OHD 1 > Tonometry > Flashcards

Tonometry Flashcards

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1
Q

What do we use Tonometry for?

A

To measure the pressure in the eye (IOP)

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2
Q

Why do we need a normal IOP?

A

To keep the eye rigid

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3
Q

What is it called when the IOP gets too low and what is there a risk of?

A

Hypotony
Risk of :
-Retinal detachment
-Hypotony maculopathy
-Suprachoroidal Haem

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4
Q

What is Hypotony maculopathy and Suprachoroidal Haem caused by?

A

Due to stuff leaking out of vessels as IOP is not high enough

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5
Q

What is it called when the IOP gets too high and what is there a risk of?

A

Ocular Hypertension
Risk of :
-Glaucoma

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6
Q

What causes raised pressure?

A

Increased production of aqueous

Reduced outflow of aqueous
Outflow resistance (POAG) - Primary Open Angle Glaucoma

Trabecular obstruction (SOAG) - Secondary Open Angle Glaucoma

Closed angle
(PCAG) - Primary Closed Angle Glaucoma

Raised pressure in episcleral vessels

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7
Q

What should the difference in IOP be when looking at both eyes?

A

Difference should be < 5mmHg

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8
Q

What can cause a thin central corneal thickness and what can this lead to?

A

Refractive surgery
Can lead to Normotensive Glaucoma (normal tension glaucoma)

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9
Q

What can lead to a thick central corneal thickness and what cab this lead to?

A

Fuchs endothelial dystrophy
Can lead to Ocular Hypertension

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10
Q

What happens in Fuchs endothelial dystrophy?

A

Endothelial doesn’t actively pump out enough water from the corneal stroma

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11
Q

What is having raised pressure without Glaucoma called?

A

Ocular Hypertension (OHT)

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12
Q

What is having Glaucoma without raised pressure?

A

Normal Tension Glaucoma (NTG)

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13
Q

What does a higher IOP mean in terms of the risk of Glaucoma?

A

Higher risk of Glaucoma

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14
Q

What are the 2 hypotheses when it comes to IOP and Glaucoma?

A

1) Direct - Mechanic
Increase in pressure affects the structure of the optic nerve head

2) Indirect - Vascular
Increase in pressure does not allow blood vessels to perfuse the nerve with oxygen

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15
Q

What are the other tests for Glaucoma?

A

-Visual Field Test
-Optic disc assessment
-Family History of Glaucoma

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16
Q

Why do we do a Visual Field Test when examining for Glaucoma?

A

Glaucoma affects the periphery of your vision first

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17
Q

When should we use Tonometry?

A

-Routinely on patients at risk of POAG
-Over 40’s
-FH of Glaucoma
-Afro-carribeans
-Diabetics
-Myopes
-Suspicious cups

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18
Q

What is Manometry?

A

-A tube is inserted into the eye

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19
Q

Advantages of Manometry

A

-Very accurate

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20
Q

Disadvantages of Manometry

A

-Clinically unacceptable (not used in practice as it is risky and painful)

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21
Q

What is Digital Palpitation?

A

-Close your eyes
-Place 2 fingers on your lid and leave one finger resting with firm contact
-With the other finger tap the eyeball
-Finger that remains in contact should feel the eyeball poop out and in slightly (high IOP = resting finger wouldn’t move)

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22
Q

Advantages of Digital Palpitation

A

-Easy to do

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23
Q

Disadvantages of Digital Palpitation

A

-Not very accurate

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24
Q

What are Indentation Tonometers?

