Tonometry

Question 1

Describe the uveoscleral pathway

Answer 1

Back into the ciliary body
Into iris blood vessels
into the choroid and sclera

Question 2

How is aqueous humour produced and how is it drained?

Answer 2

Ciliary body produces aqueous. 90% travels through the pupil to be drained at the trabecular meshwork and Schlemm’s canal. The remaining 10% takes uveosclearal pathway.

Question 3

How is aqueous humour linked to pressure?

Answer 3

There is no feedback mechanisms at the ciliary body so if too much aqueous is produced or not enough drainage, the ciliary body still continues to produce aqueous. The eye can’t expand so instead the pressure rises.

Question 4

What are the key features of glaucoma?

Answer 4

1. Loss of ganglion cells
2. Irreversible field loss
3. Linked with raised IOP (but not always)

Question 5

Why is glaucoma not always linked with IOP?

Answer 5

Some individuals have consistently high IOP but do not develop optic nerve damage or visual field loss. These individuals have strong optic nerves that can tolerate elevated pressure.

Question 6

What is normal tension glaucoma?

Answer 6

Where damage to the optic nerve and associated visual field loss occur despite having an intraocular pressure (IOP) within the normal range.

Question 7

For normal tension glaucoma (NTG) what is the method of treating this type of glaucoma?

Answer 7

Even in NTG lowering IOP is the only proven method of treating glaucoma.

Question 8

What factors influence IOP?

Answer 8

1. Race
2. Sex
3. Age
4. Time of day
5. General health

Question 9

What is diurnal variation ad factors influence it?

Answer 9

Intraocular pressure (IOP) fluctuates throughout the day in a pattern known as diurnal variation, which is influenced by physiological factors, body position, and systemic conditions.

Question 10

What is the guidance NICE set?

Answer 10

NICE set guidance in 2017 to refer everyone with pressures of 24mmHg and above

Question 11

Evaluate IOP as a screening tool?

Answer 11

Poor sensitivity - not great at spotting glaucoma Pxs

Fairly high specificity - good at spotting healthy Pxs but a lot of false positives

Question 12

What are the tonometry methods? Which one is the best most accurate one?

Answer 12

1. Manometry (piercing) -most accurate
2. Digital (touching)
3. Indentation (synringe)
4. Ballistic (hitting)
5. Applanation (flattening)

Question 13

Describe procedure of manometry

Answer 13

1. Pierce the globe with a hollow needle attached to a reservoir of water
2. adjust the pressure of the water in the needle to the point at which no aqueous escapes and no water flows into the eye
3. Take the reading in mmHg

Question 14

What is imbert fick law?

Answer 14

The pressure inside a sphere is equal to the force applied to flatten a portion of its surface divided by the area of that flattened portion. (P = W/A. P= pressure of substance inside container. W = External force required to applanate. A = area of surface applanated.)

Question 15

What criteria does imbert fick law rely on?

Answer 15

1. Perfectly spherical
2. Infinitely thin
3. Perfectly flexible
4. Elastic
5. Dry

Question 16

What applanation area did Goldmann choose and why?

Answer 16

Applanation area of 3.06

1. Diameter between 3-4 mm so M=N (surface tension and corneal rigidity cancel out)
2. Low aqueous displacement (to prevent massage effect)
3. Simple conversion : Tonometer force (g) X 10 = Pressure in mmHg

Question 17

What is the size of the probe and why?

Answer 17

Applanation area is 3.06mm. So 7 mm is chosen to protect the cornea if too much force applied or probe is misaligned.

Question 18

Why is a calibrated spring needed?

Answer 18

Calibrated so that when set to 2g we know the force exerted on the eye is 2g.
We can use it to apply force and measure accurately how much.

Question 19

Why is fluorescein used?

Answer 19

When dissolved in tears are pushed to the edge can help to see the edge of the applanated area. Can help to adjust the mires to ensure probe is in right spot.

Question 20

What is ocular pulse?

