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OPT 201 > Tonometry > Flashcards

Tonometry Flashcards

1
Q

How do Goldmann / Perkins tonometers measure IOP?

A

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

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2
Q

How does a Tonopen measure IOP?

A

Measures the area of applanation caused by a given force

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3
Q

How does a Pulsair measure IOP?

A

Measures the force of air pulse needed to applanate the cornea

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4
Q

List some disadvantages of indentation tonometry:

A

▪️ 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

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5
Q

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

A

2-3 microlitres per minute

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6
Q

What could a high IOP indicate?

A

Glaucoma

Ocular hypertension

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7
Q

What could a low IOP indicate?

A

Retinal / choroidal detachment

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

Intraocular inflammation (eg uveitis)

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8
Q

List some non- contact tonometers

A

Pulsair
ORA
Reichert

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9
Q

List some contact tonometers

A

Goldmann
Perkins
Tonopen

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10
Q

What is the normal range of IOPs?

A

10-21 mmHg

Average: 16mmHg

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11
Q

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

A

Normal tension glaucoma (NTG)

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12
Q

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

A

Ocular hypertension (OHT)

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13
Q

How could you accurately measure the IOP of an eye?

A

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

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14
Q

What law is applanation tonometry based on?

A

Imbert-Fick Law:

P = W/A

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

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15
Q

What are the limitations of the Imbert-Fick Law?

A

▪️ 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.

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16
Q

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

A

Instrument factors

Operator factors

Patient factors

17
Q

Describe some instrument factors that affect IOP measurements:

A

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

18
Q

Describe some operator factors that affect IOP measurements:

A

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
19
Q

Describe some patient factors affecting IOP measurement:

A 
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
20
Q

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

A

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

21
Q

List some common factors affecting IOPs

A 
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
22
Q

Describe the current NICE guidelines on IOP measurements

A

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.

23
Q

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

A

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

24
Q

Explain why Goldmann tonometry uses an applanation diameter of 3.06mm

A

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.

25
Q

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

A

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

26
Q

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

A

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

27
Q

List some advantages of GAT

A

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

28
Q

List some disadvantages of GAT

A 
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
29
Q

Common errors with GAT and Perkins?

A

Lack of calibration
Incorrect alignment
Probe not clean
Lids touching probe

30
Q

Advantages of Perkins tonometer?

A 
Can compare readings with Goldmann as same operation 
Portable 
Patient can be sitting or laying down 
Easier to hold eyelids open 
Accurate 
Low cost
31
Q

Disadvantages of Perkins tonometer?

A

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

32
Q

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

A

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

33
Q

List advantages of NCT

A 
Non-invasive 
No corneal anaesthesia required 
No specialist skills required 
Usually quick 
Less intimidating for px
Some portable
34
Q

List disadvantages of NCT

A

Not accepted as gold standard or NICE

Affected by arterial pulse which can spread results by about 2-4mmHg

Expensive

May overestimate high IOPs

35
Q

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

A

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.

36
Q

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

A

It measures actual air pressure instead of time.

Still uses a projection / detection system with infrared LEDs.

37
Q

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

A

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

38
Q

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

A

Pascal Dynamic contour tonometry-

Probe resembles the curve of the cornea, provides a dynamic measurement using pascals principle.

OPT 201 (1 decks)

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