What does a tonopen measurement tell you?

What a Tonopen Measurement Tells You

A Tonopen measures intraocular pressure (IOP) in millimeters of mercury (mmHg), providing a portable alternative to Goldmann applanation tonometry, though it is less accurate and should not be considered interchangeable with the gold standard, particularly in glaucoma management. 1, 2

Primary Clinical Information

The Tonopen provides:

• Numerical IOP value expressed in mmHg, which reflects the fluid pressure inside the eye 1
• Screening information for potential glaucoma risk, though IOP alone is insufficient for diagnosis 1
• Monitoring data for patients with known glaucoma or ocular hypertension, though with significant limitations 3

Key Advantages of Tonopen

The device offers practical benefits in specific clinical scenarios:

• Portability allows measurement in children, supine patients, or those unable to position at a slit lamp 1, 4
• Ease of use by ophthalmic technicians without requiring slit lamp expertise 1
• Pediatric applications where compact instruments facilitate IOP measurement in difficult-to-examine children 1
• Topical anesthesia requirement is minimal compared to Goldmann applanation 1

Critical Limitations You Must Understand

The Tonopen has substantial accuracy problems that can lead to dangerous clinical errors, particularly in glaucoma patients:

• Underestimates elevated IOP: In eyes with Goldmann IOP ≥22 mmHg, Tonopen measured significantly lower values (mean difference -3.6 mmHg), and missed elevated IOP in 33% of eyes 3
• Overestimates low IOP: In eyes with Tonopen IOP ≤10 mmHg, Goldmann measured 11-17 mmHg in 66% of cases 3
• Wide variability: IOP differences >±3 mmHg occurred in 34% of eyes, with a range of -14 to +12 mmHg compared to Goldmann 3
• Glaucoma-specific problems: In bilateral primary open-angle glaucoma patients, Tonopen consistently underestimated IOP by approximately 1 mmHg compared to Goldmann 3
• Extreme IOP unreliability: All portable tonometers, including Tonopen, are less reliable at IOP values <10 or >24 mmHg 5

Factors Affecting Tonopen Accuracy

Multiple variables influence measurement reliability:

• Central corneal thickness (CCT) significantly affects readings, with thin corneas underestimating and thick corneas overestimating true IOP 1, 2, 6
• Corneal properties including hydration, curvature, astigmatism, and scarring introduce measurement artifacts 1, 2
• Patient age and glaucoma type affect the magnitude of difference between Tonopen and Goldmann measurements 6
• Order of measurement matters—the first tonometer used gives higher readings in uncannulated eyes 7

When Tonopen May Be Appropriate

Despite limitations, specific clinical situations warrant its use:

• Pediatric examinations where children cannot cooperate with slit lamp positioning 1
• Supine patients who cannot sit upright for Goldmann tonometry 4
• Screening in high-risk populations (older adults, family history, African Americans, Hispanics) when Goldmann is unavailable 1
• Initial assessment before referral to comprehensive glaucoma evaluation 1

Critical Clinical Pitfalls to Avoid

Never rely solely on Tonopen for glaucoma management decisions:

• Do not use for treatment titration in established glaucoma patients, as it may mask inadequate IOP control 3
• Do not switch between devices during longitudinal follow-up, as this introduces variability that obscures true IOP changes 2, 8
• Do not assume "normal" Tonopen readings exclude elevated IOP—confirm with Goldmann applanation tonometry 3
• Do not use in post-refractive surgery eyes where IOP is already underestimated; alternative methods like pneumotonometry or dynamic contour tonometry are preferred 1, 2

Proper Interpretation Framework

When you receive a Tonopen measurement, consider it within this context:

• Screening tool only: Values suggesting elevated IOP (>21 mmHg) require confirmation with Goldmann applanation 1, 2
• Baseline assessment: Useful for initial evaluation but inadequate for monitoring progression 3
• Clinical correlation mandatory: Always integrate with optic nerve examination, RNFL imaging (OCT), and visual field testing rather than relying on IOP alone 1, 2, 8
• Risk stratification: Remember that approximately half of glaucoma patients have IOP <22 mmHg, so "normal" Tonopen readings do not exclude disease 1, 9

Alternative Methods for Accurate IOP Assessment

When corneal abnormalities or clinical situations demand greater accuracy:

• Pneumotonometer: Generates 40 readings/second with fenestrated silicone tip that conforms to irregular corneas 1, 2
• Dynamic contour tonometer (Pascal): Calculates IOP independent of corneal properties by sampling 100 times/second 1, 2
• Ocular response analyzer: Measures corneal biomechanical properties and provides "corneal-compensated" IOP 1, 2
• Rebound tonometry (iCare): Requires no anesthesia and shows better agreement with Goldmann than Tonopen in some studies 1, 4