Applanation Tonometry

Describe the Imbert-Fick principle

• When a flat surface with a defined area is pressed against the surface of a sphere (applanation) with a given internal pressure

• Equilibrium is reached when the force exerted is balanced by the internal pressure

Equation Imbert Fick principle

• The force required to flatten the surface of a sphere (W) Is equal to product of the pressure inside the sphere (P) and the area is applanated (A)

> W = P x A

what does the equation assume (W = P x A)

• only true assuming cornea is thin , dry , perfectly elastic and flexible

• and only force acting on the surface should be the pressure of the applanating surface

GAT uses the Imbert Fick principle but takes 2 other conditions into account - What are they ?

• Surface tension (S) of the tear film which attracts the tonometer prism towards the cornea

• Corneal rigidity (B) which resists applanation

What did Goldmann and Schmidt determine ?

• if area being applanated was 3.06mm2 these two forces cancel out

• W + S = (P xA) -B

Consent before GAT

• Explain test to px beforehand

• Obtain informed consent

• Ask about sensitivity to anaesthetic drops

Anaesthetics

• inform px drops will sting at first → this should dissipate quickly

• Add a drop to each eye

• After a minute add second drop to each eye → should not feel second drop if you’ve correctly inserted first drop

(drops act fast)

Slit lamp set up for GAT

• need to place GAT on stage

• Stage and GAT need to be secure → clicks in place

• insert and align probe with white line on carrier

What do we do if conceal astigmatism greater than 3.00D

• An error of 1mmHg for every 4.00D of corneal astigmatism

• can use keratometer

What to do if astigmatism is WTR or ATR

• move to the red line on the carrier

What to do if astigmatism is oblique ?

• needs to be 43 degrees from FLATTEST meridian

• Keratoconus px etc

What is the slit lamp set up for GAT

• Mag 10-16x

• Angle 45-60

• WIDEST beam

• Cobalt Blue Filter + Wratten

• High illumination

Fluorescein instillation

• FI need to be instilled safely

• makes mire easier to see

• Aim not to put too much as this will affect quality of mires → wait for the fluorescein to wash out

Patient instructions during GAT

• ask px to keep STILL as possible

• need to keep head right up against forehead rest

• ensure slit lamp is at right height for px prior to GAT

• ask them to look straight ahead and encourage them not to blink

• readings can be higher in anxious px - reassure px

Applanating

• set tonometer to expected result

• gently move slit lamp forward

• LOOKING AROUND SIDE of slit lamp until applanation occurred

• will see tonometer probe head move back when applanated

What type of probe is the GAT probe ?

• Biprism

GAT appearance when to adjust

What to do post GAT

• Check corneal integrity after GAT → optic section

• ensure any staining not affecting stroma

How to record GAT

Need to record :

> Techniques

> IOP measurements (each eye)

> TIME

E.g .GAT R 21, L22 @3:00pm

How to calibrate GAT

• metal thing

• check at 0,20 and 60

• should be calibrating once a month

Perkins - which px

• similar to GAT but different patient setup

• useful for px that are unable to use a slit lamp

• domiciliary px

• its handheld, portable and has head rest

What is Pachymetry

• Green word → Pachos-thick and “metry” to measure

• is the term used for measurement of corneal thickness

When to perform Pachmyetry

• Patients with ocular hypertension

• When IOP measured

• Refractive surgery → what type is most suitable for px

• screening + monitoring of

> Corneal oedema

> Corneal dystrophies

> Kerataconus

CCT and ocular hypertension

• Application based on Imbert Ficks law which assumes cornea is a perfect flexible, dry sphere which is thin

• increase in tissue in thicker cornea makes it less compliant and leads to overestimation of IOP

• thinner cornea leads to underestimation of IOP

Ocular Hypertension Treatment study published

• OHTS published that CCT important independent risk factor for progression from ocular hypertension to early glaucoma

With the Goldman Tonometer what does it assume Corneal Centre Thickness to be ?

> 520μm

What do other tonometers assume CCT?

545 or 550 μm

• If CCT higher than that assumed by tonometer device will OVERESTIMATE the px IOP

• If CTT lower than that assumed by tonometer the device will UNDERESTIMATE px IOP

What are the different types of Pachymetry ?

> Ultrasound Pacymetry

> Ultrasound Biomicroscopy

> Optical Pachymetry

> OCT

> Schemipflug imaging

What is a Tongue -P

• Uses Schemipflug Camera

• calculates a compensated IOP measure

Recording

• Techniques

• CCT measures of each eye

• Time (time of day can affect CCT)

- > E.g GAT R 555μm L 560 μm @ 3:00pm