O2B Tonometry

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Author:
sookylala
ID:
180765
Filename:
O2B Tonometry
Updated:
2012-10-30 11:37:41
Tags:
tonometry optometry
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tonometry
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  1. What is IOP?
    • dynamic balance between inflow and outflow of aqueous
    • AH exerts pressure on the globe-> inflation of the globe
  2. What causes an increase in IOP?
    • trabecular meshwork is clogged
    • or high production of AH at the pars plicana
  3. What are associated factors with high IOP?
    • high myopia
    • diabetes
    • hereditary
    • vascular disorders
  4. Why is a high IOP dangerous?
    • glaucoma
    • damage to ocular structures
    • blinding
  5. What is the range of IOP?
    10-21mmHg
  6. What is the population mean of IOP?
    • 15+/-3mmHg
    • 68% of population
  7. What is the acceptable difference between RE and LE IOP values?
    • <3
    • >2 than it should be investigated
  8. 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
  9. 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
  10. By how much does the IOP vary with arterial pulse oscillation?
    • 2-4mmHg
    • influenced by heart rate
  11. How does a thick cornea affect IOP?
    • more rigid surface too applnate
    • requires more pressure
    • >IOP readings
  12. How does age affect IOP?
    • and birth ~10
    • increases to adult values at around 4yo
    • slightly increases with age
  13. How does weight affect IOP?
    • more fatter= >5mmHg
    • fat pushing on globe
    • hypertension
  14. How does rx affect IOP?
    • high myopia 
    • >1.5-2mmHg
  15. How does lid squeezing affect IOP?
    3-8mmHg
  16. How does gender differ in IOP?
    • female >40% 
    • during menstruation
    • more bloated
    • increased weight
  17. How often is IOP measured?
    • young: 3 years
    • borderline results: last appointment of the day, return 1-2 weeks
  18. What is the principle behind applanation?
    Area deformed kept constant- measure applied force
  19. What is the principle behind indentation?
    force kept constant: measure deformation
  20. 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
  21. What is a Schiotz tonometer?
    • uses 5.5g/7.5/10g weights
    • mechanical plunger on a sleeve
  22. 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
  23. What is a differential Schiotz Tonometer?
    uses empiracally derived nomograms to get a better estimate of IOP
  24. What is a tono-pen XL?
    • indentation tonometer
    • modern schiotz with disposable rubber sheaths
  25. 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
  26. 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
  27. 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
  28. What is a keeler pulse air?
    hand held NCT
  29. 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
  30. What is the protocol for NCT recording?
    • take 3 readings- dont take averages
    • record time of day
    • record instrument used
  31. 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
  32. What is a Pascal tonometer?
    • 10mm head moulds to cornea: constant curvature
    • less affected by corneal thickness and age
    • >2mmHg than Goldmann:
  33. By how much do the readings between a Pascal tonometer and a GAT differ?
    Pascal is 2mmHg more 
  34. What is a rebound tonometer?
    • moving object momentarily collides iwth eye and motion parameters are monitored
    • >IOP: >deceleration of probe
    • no anaesthetic
  35. What is a GAT?
    Measures force per unit area required to flatten cornea= IOP
  36. 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
  37. How many readings do you take for a GAT and what range do they have to be in?
    • 3 readings
    • +/-1mmHg
  38. When do you take the reading for the GAT? 
    inner mire touches other inner mire
  39. 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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