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Cs 2 > principles of tonometry > Flashcards

principles of tonometry Flashcards

1
Q

Why is a positive intraocular pressure required in the eye?

A

To maintain the shape of the eye

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2
Q

What is intraocular pressure largely determined by?

A

Aqueous production & outflow (drainage

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3
Q

What is the mean (average) IOP?

A

15.7mmHg (approx 16mmHg)

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4
Q

What is the (one) standard deviation of 16mmHg IOP?

A

2.5mmHg

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5
Q

What is the mean IOP (16mmHg) plus 2 standard deviations (2.5mmHg)?

A

21mmHg

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6
Q

What % of the population fall into the category of ± 2 SD of 16mmHg & IOP?

A

95%

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7
Q

What should be done if IOP is22mmHg or above with non contact tonometry?

A

Check goldmann application tonometry

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8
Q

What method of checking IOP is the gold standard?

A

Goldmann Applannation Tonometry

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9
Q

List 5 factors that have an effect on IOP.

A
  • age
  • gender
  • genetically determined
  • myopia
  • race
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10
Q

How does age have an effect on IOP?

A

IOP rises by 1-2mmHg between 20-70 years old

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11
Q

Why does IOP increase by 1-2mmHg from the age of 20-70?

A

Due to systolic blood pressure increase as we age

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12
Q

How does gender affect IOP?

A

IOP is 1-2mmHg higher in older women

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13
Q

How does genetics affect IOP?

A

Family history can result in a lower or higher IOP

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14
Q

How does myopia affect IOP?

A

Myopia is associated with higher IOP

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15
Q

How does race affect IOP?

A

IOP is higher in the non glaucomatous black population than caucasian population

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16
Q

What causes the IOP to pulsate/vary?

A

The cardiac cycle, as the choroid fills with blood and is emptied, IOP varies

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17
Q

What is the term that describes the way in which time of day affects IOP?

A

Diurnal range (varies throughout the day)

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18
Q

What is a normal diurnal range?

A

3-5mmHg

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19
Q

What is considered to be a glaucomatous diurnal range?

A

13mmHg

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20
Q

What diurnal range is considered to be pathological (assess for glaucoma)?

A

10mmHg

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21
Q

When in the day does IOP peak (2)?

A

1) It is higher in the mornings (following an afternoon dip especially in males)
2) Some people have afternoon peaks (varies throughout the day)

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22
Q

How do we measure someone’s diurnal IOP range?

A

Repeat IOP measurements at different times of the day

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23
Q

What must always be recorded when measuring IOP, especially when monitoring the diurnal range?

A

The time of the day

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24
Q

How does drinking (a substantial amount of) water affect IOP?

A

It increases IOP by up to 3mmHg, with maximum effects at 20 minutes.

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25
Q

How does alcohol affect IOP?

A

It decreases IOP by up to 3mmHg, with maximum effects at 5 minutes.

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26
Q

How does coffee affect IOP?

A

It increases IOP by up to 3mmHg, with maximum effects at 20 minutes

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27
Q

How do you carry out a provocative test for glaucoma?

A

drink one litre of water and measure IOP after 15 minutes and/or lying down in the dark for 1 hour
a rise in IOP of 8mmHg suggests a risk of glaucoma due to decreased aqueous outflow

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28
Q

Why it is important to measure IOPs?

A
  • raised IOP is one of the major risk factors of glaucoma
  • glaucoma causes optic nerve fibres to atrophy/die
    eg, optic neuropathy, visual field defects & possible blindness
  • reducing IOP can slow down the progression of optic neuropathy
  • early detection is beneficial
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29
Q

What are the 4 different types of IOP measuring techniques?

A
  • manometry
  • indentation tonometry
  • applanation tonometry
  • non contact methods
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30
Q

How is manometry carried out?

A

A probe/pressure sensor is inserted into eye and pressure is measured
(not popular with patients)

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31
Q

In indentation tonometry, what is the amount of indentation proportional to?

A

The pressure inside the eye

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32
Q

What is indicated when the cornea is more indented in indentation tonometry?

