Why is the corneal diameter 3.06 mm used in Goldmann tonometry?

At exactly 3.06 mm, the surface tension of the tear film and the resistance of the cornea cancel each other out. This means the force measured by the tonometer reflects intraocular pressure only, without needing to mathematically correct for those other factors. It is derived from the Imbert-Fick law applied to the physical properties of the cornea.

What happens if you use too much fluorescein?

Excess fluorescein makes the mire rings appear thicker and brighter, which causes the technician to align the inner edges at a point that requires more force than needed. This results in a falsely elevated IOP reading. The fluorescein strip should be lightly moistened, not soaked.

Why does central corneal thickness affect IOP readings?

Goldmann tonometry assumes the cornea has an average thickness of approximately 520-555 micrometers. A thicker cornea is stiffer and requires more force to flatten, causing the device to report a higher IOP than is actually present. A thinner cornea requires less force, leading to underestimation of true IOP. CCT measurements allow clinicians to interpret tonometry results more accurately.

What drops are required before Goldmann tonometry?

Two preparations are needed: a topical anesthetic (such as proparacaine or tetracaine) to numb the cornea so the prism contact is comfortable, and fluorescein dye (applied via fluorescein strip) to make the tear film visible under the cobalt blue slit lamp filter. Without these, the mires cannot be visualized and the patient would experience significant discomfort.

Goldmann Applanation Tonometry: CPO Exam Guide

Why Goldmann Tonometry Is the Gold Standard

Goldmann applanation tonometry (GAT) is the most widely accepted method for measuring intraocular pressure (IOP). It has been the clinical benchmark for decades because it produces consistent, reproducible results when performed correctly. Almost every other tonometry device is compared against Goldmann when evaluating accuracy.

The principle behind Goldmann tonometry comes from the Imbert-Fick law, which states that the pressure inside a sphere equals the force needed to flatten a defined area of its surface divided by that area. For a standardized corneal diameter of 3.06 mm, the surface tension of the tear film and the corneal rigidity happen to cancel each other out, leaving IOP as the only variable. This is why 3.06 mm is not an arbitrary number.

Equipment and Setup

GAT uses a biprism tonometer head attached to the slit lamp. The biprism splits the reflected fluorescent mires into two semicircles. Before the measurement, two things must be applied to the patient's eye:

Topical anesthetic (e.g., proparacaine or tetracaine drops) to eliminate discomfort when the prism contacts the cornea.
Fluorescein dye, typically a moistened fluorescein strip, which makes the tear film fluoresce bright yellow-green under the cobalt blue slit lamp filter.

The cobalt blue light excites the fluorescein and reveals the two semicircular mire patterns that you will align during the measurement.

Too much fluorescein makes the mires wide and bright, causing artificially high readings. Too little fluorescein makes the mires faint and hard to see. Aim for mires that are clear but not overly thick.

The Measurement Procedure

Once anesthetic and fluorescein are instilled, the patient sits at the slit lamp with their forehead and chin secured. You advance the tonometer prism until it gently contacts the center of the cornea. Through the eyepiece you see two fluorescent semicircles.

Your task is to turn the tonometer dial until the inner edges of the two semicircles just touch. Think of aligning the inner curve of the top half with the inner curve of the bottom half to form a complete circle at the point of contact. When they are precisely aligned, you read the number on the dial and multiply by 10 to get IOP in mmHg (the dial reads in tenths of mmHg).

Mire Alignment Situation	What It Means
Inner edges just touching	Correct measurement point
Mires overlapping	Too much force applied, reading too high
Mires separated	Too little force, reading too low

Pressing the prism too firmly against the cornea compresses the globe slightly and produces a falsely elevated reading. Use the minimum force needed to achieve mire alignment.

Normal IOP Range and Clinical Significance

The statistically normal range for IOP is 10 to 21 mmHg. Values above 21 mmHg are considered elevated and warrant further investigation, particularly for glaucoma risk. However, IOP alone does not determine glaucoma. Some patients have pressures above 21 mmHg without any glaucomatous damage (ocular hypertension), and others develop glaucoma with pressures within the normal range (normal-tension glaucoma).

For this reason, IOP is always interpreted alongside other findings: optic nerve appearance, visual field testing, and central corneal thickness (CCT).

Effect of Central Corneal Thickness

Central corneal thickness (CCT) affects GAT readings. A thicker-than-average cornea (greater than ~555 micrometers) causes the tonometer to overestimate true IOP, because more force is needed to flatten a stiffer surface. A thinner cornea underestimates IOP. Clinicians use correction formulas to adjust the measured IOP based on CCT, though this adds some uncertainty.

A patient with thin corneas and IOP of 18 mmHg may actually have a higher true IOP than the reading suggests. CCT is an important modifier when interpreting tonometry.

CPO Role in Goldmann Tonometry

CPO technicians typically instill the anesthetic and fluorescein drops and position the patient at the slit lamp. Depending on state regulations and office protocols, the CPO may perform the actual measurement. Understanding the procedure well enough to prepare the patient, explain what they will feel, and recognize a properly aligned mire reading is essential for exam and clinical practice.

Key Takeaways

Goldmann applanation tonometry flattens a 3.06 mm diameter area of cornea to measure IOP.
Fluorescein and topical anesthetic are required before measurement.
Mires are aligned so their inner edges just touch; the dial reading multiplied by 10 gives IOP in mmHg.
Normal IOP range is 10 to 21 mmHg, but context (CCT, optic nerve, visual field) always matters.
Thick corneas overestimate IOP; thin corneas underestimate it.