Purpose
To compare intraocular pressure (IOP) among rebound, noncontact, and Goldmann applanation tonometry (GAT) and their relationships to central corneal thickness in children.
Design
Diagnostic protocol comparison and evaluation.
Methods
In right eyes of 419 children, mean IOP, rates of successful measurement with 3 tonometries, and intermethod agreement by Bland-Altman plot were assessed. The influences of central corneal thickness, and of average IOP of 3 tonometries on IOP differences between tonometries, were evaluated.
Results
The mean age was 8.89 ± 3.41 years (3–15 years). There was significant difference in mean IOP of each tonometry; GAT showed the lowest values ( P < .05). The IOP was successfully measured by noncontact tonometry in 89%, by rebound tonometry in 75%, and by GAT in 64% of children less than 10 years old, and in 100%, 98%, and 94% of children older than 10 years, respectively. The IOP of each tonometer positively correlated with central corneal thickness ( P < .05). The mean differences and limits of agreement were 0.81 ± 6.19 mm Hg (noncontact minus rebound), 2.56 ± 4.62 mm Hg (rebound minus GAT), and 1.81 ± 4.76 mm Hg (noncontact minus GAT). Rebound and noncontact tonometry overestimated IOP relative to GAT for thicker central corneal thicknesses. Rebound tonometry overestimated IOP relative to GAT and noncontact tonometry at higher average IOP of 3 tonometries.
Conclusions
Rebound, noncontact, and Goldmann applanation tonometries can be considered appropriate methods for children, though noncontact tonometry is the most accessible. Given the risk of false-positive diagnosis of pediatric glaucoma, attention should be devoted to children with IOP within a suspicious range or thicker cornea.
Intraocular pressure (IOP) is the only known modifiable risk factor relevant to the treatment of glaucoma. Intraocular pressure reduction is a proven therapy that slows down the progression of disease. Thus, the accurate measurement of IOP is essential for the diagnosis and treatment of glaucoma.
Goldmann applanation tonometry (GAT) has long been considered the “gold standard” for IOP measurement. Goldmann applanation tonometry also has been known to have less variation in repeated checks than noncontact tonometry. However, accuracy can be influenced by many factors related to the cornea, especially central corneal thickness. Furthermore, accurate IOP measurement in children is particularly difficult owing to poor cooperation, especially with contact-type tonometers. Instillation of a topical anesthetic prior to employment of GAT, moreover, can increase anxiety in children.
Rebound tonometry, which is also known as impact or dynamic tonometry, uses the impact rebound principle by launching a magnetized probe against the eye using a solenoid. The probe slows down faster as the IOP increases; thus, when the impact duration becomes shorter, IOP is calculated to be higher. This method is easy to use, affordable, and quick, and above all, it does not require local anesthesia. These points of measurement enable the measurement of IOP in noncompliant patients, especially a pediatric population. Several studies have evaluated rebound tonometry vs GAT, and most of them have detected higher mean IOP readings for rebound tonometry than for GAT.
Noncontact tonometry has been proven to have several favorable characteristics because it does not require any corneal contact or local anesthesia, enabling convenient usage and easy measurement. However, the instrument is designed for eyes with clear, smooth corneas; and the measurement can be difficult or even impossible in patients who cannot see the fixation target and/or anxiously squeeze eyelids. Moreover, noncontact tonometry has both underestimation and overestimation issues, as established by several studies. A couple of reports found that IOP values measured with noncontact tonometry significantly overestimated those measured with GAT; Tonnu and associates reported, moreover, that noncontact tonometry significantly both underestimated and overestimated GAT at low and high IOP, respectively.
The present study compared IOP readings by rebound tonometry, noncontact tonometry, and GAT in order to determine which type is best tolerated in clinical practices with children and which is most reliable. In addition, the influence of central corneal thickness on the IOPs measured with each tonometry was evaluated.
Methods
Patients
The medical records of 715 children (age range: 3–15 years) who visited a pediatric ophthalmology clinic between June 2013 and March 2014 were reviewed in this retrospective diagnostic protocol comparison and evaluation. All of the subjects were selected in accordance with the following exclusion criteria: parental or patient refusal to check IOP at first visit, corneal astigmatism higher than 2 diopters (D), corneal diseases, contact lens wearing, microphthalmos, any history of intraocular or refractive surgery, nystagmus, ocular inflammation, glaucoma, periocular steroid use within 3 months of enrollment or current systemic steroid use, or any ocular or systemic medication likely to affect IOP. This study adhered to the tenets of the Declaration of Helsinki, and approval from the Institutional Review Board (IRB) of Kangnam Sacred Heart Hospital was obtained.
Intraocular Pressure Measurement
Three independent examiners performed IOP measurements for each tonometry at 10-minute intervals, with the patient in the sitting position and prior to pupil dilation. To avoid the influence of the previously performed tonometry, IOP measurements were made sequentially in the following order: noncontact tonometer (Topcon CT80; Topcon, Tokyo, Japan), rebound tonometer (ICare PRO; ICare, Helsinki, Finland), and Goldmann applanation tonometer (Haag-Streit AG, Bern, Switzerland). This order was established to avoid any possible change in IOP as a result of corneal compression or aqueous massage. To minimize any potential bias, the examiners did not read the previously measured IOP. If the IOP value was recorded for each tonometry, the measurement was considered to be successful.
