Tono

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ijesc
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286246
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Tono
Updated:
2014-10-19 08:26:46
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OPTM
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tono
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  1. Define IOP
    the dynamic (steady-state) balance between aqueous formation and outflow
  2. "what" is IOP
    the aq humour exerts pressure to keep the globe inflated
  3. Aq Humour pathway
    • 1 secreted by the ciliary processes to the post. chamber
    • 2 inflow - goes through the pupil to the ant ch
    • 3 outflow - leave the ant ch through the TM to the venous system
  4. what causes IOP changes?
    any circumstance that affects the formation or outflow of aq humour
  5. what allows for normal IOP?
    equilibrium between the inflow and outflow of AH
  6. what can cause (specific) and what is the result?
    • 1 local and systemic disorders, meds (e.g. corticosteroids), ocular surgeries
    • 2 higher IOP can cause ganglion cells and NFL loss
    • 3 progression of damage causes ON damage and irreversible visual loss
  7. Normal IOP + its units
    • 1 mm/hg
    • 2 general population: 15.5+-2.5mmHg
    • 3 expect normal range: 9-21mmHg (mean +- 2SD)
    • 4 Bell curve with slight positive skew
    • 5 there is difference b/t stats and individual norms
    • 6 IOP can be influenced by corneal thickness
  8. what is ocular hypotony?
    • 1 IOP is too low
    • 2 cause: inflammation, injury
    • 3 severe/rare
  9. what is ocular hypertension and glaucoma?
    • 1 IOP too high >21mmHg
    • 2 cause is uncertain
    • 3 very common
  10. what is OHT?
    ocular hypertension
  11. define glaucoma
    a progressive optic neuropathy with raised IOP relative to the tolerance of the given eye being a major risk factor
  12. define OHT
    the presence of high IOP (>21mmHg) in the absence of optic nerve/visual field damage
  13. what is considered as high IOP?
    > 21mmHg
  14. properties of glaucoma
    • 1 no symptoms until advanced
    • 2 peripheral vision first affected; can lead to blindness
    • 3 optoms have major role in prevention and co-management
    • 4 ~3% prevalence in Aust with 50% undiagnosed
  15. What is POAG?
    primary open angle glaucoma
  16. what factors tell us it's POAG?
    • 1 IOP >21mmHg
    • 2 open filtration angle
    • 3 signs of ON damage
  17. High IOP means?
    the px doesnt necessarily have glaucoma
  18. what is NTG?
    • 1 normal tension glaucoma
    • 2 IOP is statistically normal (21mmHg or less)
    • 3 33-50% of glaucoma cases have "normal" IOP
    • 4 this IOP is not "normal" for the px
  19. conventional therapy for _______________ aims to________
    • 1 POAG and NTG
    • 2 lower IOP
  20. Glaucoma can occur _________
    with or without high IOP
  21. Other tests for glaucoma are:
    • 1 basic workup - ophthalmoscopy, visual fields
    • 2 gonio and pachymetry
  22. What is pachymetry?
    the process of measuring corneal thickness
  23. how does corneal thickness affect IOP?
    • 1 thick = overestimation
    • 2 thin = underestimation
  24. what do we observe and what are the signs of glaucoma?
    • the ON
    • 1 big C/D
    • 2 size of the disc
    • 3 colour of neural rim
    • 4 differences in RE/LE - usually asymmetrical
    • 5 ISNT rule
    • 6 NFL dropout
    • 7 haemorrhage near ON
  25. Name the condition, level of IOP and if there is VF/ON damage associated with it
  26. Determinants of IOP
    • 1 Age
    • 2 Ethnicity
    • 3 Genetics
    • 4 BP
    • 5 refractive error
    • 6 eye rubbing
    • 7 "valsalva" manouver
    • 8 obesity
    • 9 diurnal variation
    • 10 corticosteroid responders
  27. IOP variations with age
    • 1 low at birth - avg 10mmHg
    • 2 increases to adult values at ~4 years
    • 3 general increase with age during adulthood ~ <1mmHg per decade until age 70; then it decreases
    • 4 ethnicity dependent e.g. japanese decreases with age
  28. IOP variations with ethnicity
    • 1 African americans IOP > caucasians by 1.5 to 2.5mmHg
    • 2 Asian and Middle Eastern ~ = Caucasians (with exceptions)
    • 3 Japs, Eskimos, Mongolians > caucasians by 1-3mmHg
  29. IOP and genetics
    • 1 relatives have similar IOP
    • 2 IOP closer in identical twins than dizygotic twins
  30. IOP and BP
    • 1 only small effect on IOP
    • 2 normal systolic BP = 90-140mmHg
    • 3 For 50mmHG increase in BP, IOP increases by 1-2mmHg
    • 4 due to increased aqueous production (ultrafiltration)
  31. IOP and refractive error
    • 1 in axial ametropia, myopes have longer eyes than hyperopes
    • 2 myopes tend to have higher IOPs
    • 3 reason unknown - reverse relo suggested where high IOP causes refractive error
  32. IOP and eye rubbing
    • 1 increases IOP
