Lecture 1- Tonometry

Question 1

What is the measurement of intraocular pressure?

Answer 1

Tonometry

Question 2

What is a pascal?

Answer 2

A unit of pressure
= 1 newton per square metre (N/m
2)

Question 3

What unit is IOP given?

Answer 3

mmHg

examples
1mm Hg= 1.33 hPa
14mm Hg= 19 hPa

Question 4

Atmospheric pressure

1 bar=…kPa= …mmHg

Answer 4

1 Bar= 100 kPa= 750 mm Hg
It’s a metric (non- SI) unit
example; car tyre pressure = 2 Bar

Question 5

What is glaucoma

Answer 5

Glaucoma is a common eye condition where the optic nerve, which connects the eye to the brain, becomes damaged.

Question 6

What are the two main categories of glaucoma

Answer 6

Acute- closed angle
Chronic- open angle

Question 7

What are the characteristic of acute glaucoma

Answer 7

Sudden onset and very painful

Question 8

What are the characteristics of chronic glaucoma

Answer 8

-Gradual loss of peripheral visual
field is often not noticed
-Cupping of optic disc occurs over
time

Question 9

What is tonometry critical for

Answer 9

Detection and monitoring glaucoma

Question 10

What is the normal IOP range

Answer 10

10-21 mmHg
The mean is 15 mmHg

Question 11

Do all patients with high IOP have glaucoma

Answer 11

Not necessarily glaucoma if pressure > 20mm Hg

A patient could have glaucoma even when pressure is normal

Question 12

How does applanation tonometry measure IOP?

Answer 12

It measures IOP by flattening a small area of the cornea of 3.06mm. Its based on the Imbert- Flick principle which was refined by Hans Goldmann to the Goldmann tonometry

Question 13

What is the principle of the applanation tonometry method

Answer 13

The method involves balancing the meniscus forces of the tear film with corneal rigidity to estimate IOP

Question 14

Is applanation tonometry precise

Answer 14

No, because it’s non invasive it’s imprecise

Question 15

What is the principle of applanation tonometer

relies on….

Answer 15

Rely on relationship between wall tension
and pressure in an elastic sphere

Question 16

What is surface tension (γ)

Answer 16

It’s 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 17

Presure=

Answer 17

Force/ Area

Question 18

Is the Goldmann tonometer the most wideley used version is current practice

Answer 18

Yes

Question 19

How is the Goldmann tonometer performed

Answer 19

• 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 20

What is sodium fluorescein (NaF) excited by and what colour does it fluoresce?

pH?
nm?

Answer 20

Its excited by blue light (maximally at 494nm) and fluoresces green (521nm)
It depends on pH usually between 7.5- 8.5

Question 21

What is the function of colbalt blue light in Goldmann tonometry

Answer 21

• view the meniscus formed by the
  fluorescein stained tear-film around the
  probe contact area
• its split into two green semi circles (mires) by a bi-prism within the
  tonometer head.

Question 22

What happens to the mires as force is applied to the tonometer head

Answer 22

It’s adjusted using the dial until the inner edges of these green
semicircles meet

This is known as mire alignment

Question 23

What to consider during mire alignment

Answer 23

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

Question 24

What are the confounding factors

Answer 24

Corneal thickness
-corneal heterogeneity
-scar tissue etc.
Corneal curvature
e.g. KC
Variations in tear film:
-runny, gooey, dry

Question 25

Further confounding factors

6

T,A,E,P,H,D

Answer 25

Time of day
Age
Epithelial oedema
Poor cooperation
-blinking and movement
Hypertension
Drugs e.g. ßBs, ACEIs

Question 26

What is a common problem with Goldmanns tonometry

Answer 26

-Assumptions of sphericity, elasticity and
homogeneity
-Untenable following refractive surgery
-IOP measurements become unreliable

Question 27

What are the more recent tonometers like

Answer 27

They use a puff of air to deform the surface of the cornea
They use a known velocity and volume of air and measuring the resulting deformation of the corneal surface

Question 28

What are the pros of puff of air tonometry

Answer 28

Reasonably accurate measure of IOP
Less invasive
Convenient
Not gold standard

Question 29

What are the cons of applanation tonometry

Answer 29

It can be imprecise due to variations in corneal properties and other factors