A

-Indents the cornea (pushes aqueous back)
-Px lies down
-Cornea needs to be anesthetised
-Rest in the instrument vertically on the cornea
-Read measurement from the scale (indentation is proportionate to IOP)

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25
Disadvantages of Indentation Tonometers
-Difficult to do and read scale -Heavy tonometer weight -Patients dislike -Patient laid flat -Scleral rigidity
26
How is scleral rigidity a disadvantage for Indentation Tonometers?
If a Px's sclera can stretch when you indent the cornea, you are not actually pushing aqueous out as the sclera can just stretch
27
What does the term distend mean?
Get bigger
28
How do Applanation Tonometers work?
-Flatten the cornea, rather than indenting it -When the device "applanates" the cornea, it creates an applanation area e.g. pushing a wet football against glass -Around the edge of the applanation area you will get a meniscus of tears (wet ring all the way around) = MIRES
29
What does flattening the cornea do, rather than indenting it?
Displaces less aqueous
30
Advantages of Applanation Tonometery
-Sceral rigidity is insignificant
31
What are the 2 ways of Applanation Tonometry?
1) Fixed Force 2) Constant Area
32
Describe Fixed Force Applanation Tonometry
-Fixed force/weight and see how big the applanation area is by measuring it -Contact diameter/applanation area is proportional to IOP
33
What are errors with Fixed Force Applanation Tonometry?
Errors due to eye movement and alignment
34
Describe Constant Area Applantion Tonometry
-Variable force/weight (g) needed to applanate a constant applanation area -Force required = IOP using back on it
35
How do you convert from grams to mmHg?
Whatever the value is in grams, multiply it by 10 to get the IOP reading 1g = 10 mmHg
36
What area is a Goldman Applanation Tonometer calibrated to applanate and what is the significance of this area?
Area = 7.35 mm^2 Diameter = 3.06 mm At exactly 3.06 mm diameter, the force needed to applanate is proportional to IOP
37
How is the tear meniscus split/ and how does this create the mires?
Split by prisms in the probe Mires are visible with fluorescein dye
38
How do you when the setup of the mires is correct?
The inner edges of each mire (superior/inferior) will be slightly touching each other
39
What do we do if the mires are not aligned?
Chase the mires -Remove tonometer from Px's cornea Move slit lamp up/down/left/right and reapply
40
What is Goldman Applanation Tonometry based on (theory)?
Based on : Force required to applanate a sphere = Pressure inside x Applanated area
41
What does this : Force required to applanate a sphere = Pressure inside x Applanated area only apply to?
Only applies to : -A dry surface -Infinitely thin surface -Perfectly elastic surface -Perfectly spherical surface -Perfectly flexible membrane
42
What are the 2 types of probes?
1) Tonosafe Disposable probe 2) Reusable probe
43
What is the procedure for Goldman Applanation Tonometry?
-Set up slit lamp -Set up patient (anaesthetic and fluorescein) -Wide, bright cobalt blue beam -Illumination at ~45o Temporal -Add some weight to drum (e.g. 1.5g =0.0529 oz 15 mmHg) -Move forward to applanate corneal apex -Adjust drum to align mires -Disengage from cornea -Read weight from drum and convert from g to mmHg
44
What are errors that can arise with Goldman Applanation Tonometry?
-Incorrect alignment (slit lamp and/or mires) -Too much/too little fluorescein -Touching lids (will push probe back) -Astigmatism - Instead of getting a spherical applanation area, it will be oval like a rugby ball
45
Advantages of Goldman Applanation Tonometry
-Easy to use with repeated use -Accurate (gold standard) -Small force on cornea -Cheap -Px's like it
46
Disadvantages of Goldman Applanation Tonometry
-Needs Anaesthetic + Fluorescein -Disinfection of reusable probes -Can damage cornea -Acquired skill -Repeated readings reduce IOP (massages aqueous out of the Trabcular Meshwork)
47
Give the procedure on how to disinfect reusable probes before use?
-Remove from the 2% sodium hypochlorite solution -Rinse well with normal saline -Wipe the probe with an alcohol swab -Rinse well with saline
48
Give the procedure on how to disinfect reusable probes after use
-Do not allow the probe to dry -Rinse well with normal saline -Place into solution of 2% sodium hypochlorite for a minimum of 10 minutes