    • 2 during measurement, retract lid onto orbit to avoid pressure on the globe
  33. IOP and "valsalva" manouver
    • 1 attempted forced exhalation while the airways (nostril and mouth) are closed
    • 2 highly variable effect on IOP
    • 3 range observed -1 to +9.5mmHg
    • 4 nervous px may hold breath - expect a smaller error than above
    • 5 pressure increase in orbital, jugular and vortex veins increase iOP
    • 6 drop in systemic pressure drops IOP
  34. IOP and obesity
    • 1 IOP higher by up to 5mmHg or more
    • 2 due to difficult positioning of px behind slit lamp --> breath holding; use perkins
    • 3 due to venous compression by peri-orbital fat restricting aq outflow
    • 4 hypertension associated with obesity
  35. IOP and diurnal variation
    • 1 causes IOP variations throughout the day (reason unknown)
    • 2 can be misleading
    • 3 waking hrs: generally high in morning, declines during the day
    • 4 high inter-individual variation; difficult to predict
    • 5 range of 5mmHg at most
    • 6 can be higher than 10mmHg and ¬†asymmetric in glaucoma/certain eye diseases
    • 7 diurnal curve is often erratic in disease
    • 8 aq prodn roughly halved during sleep; but IOP increases at this time
    • 9 diurnal variation in resistance to outflow is possibly why
    • 10 thought to be due to BP changes
    • 11 diff b/t sitting and lying down ~1-4mmHg
    • 12 basic rule: IOP depends on position b/t eye and heart vertically
    • 13 when eye is lower, IOP is higher than sitting position
  36. IOP and corticosteroid responders
    • 1 corticosteroids is the main cause of drug-induced glaucoma
    • 2 steroids administered by any route are associated with increases in IOP
    • 3 46-92% of subjects with OAG experiences increase in IOP after admin with drug
    • 4 steroid-like substances can be found in traditional and natural meds
    • 5 prolonged inhalation use is significant risk for developing
  37. What to record when measuring IOP
    • 1 pre and post corneal check
    • 2 technique used
    • 3 drugs: name, conc, gtts
    • 4 time admin
    • 5 individual measurements
  38. What is manometry
    • 1 direct measurement of IOP; all others are indirect
    • 2 cannula connected to a water column is inserted into the ant chamber
  39. Properties of Glodmann tono
    • 1 applanation tono
    • 2 gold standard
    • 3 small plastic probe (3.06mm) flattens (applanates) part of the cornea
    • 4 force required to flatten is proportional to the IOP
    • 5 displaces small amt of aq humour
    • 6 applanated area surrounded by meniscus of tears + fluorescein (circular green wedge)
    • 7 prism-split view us if 2 adj semicircles
  40. what is the diameter of the tono probe?
    3.06mm
  41. Assumptions for tono
    • 1 object is dry, flexible, perfect sphere and infinitely thin walls
    • 2 BUT the cornea doesnt meet this: moist due to TF, has a degree of rigidity, is aspheric, has finite thickness
  42. problems with tono assumptions
    • 1 moist cornea = attraction/surface tension b/t the probe and the eye = underestimation; varies with probe dia
    • 2 corneal thickness and rigidity will increase force needed for applanation = overestimation; effect varies with probe dia
    • 3 these forces cancel out when probe dia is 3.06mm/applanated area = 7.35mmsq
    • 4 error due to asphericity is unimportant
  43. What is the imbert-fick relationship?
    if a plane surface is applied to a spherical membrane to cause a flattening of surface area, the pressure inside the spherical membrane will be equal to the applied force divided by the area of contact
  44. Assumption about the forces on the cornea
    corneal bending force and surface tension cancel
  45. what is CCT + its effect?
    • central corneal thickness
    • avg = 530.1 +- 30.5 um
    • there is a wide range of mean +- 2SD = ~476-612mmHg
    • overestimation
    • max potential error ~-5 to +7 mmHg
  46. what are the relative forces when CCT is avg, above avg and below avg?
    • s=force exerted on cornea
    • b=force exerted by the cornea
    • avg: b=s
    • below: b<s
    • above: b>s
  47. relationship b/t IOP and CCT
    • nonlinear
    • max potential error = -5 to +7mmHg in normals
  48. effect of corneal curvature
    • magnitude of error is very low
    • flatter corneas easier to applanate
    • tono will underestimate IOP: 0-1mmHg error over 3D of corneal curvature
  49. avg CCT
    42.50+- 1.5D
  50. abt the goldmann tono
    • 1 assume standard width of mires
    • 2 desired endpoint is inner to inner
    • 3 take 3 readings
    • 4 record all readings to establish personal repeatability and reveal effects of px apprehension etc
  51. Other tono methods
    • 1 non-contact tono
    • 2 tonopen
    • 3 pascal tono
    • 4 iCare tono
    • 5 ocular response analyser
    • 6 pneumotonometry
    • 7 Schiotz
  52. what is NCT?
