Goldmann Applanation Tonometry

Purpose

Goldmann applanation tonometry serves as the reference standard for measuring intraocular pressure in ophthalmology. IOP measurement is fundamental to:

• Screen for and diagnose glaucoma
• Monitor disease progression in patients with established glaucoma
• Assess treatment response to IOP-lowering therapies
• Evaluate IOP in patients at risk for angle-closure or secondary glaucomas
• Guide surgical decision-making in glaucoma management

GAT has been compared to alternative tonometry methods including rebound tonometry and non-contact tonometry, with studies showing variable agreement depending on patient populations and corneal characteristics.^1,2

Equipment

Required equipment includes:

• Goldmann applanation tonometer mounted on a slit lamp biomicroscope
• Cobalt blue filter for the slit lamp illumination system
• Topical anesthetic drops (e.g., proparacaine, tetracaine)
• Fluorescein sodium solution or fluorescein-anesthetic combination drops
• Sterile tonometer prisms (or disinfection system for reusable prisms)
• Alcohol wipes or appropriate disinfectant for the tonometer tip
• Tissues for patient comfort

The tonometer consists of a measuring drum calibrated in grams of force, with each 0.1 gram corresponding to 1 mmHg of IOP when using the standard conversion factor.

Steps

Patient Preparation:

1. Explain the procedure to the patient to reduce anxiety and ensure cooperation
2. Position the patient comfortably at the slit lamp with chin and forehead properly placed
3. Instill topical anesthetic in both eyes
4. Apply fluorescein dye—either separate fluorescein drops or combined anesthetic-fluorescein preparation; fluorescein is necessary for accurate measurement as omitting it results in significantly lower readings (averaging 7.01 mmHg lower) with errors increasing at higher IOP levels³

Measurement Technique: 5. Set the tonometer measuring drum to 10 mmHg (1.0 grams) as a starting point 6. Engage the cobalt blue filter on the slit lamp 7. Clean and mount a sterile tonometer prism on the tonometer head 8. Instruct the patient to look straight ahead at a distant target and breathe normally 9. Advance the tonometer toward the cornea using the joystick until the prism just touches the central cornea 10. Observe the fluorescein tear film through the eyepiece—two semicircular mires will appear 11. Adjust the tonometer height and angle so the mires are centered and equal in size 12. Turn the measuring drum to bring the inner edges of the two semicircles into alignment, forming an "S" pattern when properly positioned 13. Read the measurement from the drum (multiply the drum reading by 10 to obtain IOP in mmHg) 14. Withdraw the tonometer and repeat for the fellow eye 15. Consider taking multiple readings and averaging if there is variability

Post-Procedure: 16. Clean and disinfect the tonometer prism according to institutional protocol 17. Advise the patient not to rub their eyes until the anesthetic has worn off

Normal vs. Abnormal Findings

Normal IOP Range:

• Generally considered to be 10-21 mmHg in the general population
• Significant diurnal variation can occur, with pressures typically higher in the morning
• IOP should be similar between eyes; asymmetry >3 mmHg warrants further investigation

Elevated IOP:

• IOP >21 mmHg is traditionally considered elevated, though not all patients with elevated IOP have glaucoma (ocular hypertension)
• Markedly elevated IOP (>30 mmHg) requires urgent evaluation for acute angle-closure or secondary causes
• Serial measurements over time are more valuable than single readings

Low IOP:

• IOP under 8 mmHg may indicate ocular hypotony from various causes including overfiltering glaucoma surgery, uveitis, retinal detachment, or globe perforation

Factors Affecting Accuracy:

• Central corneal thickness significantly influences GAT readings; thicker corneas lead to overestimation of true IOP, while thinner corneas result in underestimation^2,4
• Corneal curvature affects measurements, with steeper corneas potentially yielding higher readings²
• Previous corneal refractive surgery causes systematic underestimation of IOP due to altered corneal biomechanics and thickness⁴
• Systolic blood pressure and body mass index have been associated with IOP measurements in some populations²