49
What is a Perkins Tonometer?
-Hand held Goldman -Comparable accuracy -Can be used in domicillary visits/used for Px's that can't get to the slit lamp
50
What is a Schiotz Tonometer?
-Same as an Indentation Tonomter but it has a digital scale readout -Still subject to same errors
51
Is Mackay-Marg the type of tonometer or the commercial name?
Type of tonometer
52
Is Tonopen the type of tonometer or the commercial name?
Commercial name
53
What type of tonometer is a Tonopen?
Constant Area Applanation Tonometer Hand held Mackay-Marg
54
What are the 3 components of a Non-Contact Tonometer (NCT)?
Alignment system Pneumatic system Applanation detector
55
Describe the Alignment system on an NCT
-Ensures correct positioning (approximately 11 mm from the eye) -Cannot operate machine unless properly aligned (centre of the cornea)
56
Describe the Pneumatic system on an NCT
-Creates the puff of air (narrow collimated puff) -Puff of air gradually gets stronger within a few milliseconds -Indents the cornea slightly -Applanation reached before indentation
57
Describe the Applanation system on an NCT
-IR detector detects the collimated IR beam once the cornea has been flattened
58
What do modern NCTs measure?
Measure pressure of air puff at applanation
59
What is Pulsair and how does it work?
-Hand held NCT -Automatic activation system when in the right position -Measures system pressure at apllanation -Display shows running mean, so press R button (review) at the end to see individual readings
60
What are the types of Pulsair NCT?
Desktop Handheld
61
How many readings do you take with a Pulsair NCT?
Take 4 readings unless you get 2 in a row within ±2 mmHg However in labs we have been taking 3 readings
62
What type of tonometer is a Reichert Xpert?
NCT
63
How does a Reichert Xpert work?
-Manual and automatic modes -Measures system pressure -Pulse cut off at applanation -TV monitor -Results printed
64
Errors with the Reichert Xpert
-Ocular pulse -Distorted corneal surface -Eye movement -Lids/lashes -Not very good with high IOP measures
65
Why do we need to take multiple readings with the Reichert Xpert?
Because of the Px's ocular pulse It could be at the top or bottom of the pulsar range Also the heart beat could give a higher/lower IOP reading WE DON'T TAKE MULTIPLE READINGS BECAUSE THE MACHINE IS INACCURATE THE MACHINE IS ACCURATE
66
Advantages of the Reichert Xpert NCT
-Quick and easy to use -Repeated readings are possible -No contact with the cornea -Accurate as it does not massage aqueous out of the eye because it doesn't touch the eye
67
Disadvantages of the Reichert Xpert NCT
-Patients dislike -Expensive -Not good on scarred corneas (need a wet and smooth cornea) -Complicated to calibrate
68
How do Rebound Tonometers work?
-Induction coil propels a magnetized steel wire (with plastic tip) towards the cornea (you press a button to fire the needle) -Probe hits cornea and rebounds -Returning movement induces current in coil -Speed at which the needle bounces back into the tonometer is proportional to the IOP (Bouncier cornea = Higher IOP) -Records 6 measurements and automatically discards highest/lowest
69
Give an example of a Rebound Tonometer
iCare
70
Advantages of Rebound Tonometers
-Patients like it (often cannot feel it) - they prefer it to NCT -No drugs/anesthetics/fluorescein required -Self-assessment versions available -Less affected by corneal thickness
71
Disadvantages of Rebound Tonometers
-Ongoing costs (probes) -Less reliable at high IOPs (same as NCT's) -Affected by corneal rigidity (elasticity of the cornea - how bouncy your cornea is)
72
What weights do we use when calibrating a Perkins Tonometer and how do we calibrate it?
2g and 5g -Lie tonometer flat and place top of machine on a plastic disc to ensure that the tonometer is level -Place the weights on the probe
73
What weights do we use when calibrating a Goldman Applanation Tonometer?
2g and 6g
74
What criteria must be met when compensating calibration?
Can compensate ONLY if the calibration is out by a linear amount e.g. 1 mmHg out at 2g and 1 mmHg out at 6g
75
When should you contact the manufacturer?
If the reading is out by 3 mmHg or more
76
What to record for each eye?
-Record each measurement and average -Record time of day
OHD 1
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