    non contact tono
  53. Abt the NCT
    • 1 puff of air applanates the cornea (based on imbert-fick principle)
    • 2 in private practice more popular than GAT - commercial reasons= quicker + little skill required
    • 3 many new NCT are fully auto = position px + push button, no anaesthetic required, less infection control requirements
  54. NCT process
    • 1 central cornea applanated by puff of air of known pressure increasing linearly with time
    • 2 usually lasts less than 25ms - eliminates errors due to eye mvts + blink reflex (~100ms)
    • 3 in the 25ms, puff is weak and increases in strength until applanation is detected
    • 4 applanation detected by infra-red light
    • 5 oblique infra-red beam aimed at cornea will max reflect to symm placed telecentric receiver --> cuts off air
  55. Other NCT facts
    • 1 electronic clock measures time lapse b/t initiation of pistol mvt and max light on the infra-red receiver (stops the piston)
    • 2 time elapsed is proportional to IOP
    • 3 IOP read off digital display panel - objective measure
    • 4 NCT has self-monitoring 'quality' function which warns of poor confidence in any particular measurement
  56. tonopen
    • surface and annular ring alignment only when corneal surface is plane (not convex or concave)
    • alignment triggers voltage signalling the force needed to achieve this
  57. pascal tono mechanism
    • 1 also known as dynamic contour tono (DCT)
    • 2 uses contour matching based on Pascal's law
    • 3 SL mounted
    • 4 touches cornea for a few secs with load and spreads across greater area
    • 5 measures pulsatile IOP directly and continuously
    • 6 numeric output of ocular pulse amplitude (OPA)
  58. What is OPA?
    • ocular pulse amplitude
    • measured in pascal tono
  59. what is DCT
    • dynamic contour tono
    • aka pascals tono
    • first introduced early 2000s
  60. pascal tono features
    • 1 probe is 7mm, with rad of curvature = 10.5mm
    • 2 this allows it to 'fit' over most corneas (contour matching)
    • 3 allows pressure to be estimated without corneal interference
  61. iCare tono
    • 1 also known as rebound tono
    • 2 small blunt probe is projected at eye
    • 3 IOP determined by how speed of probe changes after impact
    • 4 no anaesthetic required
    • 5 first available early 2000s
  62. ocular response analyser
    • 1 first available in 2005
    • 2 basically functions as NCT
    • 3 difference - 2 applanation pts are considered
    • 4 it looks at applanation from undisturbed state (from initial convex state) and that after concavity is achieved
  63. what is ORA
    ocular response analyser
  64. the NCT looks at applanation from the...
    undisturbed state: from initial convex state
  65. ORA calculations
    • calculates 4 parameters using P1 and P2
    • -IOPg: similar to standard NCT reading; the avg of P1 and P2
    • -IOPcc: an IOP measurement without corneal error
    • -CH: diff b/t P1 and P2; represents corneal viscoelasticity
    • -CRF: represents material property related to elastic resistance
  66. what is IOPg?
    • goldmann correlated IOP
    • avg of P1 and P2
    • similar to NCT standard reading
  67. what is IOPcc?
    • corneal compensated IOP
    • IOP measurement without corneal error
  68. What is CH?
    • corneal hysteresis
    • diff b/t P1 and P2
    • represents corneal viscoelasticity
  69. What is CRH?
    • corneal resistance factor
    • material property related to elastic resistance
  70. What is GAT?
    goldmann applanation tonometry
  71. why is GAT the standard?
    • 1 reliability and precision
    • 2 manometry is too invasive
    • 3 most tonos affected by same errors but to diff degrees
    • 4 NCT more affected by CCT than GAT
    • 5 DCT seems unaffected by CCT, but GAT better
    • 6 short term IOP variation dependent on tono type
  72. tonography
    • based on observation that repeated tono results in lowering of IOP
    • rate of decrease influenced by outflow facility
  73. Indentation tono
    • 1 Schiotz
    • 2 based on depth of corneal indentation made by plunger of known weight
  74. pneumotonometry
    • central sensing device measures force required to bend cornea
    • can be used on scarred corneas
  75. thin corneas associated with _______ IOP __________
    • greater¬†
    • than is measured by tono
    • people with thin corneas may obtain less accurate readings
  76. what can influence CCT?
    • 1 corneal oedema
    • 2 time of day
    • 3 diseases e.g. keratoconus, keratoglobus and pellucid marginal degeneration
    • 4 refractive surgery
  77. CCT measurements are important in:
    • 1 refractive surgery (pre and post)
    • 2 in the diagnosis of keratoconus, keratoglobus and pellucid marginal degeneration
    • 3 a direct correlate of the physiologic condition of the corneal endo - a poorer function of endo = ocular oedema + thicker CCT
  78. pachymetry
    • 1 ultrasonic = gold std
    • 2 operates at freq 20-50MHz
    • 3 emits short acoustic pulses and detects reflections from the ant and post surfaces of the cornea
    • 4 CCT calculated from the measured time of flight b/t these reflections
    • 5 requires anaesthetic
    • 6 can be influenced by variations in sound wave due to degrees in corneal hydration
  79. instruments that can measure CCT
    • ultrasonic pacymetry = gold std
    • scanning slit topography (Orbscan II)
    • Scheimpflug photography (pentacam)
    • measurements b/t instruments can vary significantly

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