Comparison of Tono-Pen and Goldmann applanation tonometers for measurement of intraocular pressure in healthy children

Abstract

Purpose: To assess the agreement of intraocular pressure (IOP) measured with the Tono-Pen and the Goldmann applanation tonometer (GAT) in normal children and adolescents.

Methods: A total of 439 subjects from birth to <18 years of age without anterior segment anomalies or glaucoma had their IOP measured with the two instruments by separate, masked examiners in the office or under general anesthesia.

Results: On average, the Tono-Pen measured values slightly lower than the GAT for IOP <11 mm Hg and slightly higher than the GAT for IOP >11 mm Hg in the office setting. Using the average of GAT and Tono-Pen IOPs to estimate the true IOP, the average difference (GAT - Tono-Pen) was 0.4 mm Hg at IOP of 10 mm Hg and -3.0 mm Hg at IOP of 20 mm Hg. The 95% limits of agreement on the average difference between instruments were ± 6.4 mm Hg in the office setting and ± 6.8 mm Hg under general anesthesia. Larger differences between instruments were found with younger age. Standard error of measurement with the Tono-Pen was 1.44 mm Hg and 1.82 mm Hg for the office and anesthesia settings, respectively. Thicker corneas were associated with higher IOP with both the GAT and the Tono-Pen.

Conclusions: In normal children, average differences between IOP measured by Tono-Pen and GAT were small, although there was substantial test-retest variability. Younger age was associated with larger average differences, as was higher IOP in the office setting.

FIG 1

Difference between Tono-Pen and slit-lamp-mounted Goldmann applanation tonometer (GAT) by average intraocular pressure (IOP) in subjects measured in an office setting. The middle line gives the estimated mean GAT–Tono-Pen difference as a function of the average of the Tono-Pen and GAT measurements (x). The upper and lower lines are the 95% limits of agreement for the difference. The Tono-Pen underestimated IOP relative to GAT for IOP <11 mm Hg and overestimated IOP relative to GAT for IOPs >11 mm Hg.

FIG 2

Difference between Tono-Pen and handheld Goldmann applanation tonometer (GAT) by average intraocular pressure (IOP) in subjects measured under general anesthesia. The middle line gives the estimated mean GAT–Tono-Pen difference in IOP. The upper and lower lines are the 95% limits of agreement for the difference. On average, the Tono-Pen overestimated the IOP relative to the handheld GAT by 1.6 mm Hg across the range of IOP studied, with 95% of GAT–Tono-Pen differences falling within −8.4 and +5.2 mm Hg.

FIG 3

Goldmann applanation tonometer (GAT) determined intraocular pressure (IOP) by central corneal thickness (CCT). Slope (β) of the regression line is 0.019. Partial coefficient of determination (R²) for CCT is 0.04. The IOP determined by the GAT was 1.9 mm Hg higher for every 100 micrometer increase in CCT (P < 0.0001).