Non-Contact Tonometry, iCare, and Tono-Pen

Beyond Goldmann: Alternative Tonometry Methods

While Goldmann Applanation Tonometry (GAT) remains the gold standard, several alternative tonometers serve important clinical roles. These instruments are particularly useful for screening, pediatric patients, portable settings, or situations where a slit lamp is not available or practical.

Non-Contact Tonometry (NCT)

The non-contact tonometer, commonly called the "air puff" tonometer, measures IOP without touching the eye. It releases a precisely calibrated puff of air that flattens the central cornea. Sensors detect the moment of applanation and calculate the IOP based on the air pressure required.

Advantages

• No corneal contact means no risk of cross-contamination
• No anesthetic drops required
• Quick and easy to perform, minimal training needed
• Excellent for high-volume screening

Limitations

• Less accurate than GAT, especially at higher pressures
• Tends to overestimate IOP in some patients
• Affected by corneal thickness like all applanation methods
• The air puff startles some patients, particularly children and anxious individuals

NCT is best used as a screening tool. Elevated readings should be confirmed with GAT before clinical decisions are made.

iCare Rebound Tonometry

The iCare tonometer uses rebound technology. A small, lightweight magnetized probe is launched toward the cornea at low velocity. The instrument measures how quickly the probe decelerates and rebounds after briefly contacting the corneal surface. Higher IOP causes the probe to decelerate and rebound more quickly.

Advantages

• No anesthetic needed: The probe contact is so brief and gentle that patients barely feel it
• Excellent for children: The handheld design and painless measurement make it ideal for pediatric populations
• Portable: Battery-operated and lightweight
• Single-use, disposable probes eliminate cross-contamination risk
• Takes multiple readings and calculates an average automatically

Limitations

• Affected by corneal thickness (overestimates in thick corneas)
• Less reliable at very high IOP values
• Requires proper probe alignment with the central cornea
• Positioning must be horizontal (patient seated upright looking straight ahead)

Tono-Pen

The Tono-Pen is a handheld electronic tonometer that uses a small, disposable latex cover over its tip. It contacts the cornea briefly multiple times (typically four acceptable readings) and calculates an averaged IOP. It also provides a coefficient of variation indicating the consistency of the readings.

Advantages

• Portable: Can be used at the bedside, in the operating room, or with patients who cannot sit at a slit lamp
• Works on irregular corneas where GAT mires may be distorted
• Can measure IOP through bandage contact lenses
• Useful for patients in supine position (wheelchair, stretcher, operating table)

Limitations

• Requires topical anesthetic
• Less accurate than GAT at extreme IOP values
• User technique affects accuracy (perpendicular approach, no excess pressure)
• Disposable covers add ongoing cost

Comparison of Tonometry Methods

Key Takeaways

• NCT (air puff) is a no-touch screening tool best used for high-volume IOP screening, not definitive diagnosis
• iCare rebound tonometry requires no anesthetic and is ideal for children and uncooperative patients
• The Tono-Pen is portable and works well on irregular corneas, supine patients, and through bandage lenses
• All alternative methods are affected by corneal thickness
• Elevated readings from any screening tonometer should be confirmed with Goldmann applanation tonometry