Purpose: To find out the effect of central corneal thickness (CCT) and radius of the corneal curvature on intraocular pressure (IOP) measurements using rebound tonometer (RBT) and Tono-Pen in healthy schoolchildren.
Methods: IOP was measured with Tono-Pen and RBT, respectively, in 165 healthy schoolchildren with a mean age 9.8±3.1 (range: 7 to 12 y) years. Corneal radius of curvature (in mm) was determined using a keratometer before CCT and IOP measurements. CCT was measured using an ultrasonic pachymeter after all IOP determinations had been made. The effect of CCT, radius of the corneal curvature, and sex on measured IOP was evaluated by linear regression analysis.
Results: The mean IOP readings were 17.47±2.7 mm Hg using Tono-Pen, and 16.81±3.1 mm Hg using RBT. Tono-Pen measured IOP values slightly greater than that of RBT (P=0.006). Mean CCT was found to be 561.37±33 μm. A significant association between measured IOP and CCT was found with each device (r=0.220 for the Tono-Pen, r=0.373 for the RBT; P=0.006 for the Tono-Pen and P<0.0001 for the RBT). The IOP increased 2.2 and 3.7 mm Hg for every 100-μm increase in CCT for the Tono-Pen and the RBT, respectively. The relation between IOP and CCT was not different for boys and girls. Mean radius of the corneal curvature readings was 7.68±0.41 mm (42.75±1.37 D) for both sexes. There was no significant relationship between either the mean corneal curvature readings, or CCT and IOP (r=0.02; P=0.4 for CCT and r=0.01; P=0.5 for IOP).
Conclusions: Both the Tono-Pen and RBT have a systematic error in IOP readings caused by its dependence on CCT. The CCT measurements should be considered to ensure proper interpretation of IOP measurements in children, like in adults. The corneal radius of curvature had no significant effect on measured IOP with each device.
Accurate intraocular pressure (IOP) measurement is an important step in the diagnosis and management of glaucoma. The Goldmann applanation tonometer is currently the most widely used device in clinical practice,1 and is considered the gold standard for IOP measurement. However, it is not always suitable and easy-to-use for younger children who are noncompliant and resisting the examination. Because of this, there is a need for a rapid, accurate, highly reproducible, and easy-to-use tonometer that allow IOP measurements to be made in the pediatric population.
There are several reports about IOP measurement in children.2–8 However, the instruments used and clinical conditions have varied in these studies. Tono-Pen and recently introduced rebound tonometer (RBT) are increasingly being used in pediatric population. However, there has been growing knowledge that corneal thickness and curvature also affect the IOP measurements obtained with these devices.9–15 Most studies studying the effect of CCT and corneal curvature on measured IOP were performed in adult population, and there is scarce available data in the literature about the relationship of CCT and corneal curvature to measured IOP in the pediatric population.14,16–19
In the current study, we evaluated the effect of CCT and corneal curvature on IOP measurements obtained with the Tono-pen and the RBT in healthy schoolchildren and we tried to assess if there is a relationship of CCT and corneal curvature to measured IOP in schoolchildren, as has been reported in adults.
A total of 165 primary schoolchildren were enrolled in this study. Informed consent was obtained from children's parents before the study. The local medical ethics committee approved the study and the tenets of the Declaration of Helsinki were followed.
Each subject underwent a complete ophthalmologic examination including cycloplegic refraction, slitlamp biomicroscopy, and fundus examination. Individuals with a spheric equivalent error more than ±5 D and with corneal diseases or with history of intraocular surgery were not included in the study. All study measurements were performed before the pupil dilatation required for refraction.
Corneal radius of curvature (in mm) was determined using a keratometer before CCT and IOP measurements. The mean of the 2 principal corneal meridians was calculated to give the mean radius of the corneal curvature.
All patients were checked in the sitting position. Subjects were given topical anesthesia (Alcaine, Alcon, Ft Worth, TX) bilaterally before IOP and CCT measurements. IOP was measured with the RBT (Tiolat Oy, Helsinki, Finland), and the Tono-Pen XL (Mentor Ophthalmics, Inc, Norwell, MA), respectively. All the RBT measurements were performed by examiner 1 (A.S.) and all the Tono-Pen measurements were performed by examiner 2 (H.B.). The RBT measurements were performed always first. Both observers were masked to the readings obtained. After then, CCT was measured by examiner 3 (N.Y.) with an ultrasonic pachymeter (Optikon 2000, Pacline, Rome, Italy). Three acceptable readings within a range of ±2 mm Hg for IOP and ±5 μm for CCT were recorded. Mean of the 3 readings was used in the statistical analysis. The time interval between tests of each tonometer was about 5 minutes. All the instruments were calibrated daily during the study period.
The RBT contains an assembly of 2 coils coaxial to a probe shaft that bounce a magnetic probe off the cornea and detect the deceleration of the probe caused by the eye. The moving magnet in the probe induces voltage in the solenoid and the motion parameters of the object are monitored. The probes used by the tonometer are disposable with a 1-mm diameter plastic cover and are 24-mm long and weigh 11 mg.
