Tonometry

Question 1

How do Goldmann / Perkins tonometers measure IOP?

Answer 1

They measure the force needed to flatten a given area of the cornea

Question 2

How does a Tonopen measure IOP?

Answer 2

Measures the area of applanation caused by a given force

Question 3

How does a Pulsair measure IOP?

Answer 3

Measures the force of air pulse needed to applanate the cornea

Question 4

List some disadvantages of indentation tonometry:

Answer 4

▪️ The indentation reduces intraocular volume; Causes IOP to rise initially then decrease as the weight increases aqueous outflow.

▪️ Affected by ocular rigidity

▪️ Affected by supine position

▪️ Contact procedure so requires anaesthetics and puts eye at risk of corneal abrasion

Question 5

What is the rate of aqueous production by the ciliary body?

Answer 5

2-3 microlitres per minute

Question 6

What could a high IOP indicate?

Answer 6

Glaucoma

Ocular hypertension

Question 7

What could a low IOP indicate?

Answer 7

Retinal / choroidal detachment

Leakage of ocular fluid (due to surgery / penetrating injury)

Intraocular inflammation (eg uveitis)

Question 8

List some non- contact tonometers

Answer 8

Pulsair
ORA
Reichert

Question 9

List some contact tonometers

Answer 9

Goldmann
Perkins
Tonopen

Question 10

What is the normal range of IOPs?

Answer 10

10-21 mmHg

Average: 16mmHg

Question 11

What is the name for glaucoma in patients with normal range IOP?

Answer 11

Normal tension glaucoma (NTG)

Question 12

What is the name for high IOP with no signs of glaucomatous damage?

Answer 12

Ocular hypertension (OHT)

Question 13

How could you accurately measure the IOP of an eye?

Answer 13

Using manometric methods - inserting a probe directly into the anterior chamber

Question 14

What law is applanation tonometry based on?

Answer 14

Imbert-Fick Law:

P = W/A

Pressure (mmHg) = Force (g) / Area (mm*2)

Question 15

What are the limitations of the Imbert-Fick Law?

Answer 15

▪️ The eye is not perfectly spherical.
▪️ Cornea is not infinitely thin (540nm thick approx)
▪️ Cornea not perfectly elastic and flexible, it has some rigidity which causes resistance to the applanation.
▪️ Eye is not dry - the tear meniscus creates surface tension which pulls the tonometer head towards it.

Question 16

Three main groups of factors that can affect accuracy of IOP measurements:

Answer 16

Instrument factors

Operator factors

Patient factors

Question 17

Describe some instrument factors that affect IOP measurements:

Answer 17

The imbert - fick principle only applies to spherical, dry, thin, elastic objects

Tonometers were calibrated using a limited number of eyes - therefore measurements are just a comparison to a test population

Question 18

Describe some operator factors that affect IOP measurements:

Answer 18

Calibration (ideally once a month)

Measurement technique:

• pressure on globe when holding eyelids
• massaging effect (prolonged / repeated measurements can cause IOP to fall)
• probe position incorrect
• subjective end point
• inconsistent technique

Question 19

Describe some patient factors affecting IOP measurement:

Answer 19

Corneal thickness 
Corneal curvature 
Corneal rigidity / elasticity 
Corneal disease 
Foreign bodies / scarring 
Irregular corneal surface 
Consistency of tears 
Wetability of tonometer head 
Amount of tears
Visibility of mires

Question 20

What are the risks associated with central corneal thickness? (CCT)

Answer 20

Healthy CCT is around 540nm, thin corneas are a risk factor as IOP can be measured low and are more susceptible of optic nerve damage or glaucoma due to high IOPs

Question 21

List some common factors affecting IOPs

Answer 21

Ocular pulse 
Tight clothing around neck
Holding breath 
CL removal 
Repeated IOPs
Blinking 
Genetic factors 
Gender
Accommodation 
Recent exercise 
Food and drugs 
Age 
Eye rubbing 
Straining 
Posture change 
Seasonal variation

Question 22

Describe the current NICE guidelines on IOP measurements

Answer 22

If IOP is 24mmHg or above in either eye you need to refer (measurement taken with GAT)

Should not refer patients discharged from HES after glaucoma screening unless condition has changed and a new referral needed.

