Non-Contact & Other Tonometry Methods: CPO Exam Guide

Beyond Goldmann: Why Multiple Tonometry Methods Exist

Goldmann applanation tonometry is the reference standard, but it has real-world limitations: it requires fluorescein drops, a topical anesthetic, and a slit lamp. In busy practices, with young children, or for patients who cannot cooperate with slit lamp positioning, alternative methods become essential. Understanding each device's strengths and weaknesses helps you choose the right tool for the right patient.

Non-Contact Tonometry (NCT)

Non-contact tonometry, widely called the "air puff" test, uses a rapid pulse of air to temporarily flatten the cornea. A detector measures the time it takes for the air puff to flatten the cornea to the standard endpoint, and the machine converts that into an IOP estimate.

The major advantages are speed and patient comfort: no drops are needed, the probe never touches the eye, and the measurement takes seconds. NCT is excellent for screening purposes in high-volume settings, particularly for patients who are apprehensive about eye drops or eye contact.

The drawback is accuracy. NCT tends to overestimate IOP, especially in patients with thicker corneas, and it is more variable than Goldmann. Results outside the normal range should be confirmed with a more precise method before clinical decisions are made.

Rebound Tonometry (iCare)

Rebound tonometry, most commonly represented by the iCare device, works by bouncing a tiny, lightweight probe off the corneal surface. The probe's deceleration upon contact is measured electronically and converted to IOP. Because the probe is so light and moves so quickly, most patients feel almost nothing.

This makes iCare particularly valuable for:

• Pediatric patients who cannot cooperate with air puffs or slit lamp measurements.
• Patients with needle anxiety who may resist even anesthetic drops.
• Home monitoring in select glaucoma patients (some iCare devices are designed for patient self-testing).

No anesthetic drops are required, which is a significant practical advantage. Accuracy is generally comparable to NCT, and it is considered a reliable screening device.

Tono-Pen

The Tono-Pen is a portable, handheld applanation tonometer. It makes multiple micro-applanation measurements across the cornea, averaging the results to report IOP. Because it involves direct contact with the cornea, a topical anesthetic is required.

The Tono-Pen is most useful when the patient cannot reach a slit lamp:

• Patients in wheelchairs or with limited mobility.
• Bedridden patients in hospitals or care facilities.
• Patients in the supine position (note: IOP is slightly higher when lying down than sitting up).

While not as accurate as Goldmann, the Tono-Pen offers more clinical precision than NCT and is appropriate for follow-up measurements when the gold standard is impractical.

Comparing Tonometry Methods

Positioning and Technique Tips

For NCT, position the patient at the machine with the chin on the rest and forehead against the bar. Instruct them to keep both eyes open and look at the fixation target. The device auto-fires when alignment is detected. Obtain at least three readings per eye and average them.

For iCare, hold the probe perpendicular to the corneal surface, within 4 to 8 mm of the eye. The device takes six measurements automatically and provides an average and standard deviation. A high standard deviation indicates poor repeatability, which may mean the probe was not held steadily.

Key Takeaways

• NCT is a non-contact screening tool that requires no drops; results should be confirmed with Goldmann if elevated.
• iCare (rebound tonometry) is ideal for children and anxious patients; no anesthetic needed.
• The Tono-Pen is a portable applanation device used for bedside or mobility-limited patients; requires anesthetic drops.
• Goldmann remains the gold standard for diagnostic IOP measurement.
• Screening devices identify potential problems; diagnostic devices confirm them.