Goldmann Applanation Tonometry: The Gold Standard for IOP Measurement

What Is Goldmann Applanation Tonometry?

Goldmann Applanation Tonometry (GAT) is considered the gold standard for measuring intraocular pressure (IOP). It is the reference method against which all other tonometers are compared. The instrument mounts directly onto a slit lamp biomicroscope, making it a standard component of every comprehensive ophthalmic examination.

How It Works: The Imbert-Fick Principle

GAT operates on the Imbert-Fick principle, which states that the pressure inside a perfectly flexible, dry, thin-walled sphere equals the force required to flatten a given area of the sphere divided by that area. The human cornea is not perfectly flexible or dry, but the Goldmann tonometer was specifically designed to minimize these sources of error.

The instrument applies a known force to flatten (applanate) a circular area of the cornea exactly 3.06 mm in diameter. At this specific diameter, the surface tension of the tear film pulling the tonometer tip toward the cornea roughly equals the corneal rigidity pushing it away, effectively canceling both sources of error.

Step-by-Step Procedure

1. Instill fluorescein and anesthetic: Place a drop of topical anesthetic (proparacaine or tetracaine) and fluorescein dye into the patient's eye. Some practices use combination strips.
2. Set up the slit lamp: Mount the tonometer on the slit lamp. Set the blue cobalt filter to illuminate the fluorescein. Set the dial to 1 (10 mmHg) as a starting point.
3. Position the patient: Have the patient sit comfortably with forehead and chin on the slit lamp rests.
4. Approach the cornea: Using the slit lamp joystick, gently advance the tonometer prism until it contacts the center of the cornea.
5. Align the mires: Through the oculars, you will see two glowing green semicircles (mires) created by the fluorescein-stained tear film. Adjust the tonometer dial until the inner edges of the two semicircles just touch.
6. Read the pressure: The dial reading multiplied by 10 gives the IOP in mmHg. For example, a reading of 1.6 on the dial equals 16 mmHg.
7. Withdraw and clean: Retract the tonometer from the cornea and clean the prism tip per infection control protocols.

Reading the Mires

Correct mire alignment is the most critical skill in GAT:

• Mires too wide apart: Too much fluorescein was used, or the dial reading is too high. The width of each semicircle should be approximately 10% of the total diameter.
• Mires too narrow: Too little fluorescein. The semicircles appear thin and difficult to read.
• Inner edges just touching: This is the correct endpoint. The inner edges should barely overlap.
• Pulsating mires: Normal. The mires pulse slightly with each heartbeat. Take the reading at the midpoint of the pulsation.

Sources of Error

Infection Control

The tonometer prism contacts the cornea directly, creating a risk of transmitting infections. After each use:

• Wipe the prism tip with an alcohol swab or soak in dilute bleach solution (per your facility's protocol)
• Allow adequate contact time for disinfection
• Rinse and dry before the next patient

Key Takeaways

• GAT is the gold standard for IOP measurement, based on the Imbert-Fick principle
• It flattens a 3.06 mm diameter area of the cornea using a prism mounted on a slit lamp
• The correct endpoint is when the inner edges of the two fluorescein mires just touch
• Central corneal thickness significantly affects accuracy: thick corneas read high, thin corneas read low
• Proper technique avoids errors from lid pressure, incorrect fluorescein amount, or patient squeezing