Question 23

What should you do with a patient with IOPs of 30mmHg or over?

Answer 23

Refer urgently!
Risk of closed angle glaucoma, central retinal vein occlusion
Note any additional symptoms

Question 24

Explain why Goldmann tonometry uses an applanation diameter of 3.06mm

Answer 24

Because for this diameter, the surface tension caused by the tear meniscus is equal and opposite to the force from corneal rigidity - therefore they cancel each other out and IOP = W/A

Also because this area causes minimal fluid displacement and therefore only a small rise in IOP.

Question 25

During Goldmann tonometry; if one mire is appearing larger than the other...

Answer 25

Move the tonometer head towards the larger mire till they appear equal.

Question 26

How do you record an accurate IOP of a corneal with significant astigmatism?

Answer 26

Rotate the probe so that the biprism line is 43 degrees to the lowest power meridian reading

Question 27

List some advantages of GAT

Answer 27

Gold standard Only method recognised by NICE Accurate and repeatable Steady and secure (due to slit lamp) Mag and lighting show mires clearly (due to slit lamp) Ocular pulse visible therefore adj can be made Inexpensive if you already have a slit lamp

Question 28

List some disadvantages of GAT

Answer 28

``` Requires corneal anaesthetic Skilled operator required Needs a slit lamp Patient must sit up Subjective end point Potential corneal scarring Potential transmission for infection Not portable Problems with px’s with nystagmus / scarred corneas ```

Question 29

Common errors with GAT and Perkins?

Answer 29

Lack of calibration Incorrect alignment Probe not clean Lids touching probe

Question 30

Advantages of Perkins tonometer?

Answer 30

``` Can compare readings with Goldmann as same operation Portable Patient can be sitting or laying down Easier to hold eyelids open Accurate Low cost ```

Question 31

Disadvantages of Perkins tonometer?

Answer 31

Corneal anaesthetic required Requires skilled operator Mag and lighting not as clear as Goldmann Less stable than Goldmann Subjective endpoint Problems in px’s with nystagmus and scarred corneas

Question 32

How do NCTs (non-contact tonometers) measure IOPs?

Answer 32

An alignment system makes sure reading is taken at central cornea An air pulse is used to applanate the cornea, the force is increased till it flattens Photoelectric cell is used to detect when the applanation has occurred A processor and calibration database converts the air pulse force to an IOP reading

Question 33

List advantages of NCT

Answer 33

``` Non-invasive No corneal anaesthesia required No specialist skills required Usually quick Less intimidating for px Some portable ```

Question 34

List disadvantages of NCT

Answer 34

Not accepted as gold standard or NICE Affected by arterial pulse which can spread results by about 2-4mmHg Expensive May overestimate high IOPs

Question 35

How does a NCT such as the reichert tonometer measure IOP?

Answer 35

Air is pumped with increasing intensity at the cornea till applanation with a diameter of 3.6mm is reached. This is detected by infrared light being detected by a photoreceptor diode. The time at which the applanation occurs is measured and converted to IOP using a table.

Question 36

How does the Pulsair measure IOP differently to most other NCTs?

Answer 36

It measures actual air pressure instead of time. Still uses a projection / detection system with infrared LEDs.

Question 37

How does the ORA (ocular response analyser) differ from other NCTs?

Answer 37

It records two applanation pressure measurements instead of one. This helps consider any corneal biomechanics.

Question 38

Describe a form of tonometry which may one day replace Goldmann as the gold standard?

Answer 38

Pascal Dynamic contour tonometry- Probe resembles the curve of the cornea, provides a dynamic measurement using pascals principle.