Answer 20

Refers to the small, rhythmic fluctuations in intraocular pressure (IOP) that occur with each heartbeat. These fluctuations are caused by changes in blood flow within the choroidal vasculature and other ocular tissues during the cardiac cycle.

Question 21

How do you overcome ocular pulse in tonometry?

Answer 21

Will need to average it out and find a mean by taking multiple measurements.

Question 22

What is perkins tonometry?

Answer 22

Another contact tonometry method.
Is a portable hand held version of Goldmann.

Question 23

What are the disadvantages of Contact tonometry?

Answer 23

1. Hard to master so operator errors occur
2. Potential for infection from probe (this is reduced since introduction of disposable probe heads)
3. Potential for allergic reaction to drops
4. Has to be performed by optometrist, can’t be delegated
5. Chance of corneal abrasion

Question 24

What are the ways someone could get corneal abrasion from contact tonometry?

Answer 24

1. Abrasion from the probe
2. Abrasion due to the anaesthesia making the eye numb and so a foreign object wasn’t felt

Question 25

What is the ideal width of mires? And what to do if too thick/thin?

Answer 25

0.2mm Too thick = too many tears, let them blink it out or else too high IOP result Too thin = less tears, add more local anaesthetic / fluorescein or else too low IOP result (could use artifical tears but use the other 2 first as tears can dilute fluorescein and could alter tear film composition)

Question 26

How to carry out Goldmann applanation on those with astigmatism?

Answer 26

The mires will be oval not circular so this is necessary for all Pxs with over +/- 3.00DC Take a reading with the probe in horizontal position and then repeat with vertical and find the average.

Question 27

For what central corneal thickness (CCT) did Goldmann calibrate it for?

Answer 27

Goldmann calibrated it for 520 μm CCT

Question 28

What happens to the IOP readings if there are variations in corneal structure?

Answer 28

1. High or low CCT can affect N. M is no longer equal to N. 2. Thick cornea = IOP overestimated 3. Thin cornea = IOP underestimated

Question 29

How does accommodation affect IOP readings?

Answer 29

Short term = increases pressure as there is more space Long term = Decreases pressure as drainage occurs

Question 30

Name all the contact tonometers

Answer 30

1. Goldmann applanation (Applanation) 2. Perkins tonometry (Applanation) 3. I - care (Ballistic) 4. Tono-pen (applanation and indentation) 5. Triggerfish contact lenses

Question 31

What are the similiarities and differences between Grolman's A/O Reichert NCT and Goldmann applanation?

Answer 31

Similarity = we are still finding how much force is required to flatten a fixed area of cornea Differences= 1. The force is applied with puff of air 2. The point of applanation is measured using light reflected from the cornea 3. Diameter of applanated area is larger, now is 3.6 4. Brief so no massaging effect

Question 32

What are the components of Grolman's A/O Reichert?

Answer 32

1. Air puff mechanism to flatten cornea 2. Column of infared light beam 3. Detector to establish when cornea is flat 4. Targeting system for the operator 5. Separate IR system to ensure correct distance front the eye

Question 33

What are the advantages of Pulsair over A/O Reichert?

Answer 33

1. Shorter puff, more comfy for Px 2. Calibration is less of an issue and doesn't rely on force/time relationship 3. Px doesn't have to sit up at the instrument 4. Easier to operate

Question 34

How to prevent misreading for pulsair tonometry?

Answer 34

1. Don't write down the running average values, we need the absolute values, this is so we know that we haven't take a misreading 2. Always take a minimum of 3 readings per eye

Question 35

Name all non contact tonometers

Answer 35

1. A/O Reichert 2. Pulsair tonometer 3. Table top NCT

Question 36

What are the disadvantages of NCT?

Answer 36

1. More variability 2. NCT is less accurate at measuring higher IOPs 3. Variations due to ocular pulse are not averaged out 4. Pxs hate it

Question 37

How should IOP be recorded down?

Answer 37

1. Note time of day 2. Record method/instrument used 3. Always write down the units (mmHg) 4. Record date, Px and which eye.