure ( force exerted across a membrane by a fluid) between that in the vessel and in eye favours movement into the eye , oncotic pressure favours the reverse
  • hp in the veins in the cillary body is greater than the pressure in the posterior chamber - hydrostatic pressure favours transfer of fluid across the semi permeable membrane into the posterior chamber - oncotic pressure favours the reverse
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17
Q

how is simple diffusion involved in aqueous humour production

A
  • movement of particles freely along a concentration gradient
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18
Q

what are the two routes of aqueous outflow

A
  • conventional route - trabecular pathway - 90%- Drains via the trabecular meshwork
  • unconventional route - uveoscleral pathway (10%) - uvea = the cillary body and the iris
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19
Q

describe the conventional/trabecular route of aqueous outflow

A
  • 90%
  • drains through the trabecular meshwork
  • flows though the canal of schlemm

drains into the episcleral veins

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20
Q

describe the unconentional/uveoscleral route (10%)

A

aqueous passes through the anterior face of the cillary body/iris root into the cillary muscle and then suprachoroidal space

  • drained by the uveal and scleral veins
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21
Q

what is aqueous humour

A
  • aqueous humour is predominantly water (98%)
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22
Q

which avascualr tissues does the aqueous humour provide nutrition to

A
  • aqueous humour provides nutrition to posterior cornea , trabecular meshwork , lems and anterior virteous
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23
Q

what process is aqeuous humour predominantly produced by

A
  • active secretion (90%) - non- pigmented epithelium of the pars plicata of the cillary body
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24
Q

why is the blood aqueous barrier significant

A
  • signifciant in preventing blood entering the aqueous to maintain transparency and immune privlegde
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25
Q

what are the two routes for aqueous outflow

A
  • conventional /trabecular route (90%)

- unconventional/uveoscleral route (10%)

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26
Q

what is the conventional route of aqueous outflow via

A

the conventional route is via the trabecular meshwork, canal of schlemm then the episcleral veins

27
Q
  • describe the structure of the trabecular meshwork
A
  • 3 paralell sheets of connective tissue
  • fenestrated
  • pores allow for movement of aqueous

drains into schlemms canal - goes around the angle behind the trabecular meshwork

  • majority of resistance to aqueous outflow (75%)- remaining 25% is beyond schlemms canal - vascular resistance
28
Q

what is the canal of schlemm

A

endothelium lined vessel

runs circumfrientially around the globe at conreoscleral junction

no direct communication between the canal and meshwork

29
Q

where is the anterior chamber angle

A
  • the angle of the anterior chamber is the angle between the cornea and the iris
  • when the angle is open the drainage system works - able to drain via the uvea scleral / trabecular meshwork pathway
  • when the angle closes, aqueous becomes unable to drain

asess angle clinically using gonioscopy

30
Q

what is the normal range of intraocular pressure

A

in normal people it is 10-21 mmhg

31
Q

what are the three factors that maintain introcular pressure

A

3 maintaining factors

  • aqueous production- i.e. you are producing too much - the pressure will go up
  • aqueous outflow - if it is not draining properly the pressure will go up
  • pressure in the episcleral vessels (25%)- conventional pathway is pressure dependent - uncoventional is not
32
Q

which factors affect the iop

A

extertional- HR, BP , RR

Straining (valsava) (increasing intrathoracic pressure under pressure)

prolonged exercise

enviormental

cold air

general anesthesia

structural

postural

diurnal variation (peaks late morning) - iop = highest late morning

lid and eye movement

central corneal thickness

33
Q

what can cause an increase in iop

A

steroids

diabetes

hyperthermia

hypertension

34
Q

what can cause a decrease in iop

A

pregnancy

hypothyroid

hiv

35
Q

how do you measure iop

A 
applanation tonometry 
(gold standard) - in practice you put flourcein on the patients eye - you illuminate it with a blue light and their is a probe

electronic indendation tonometry

rebound tonomery

pneumatonmetry

dynamic colour tonometry

nb- these are all dependent on the shape of the cornea - therefroe people with abnormally shaped corneas can produce unreliable results