A

Low pressure

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33
Q

What is indicated when the cornea is not indented much in indentation tonometry?

A

High pressure

34
Q

What is the tool in indentation tonometry which creates the indentation?

35
Q

What is the name of an indentation tonometer?

A

Schiotz tonometer

36
Q

What is the probe attached to with the indentation tonometer?

A

A scale where you can add variable amounts of weight

37
Q

What must be taken into account which resists the indentation during indentation tonometry?

A

The rigidity of the globe

38
Q

What is the calculation of the pressure with indentation tonometry (e.g. when using the schiotz tonometer)?

A

Pt = F/a + b.s

Pt = pressure
F = force applied
a & b = constants
s = scale (amount of indentation occurred)

39
Q

Why is the calculation for indentation tonometry not accurate?

A

b & a vary between people as everyone has different scleras i.e. variance between eyes

40
Q

Where is indentation tonometry still used?

41
Q

What is the name of the dynamic or rebound tonometer?

A

Icare tonometer- basically with this device the probe hits the cornea and bounces back.

42
Q

What is not required when using the icare tonometer?

A

Anaesthetic

43
Q

How does the icare tonometer work?

A

a magnetised probe is launched towards the cornea and bounces back
very rapid, px won’t feel the probe hit there cornea

44
Q

What occurs with the icare tonometer with someone with a high IOP?

A

probe will decelerate more rapidly

45
Q

What occurs with the icare tonometer with someone with a low IOP?

A

probe will decelerate less rapidly

46
Q

What does the tonopen require the px to have done before use?

A

Anaesthesise of the cornea

47
Q

How does the tonopen work to measure IOP?

A

It has an applanation service that flattens the cornea & a tiny plunger protruding microscopically in the centre, the transducer senses the indentation produced by the probe.

48
Q

What are the 3 advantages of the tonopen?

A

Its portable, accurate and easy to use

49
Q

Lists the 3 instruments used for indentation tonometry

A
  • schiotz tonometer
  • iCare tonometer: dynamic or rebound tonometry
  • Tonopen: involved applanation and indentation
50
Q

State the theory behind the imbert-fick principle:

A

for a spherical container which has an infinitely thin, perfectly elastic and dry surface, equilibrium is achieved for an object placed in contact when:
Pt = F/A

Pt = pressure (within the spherical object)
F = force applied 
A = area of applanation
51
Q

State 3 reasons why the cornea/eye does not fulfil the required criteria of the imbert-fick principle and state the modified imbert-fick calculation that applies to a surface like the cornea

A
  • the cornea is not infinitely thin
  • the cornea is not perfectly elastic
  • the corneal surface is moistened by the tear film

Pt = F/A - k + t

Pt = pressure 
F = force applied 
A = area of applanation 
k = rigidity of eyeball (which pushes tonometer AWAY) 
t = surface tension (force to tears which pushes tonometer TOWARDS the eye)
52
Q

which two components of the eye exert their own pressure when doing applanation tonometry

A
  • force of the cornea k (pushes away)

- tear film/surface tension t (pulls towards)

53
Q

When are k and t of the imbert-fick principle cancelled out (i.e. approx. equal) and state the new calculation

A

when the diameter of the circular area of applanation is between 3-4 mm

then rigidity of eyeball and surface tension are approx. equal to one another and cancel each other out

Pt = F/A

Pt = pressure 
F = force applied 
A = area of applanation
54
Q

How much is the circular area of applanation with the goldmann tonometer, and what does this mean?

A

7.35mm2
diamater = 3.06m

this means the imbert-fick principle applies, as the diameter is between 3-4mm, so we can assume that surface tension of fluid and rigidity of the cornea cancel put

55
Q

What is the advantage to the area of applanation being 3.06mm in goldmann tonometry?

A

1g = 10mmHg (if the diameter is 3.06mm)

so no conversion tables are needed. Basically easy maths.

56
Q

What does the observer look through when using the goldmann tonometer?

A

The tonometer head

57
Q

When does the circular flattened tonometer head of the goldmann tonometer become larger, and when do you know that you have full applanation

A

When you increase the force on the area applanated

full applanation is when the diameter of the circular flattened area = 3.06mm

58
Q

What does the bi prism tonometer head do in goldmann tonometry?

