Tonometry

Question 1

How do we measure units of pressure?

Answer 1

Pascal (Pa) = 1 newton per square metre (N/m2)

IOP traditionally given in mm Hg (mm of Mercury)

Question 2

What should we be using to measure units of pressure opposed to mm Hg?

Answer 2

Hectopascals (hPa)!

Don’t use because conversion is too similar to N/m2

1 mm Hg = 1.22 hPA

Question 3

How do we measure atmospheric pressure?

Answer 3

Atmospheric pressure
= 1 Bar = 100 kPa = 750 mm Hg
Metric (but non-SI) unit
Car tyre pressure ≈ 2 Bar

Question 4

What is Glaucoma?

Answer 4

Raised intra-ocular pressure

• Impaired drainage of aqueous fluid from the anterior chamber
• Permanent damage to optic nerve leading to loss of ganglion cells

Question 5

What is Glaucoma considered a form of?

Answer 5

Form of optic neuropathy

Question 6

How many people does Glaucoma affect?

Answer 6

1 in 200 people <50

1 in 10 people >80yo

Question 7

What are the two main categories of glaucoma?

Answer 7

• Acute (closed angle)
  Sudden onset & very painful
• Chronic (open angle)
  Gradual loss of peripheral visual field not often noticed
  Cupping of optic disc occurs over time

Question 8

What is Tonometry critical for?

Answer 8

Detection & Monitoring

Question 9

What is the normal range for Intraocular Pressure?

Answer 9

Normal range = 10 - 20mm Hg

Mean = 15mm Hg

Not necessarily glaucoma if pressure > 20mm Hg

A patient could have glaucoma even when pressure is normal

Question 10

What is Applanation Tonometry?

Answer 10

Applanation tonometry infers intraocular pressure from the force required to flatten (applanate) a constant area of the cornea, as per the Imbert-Fick principle/”law”

By flattening an area of 3.06 mm Ø so that the meniscal forces of the tear film become equivalent to those of corneal rigidity, the intraocular pressure can be estimated from the force applied

Question 11

How do applanation tonometers work?

Answer 11

• Rely on relationship between wall tension and pressure in an elastic
• Surface tension γ is related to the pressure difference ∆P across the curved wall by
  ∆P = 2γ/r, where r is the radius
• If the wall of the sphere is flattened (i.e. r = ∞), the pressure difference will be zero, and the pressure within the sphere can be related to the force applied to the wall by pressure = force / area

Question 12

What is the equation for applanation tonometry?

Answer 12

Pressure = Force / Area

Question 13

How does the Goldmann Tonometer work?

Answer 13

• A special disinfected biprism is mounted on the tonometer head and then placed against the cornea
• Topical anaesthesia is obviously required for comfort – typically oxybuprocaine 0.4%
• A drop of sodium fluorescein is instilled into the eye at the same time

Question 14

Why do we use Sodium Fluorescein NaF in Tonometry?

Answer 14

• Excited by blue light
  maximally at 494 nm
• Fluoresces green
  ~ 521 nm
  depends on pH (7.5 – 8.5

Question 15

How do we get the measurement in Goldmann Tonometry?

Answer 15

The examiner then uses ‘cobalt blue’ light to view the meniscus formed by the fluorescein stained tear-film around the probe contact area, split into two green semi circles (mires) by a bi-prism within the tonometer head.

The force applied to the tonometer head is then adjusted using the dial until the inner edges of these green semicircles meet

Question 16

What is Mire Alignment in Tonometry?

Answer 16

Care is needed to avoid injury; planar movement and excessive pressure can abrade the cornea

Question 17

What are some confounding factors in Tonometry?

Answer 17

• Corneal Thickness
  Corneal heterogeneity such as scar tissue
• Corneal Curvature
• Variations in Tear Film
• Time of Day
• Age
• Epithelial Oedema
• Poor Cooperation (blinking and movement)
• Hypertension
• Drugs

Question 18

What is the biggest problem for IOP measurements?

Answer 18

Assumptions of sphericity, elasticity and homogeneity are tentative at best, but become untenable following refractive surgery

Question 19

LOOK AT SLIDES 141 & 142 FOR TASKS TO DO

Question 20

How do we define ‘normal’ IOP?

Answer 20

Unfortunately, the only definition of ‘normal’ for intraocular pressure is the pressure that does not lead to glaucomatous damage of the optic nerve head.

Question 21

Why can’t we rely on IOP alone to diagnose glaucoma?

Answer 21

The overlap between the two populations (non-glaucomatous and glaucomatous), which demonstrates that diagnosis cannot be purely reliant on IOP.

The standard deviation should also be treated with caution as the distributions are skewed.

It is important to remember that high IOP does not nessarily result in glaucoma, and conversely, low IOP doesn’t automatically rule out glaucoma

Question 22

What is tonometry performed with?

Answer 22

Tonometers

Question 23

How can we measure the pressure in the eye?

Answer 23

The eye is a fluid-filled globe, and the pressure within it can be measured by relating the deformation of the globe (unknown) to an externally applied force (known).

Question 24

What are the two types of Tonometer?

Answer 24

There are two types of tonometer: indentation, which are no longer used, and applanation (flattening).

Question 25

How do Applanation Tonometers work?

Answer 25

Applanation tonometers rely on the relationship
between wall tension and pressure in an elastic sphere.

Question 26

What maths is there in Applanation Tonometry?

Answer 26

The surface tension g (gamma) is related
to the pressure difference, Δp, across the curved
wall by the following equation:
Δp = 2y/r

Where r is the radius. If the wall of the sphere is
locally flattened (i.e. r = infinity symbol ), Δp will be zero, and
the pressure within the sphere can be related to
the force applied to the wall by:
Pressure = Force / Area

Question 27

How does Applanation Tonometry work, as in how does the mechanism itself work?

Answer 27

The cornea has a central thickness of approximately 0.55 mm, the outer contact area is therefore less than the internal flattening. The wet surface results in surface tension effects of cohesion and adhesion acting on the piston.

Application of pressure through the piston bends the cornea, and the surface will attempt to “push back” against any bending. As such, surface tension has a component acting in the same direction as the applied force, which is opposed by the bending force.

Question 28

How does the Goldmann-type applanation tonometer work?

Answer 28

The Goldmann-type applanation tonometer applies a force to the surface of the cornea and uses an optical system to
determine when the applanated area is 3.06 mm in diameter.

The cornea is anaesthetised with a topical anaesthetic, and the tear film stained with sodium fluorescein.

Fluorescence allows visualisation of the meniscus surrounding the applanated area, which is split into two semicircles by biprisms.

The geometry is such that the inner edges of the semicircles touch when the area applanated is correct

Question 29

What Tonometers do they use at opticians?

Answer 29

Tonometers are now available that use a puff of air to deform the surface of the cornea. By using a known velocity and volume of air, and measuring the resulting deformation of the corneal surface, it is possible to provide a reasonably accurate measurement of intraocular pressure in a much less invasive manner - convenient, but not a gold-standard.