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OV2004 clinical skills > principles of tonometry > Flashcards

principles of tonometry Flashcards

1
Q

what is intraocular pressure ?

A

. pressure exerted by fluid inside the eye in the walls of globe
. positive intraocular pressure required to maintain the shape of the eye

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2
Q

what is the pressure determined by ?

A

. pressure is largely determined by the balance between aqueous production and outflow §

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3
Q

what is the distribution of IOP within the population ?

A

. mean of IOP = 15.7mmHg
. standard deviation = 2.5mmHg
. 95% of population within 2SD of mean , so < or equal 21mmHg
. so 11-21 is normal range

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4
Q

when should Goldmann applanation tonometry be used ?

A

optometrist should recheck IOPs in excess of 21mmHg using GAT

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5
Q

is IOP constant ?

A

. IOP is determined by 3 factors , which are all fluctuating independently over differing timescales

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6
Q

what are the 3 factors that IOP is determined by?

A

1 . the amount of fluid within the eye
2 . external forces acting upon the eye, including the tension within the ocular walls
3 . the intraocular volume

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7
Q

what are demographic and clinical factors that can affect IOP ?

A

1 . age : elevation with increasing age, especially in people of African descent. Related to other factors changing with age e.g. systemic hypertension , diabetes
2 . race ( higher in non glaucomatous black population )
3 . genetics
4 . gender : IOP 1-2mmHg higher in older women
5 . myopia : is associated with higher IOP

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8
Q

what are non-demographic and non- clinical factors that can affect IOP?

A
  1. IOP pulsates with cardiac cycle , so from one moment to next IOP can vary by as much as 3-4mmHg
  2. also affected by lid squeezing , apprehension , breath holding , posture , accommodation , caffeine ( lowers ) , alcohol ( lowers) and water ( higher )
  3. time of day
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9
Q

what is normal time of day (diurnal range) ?

A

normal range 3-5mmHg

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10
Q

what is glaucomatous average

diurnal range?

A

glaucomatous average range is 13mmHg

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11
Q

what is considered pathological diurnal range?

A

anything greater than 10mmHg

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12
Q

what are the 2 groups of patients in terms of IOP peak?

A

. most higher in morning but some patients have afternoon peaks
. repeat IOP measurement at a different time of day
. always record time of day

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13
Q

why is IOP clinically important ?

A

. Raised IOP is one of the major risk factors in glaucoma
. reducing IOP can slow down the progression of optic neuropathy
. early detection is beneficial

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14
Q

what does Glaucoma cause?

A

. glaucoma causes optic nerve fibres to atrophy

  • optic neuropathy
  • visual field defect
  • possible blindness
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15
Q

what is manometry ?

A

. it’s an intraocular measurement
. pressure sensor inserted into the eye
. not popular with patients

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16
Q

what is indentation tonometry ?

A

1 . probe is inserted into surface of eye
2 . the amount by which the globe is indented is proportional to the intraocular pressure inside eye and ocular rigidity
. Schiotz tonometer is mostly used

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17
Q

how does Schiotz tonometer work?

A

. patient lies down
. anesthesie the cornea
. lower plunger into eye ball
. footplate rests gently onto the cornea
. conversion tables , supplied with the instrument, translate scale readings into estimates of IOP

18
Q

what are the draw backs of schiotz tonometry ?

A

. assumes everyone has same ocular rigidity
. tonometer weighs > 16.5g, therefore raises IOP during measurement
. anaesthetic required

19
Q

what is tonopen ?

A

. uses a combination of applanation and indentation
. transducer senses indentation produced by probe
. digital readout
. portable ,accurate and easy to use
. anaesthetic required

20
Q

what is Icare tonometer ?

A

. uses dynamic or rebound tonometry
. a magnetised probe with a round plastic tip at the end is launched towards the cornea
. probe hits the cornea, decelerates and bounces back
. probe decelerates more rapidly if IOP is high
. no anaesthetic is required

21
Q

what is Imbert-Fick principle ?

A

. all applanation tonometry is based on this principle
. principle states that for a spherical container which has an infinitely thin , perfectly elastic and dry surface, equilibrium is achieved for an object placed in contact when

pressure = force/area

22
Q

why does the Imbert -Fick principle not apply to the cornea ?