Common Mistakes

• Omitting fluorescein: Attempting GAT without fluorescein results in significantly lower readings (mean 7.01 mmHg lower) with greater errors at higher IOP levels³
• Excessive fluorescein: Too much dye creates thick, fuzzy mires that are difficult to align accurately
• Poor patient positioning: Inadequate chin or forehead contact with the slit lamp causes head movement and measurement variability
• Patient holding breath or Valsalva: Straining increases episcleral venous pressure and falsely elevates IOP
• Measuring through contact lens: Remove contact lenses before measurement
• Excessive pressure on globe: Pressing on the eyelid or globe during lid opening elevates IOP
• Prism misalignment: Off-center applanation or tilted prism creates asymmetric mires and inaccurate readings
• Multiple repeated attempts: Excessive corneal touch stimulates aqueous suppression and artificially lowers subsequent readings
• Ignoring corneal factors: Failing to consider central corneal thickness, especially in post-refractive surgery patients where GAT systematically underestimates IOP⁴
• Reading the drum incorrectly: Remember to multiply the drum reading by 10 to obtain mmHg

OSCE Script

Introduction (30 seconds):

"Hello, I'm [name], and I'll be measuring the pressure inside your eyes today using a technique called applanation tonometry. This is an important test to screen for glaucoma. I'll need to put some numbing drops in your eyes, then gently touch the surface of each eye with a small probe while you're positioned at this microscope. You'll feel slight pressure but no pain. Do you have any questions before we begin?"

Procedure (3-4 minutes):

"I'm going to instill numbing drops in both eyes now. You may feel a brief sting, but it will pass quickly." [Instill anesthetic and fluorescein]

"Please place your chin here and forehead against this bar. Try to keep your eyes open wide and look straight ahead at [indicate fixation target]. You can blink normally until I'm ready to take the measurement."

[Position tonometer, engage cobalt filter]

"Now I need you to keep both eyes open and look straight ahead. Try to breathe normally and stay relaxed."

[Advance tonometer to corneal contact, align mires, take measurement]

"Good, that's the first eye done. Now we'll do the other side. Same thing—eyes open, look straight ahead."

[Repeat for second eye]

Conclusion (30 seconds):

"All done. Your eye pressures today are [X] in the right eye and [Y] in the left eye. [Interpret based on findings]. The numbing drops will wear off in about 15-20 minutes. Until then, please avoid rubbing your eyes as you won't feel if you injure them. Do you have any questions?"

Clinical Pearls

• GAT remains the reference standard for IOP measurement despite the development of newer tonometry technologies; comparative studies show variable agreement between GAT and alternative devices depending on patient characteristics^1,2
• Always use fluorescein—omitting it produces clinically significant underestimation of IOP, with errors magnifying at higher pressure ranges where accurate measurement is most critical³
• Measure IOP before dilating pupils or performing gonioscopy, as these maneuvers can transiently affect pressure
• Central corneal thickness should be measured in glaucoma suspects; thick corneas (over 580 μm) cause overestimation while thin corneas (under 520 μm) cause underestimation of true IOP²
• In post-refractive surgery patients (LASIK, PRK), GAT systematically underestimates IOP due to altered corneal biomechanics; multiple correction formulas have been proposed though none is universally accepted⁴
• Take IOP measurements at the same time of day for longitudinal comparisons, as diurnal variation is common
• If readings differ significantly between attempts, take additional measurements and average the results
• Document which eye was measured first, as repeated tonometry can lower IOP
• In highly myopic patients, be aware that non-contact tonometry may overestimate IOP compared to GAT by approximately 3.75 mmHg on average²
• Irregular corneas (keratoconus, scars, edema) make GAT unreliable; consider alternative tonometry methods

Common Pitfalls

• Failure to use fluorescein: Results in systematic underestimation averaging 7.01 mmHg, with larger errors at elevated IOP levels when accuracy is most clinically important³
• Not accounting for corneal thickness: GAT was calibrated for average corneal thickness (~520 μm); significant deviations cause measurement error⁴
• Uncorrected interpretation after refractive surgery: Post-LASIK and post-PRK corneas yield falsely low GAT readings; using unadjusted values may delay glaucoma diagnosis⁴
• Excessive applanation force: Pressing too hard creates overlarge mires and overestimates IOP
• Insufficient applanation: Barely touching the cornea creates small mires and underestimates IOP
• Ignoring patient factors: Tight collar, breath-holding, or squeezing eyelids closed artificially elevates pressure
• Measuring through corneal pathology: Edema, scarring, or irregular astigmatism compromises accuracy
• Failing to clean prism between patients: Infection control breach and potential disease transmission
• Single measurement reliance: IOP varies throughout the day; isolated readings may miss pressure spikes
• Not correlating with clinical context: IOP is only one component of glaucoma assessment; normal pressure does not exclude glaucoma, and elevated pressure does not confirm it