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Eye Disease > GLAUCOMA 2 - tonometry + pachymetry > Flashcards

GLAUCOMA 2 - tonometry + pachymetry Flashcards

1
Q

why is early detection of glaucoma needed?

A

treatment is more effective
allows for identification and follow up of at risk px’s

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2
Q

which is the only treatment that is effective in glaucoma?

A

lowering IOP

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3
Q

demographic and clinical factors that can affect IOP in long term

A
  • age
  • race (african)
  • genetics
  • gender (women)
  • myopia
  • systemic HTN and diabetes
  • ocular disease
  • corneal characteristics
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4
Q

which ocular disease can raise IOP? (an example)

A

pigment dispersion syndrome

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5
Q

which ocular disease can lower IOP? (an example)

A

retinal detachment

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6
Q

what are the 2 factors that affect IOP measurement?

A

corneal thickness
corneal curvature

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7
Q

steep corneas…

A

over estimate true IOP

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8
Q

flat corneas…

A

under estimate true IOP

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9
Q

according to ocular HTN treatment study, does a thick or thin cornea pose as a risk factor for OAG?

A

thin

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10
Q

on average, do people with ocular HTN have thin or thick corneas?

A

thick

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11
Q

on average, do people with NTG have thin or thick corneas?

A

thin

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12
Q

how do you measure CCT?

A

pachymetry

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13
Q

what is the suggested correction factor for IOP measurement using CCT?

A

~0.2 - 0.7 mmHg per 10um difference from an average CCT

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14
Q

what is the average CCT?

A

535um

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15
Q

what are the 3 ways pachymetry measures CCT?

A
  1. optical
  2. interferometric
  3. ultrasound techniques
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16
Q

what is the gold standard for CCT measurement>

A

hand-held ultrasound pachymeter

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17
Q

what are 2 examples of hand-held ultrasound pachymeter?

A

pachmate
PachPen

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18
Q

why can’t you use hand-held ultrasound pachymeter on everyone?

A

need to use anaesthetic and touch the px’s eye

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19
Q

what frequencies do ultrasound pachymeters operate at?

A

20-50MHz

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20
Q

how do ultrasound pachymeters work?

A

they emit short acoustic pulses and detect reflections from anterior and posterior surfaces of the cornea

CCT is calculated from the measured time-of-flight between the reflections and the accepted speed of sound

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21
Q

what is the accepted speed of sound in the cornea?

A

1636-1640 m/s

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22
Q

name 6 sources of error in pachymetry?

A
  • decentration
  • oblique angle
  • central compression by the probe
  • effects of local anaesthetic on cornea up to 10um difference
  • variation in the speed of sound between healthy and diseased tissue
  • inter and intra observer variability
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23
Q

what are some short term factors that affect IOP (there are BARE)

A
  • time of day
  • cardiac cycle
  • body position
  • blinking, force blinking or hard squeezing
  • fluid intake
  • px holding their breath
  • accommodation
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24
Q

what is the normal diurnal range in IOP?

A

3-5 mmHg

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25
Q

what is the glaucomatous average diurnal range?

A

13mmHg

26
Q

what diurnal range is considered pathological?

A

10 mmHg or greater

27
Q

why should you repeat IOP measurements at diff time of day?

A

most people’s IOP higher in morning, but some people can have afternoon peak instead

28
Q

what is phasing?

A

measuring IOP over 24 hours

29
Q

at what time of day is IOP the lowest?

A

middle of the night (lines up with BP)

30
Q

what is the average variation in cardiac cycle?

A

2-3 mmHg

31
Q

how many readings of blood pressure should you take?

A

3-4

32
Q

what is the effect of body position on IOP?

A

erect to supine = 0.3 - 6 mmHg increase

33
Q

when you are upside down, what is the increase in IOP in normals AND glaucomatous px’s

A

30-35 mmHg

34
Q

why wouldn’t you get glaucoma from yoga (being upside down)?

A

blood pressure in your head also increases therefore provides some sort of protection

35
Q

how does regular blinking affect IOP?

A

increases IOP by 3 mmHg

36
Q

how does forced blinking affect IOP?

A

increases IOP by 10 mmHg

37
Q

how does hard squeeze blinking affect IOP?

A

IOP can go up to more than 50 mmHg

38
Q

how does REPEAT hard squeeze blinking affect IOP?

A

potentially lower IOP

39
Q

how does drinking water affect IOP ?

A

increases IOP by up to 3 mmHg with max effect at 20 min

40
Q

how does drinking alcohol affect IOP?

A

decreases IOP by up to 3 mmHg with max effect at 5 mins

41
Q

how does dirnking coffee affect IOP?

A

increases IOP by up to 3 mmHg with max effect at 20 min

42
Q

how does holding your breath affect IOP?

A

increases IOP by roughly 5-20mmHg
IOP CAN DOUBLE !

43
Q

how does sustained accommodation affect IOP?

A

reduction by about 3 mmHg

44
Q

how does the onset of accommodation affect IOP?

A

can increase IOP

45
Q

what are the advantages of NCT?

A
  • not operator dependent
  • repeat measures do not affect IOP
  • no anaestheic
  • can be used by para-professionals
  • little (if any) risk of cross infection
  • can be quicker
46
Q

disadvantages of NCT

A
  • accuracy
  • need multiple measures
  • initial cost
  • opthalmologists don’t like NCTs but optoms still use them LOOL
47
Q

NICE 2017 guideline on NCT and referral

A

if IOP is >= 24 mmHg is the only sus finding - DO GAT before referral

48
Q

what is the regression to mean effect?

A

the second reading will be more likely to measure a lower IOP when the px is examined in hospital

49
Q

how does the reichart ocular response analyser work?

A

determines the total corneal resistance and includes combines effects of CCT, rigidity and hydration

50
Q

how does Dynamic Contour Tonometry work?

A

used on slit lamp like GAT
IOP sampled continuously
said to produce readings unaffected by CCT

51
Q

how does iCare “rebound” tonometry work?

A

magnetised probe is launched towards the cornea
probe consists of a magnetized steel wire shaft covered with round plastic tip at the end
probe hits the cornea and bounces back

52
Q

how many readings does the iCare tonometer take?

A

6 measurements

53
Q

how does iCare tell the IOP ??

A

a solenoid inside the probe is used to detect motion and impact, the probe slows down faster as the IOP increases

THE HIGHER THE IOP, THE SHORTER THE DURATION OF IMPACT

54
Q

what 3 questions were asked in the iCare tonometer questionnaire?

A

1 - how comfortable did you find this test
2 - how anxious did the test make you feel
3 - could you descrive the test in 3 words?

55
Q

what should patients be told before performing NCT?

A

loosen neck ties and dont hold your breath

56
Q

how many readings should you take for NCT>

A

4 per eye

57
Q

what is the normal inter-eye difference in IOP?

A

<4mm

58
Q

what is suspect inter-eye difference in IOP ?

A

5-7 mmHg

59
Q

what is considered abnormal inter-eye difference in IOP?

A

> 8 mmHg

60
Q

what is the referral speed for suspect glaucoma?

A

routine

61
Q

what is the referral speed for IOP >40 mmHg

A

emergency (within 24 hours)

62
Q

what is the referral speed for IOP between 35-40 mmHg with visual field loss?

A

urgent (1 week)

Eye Disease (48 decks)

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