A

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

23
Q

how can we make the Imbert-Fick principle apply to eye ?

A

. forces acting downwards is force by tonometer and surface tension (t)
. force acting against probe is rigidity ( K ) and pressure ( Pt)
. when the diameter of the circular area of applanation is between 3mm and 4mm then K and t are approximately equal and cancel each other
.Pt=F/A

24
Q

what is method A of applanation tonometry ?

A

. apply a known force
. measure the area applanated
. e.g. tonomat

25
Q

what is method B of applanation tonometry ?

A

. increase force until

. known area is applanated

26
Q

what is the circular area of GAT ?

A

. the circular area of applantion is 7.35mm square ( dimater = 3.06 mm )
. as diameter is between 3-4 mm , we can assume that surface tension of fluid and rigidity of the cornea cancel out
. 3.06 mm gives the huge advantage that 1g force = 10 mmHg IOP and no conversion tables are needed

27
Q

what is the principle of GAT?

A
  1. instil fluorescein
  2. tonometer head is pushed forward and makes contact with the cornea
  3. around the edge of tonometer head , there will be a circle of fluorescein where tonometer head touches the cornea this is the applanated area
  4. keep pushing forward on tonometer head until size of applanated area is 3.06mm
  5. biprism in tonometer head splits fluorescein ring of applanated area into two semi circles known as mires separated by 3.06mm
  6. full applanation is when the diameter of circular flattened area = 3.06mm
28
Q

what is the clinical technique of GAT ?

A
  1. use disposable probe
  2. position patient
  3. anaesthetise cornea
  4. apply fluorescein
  5. set 1.0gm or 1.5gm force
  6. inset cobalt blue filter
  7. bring probe in contact with cornea by moving the slit lamp forward ( looking from each side and above to keep probe centred on cornea)
29
Q

what happens if mires are overlapping ?

A

. reduce pressure

30
Q

what happens if mires are thin ?

A

add more fluorescein

31
Q

what to do if one semi-circle is bigger?

A

. if one semi-circle is bigger than the other , remove probe from eye and raise or lower slit lamp in direction of bigger circle
. if mires are off to the side, move horizontally towards them to centre up

32
Q

what is perkins tonometer ?

A

. hand held

. look through viewing hole

33
Q

what is the the theory behind non-contact tonometers?

A

. theory is the same as applanation tonometry, but puff of air is applanating force
. air puff of gradually increasing force applied. either measure time to achieve applanation, or force of air when applanation achieved

34
Q

what are advantages of non-contact tonometers?

A

. no physical contact with cornea
. no anaesthetic required
. can be performed by non optometric staff

35
Q

how does keeler Pulsair work ?

A

. handheld
. uses transducer to sense air pressure at moment of applanation
. has an infrared emitter to one side of device and an infrared detector to the other side
. when infrared light shines onto the cornea , light bounces in all directions
. when a puff of air is applied to flatten , infrared light bounces of this flattened surface to hit infrared detector
. area applanated is 3.06mm

36
Q

disadvantages of non-contact tonometer ?

A

. require multiple readings

37
Q

what is IOP measurement influenced by?

A

. corneal thickness
. corneal curvature
- steep corneas over-estimate true IOP
- flat corneas under-estimate true IOP

38
Q

what is average corneal thickness ?

A

. average CCT is approximately 0.54mm

. but CCT can range from around 0.44mm to 0.64mm

39
Q

what is the relationship between corneal thickness and IOP?

A

. thin corneas - underestimate IOP

. thick corneas - overestimate

40
Q

when is GAT closest to true IOP ?

A

. when corneal thickness is around 0.52 mm

41
Q

how to measure corneal thickness?

A

. pachymetry

42
Q

how to decide who to refer for primary open angle glaucoma?

A

. there is optic nerve head damage on stereoscopic slit lamp
. there is a visual field defect consistent with glaucoma
. IOP is 24mmHg or more using GAT
. need to check IOP ( GAT ) , fields and evaluate disc

OV2004 clinical skills (12 decks)

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