36
Q

what is glaucoma

A

progressive optic neuropathy - damage to the nerve and retinal nerve fibre layer

( you can have normal pressure glaucoma)

but is usually secondary to a raised iop

a high iop on its own would be ocular hypertension

characterstic optic nerve head changes

visual field loss (arcuate scotoma) - curved area of visual field loss

37
Q

why is iop important in glaucoma

A

it is the only modifiable risk factor

38
Q

what are other risks of galucoma

A

family history

ethnicity

degeneration of the trabecular meshwork

hypertension

steroid use

refractive error - highly myopic patients more likely to have glaucoma

diabetes

39
Q

what are the three subtypes of glaucoma

A

primary open angle glaucoma

acute angle closure glaucoma

normal tension glaucoma - you dont have a raised iop

40
Q

how does glaucoma develop

A

drainage canal blocked

build up of fluid

increased pressure damages blood vessels and optic nerve

41
Q

how does glaucoma affect the optic disc

A

increased cupping - fewer nerves run through the optic disc

vertical thinning and notching of neural rim - neural rim= space between the inner cup and the surrounding disc

optic atrophy - optic disc becomes pale as damage progresses

42
Q

how do you treat glaucoma

A

for chronic open angle glaucoma and acute angle closure glaucoma

you can treat it using drops/tablets/iv fluid

surgical

increases aqueous outflow

reduces aqueous production

43
Q

what is hypotony

A

iop too low

eye shape is not maintained

can cause folding of retina resulting in visual loss

causes

  • trauma
  • post glaucoma surgery

retinal dettachemnt (disruption to blood retinal barrier)

inflammatory

44
Q

what does the trabecular meshwork form

A

the trabecular meshowrk forms part of the conventional route of aqueous outflow and is formed by three parallell sheets of fenestrated tissue

45
Q

why is the anterior chamber angle significant

A

the anterior chamber angle is significant in iop maintence because a closed angle can block aqueous drainage - angle closure glaucoma

46
Q

how do we asess the angle clinically

A

we asess the angle clinically using gonioscopy

47
Q

what is the usual range of iop

A

iop usual range is 10-21 mmhg and is affected by aqueous production , outflow and episcleral resistance

48
Q

what is glaucoma

A

glaucoma is progressive optic neuropathy usually (but not always_ secondary to raised iop and produces characteristic optic disc changes from damage to the retinal nerve fibre layer

49
Q

what can hypotony result in

A

loss of vision

50
Q

where is aqueous humour produced

A
  • non- pigmented epithelium of the pars plicata
51
Q

the majority of aqeuous production is via which process

A

active seretion

non pigmented epithelium has a high concentration of mitchondria

52
Q

what are the four avascular structures that are nourshied by aqueous humour

A

lens, zonules, posterior cornea , anterior virteous ( is most in commununication with aqueous humour)

53
Q

what are the functions of the cillary body

A
  • produce aqueous
  • maintain zonules
  • accomodation
54
Q

what maintains the integrity of the blood aqueous barrier

A
  • tight junctions
55
Q

what is the normal range for iop

A

10-21 mmhg

56
Q

what condition would cause a fall in iop

A

pregnancy

57
Q

describe the connective tissue of the trabecular meshwork

A

the connective tissue layers are fenestrated - allows aqueous to drain in the conventional route

58
Q

describe the conventional route of aqueous outflow

A

90% is via the conventional route through the trabecular meshowrk, canal of schlemm and episcelral veins

59
Q

which of the routes of aqueous outflow is pressure dependent

A

pressure in the episcleral veins reduces aqueous outflow - uvea and scleral veins in the uncoventional route is unaffected by pressure

60
Q

which veins are used by the conventional route and which are used by the unconventional route

A

conventional route = episcleral veins

uveoscleraal/unconventional route = uveo and scleral veins

61
Q

what would you not expect to see in the anterior chamber

A

blood (due to blood aqueous barrier)

this is called a hyphema

62
Q

what is anterior uveitis

A

imflammation of the anterior uvea

uvea = iris and cillary body

  • primary site of imflammation is the anterior chamber and/or is anterior virteous

breakdown of blood aqueous barrier- extrusion of proteins, flare, presence of WBCs cells

often present with painful, photophobic, red eye and blurred vision

iop is ofetn raised - wbcs block the trabecular meshwork- blocking outflow and increasing the iop

63
Q

how does anterior uveitis cause a raised iop

A

inflammatory cells may block the trabecular meshwork

increased aqeuous viscosity may reduce outflow

secondary structural changes such as peripheral anterior sychnaie or posterior synchenaie with iris bombe may cause mechanical blockage

corticosteroid treatment is the most common cause of iop elevation in uveitis - corticosteroids may promopt iop elevations both by an increase in aqueous secretion and a reduction in outflow

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