3.1.6 Uses both a non-contact and contact tonometer to measure intra-ocular pressure and analyses and interprets the results.

Currently, NICE guidelines [NG81], last updated on 26th January 2022, states…

that an IOP measurement using Goldmann-type applanation tonometry must be recorded before referring patients for chronic open angle glaucoma.

Principles of GAT

The Imbert-Fick principle

“A small force applied to a sphere will exert a force back related to the internal pressure of the sphere.”

The Imbert-Fick principle relies on several assumptions regarding the properties of the eye:

1. The cornea is infinitely thin and flexible

2. The tear film has no pulling effect on the probe

3. Changes in the glove volume and IOP from contact with the probe are negligible

4. All anterior eye structures have the sae uniform elastic properties

In 1957 Goldmann concluded that …

A probe tip of 3.06mm in diameter allowed for 1g of pressure to equal 10mmHg.

This sized probe was chosen so the surface tension attraction from the tear film is equal and opposite to the corneal rigidity force.

Limitations of GAT

No account for CCT

Thin corneas underestimate IOP

Thick corneas overestimate IOP

Requires local anaesthetic 1mmHg error for every 4D of corneal astigmatism.

Incorrect NaFL conc. affects results

Interpreting IOP values

The normal range is between 10-21mmHg and varies diurnally.

NICE guidance requires IOP >24mmHg to be referred for specialist review.

IOP >21mmHg on two occasions using contact tonometry can be classified as ocular hypertension.

IOP needs testing at different times and is often higher in the morning than later in the day.

Suspicious IOPs are values about 25mmHg in the absence of any other ocular signs, and/or a difference of >4mmHg between the eyes.

GAT

Patient Records:

Record what type of contact tonometry you used. Drug information!

Example:

1% oxybuprocaine, 1 drop per eye, inserted at 10:55am BN H8978, Expiry 05/2027

• You need to check any staining before and after applanation.

• Check to make sure the patient hasn’t had any adverse reactions to drops previously

• Note that you have explained to the patient that the “Drops last for 30 minutes, and you are advised not to rub your eyes in case they get scratched.”

• Important to show you have given the patient an information leaflet about the drops

“Why do we take 3 readings for NCT and 1 reading for GAT?”

GAT takes into account the ocular pulse which is why you see the mires pulsating at you so the one measurement you get is already an average reading.

With NCT it doesn’t take into account the ocular pulse so you have to take 3-4 readings to get an average.

Performing GAT

Procedure

• Preparation

• Calibration

• Patient explanation & consent

• Anaesthetic & Fluorescein Instillation

• Slit lamp set up

• Applanation

• Repeatability

• Aftercare

Performing GAT

Preparation

• Wash hands and sanitise station

• Insert disposable probe onto Goldmann tonometer, aligning the biprisms horizontally

• Check tonometer calibration (often done at start of clinic)

Performing GAT

Calibration

• With no calibration bar attached, set dial at 0mmHg

• The arm should just oscillate freely either side of zero

• Attach calibration bar at “20” notch

• With the dial at 20mmHg, the arm should balance evenly, falling equally either side

• Attach calibration bar at “60” notch

• Set dial to 60mmHg and repeat same test

Interpretation

Acceptable error margin is usually:

±0.5 mmHg at 0

±1 mmHg at 20

±2 mmHg at 60

If outside this, the tonometer must be serviced before use.

GAT

Patient explanation & consent

Explain procedure to patient

“We’ll measure the pressure inside your eye using a blue light and a gentle instrument. It may feel slightly cold or uncomfortable, but it won’t hurt. We need to put a small drop of local anaesthetic in both your eyes, this will stingy briefly for a couple seconds and will last for 30 minutes, its important you don’t rub your eyes for 30 minutes so you don’t damage you eyes.”

GAT

Anaesthetic & Fluorescein Instillation

• Instil topical anaesthetic (oxybuprocaine, proxymetacaine, lidocaine)

• Instil fluorescein

GAT

Slit lamp set up

• Sit patient at slit lamp and the eye level marker is perfectly aligned.

• Adjust illumination

  • Cobalt blue filter

  • Moderate beam width

  • 45-60º angle to tonometer arm

  • Magnification 10x is sufficient to view mires clearly

• Set tonometer dial to around 10mmHg

• Ask pt to look straight ahead and keep both eyes open

GAT

Applanation

• Pull the joystick of the slit-lamp back and slowly move the entire slit-lamp toward the cornea

• When the probe biprisms are approximately 3-5mm away from the cornea, look through eyepieces and gently push the joystick to bring the biprisms into contact with the cornea

• Observe the fluorescein mires

• Adjust the dial until the inner edges of the two mires just touch (overlap by one thickness of the line)

• Read the scale (dial x10 = IOP in mmHg)

GAT

Repeatability

Take at least two readings, especially if values differ

GAT

Aftercare

• Tell patient vision may be slightly blurred from anaesthetic and fluorescein

• Check staining from probe contact

• Reaffirm how the anaesthetic lasts 30 minutes and patients must not rub their eye for this duration.