Lecture 1- Tonometry Flashcards
What is the measurement of intraocular pressure?
Tonometry
What is a pascal?
A unit of pressure
= 1 newton per square metre (N/m
2)
What unit is IOP given?
mmHg
examples
1mm Hg= 1.33 hPa
14mm Hg= 19 hPa
Atmospheric pressure
1 bar=…kPa= …mmHg
1 Bar= 100 kPa= 750 mm Hg
It’s a metric (non- SI) unit
example; car tyre pressure = 2 Bar
What is glaucoma
Glaucoma is a common eye condition where the optic nerve, which connects the eye to the brain, becomes damaged.
What are the two main categories of glaucoma
Acute- closed angle
Chronic- open angle
What are the characteristic of acute glaucoma
Sudden onset and very painful
What are the characteristics of chronic glaucoma
-Gradual loss of peripheral visual
field is often not noticed
-Cupping of optic disc occurs over
time
What is tonometry critical for
Detection and monitoring glaucoma
What is the normal IOP range
10-21 mmHg
The mean is 15 mmHg
Do all patients with high IOP have glaucoma
Not necessarily glaucoma if pressure > 20mm Hg
A patient could have glaucoma even when pressure is normal
How does applanation tonometry measure IOP?
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
What is the principle of the applanation tonometry method
The method involves balancing the meniscus forces of the tear film with corneal rigidity to estimate IOP
Is applanation tonometry precise
No, because it’s non invasive it’s imprecise
What is the principle of applanation tonometer
relies on….
Rely on relationship between wall tension
and pressure in an elastic sphere
What is surface tension (γ)
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
Presure=
Force/ Area
Is the Goldmann tonometer the most wideley used version is current practice
Yes
How is the Goldmann tonometer performed
- 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
What is sodium fluorescein (NaF) excited by and what colour does it fluoresce?
pH?
nm?
Its excited by blue light (maximally at 494nm) and fluoresces green (521nm)
It depends on pH usually between 7.5- 8.5
What is the function of colbalt blue light in Goldmann tonometry
- 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.
What happens to the mires as force is applied to the tonometer head
It’s adjusted using the dial until the inner edges of these green
semicircles meet
This is known as mire alignment
What to consider during mire alignment
Care is needed to avoid
injury; planar movement
and excessive pressure can
abrade the cornea
What are the confounding factors
Corneal thickness
-corneal heterogeneity
-scar tissue etc.
Corneal curvature
e.g. KC
Variations in tear film:
-runny, gooey, dry
Further confounding factors
6
T,A,E,P,H,D
Time of day
Age
Epithelial oedema
Poor cooperation
-blinking and movement
Hypertension
Drugs e.g. ßBs, ACEIs
What is a common problem with Goldmanns tonometry
-Assumptions of sphericity, elasticity and
homogeneity
-Untenable following refractive surgery
-IOP measurements become unreliable
What are the more recent tonometers like
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
What are the pros of puff of air tonometry
Reasonably accurate measure of IOP
Less invasive
Convenient
Not gold standard
What are the cons of applanation tonometry
It can be imprecise due to variations in corneal properties and other factors
