Card Set Information
What is IOP?
dynamic balance between inflow and outflow of aqueous
AH exerts pressure on the globe-> inflation of the globe
What causes an increase in IOP?
trabecular meshwork is clogged
or high production of AH at the pars plicana
What are associated factors with high IOP?
Why is a high IOP dangerous?
damage to ocular structures
What is the range of IOP?
What is the population mean of IOP?
68% of population
What is the acceptable difference between RE and LE IOP values?
>2 than it should be investigated
What is the diurnal variation of IOP and why does it vary?
40% of population
the eye is more hydrated in the morning
How does IOP vary when either lying down or in supine position?
this is because the heart is at the same level as the head
By how much does the IOP vary with arterial pulse oscillation?
influenced by heart rate
How does a thick cornea affect IOP?
more rigid surface too applnate
requires more pressure
How does age affect IOP?
and birth ~10
increases to adult values at around 4yo
slightly increases with age
How does weight affect IOP?
more fatter= >5mmHg
fat pushing on globe
How does rx affect IOP?
How does lid squeezing affect IOP?
How does gender differ in IOP?
How often is IOP measured?
: 3 years
: last appointment of the day, return 1-2 weeks
What is the principle behind applanation?
Area deformed kept constant- measure applied force
What is the principle behind indentation?
force kept constant: measure deformation
What is indentation tonometry?
measures depth of area of constant size that deforms the cornea
forces aqueous out at the trab mesh
: lower IOP reading
: affects indentation
: >IOP reading
>age, ocular disease, high myopia, eye surgery-scar tissue tensive, miotics thyroid disease will provide a higher IOP reading
What is a Schiotz tonometer?
uses 5.5g/7.5/10g weights
mechanical plunger on a sleeve
What are the adv and disadv of a Schiotz?
inexpensive, no slitlamp needed
small scale errors= > IOP errors
scale not linear
: use conversion table
corneal curvature assumed average
must be held exactly vertical
foot plate bears some weight/pressure
parallel errors in reading the scale
What is a differential Schiotz Tonometer?
uses empiracally derived nomograms to get a better estimate of IOP
What is a tono-pen XL?
modern schiotz with disposable rubber sheaths
What iare the adv and disadv of a tono-pen xl?
no anaesthetic required
average readings are inaccurate so you have to write down each one
What is a NCT?
central cornea applanated with air of known pressure increasing linearly with time
at the point of applanation, infrared beam is projceted onto the area and reflected maximally to a symmetrically placed telecentric receiver
time elasped for beam to return is the IOP
What are the 3 componenets of the NCT?
: delivers air pulse
Applanation monitoring system
: detects when cornea is flattened
Opticoelectronic instrument to cornea monitoring alignment system
: autopuff triggering system, recognises good readings
What is a keeler pulse air?
hand held NCT
What are the adv and disadv of an NCT>
doesnt touch eye
no alteration of corneal integrity/IOP
no risk of microbial contamination
irregular cloudy cornea- inaccurate beam cannot bounce properly of non flat surface
inaccurate if no proper fixations
1st reading 1mmhg apprenhension
lid squeezing >IOP
off axis measurement= overestimation of IOP
stage of ocular pulse
What is the protocol for NCT recording?
take 3 readings- dont take averages
record time of day
record instrument used
What is contour tonometry?
touches the cornea for a few seconds
measures pulsatile IOP directly and continuously
numeric output of IOP OCA
independent of corneal characteristics
What is a Pascal tonometer?
10mm head moulds to cornea
: constant curvature
less affected by corneal thickness and age
>2mmHg than Goldmann:
By how much do the readings between a Pascal tonometer and a GAT differ?
Pascal is 2mmHg more
What is a rebound tonometer?
moving object momentarily collides iwth eye and motion parameters are monitored
: >deceleration of probe
What is a GAT?
Measures force per unit area required to flatten cornea= IOP
What are the conditions of a GAT that are solved?
surface tension addes to the force but bending force of the cornea opposed force when area applantaed is 3.06mm
How many readings do you take for a GAT and what range do they have to be in?
When do you take the reading for the GAT?
inner mire touches other inner mire
What is a rebound icare tonometer?
used as a screening tool
mean value same as goldmann but more variability
risk of false positives