Why is fluorescein used during Goldmann applanation tonometry?

Fluorescein stains the tear film and fluoresces green under cobalt-blue light, creating the two semicircular mires visible through the slit lamp during tonometry. Without fluorescein, the examiner cannot see the applanation contact zone and cannot judge when the inner edges of the two mires are properly aligned (the endpoint of the measurement). The mire alignment indicates the exact force at which the required area of cornea is being applanated.

What is the normal range for intraocular pressure?

The normal range for IOP by Goldmann applanation tonometry is approximately 10-21 mmHg, with a population mean of about 15-16 mmHg. Values above 21 mmHg are called ocular hypertension and are a risk factor for glaucoma. However, glaucoma can also occur at normal IOP (normal-tension glaucoma), and many people with IOP above 21 mmHg never develop glaucoma.

How does the CPOA prepare a patient for Goldmann tonometry?

The CPOA instills one drop of topical anesthetic (proparacaine or tetracaine) in each eye and waits 30-60 seconds for the cornea to become numb. Then, fluorescein is applied using a strip touched to the inferior fornix (not the cornea), and the patient blinks to distribute the dye in the tear film. The patient is positioned at the slit lamp and reminded to breathe normally and keep both eyes open.

How should a tonometer prism be disinfected between patients?

The applanation prism should be wiped with a 70% isopropyl alcohol swab and allowed to air dry completely for at least 5 minutes, or soaked in 3% hydrogen peroxide or 1:10 bleach solution per facility protocol. After soaking, rinse with sterile saline before use. Adequate drying or rinsing is critical -- residual alcohol or disinfectant on the prism will sting the patient's eye and may affect the IOP reading.

How does corneal thickness affect IOP readings?

Standard Goldmann tonometry assumes an average central corneal thickness (CCT) of about 520-540 micrometers. A thinner cornea is more easily applanated, so the tonometer underestimates true IOP. A thicker cornea resists applanation, leading to overestimation of IOP. This is clinically important because patients with thin corneas (including some normal-tension glaucoma patients) may have dangerously high 'true' IOP despite apparently normal Goldmann readings. CCT measurement allows clinicians to apply a correction factor.

Goldmann Applanation Tonometry for the CPOA Exam

Goldmann Applanation Tonometry: The Gold Standard

Goldmann applanation tonometry (GAT) is considered the gold standard for measuring intraocular pressure (IOP). It measures the pressure of the aqueous humor inside the eye by determining the force required to flatten (applanate) a specific area (3.06 mm diameter) of the corneal surface. Because the surface tension of the tear film and the corneal rigidity produce equal and opposite forces that cancel each other out at this exact diameter, the resulting reading reflects true IOP with high accuracy.

Normal IOP ranges from approximately 10-21 mmHg, with a mean of about 15-16 mmHg. Elevated IOP is a major risk factor for glaucoma, though glaucoma can occur at any IOP.

Patient Preparation: Role of the CPOA

Goldmann tonometry requires fluorescein dye and a topical anesthetic, both of which the CPOA typically instills under the doctor's direction.

Step 1: Instill Topical Anesthetic

Use a single-use topical anesthetic (proparacaine 0.5% or tetracaine 0.5%).
Instill 1 drop in each eye (or the eye to be tested).
Allow 30-60 seconds for full anesthesia before proceeding.
Warn the patient: "You may feel a brief sting, then your eye will feel numb -- that is normal."

Step 2: Apply Fluorescein

Fluorescein stains the tear film orange-yellow and fluoresces green under cobalt-blue light, creating the characteristic semicircular mires used during measurement.

Use fluorescein paper strips (preferred, lowest contamination risk) or sodium fluorescein drops (with proparacaine combination).
Touch the fluorescein strip gently to the inferior fornix while the patient looks up. Do not touch the cornea.
Ask the patient to blink to distribute the fluorescein in the tear film.
Excess fluorescein makes the mires too thick; insufficient fluorescein makes them too thin -- both reduce accuracy. Aim for a moderate, even coat.

💡 Clinical Tip: If you use too much fluorescein, the mire rings will be wide and indistinct. Instruct the patient to blink rapidly several times before the doctor begins to thin out the fluorescein layer. A well-applied tear film gives sharp, crisp semicircular mires that are easy to align.

The Goldmann Tonometer: Key Components

Tonometer prism (applanation head): The biprism that contacts the cornea, splitting the fluorescein ring into two semicircles.
Pressure dial: Displays the force (in grams) being applied to applanate the cornea. The CPOA should not touch this during measurement.
Slit lamp cobalt-blue filter: The blue light illuminates the fluorescein mires so the examiner can see them.

The Measurement Process (Doctor's Technique, CPOA Awareness)

The doctor centers the prism on the corneal apex and applies it gently.
Through the slit lamp, two green semicircles are visible. The doctor adjusts the pressure dial until the inner edges of the two semicircles just touch.
The IOP reading in mmHg = the dial reading x 10 (or read directly from the digital display on modern instruments).
Three readings are taken and averaged for accuracy.

Disinfection of the Tonometer Prism

Between patients, the tonometer prism (applanation head) must be properly disinfected to prevent transmission of pathogens including adenovirus, herpes simplex, and other ocular surface organisms.

Wipe the prism with 70% isopropyl alcohol swab and allow to air dry (minimum 5 minutes).
Or soak in 1:10 bleach solution or 3% hydrogen peroxide per manufacturer and facility protocol.
Do not use paper towels to wipe, as they can scratch the prism.
Rinse with sterile saline or irrigate before use if soaked in a chemical agent.

⚠️ Common Mistake: Returning the tonometer prism to the slit lamp without allowing adequate drying after alcohol wipe. Residual alcohol on the prism contacts the patient's cornea, causing stinging, tearing, and artificially low IOP readings due to disruption of the tear film. Always allow complete drying before use.

Factors Affecting IOP Readings

Corneal thickness: Thin corneas underestimate IOP; thick corneas overestimate. Central corneal thickness (CCT) is often measured as a correction factor in glaucoma management.
Corneal rigidity and scarring: Rigid corneas may give falsely high readings.
Squeezing or breath-holding: Squeezing the eyelids or Valsalva maneuver artificially elevates the reading. Encourage the patient to relax, breathe normally, and look straight ahead.
Excessive fluorescein: Artificially lowers readings by creating a thick film that compresses.

Key Takeaways

Goldmann applanation tonometry is the gold standard for IOP measurement, normal range 10-21 mmHg.
The CPOA instills topical anesthetic and applies fluorescein dye before the doctor measures.
Fluorescein should coat the tear film evenly -- not too thick, not too sparse.
Tonometer prisms must be disinfected between patients: 70% alcohol wipe with complete drying.
Corneal thickness, patient squeezing, and fluorescein amount affect accuracy.
Central corneal thickness is used clinically to correct for IOP measurement error in thin-cornea glaucoma suspects.