tonometry to measure IOP - clinical Flashcards
What contact tonometers are there?
-goldmann applanation tonometer
-perkins tonometer
-pascal tonometer
What priniciple does IOP work under? What is the calculation used?
the imbert-fick prinicple,
weight applied (of tonometer) + surface tension (of tear film meniscus) = pressure x area x corneal rigidity
What determines the IOP?
-rate of aqueous production
-rate of aqueous outflow
what is aqueous humour made of?
plasma
what is aqueous humour produced by?
the pars plicata of the ciliary body
what us the rate of aqueous production and what % of the anterior chamber volume per minute is this?
-1.5-4.5 microlitres/min
-approx 1-1.5% of the anterior chamber volume per minute
where does the aqueous humour go once its been produced?
passes from the posterior chamber between the iris and lens into the anterior chamber where it circulates in a convection current as cornea is cooler so moves down and iris is warmer so moves up
In what ways does the aqueous humour drain out?
- trabecular meshwork into the Schlemm’s canal and onto the episcleral veins
- uveoscleral pathway
- a very small amount drains through the iris back into the posterior chamber
what do tonometers measure?
The amount of force required to cause a corneal deformation produced by a given amount of force
what units is IOP measured in?
millimetres of mercury (mmHg)
What does the imbert-fick principle directly state?
IOP = W weight of tonometer (g)/ A applanted area (mm squared)
what causes the potential for error in contact tonometry?
-a thin cornea
-thick cornea
-astigmatism > 3 D
-inadequate fluorescein
-too much fluorescein
-irregular cornea
-tonometer out of calibration
-elevating the eyes >15
-pressing on the eyelids/ globe
-observer bias
how does non-contact tonometry work?
- air is sent towards the eye increasing in speed over time used to flatten the cornea
- simultaneous beam of light is shone at the eye
- when an area of 3.6mm is applanted, light is reflected back to the machine is at its maximum
- this allows for IOP to be calculated
how is NCT different to Goldmann?
in NCT:
-high upfront cost
-take up alot of space
-repeat findings do not affect the result
-no anaesthetic
how is iCare carried out?
- use a small light sterile probe tp indent the eye by 1.8mm with no anaesthetic
- the probe touches the eye for 0.1 seconds
- time taken for the probe to move out the machine, touch the cornea and return is measured and the IOP is calculated
- 6 measurements are taken on the iCare
whats the main downside of iCare?
the probes are expensive at nearly £1 each
what do you need to do when recording the results of an IOP test?
-record the device used
-time of day
-eye tested
-each valid result including the unit
-average results
what is a normal range of IOP?
10-21mmHg (4-7% of individuals have an IOP of over 21mmHg without having glaucoma)
how is IOP affected by age?
it increases slightly with age
what is the average upper IOP for over 70s
23mmHg
what can an IOP of >21mmHg indicate?
glaucoma
what can an IOP of over 30 cause?
high eye pressure causing blood vessels in the retina to block in retinal vein occlusion
what does low IOP cause?
choroidal or retinal detachment
what can low IOP be a sign of?
-inflammation
-fluid loss from the eye following a penetrating injury
what could unequal IOPs in the eyes be a sign of?
-glaucoma
-retinal detachment
-inflammation in one eye
what short term factors cause IOP to be increased?
-blink
-eye movement
-accommodation
-tight neck ties
-breath holding
what medium term (minutes or hrs to happen and reverse) factors cause IOP to increase?
-caffeine
-smoking
-sitting to lying
-water intake
-corticosteroids
what medium term (minutes or hrs to happen and reverse) factors cause IOP to decrease
-alcohol
-beta blockers
-cannabis
-heroin
what long term factors cause IOP to be affected?
-season as IOP increases in winter
-age
-sex
whos IOP should we meausure?
those at high risk of glaucoma:
-certain ethnic groups e.g. african - caribbean groups that have first degree relatives with glaucoma
-those over age 40
-thinner corneas
-myopia of >6D
-diabetes
-systemic hypertension
-topical/ systemic steroid takers
