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What is IOP?
- dynamic balance between inflow and outflow of aqueous
- AH exerts pressure on the globe-> inflation of the globe
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What causes an increase in IOP?
- trabecular meshwork is clogged
- or high production of AH at the pars plicana
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What are associated factors with high IOP?
- high myopia
- diabetes
- hereditary
- vascular disorders
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Why is a high IOP dangerous?
- glaucoma
- damage to ocular structures
- blinding
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What is the range of IOP?
10-21mmHg
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What is the population mean of IOP?
- 15+/-3mmHg
- 68% of population
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What is the acceptable difference between RE and LE IOP values?
- <3
- >2 than it should be investigated
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What is the diurnal variation of IOP and why does it vary?
- 3-5mmHg
- 40% of population
- the eye is more hydrated in the morning
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How does IOP vary when either lying down or in supine position?
- supine +2-4mmHg
- this is because the heart is at the same level as the head
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By how much does the IOP vary with arterial pulse oscillation?
- 2-4mmHg
- influenced by heart rate
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How does a thick cornea affect IOP?
- more rigid surface too applnate
- requires more pressure
- >IOP readings
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How does age affect IOP?
- and birth ~10
- increases to adult values at around 4yo
- slightly increases with age
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How does weight affect IOP?
- more fatter= >5mmHg
- fat pushing on globe
- hypertension
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How does lid squeezing affect IOP?
3-8mmHg
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How does gender differ in IOP?
- female >40%
- during menstruation
- more bloated
- increased weight
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How often is IOP measured?
- young: 3 years
- borderline results: last appointment of the day, return 1-2 weeks
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What is the principle behind applanation?
Area deformed kept constant- measure applied force
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What is the principle behind indentation?
force kept constant: measure deformation
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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
- ocular rigidity: affects indentation
- >rigidity: >IOP reading
- >age, ocular disease, high myopia, eye surgery-scar tissue tensive, miotics thyroid disease will provide a higher IOP reading
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What is a Schiotz tonometer?
- uses 5.5g/7.5/10g weights
- mechanical plunger on a sleeve
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What are the adv and disadv of a Schiotz?
- Adv:
- mech simple
- inexpensive, no slitlamp needed
- Disadv
- supine
- 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
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What is a differential Schiotz Tonometer?
uses empiracally derived nomograms to get a better estimate of IOP
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What is a tono-pen XL?
- indentation tonometer
- modern schiotz with disposable rubber sheaths
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What iare the adv and disadv of a tono-pen xl?
- adv:
- quick
- no anaesthetic required
- disav:
- <IOP readings
- average readings are inaccurate so you have to write down each one
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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
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What are the 3 componenets of the NCT?
- Pneumatic system: delivers air pulse
- Applanation monitoring system: detects when cornea is flattened
- Opticoelectronic instrument to cornea monitoring alignment system: autopuff triggering system, recognises good readings
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What is a keeler pulse air?
hand held NCT
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What are the adv and disadv of an NCT>
- ADV:
- doesnt touch eye
- no alteration of corneal integrity/IOP
- no anaesthetic
- extremely rapid
- no risk of microbial contamination
- objective
- DISAD:
- 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
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What is the protocol for NCT recording?
- take 3 readings- dont take averages
- record time of day
- record instrument used
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What is contour tonometry?
- slitlamp mounted
- touches the cornea for a few seconds
- measures pulsatile IOP directly and continuously
- numeric output of IOP OCA
- independent of corneal characteristics
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What is a Pascal tonometer?
- 10mm head moulds to cornea: constant curvature
- less affected by corneal thickness and age
- >2mmHg than Goldmann:
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By how much do the readings between a Pascal tonometer and a GAT differ?
Pascal is 2mmHg more
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What is a rebound tonometer?
- moving object momentarily collides iwth eye and motion parameters are monitored
- >IOP: >deceleration of probe
- no anaesthetic
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What is a GAT?
Measures force per unit area required to flatten cornea= IOP
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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
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How many readings do you take for a GAT and what range do they have to be in?
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When do you take the reading for the GAT?
inner mire touches other inner mire
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What is a rebound icare tonometer?
- used as a screening tool
- mean value same as goldmann but more variability
- risk of false positives
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