A

It splits the fluorescein ring into two by 3.06mm (a top half and bottom half) when the tonometer head is correctly centred on the cornea.

59
Q

What happens to the two semicircular images on the bi prism goldmann tonometer when force is applied to the cornea?

A

The two semicircular images get closer together

60
Q

How do you know you have achieved full applanation with the bi prism goldmann tonometer?

A

When the inside edges of the two semicircles are just touching

61
Q

State the full method for carrying out goldmann applanation tonometry in practice

A
  • use a disposable probe
  • position the patient on the chin rest of the slit lamp
  • anaesthetise the cornea
  • apply flourescein using a flourescein paper strip
  • set 1.0gm or 1.5gm force
  • insert cobalt blue filter in slit lamp, so that the flourescein glows
  • bring probe into contact with the cornea by moving the slit lamp forward (looking from each side and above to keep the probe entered onto the cornea)
  • when correctly centred on cornea, the semicircle will be of equal size to each other.
62
Q

What must you do if you observe one semicircle to be larger/unequal to the other semicircle i.e. not centred on the cornea?

A

Move the tonohead/probe towards the larger semicircle, to equalise them WITHOUT touching the eye/cornea

63
Q

What is the hand held version of the goldmann tonometer called?

A

Perkins tonometer

64
Q

What are advantages of the perkins tonometer?

A
  • can use on a wheelchair px

- domiciliary visits

65
Q

What are 3 advantages of non-contact tonometers?

A
  • no ‘physical’ contact with cornea = less risk of corneal abrasion
  • no anaesthetic required
  • can be performed by non optometric staff
66
Q

State a disadvantage to the non-contact tonometer

A

It can over estimate high or low pressures

67
Q

What are the three main components used to measure IOP in a non-contact tonometer and how does it measure the IOP?

A
  • IR emitter
  • Air puff nozzle
  • IR detector

IR light is bounced off the cornea and picked up by the IR detector by the other side of the cornea, but a spherical cornea will bounce some IR away and it won’t reach the IR detector, so thats why the air force is applied to the cornea by the air puff nozzle, so an area of the cornea is applanated/flattened. the flattened bit of the cornea emits a greater amount of IR light into the detector, so the tonometer knows the applanation has occurred

68
Q

What is the name of the handheld non-contact tonometer and which type of measurement of IOP does it use?

A

keeler pulsair

uses transducer to sense the air pressure at the moment of applanation (i.e. force of air used when applanation achieved)

69
Q

Why are multiple readings required when using a non-contact tonometer

A

at least 3-4 readings should be taken due to cardiac cycles as it has peaks and troughs, so needs to be averaged and time should be noted

70
Q

What two things are IOP measurements influenced by?

A
  • corneal thickness

- corneal curvature

71
Q

How does a steep cornea affect IOP estimates?

A

It causes an over estimate IOP

72
Q

How does a flat cornea affect IOP estimate?

A

It causes an IOP underestimate.

73
Q

How does a thin central cornea affect IOP estimate?

A

It causes an IOP underestimate.

74
Q

How does a thick central cornea affect IOP estimate?

A

It causes an over estimate IOP.

75
Q

What is the average central corneal thickness and the range?

A

approx. 0.54mm

range: 0.44mm - 0.64mm

76
Q

What is used to measure corneal thickness in optometric practice?

A

pachymetry

77
Q

What must be instilled into the px’s eye before carrying out pachymetry and contact tonometry?

A

anaesthesia

78
Q

What three things must you check on the anaesthetic, before using it on a px

A
  • expiry date
  • drug concentration
  • batch number
79
Q

How long does it take for the cornea to anaesthetise and how does it last for?

A

1 min to anaesthetise and lasts for 25 mins

80
Q

List corneal anaesthetics that can be used

A

ester type:

  • tetracaine (amethocaine) hydrochloride 0.5%
  • oxybuprocaine hydrochloride (benoxinate) 0.4%
  • proxymetacaine hydrochloride 0.5%

Cs 2 (23 decks)

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