Tonometry Flashcards
How do we measure units of pressure?
Pascal (Pa) = 1 newton per square metre (N/m2)
IOP traditionally given in mm Hg (mm of Mercury)
What should we be using to measure units of pressure opposed to mm Hg?
Hectopascals (hPa)!
Don’t use because conversion is too similar to N/m2
1 mm Hg = 1.22 hPA
How do we measure atmospheric pressure?
Atmospheric pressure
= 1 Bar = 100 kPa = 750 mm Hg
Metric (but non-SI) unit
Car tyre pressure ≈ 2 Bar
What is Glaucoma?
Raised intra-ocular pressure
- Impaired drainage of aqueous fluid from the anterior chamber
- Permanent damage to optic nerve leading to loss of ganglion cells
What is Glaucoma considered a form of?
Form of optic neuropathy
How many people does Glaucoma affect?
1 in 200 people <50
1 in 10 people >80yo
What are the two main categories of glaucoma?
- 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
What is Tonometry critical for?
Detection & Monitoring
What is the normal range for Intraocular Pressure?
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
What is Applanation Tonometry?
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
How do applanation tonometers work?
- 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
What is the equation for applanation tonometry?
Pressure = Force / Area
How does the Goldmann Tonometer work?
- 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
Why do we use Sodium Fluorescein NaF in Tonometry?
- Excited by blue light
maximally at 494 nm - Fluoresces green
~ 521 nm
depends on pH (7.5 – 8.5
How do we get the measurement in Goldmann Tonometry?
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
What is Mire Alignment in Tonometry?
Care is needed to avoid injury; planar movement and excessive pressure can abrade the cornea
What are some confounding factors in Tonometry?
- 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
What is the biggest problem for IOP measurements?
Assumptions of sphericity, elasticity and homogeneity are tentative at best, but become untenable following refractive surgery
LOOK AT SLIDES 141 & 142 FOR TASKS TO DO
How do we define ‘normal’ IOP?
Unfortunately, the only definition of ‘normal’ for intraocular pressure is the pressure that does not lead to glaucomatous damage of the optic nerve head.
Why can’t we rely on IOP alone to diagnose glaucoma?
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
What is tonometry performed with?
Tonometers
How can we measure the pressure in the eye?
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).
What are the two types of Tonometer?
There are two types of tonometer: indentation, which are no longer used, and applanation (flattening).
How do Applanation Tonometers work?
Applanation tonometers rely on the relationship
between wall tension and pressure in an elastic sphere.
What maths is there in Applanation Tonometry?
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
How does Applanation Tonometry work, as in how does the mechanism itself work?
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.
How does the Goldmann-type applanation tonometer work?
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
What Tonometers do they use at opticians?
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.
