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Year 2 Investigative Techniques > Tonometry/IOP > Flashcards

Tonometry/IOP Flashcards

1
Q

What is the average CCT?

A

~530-540um
can vary by 60-70um either side

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2
Q

What time of day are IOPs higher?

A

Morning

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3
Q

On GAT, what should the difference between the eyes be at most?

A

4mmHg

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4
Q

What are the advantages of GAT?

A

Gold standard
Large research base and accepted by UK guidelines
Good illumination and magnification
Steady and secure (as on SL)
Relatively inexpensive
Can compensate for ocular pulse

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5
Q

What are the disadvantages of GAT?

A

Need a SL!
Unable to delegate
Px has to sit up
Potential to cause corneal trauma
Risk of infection transmission
Accuracy of measurement can be affected by corneal and tear film properties

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6
Q

What is the ocular pulse and what causes it?

A

IOP fluctuation in time with heartbeat
Heartbeat causes increase in choroidal volume which pushes pressure up (smaller space for fluid)

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7
Q

What is manometry?

A

Measurement of IOP by insertion of a needle into the AC

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8
Q

What is the Imbert-Fick principle?

A

Using a known area of applanation, IOP can be estimated by measuring the force needed to ‘flatten’ the cornea

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9
Q

What assumptions does the Imbert-Fick principle make?

A

The cornea is:
Perfectly elastic
Dry
Infinitely thin
Spherical

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10
Q

What factors can affect IOP measurement?

A

Corneal: thickness, biomechanics, hydration, curvature and astigmatism
Tear film: viscosity, surface tension, volume of fluid
Probe wettability

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11
Q

What is pachymetry?

A

Measurement of CCT

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12
Q

If a cornea is thicker than average, what is likely to happen to the IOP measurement?

A

Higher IOP (more resistance)

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13
Q

If a cornea is thinner than average, what is likely to happen to the IOP measurement?

A

Lower IOP (less resistance)

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14
Q

How does corneal hydration relate to corneal thickness?

A

Linearly - if the water content is higher, the cornea is thicker

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15
Q

How does corneal curvature/astigmatism affect IOP readings?

A

Steeper corneas will have higher readings bc more fluid is displaced over the area measured
Flatter corneas will have lower readings bc less fluid is displaced

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16
Q

What is the impact of corneal curvature/astigmatism on readings? Should it be taken into account?

A

Equal to or less than 1mmHg
Can be ignored most of the time

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17
Q

How can corneal biomechanics affect IOP readings?

A

More rigid corneas can cause higher readings than corneas of the same thickness which are softer

18
Q

How can changes in corneal structural integrity affect IOP measurements?

A

Weakened corneas (e.g. if had LASIK or has keratoconus) result in lower IOP readings due to curvature change and/or weakening of collagen fibrils

19
Q

How does tear film surface tension affect IOP readings?

A

Increased surface tension results in a lower IOP (reduced force attracting probe to cornea, less force used to applanate, reduced IOP)
Reduced surface tension = increased IOP

20
Q

What is the impact of tear film surface tension on readings?

A

Less than or equal to 1mmHg

21
Q

How can the wettability of the probe affect IOP readings?

A

Can change the attractive force between the probe and the tear film (reduced wettability = less force = reduced IOP)
Generally not an issue with reusable probes

22
Q

How can the amount of fluid affect IOP readings?

A

Increased fluid results in thicker mires - IOP can be increased by 4-5mmHg due to increase in attractive forces of the tear film
Reduced fluid can result in ~0.4mmHg reduction in IOP

23
Q

What is intraobserver variability?

A

Variation in measurements between the same practitioner due to a change in judgement

24
Q

What is interobserver variability? What can it cause?

A

Variation in measurement between different practitioners due to differences in judgement.
Can cause systematic errors

25
Q

Can IOP be changed if GAT is performed repeatedly within a short window of time or if the cornea is applanated for an extended time?

A

Yes - reduced

26
Q

What are the advantages of Perkins tonometry?

A

Accuracy is directly comparable to GAT
Portable
Can be used for px’s who can’t get to a SL
Can be used to hold lids open

27
Q

What are the disadvantages of Perkins tonometry?

A

Reduced mag and illumination so harder to see mire edges
Clinical guidelines for IOP apply to px’s sitting up, so measurements can be inaccurate
Reduced stability affects view of mires and increases risk of corneal scratches

28
Q

What are the advantages of NCT?

A

Non-invasive
No anaesthetic needed
Can be delegated
No risk of corneal abrasions
Reduced risk of infection transmission
Repeated measurements don’t reduce IOP
Some portable

29
Q

What are the disadvantages of NCT?

A

Expensive
Can’t use to refer
Sensitive to ocular pulse (can’t compensate)

30
Q

How does an NCT measure IOP?

A

Infrared light shone onto cornea, detector on opposite side - measures amount of light reflected (when most light is reflected is when cornea is ‘flat’). Force of air puff used to flatten cornea recorded (sometimes time also recorded) and converted into IOP.
Measurement calibration based on known pressures of fluid-filled spheres, not real eyes.

31
Q

How does a Pascal dynamic contour tonometer work?

A

Contact but non-applanation (has curved probe with sensor)
Records ocular pulse amplitude and gives a quality assessment of the measurement

32
Q

What are some advantages of PDCT?

A

Measures true IOP (not affected by corneal properties)
High precision and repeatability
Not affected by corneal surgery/treatments or meds
Compensates for ocular pulse
Non applanating

33
Q

How does iCare rebound tonometry work?

A

Contact but non-applanation
Measures deceleration and rebound time of probe x6

34
Q

What are some advantages of iCare?

A

Disposable probes
No anaesthetic needed
Within 2mmHg of GAT
Good in community care and for paramedics and paeds

35
Q

What are the advantages of tonopen?

A

Handheld and portable
Doesn’t need to be as precisely central as other methods
Poss more accurate due to smaller applanation area
Objective
Battery powered

36
Q

What are the disadvantages of tonopen?

A

Anaesthetic required
Several readings needed to counteract for ocular pulse

37
Q

How does the Ocular response analyser take corneal biomechanical measurements?

A

Uses corneal hysteresis to provide info on corneal biomechanical behaviour

38
Q

What is corneal hysteresis?

A

Difference between the pressure at which the cornea bends in and the pressure at which the cornea bends back out again

39
Q

How do the corneal biomechanical measurements help on the ORA?

A

Can correct IOP measurements more accurately than CCT alone, as all corneal properties considered

40
Q

How does the Corvis tonometer work?

A

Film the cornea during applanation, and calculates IOP from deformation of the cornea

Year 2 Investigative Techniques (9 decks)

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