Aqueous humour and IOP Flashcards
what is aqeuous humour
transparent watery fluid
total voloume = 0.2ml
it is in the anterior and posterior chambers
what is aqeuous humour secreated by
the non pigmented epithelium of the pars plicata cillary body
what is aqeuous humour made from
- 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
what are the functions of aqueous humour
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
where is the aqueous humour produced
- non- pigmented glandular epithelium of pars plicata of the cillary body
where is aqeuous humour secreated into
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
describe the cillary bodies anatomy
- 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
what epithelium secreates aqeuous humour
the non pigmented epithelium activley secreates aqeuous humour
what is the cillary body connected
connected to zonules - the zonules of zinn which attach to the lens
describe the site of aqeuous humour secreation
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
what are the three main functions of the cillary body
3 main functions
accomodation
aqueous humour production
zonule maintence
when the cillary msucle contracts what happens to the shape of the lens
lens becomes more convex which allows you to accomodate on near objects
the cillary muscle comtracts and the zonular fibres relax
what is the blood aqueous barrier
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
what are the three processes involved in the production of aqueous humour
- active secretion - 80- 90%
ultrafiltration
simple diffusion
how is active secretion involved in the production of atp
uses atp to move fluid from the plasma in blood vessels
accounts for the majority of aqueous production
how is ultrafiltration involved in aqueous humour production
- 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
how is simple diffusion involved in aqueous humour production
- movement of particles freely along a concentration gradient
what are the two routes of aqueous outflow
- conventional route - trabecular pathway - 90%- Drains via the trabecular meshwork
- unconventional route - uveoscleral pathway (10%) - uvea = the cillary body and the iris
describe the conventional/trabecular route of aqueous outflow
- 90%
- drains through the trabecular meshwork
- flows though the canal of schlemm
drains into the episcleral veins
describe the unconentional/uveoscleral route (10%)
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
what is aqueous humour
- aqueous humour is predominantly water (98%)
which avascualr tissues does the aqueous humour provide nutrition to
- aqueous humour provides nutrition to posterior cornea , trabecular meshwork , lems and anterior virteous
what process is aqeuous humour predominantly produced by
- active secretion (90%) - non- pigmented epithelium of the pars plicata of the cillary body
why is the blood aqueous barrier significant
- signifciant in preventing blood entering the aqueous to maintain transparency and immune privlegde
what are the two routes for aqueous outflow
- conventional /trabecular route (90%)
- unconventional/uveoscleral route (10%)
what is the conventional route of aqueous outflow via
the conventional route is via the trabecular meshwork, canal of schlemm then the episcleral veins
- describe the structure of the trabecular meshwork
- 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
what is the canal of schlemm
endothelium lined vessel
runs circumfrientially around the globe at conreoscleral junction
no direct communication between the canal and meshwork
where is the anterior chamber angle
- 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
what is the normal range of intraocular pressure
in normal people it is 10-21 mmhg
what are the three factors that maintain introcular pressure
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
which factors affect the iop
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
what can cause an increase in iop
steroids
diabetes
hyperthermia
hypertension
what can cause a decrease in iop
pregnancy
hypothyroid
hiv
how do you measure iop
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
what is glaucoma
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
why is iop important in glaucoma
it is the only modifiable risk factor
what are other risks of galucoma
family history
ethnicity
degeneration of the trabecular meshwork
hypertension
steroid use
refractive error - highly myopic patients more likely to have glaucoma
diabetes
what are the three subtypes of glaucoma
primary open angle glaucoma
acute angle closure glaucoma
normal tension glaucoma - you dont have a raised iop
how does glaucoma develop
drainage canal blocked
build up of fluid
increased pressure damages blood vessels and optic nerve
how does glaucoma affect the optic disc
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
how do you treat glaucoma
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
what is hypotony
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
what does the trabecular meshwork form
the trabecular meshowrk forms part of the conventional route of aqueous outflow and is formed by three parallell sheets of fenestrated tissue
why is the anterior chamber angle significant
the anterior chamber angle is significant in iop maintence because a closed angle can block aqueous drainage - angle closure glaucoma
how do we asess the angle clinically
we asess the angle clinically using gonioscopy
what is the usual range of iop
iop usual range is 10-21 mmhg and is affected by aqueous production , outflow and episcleral resistance
what is glaucoma
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
what can hypotony result in
loss of vision
where is aqueous humour produced
- non- pigmented epithelium of the pars plicata
the majority of aqeuous production is via which process
active seretion
non pigmented epithelium has a high concentration of mitchondria
what are the four avascular structures that are nourshied by aqueous humour
lens, zonules, posterior cornea , anterior virteous ( is most in commununication with aqueous humour)
what are the functions of the cillary body
- produce aqueous
- maintain zonules
- accomodation
what maintains the integrity of the blood aqueous barrier
- tight junctions
what is the normal range for iop
10-21 mmhg
what condition would cause a fall in iop
pregnancy
describe the connective tissue of the trabecular meshwork
the connective tissue layers are fenestrated - allows aqueous to drain in the conventional route
describe the conventional route of aqueous outflow
90% is via the conventional route through the trabecular meshowrk, canal of schlemm and episcelral veins
which of the routes of aqueous outflow is pressure dependent
pressure in the episcleral veins reduces aqueous outflow - uvea and scleral veins in the uncoventional route is unaffected by pressure
which veins are used by the conventional route and which are used by the unconventional route
conventional route = episcleral veins
uveoscleraal/unconventional route = uveo and scleral veins
what would you not expect to see in the anterior chamber
blood (due to blood aqueous barrier)
this is called a hyphema
what is anterior uveitis
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
how does anterior uveitis cause a raised iop
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
