Can a tonopen (portable tonometer) be used to measure intraocular pressure in the event of an eye injury?

Use of Tonopen for Eye Injury

Contact tonometry should be deferred in the setting of corneal trauma, and if intraocular pressure (IOP) measurement is absolutely necessary, a Tonopen or other portable tonometer can be used with extreme caution—but IOP measurement is absolutely contraindicated in penetrating eye injuries. 1, 2

When to Avoid IOP Measurement Entirely

• In penetrating or open-globe injuries, any tonometry is absolutely contraindicated, as pressure on the globe can cause extrusion of intraocular contents through the wound 2, 3
• IOP measurement should be deferred unless absolutely necessary in any acute eye injury with suspected corneal trauma or infection 1
• The American Academy of Ophthalmology explicitly states that contact tonometry may be deferred in the setting of suspected ocular infection or corneal trauma 1

When Tonopen Can Be Used

If IOP measurement is clinically essential in a non-penetrating eye injury, portable tonometry with a Tonopen is a reasonable alternative to Goldmann applanation tonometry. 2, 4

Advantages of Tonopen in Trauma Settings:

• Portable and slit-lamp independent, allowing measurement in various patient positions and emergency settings 4
• Does not require the patient to be positioned at a slit lamp, which may be difficult or impossible with acute trauma 4
• Can be used with disposable covers to reduce cross-contamination risk 1
• Comparable accuracy to Goldmann tonometry when averaging 3 measurements (mean IOP 16.9 mmHg vs 17.7 mmHg for Goldmann, not statistically significant) 4

Critical Limitations in Trauma:

• Tonopen significantly underestimates true IOP in edematous corneas, with mean absolute error increasing as true IOP increases (ranging from 1.67 to 13.33 mmHg error) 5
• Requires a learning curve of approximately 10 measurements even for experienced ophthalmologists 4
• Reproducibility is inferior to Goldmann tonometry by a factor of 2 4
• Three measurements should be averaged for acceptable accuracy 4

Alternative Tonometry Methods for Injured Eyes

For corneal injuries with edema, scarring, or irregular surfaces, alternative tonometry methods are strongly advised over both Goldmann and Tonopen. 1, 2

Preferred alternatives include:

• Pneumotonometer: Uses a 5-mm fenestrated silicone tip that conforms to irregular corneal surfaces, generating 40 readings per second 1, 2
• Rebound tonometry (iCare): Does not require topical anesthesia and may perform better at the limbus than central cornea in edematous corneas (mean absolute error 5.22 mmHg at limbus vs 10.83 mmHg centrally) 1, 2, 5
• Dynamic contour tonometer (Pascal): Calculates IOP independent of corneal properties, though requires slit-lamp mounting 1, 2

Clinical Algorithm for Eye Injury

1. First, rule out penetrating injury through careful history and examination—if penetrating injury is suspected or confirmed, do NOT measure IOP 2, 3

2. If non-penetrating trauma with intact globe, assess whether IOP measurement will change immediate management:

   • If not immediately necessary, defer measurement 1
   • If essential (e.g., suspected traumatic hyphema with risk of elevated IOP), proceed cautiously 2

3. For corneal edema or irregularity, consider limbal measurement with Tonopen (mean absolute error 5.47 mmHg) or iCare (mean absolute error 5.22 mmHg) rather than central corneal measurement 5

4. Always average 3 measurements with Tonopen for acceptable accuracy 4

Key Pitfalls to Avoid

• Never attempt tonometry on a penetrating injury—this is the most critical error that can result in devastating vision loss 2, 3
• Do not rely on a single Tonopen measurement; the device requires averaging multiple readings 4
• Be aware that Tonopen will underestimate true IOP in edematous or traumatized corneas, potentially missing dangerously elevated pressures 5
• Ensure proper disinfection after use (5-10 minute dilute bleach soak at 1:10 concentration) to prevent cross-contamination 1