Applanation tonometer simulator

Goldmann applanation tonometry is a slit-lamp test that measures intraocular pressure (IOP, the fluid pressure inside the eye) by gently flattening a 3.06 mm circle of the numbed cornea and reading the force needed.

Parts you use

• Tip (prism): the clear cone that touches the cornea. A split prism inside cuts the tear ring into two half circles.
• Mires: the two green half circles you line up.
• Measuring dial: the knob on the side, convert its reading to mmHg (millimeters of mercury, the unit eye pressure is recorded in) as the instrument's instructions describe.
• Cobalt blue filter: the slit lamp's blue light that makes fluorescein glow.
• Joystick and height control: move the slit lamp forward, back, up and down.

Steps

1. Clean the tip and set the dial. Use a cleaned, disinfected tonometer tip (prism). Start with the dial at a low setting.
2. Numb and stain the eye. Put in a topical anesthetic (numbing drop) and fluorescein (a yellow-green dye) as your clinic directs. The dye is what makes the tear ring visible.
3. Turn on the cobalt blue light. Set the slit lamp to the cobalt blue filter with the beam angled toward the tip. Under blue light the fluorescein glows green.
4. Touch the cornea. Ask the patient to look straight ahead and keep both eyes open. Slowly move the slit lamp forward until the tip just touches the center of the cornea.
5. Center the mires. Look through the eyepiece. Mires are the two green half circles. Keep the two half circles centered in view and the same size.
6. Check the mire thickness. The amount of fluorescein changes the mire width, and mire width affects the reading. Fix mires that look too thick or too thin before you read.
7. Turn the dial. Adjust the dial until the inside curves of the two green half circles meet, with no gap and no overlap. A gap means turn up; overlap means turn down.
8. Read and record. Convert the dial reading to mmHg as the instrument's instructions describe. Record the eye, the pressure, the method and the time, then clean the tip.

Reading the mires

• Apart: not enough force yet. Turn the dial up.
• Inside curves just meeting: the endpoint. Read the dial.
• Overlapping: too much force. Turn the dial down.
• Top and bottom unequal: re-center the tip before reading; the dial does not fix this.
• Too thick or too thin: mire width affects the reading, so fix it first. (In this simulator, thick reads high and thin reads low.)

About 10–21 mmHg is the commonly cited normal range. In this simulator, readings outside it are flagged as a study cue only; the doctor interprets IOP. The simulator's thick/thin error (3 mmHg, and its direction) is its own model for practice; real errors vary.

FAQ

How do you read a Goldmann tonometer dial?

Multiply the dial number by 10, so 1.6 on the dial is 16 mmHg and 2 is 20 mmHg.

What is normal eye pressure?

About 10 to 21 mmHg is the commonly cited normal range. A single number does not diagnose glaucoma; the doctor reads it with the rest of the exam.

Why do you need fluorescein and cobalt blue light for applanation?

The mires are the fluorescein-stained tear ring around the tip. Without fluorescein and the cobalt blue filter you cannot see them, so you cannot find the endpoint.

Where should the mires line up?

At the endpoint, the inside curves of the two green half circles meet with no gap and no overlap. A gap means the dial needs to go up; overlap means it needs to come down.

What happens if the mires are too thick or too thin?

Mire width affects the reading. Mires that look too thick or too thin come from the wrong amount of fluorescein in the tear film, so fix them before reading. In this simulator, thick mires read high and thin mires read low (the simulator's own model).

What can make applanation readings inaccurate?

Pressing on the globe while holding the lids open can make the reading falsely high.

Do you need to clean the tonometer tip?

Yes. The tip touches the eye, so it is cleaned and disinfected between patients according to the maker's instructions and your clinic's infection-control policy.

Studying for the COA exam? See the COA certification guide. More clinic simulators: visual acuity test, swinging flashlight test, lensometer and keratometer.