Tonometry

Question 1

What is IOP

Answer 1

Measurement of pressure within the eye (measured in mmHg)

Question 2

What caused IOP

Answer 2

Aqueous humour produced at ciliary body drains away through trabecular meshwork/canal of schlemm - the more resistance to that drainage causes pressure

Question 3

What causes raised IOP

Answer 3

Increased production of aqueous or less aqueous being drained away

Question 4

Why measure IOP

Answer 4

Normal IOP required to keep eye rigid

Question 5

When IOP is low, what is this called and what are the risks of it?

Answer 5

Hypotony
Risk of:
Retinal detachment
Hypotony maculopathy - due to stuff leaking out vessels due to low pressure
-Suprachoroidal Haem - due to stuff leaking out vessels due to low pressure

Question 6

When IOP is too high, what is this called and what are the risks of it?

Answer 6

Ocular hypertension
Risk of glaucoma

Question 7

Normal IOP

Answer 7

Mean = 16.3mmHg (SD +/- 3.6mmHg)

9.4 - 23.5mmHg (2 SD’s)

interocular difference should be <5mmHg (difference between both eyes)

Average IOP higher for those with glaucoma

Question 8

Problems measuring IOP

Answer 8

IOP changes every few seconds

Question 9

Factors increasing IOP

Answer 9

All these factors may increase IOP

Diurnal variation (3-6mmHg)
Pulse (4mmHg)
Lid pressure (10mmHg)
Apprehension (4mmHg)
Drinking (not alcohol)
Lying Down

Question 10

Factors reducing IOP

Answer 10

Accommodation (-1mm Hg)
Exercise
Seasonal (-1mmHg in summer)
(some) Drugs

Question 11

Correlations between IOP & Glaucoma

Answer 11

Can have raised pressure without glaucoma- ocular hypertension
Can have glaucoma without raised pressure - normal tension glaucoma

Question 12

Does higher pressure mean you have glaucoma

Answer 12

Higher pressure = higher risk of glaucoma but shouldn’t be the only test to identify glaucoma

Question 13

Increased IOP damages ganglion cell axons at the optic nerve head, what are the direct and indirect hypothesis around this

Answer 13

Direct - mechanical pressure on optic nerve head damaged physiology of it
Indirect - pressure goes up, doesn’t provide blood vessel enough oxygen

Question 14

Which other tests is tonometry used with

Answer 14

Visual fields
Optic disk assessment
Case history

Question 15

When to use tonometry

Answer 15

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

Question 16

What is manometry

Answer 16

A way of measuring IOP, tube inserted into the eye - clinically unacceptable due to risks however is very accurate

Question 17

Measuring IOP with digital palpitations method:

Answer 17

Press lightly with index finger
Feel reaction with second finger
Above tarsal plate
Easy but inaccurate
Experienced claim 5 point scale accuracy

Question 18

Indentation tonometry: measuring IOP

Answer 18

Indent cornea
Central plunger
The amount it indents the cornea is proportional to the pressure pushing back against it (IOP)

Question 19

Procedure of Schiotz Tonometry

Answer 19

Patient lies down
Cornea anaesthetised (4 drops of Oxybuprocaine)
Hold instrument collar
Rest vertically on cornea
Read indentation from scale

Question 20

How does Scleral rigidity affect tonometry

Answer 20

everyone has a slightly different elasticity of sclera, if you indent cornea, you are pushing aqueous back, with someone with a very elastic sclera, their sclera will get bigger by distending so we can’t fully know how rigid sclera is

Question 21

Applanation tonometers

Answer 21

Flatten rather than indent cornea
Displace less aqueous
Scleral rigidity insignificant
Corneal rigidity approx same for all eyes

When you flattern patients cornea and look at the other side, you’re creating an applanation area - when you applanate a wet spherical surface, you see a ring/meniscus where you applanate

Question 22

2 ways to use applanation tonometry

Answer 22

Use a fixed force/weight and see how big applanation area is - Maklakow & Tonometry - applanation area proportional to IOP

Constant area - Goldmann - variable force/weight and constant applanation area - always small applanation area - force required proportional to IOP

Question 23

Principles of Goldmann Tonometer

Answer 23

At approx 3mm applanation diameter
Surface tension = Corneal rigidity
Corneal rigidity approx same for all eyes
At exactly 3.06mm diameter
Force proportional to IOP (linear)
1g=10mmHg

Question 24

How do we know when it’s equal to 3.06mm

Answer 24

Using prisms - prisms move image 3.06mm in respect of eachother

Question 25

Goldmann tonometer procedure

Answer 25

Set up patient (Anaesthetic & Fluorescein) Wide, bright cobalt blue illumination at ~45 degrees (temporal) Set drum to ~1.5g Move forward to applanate corneal apex Adjust drum to align rings Disengage from cornea Read weight from drum

Question 26

Errors with Goldmann tonometer

Answer 26

Incorrect alignment Too much/too little fluorescein Failure to focus slit lamp Lid touch e.g., touching lashes High astigmatism

Question 28

Procedure of Goldmann tonometer in astigmatism

Answer 28

Corneal astigmatism: not circular applanation area as cornea is rugby shaped so get oval applanation area Rotate to 43° from flattest meridian Gives close to 7.35mm2 applanation area Large cyls only e.g., 4.00D of cyl Astigmatism creates errors of 1-2mmHg

Question 29

Disinfection procedure

Answer 29

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 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.

Question 30

What is the Perkins tonometer

Answer 30

Hand held version of the Goldmann Counterbalanced to use at all angles e.g., can use on a patient in bed Comparable accuracy benefit - portable

Question 31

Electronic tonometers

Answer 31

Schiotz Digital scale readout Subject to same errors Mackay-Marg Developed in 60’s to be used without drugs 1.5mm diameter probe in tonometer head Movements create electrical current Know how much force required to applanate 1.5mm2 Tonopen Hand held version of Mackay-Marg Battery Operated Read IOP directly - Digital display Grolmann - Introduced 1972 (most common) - Constant Area Applanation - No contact with cornea (non-contact tonometry) - No need for anaesthetic - Little risk of corneal damage

Question 32

3 components they all have

Answer 32

Alignment system Pneumatic system that creates the air puff Applanation detector - detects at what point you have a flat cornea using reflection and infrared light

Question 33

Errors with NCT

Answer 33

Ocular pulse pressure goes up & down so take multiple readings - you may get a reading when you are at the top/botttom of their pulse range so it may be higher or lower Distorted corneal surface - machine may never fire if patient doesnt have a perfect corneal surface Eye movement Lids/lashes get in the way Not good at High IOP as you need a high pressure pulse of air e.g., if checking on patient if ocular hypertension turning into glaucoma, not good to use this

Question 34

Rebound tonometry

Answer 34

Induction coil propels a magnetized steel wire (with plastic tip) towards the cornea Probe hits cornea and rebounds Returning movement induces current in coil Bouncier cornea = Higher IOP 6 measurements. Discards highest/lowest practitioner presses button, not automatic

Question 35

Calibrating tonometers

Answer 35

**GOLDMANN** Calibration bar (weighted bar you plug into the side) Marked at 0.2 and 6gms 20 and 60 mmHg Can compensate if calibration out Must be linear **PERKINS** Lie on plastic disc Put weights on cone – 2g and 5g 20mmHg and 50mmHg Can compensate if calibration out Must be linear Non Contact Self check when start up on electronic tonometers Won’t work unless calibrated Technician sets using silicone spheres