For noncontact tonometry, the IOP was recorded according to the mean value of 3 consecutive measurements. Rebound tonometry was measured by operating the button carefully, avoiding shaking the device and hitting the central cornea with the tip of the probe. Six measurements were taken consecutively, and the average value was automatically calculated. A new disposable probe was used for each patient. To prepare for GAT, topical anesthetic (Alcaine proparacaine hydrochloride; Alcon, Fort Worth, Texas, USA) and fluorescein dye (Norvatisfluorescein; Norvatis, Basel, Switzerland) were instilled. Before the measurement of GAT, the drum was reset to approximately 10 mm Hg after each reading, and the biprism was cleaned with 3% hydrogen peroxide.
After all of the IOP readings had been obtained, central corneal thickness was measured using an ultrasound pachymeter (SP-100; TOMEY, Nagoya, Japan). The pachymeter probe was placed on the center of the cornea, and 5 consecutive central corneal pachymetry values were recorded. The mean of the readings also was calculated and recorded.
Outcome Measures
The main outcome measures were the mean IOP and the rates of successful measurement by the 3 tonometries. If the IOP value was recorded as guided, the measurement was considered to be successful. The intermethod agreement was assessed, and the influences of age, sex, and central corneal thickness, and of the average IOP of the 3 tonometries, on the IOP differences between the tonometries were evaluated.
Statistical Analysis
All analyses were performed using statistical software (SPSS for Windows V.12.0.K; SPSS Inc, Chicago, Illinois, USA). A P value less than .05 was considered statistically significant. The mean IOP obtained by each tonometer was compared using analysis of variance, and a Pearson χ 2 test was conducted to compare the rates of successful measurement between tonometries. The intermethod agreement was assessed by Bland-Altman plot, and the calculation of the mean difference among the measurements, the standard deviation (SD), and the 95% confidence interval (CI) of the differences was included. The deviation of each tonometry reading from the other types’ was calculated and correlated with age, sex, and central corneal thickness using a linear regression model. The correlation between the intermethod IOP difference and the average IOP of the 3 tonometries also was evaluated using a linear regression model.
Results
Among a total of 715 children whose medical records were reviewed, 419 were included in this study: 198 boys (47.3%) and 221 girls (52.7%). The measurements of only right eyes were used for the analysis. The mean age of the subjects was 8.89 ± 3.41 years (range: 3–15 years). The mean IOP obtained was 17.69 ± 2.44 mm Hg by noncontact tonometry, 16.87 ± 3.06 mm Hg by rebound tonometry, and 15.21 ± 2.06 mm Hg by GAT. There were significant differences in the measured IOP for each tonometry ( P < .05). The mean central corneal thickness was 560.91 ± 33.04 μm ( Table 1 ).
| Variables | Measurement |
|---|---|
| Age (mean ± SD, y) | 8.89 ± 3.41 (range: 3–15) |
| Sex (male/female) | 198/221 |
| NCT (IOP, mean ± SD, mm Hg) | 17.69 ± 2.44 |
| RT (IOP, mean ± SD, mm Hg) | 16.87 ± 3.06 |
| GAT (IOP, mean ± SD, mm Hg) | 15.21 ± 2.06 |
| CCT (mean ± SD, μm) | 560.91 ± 33.04 |
The rates of successful measurement of IOP were 93% for noncontact tonometry, 85% for rebound tonometry, and 77% for GAT, with the lowest success rates in GAT among the 3 tonometries. For children less than 10 years old, the rates were 89% for noncontact tonometry, 75% for rebound tonometry, and 64% for GAT. Meanwhile, for children older than 10 years, the rates were 100% for noncontact tonometry, 98% for rebound tonometry, and 94% for GAT ( Table 2 ).
| Tonometry | Total (n = 419) | <10 Years (n = 244) | ≥10 Years (n = 175) |
|---|---|---|---|
| NCT | 391 (93%) | 216 (89%) | 175 (100%) |
| RT | 356 (85%) | 184 (75%) | 172 (98%) |
| GAT | 321 (77%) | 156 (64%) | 165 (94%) |
The mean differences between each of the tonometry pairs were 0.81 ± 2.74 mm Hg (noncontact minus rebound), 1.81 ± 2.43 mm Hg (rebound minus GAT), and 2.56 ± 2.36 mm Hg (noncontact minus GAT). These differences all were statistically significant ( P < .05). On a Bland-Altman scatterplot, the values of the 95% levels of agreement were within the −4.56 to +6.19 mm Hg (noncontact minus rebound), −2.06 to +7.18 mm Hg (rebound minus GAT), and −2.95 to +6.57 mm Hg (noncontact minus GAT) ranges, respectively ( Figure 1 ).
The measured IOPs of the 3 tonometries showed a significant positive correlation with central corneal thickness: the thicker the central corneal thickness, the higher the IOP reading ( P < .05) ( Figure 2 ). With linear regression analyses, both rebound and noncontact tonometry overestimated IOP relative to GAT at thicker central corneal thicknesses and underestimated the value at thinner central corneal thicknesses. In correlation with the average IOP of the 3 tonometries, rebound tonometry overestimated IOP relative to both GAT and noncontact tonometry at the higher average IOP of 3 tonometries ( Figure 3 ). No statistically significant difference was observed in relation to age and sex.