All statistical analyses were performed using SPSS for Windows, Version 11.0 (SPSS, Inc, Chicago, IL). The level of significance was set at P<0.05. For all statistical tests, the mean IOP and CCT of the 3 values within the preferred range were used. Because the eyes for one subject are not independent when considering ocular parameters such as CCT and IOP, only the right eyes were used for all statistical analyses. For each method of measurement, a linear regression equation was calculated after plotting IOP against CCT. From the graphs, the apparent increase in IOP per 100-μm increases in CCT was calculated for each device. In addition, linear regression analysis was performed to evaluate the influence of corneal curvature and sex on the measured IOP. The mean IOP measurement by Tono-pen was compared with the measurement by the RBT, by student t test. The correlation between the IOP measurements obtained with different methods was analyzed by Pearson correlation method. Bland-Altman analysis was performed to assess the clinical agreement between the 2 methods.20
One hundred sixty-five eyes of 165 children comprised the study group. Eighty-five (51.5%) subjects were boys. The mean age of the subjects were 9.8±3.1 years (range, 7 to 12 y). The age distribution between boys and girls was not statistically different (mean age for boys: 9.6±3; mean age for girls: 9.9±2.9; P=0.4).
The CCT values were normally distributed (Fig. 1). The mean CCT value was 561.37±33 μm.
The distribution of IOP values was normal for both the Tono-Pen and the RBT (Figs. 2, 3). The mean IOP readings were 17.47±2.7 mm Hg using the Tono-Pen and 16.81±3.1 mm Hg using the RBT. There was a significant correlation between Tono-Pen and RBT readings (r=0.512, P<0.0001). Mean IOP readings for the girls obtained by the Tono-pen and the RBT were 17.70±2.81 mm Hg and 16.86±3.21 mm Hg, respectively (Table 1). Mean IOP readings for the boys obtained by the Tono-pen and the RBT were 17.26±2.63 mm Hg and 16.76±3.08 mm Hg, respectively (Table 1). Tono-Pen measured IOP values slightly greater than those of RBT in girls (P=0.008). However, the difference was not statistically significant for boys (P=0.16).
The IOP measurements with both devices were correlated with CCT (r=0.220 for the Tono-Pen, r=0.373 for RBT; P=0.006 for the Tono-Pen and P<0.0001 for the RBT). The relationship between IOP and CCT was explored using linear regression analysis. The linear regression graphs for Tono-Pen and RBT are shown in Figures 4A and B. In this model, the IOP increases 2.3 mm Hg per 100-μm increase in CCT for the Tono-Pen and 3.5 mm Hg per 100-μm increase in CCT for the RBT for both sexes. The dependence of IOP on CCT was not different for boys and girls with both methods.
Figure 5 shows the mean difference between 2 devices and 95% limits of agreement plotted against the mean IOP values. Most values were within 6.36 to −5.04 mm Hg (95% limits of agreement) between the 2 devices. The mean difference was 0.66 mm Hg.
Mean radius of the corneal curvature readings was 7.68±0.41 mm (42.75±1.37 D) for both sexes. There was no significant difference between boys and girls in terms of mean corneal curvature (P<0.05). A linear regression analysis revealed no relationship between either the mean corneal curvature readings, or CCT and IOP (r=0.02; P=0.4 for CCT and r=0.01; P=0.5 for IOP).
There are numerous studies about the effect of central corneal thickness (CCT) on IOP measurements obtained by the Tono-Pen and the RBT in adults.9–11,13,14,21–27 However, there is lack of data regarding IOP values obtained by the Tono-Pen and the RBT in children and its relationship with CCT and corneal curvature.2,7,9,16–19 According to the medline survey, our study is the only report regarding IOP measured by the Tono-Pen and the RBT and its relationship with CCT and corneal curvature in healthy schoolchildren.
In the current study, we found that the mean CCT was 561.37±33 μm in healthy schoolchildren aged 7 to 12 years old. This value is similar to that reported previously for children and adults.16–19,28–30 There was no significant difference in terms of mean CCT between boys and girls. However, there is inconsistency about the relation between sex and CCT in the literature and no possible explanation was found.16
In our study, we tried to find out the possible impact of corneal thickness and corneal curvature on IOP measurements by Tono-Pen and RBT in schoolchildren. The measured IOP increased 2.3 and 3.5 mm Hg for every 100-μm increase in CCT for the Tono-Pen and RBT, respectively. These IOP increases fall within the range of data previously reported in children and adults.6,9,11,12,14–19,31,32 The Tono-Pen appeared to be less affected than the RBT. A possible explanation of the greater effect of CCT on RBT measured IOP lies in the viscoelastic property of the cornea,33 in which stiffness is related to the rate of application of strain. Similar results in previous studies performed in adults also showed that RBT was significantly more susceptible to the effects of CCT than Goldmann applanation tonometer and Tono-Pen.13,25,27
Despite the significant correlation coefficients obtained when comparing the RBT with the Tono-Pen, the instruments show considerable discrepancy in their measurements. The Bland-Altman plot indicates that the 95% confidence interval of the differences in measurements made with the 2 devices was between 6.36 and −5.04 mm Hg (Fig. 5). This indicates that in some cases IOP measurements with Tono-Pen were much greater and in some they were less than those measured by RBT. This value is similar to that reported previously in adults.13,22,25,27
As there are studies that discussed the correlations between corneal curvature and IOP in adults,17,21 there is no published data in the literature regarding this topic in healthy schoolchildren. The current study represents that there is no significant correlation between radius of the corneal curvature and IOP readings obtained by the Tono-Pen and the RBT in children. None of the subjects, however, had high refractive errors or substantial astigmatism. It is, therefore, still possible that high refractive errors can affect IOP measurements.
One of the limitations of our study is that we did not use Goldmann applanation tonometer to compare with the Tono-Pen and the RBT, because Goldmann applanation tonometry is difficult in some children who are uncooperative, crying, squeezing, and resisting the examination. It is more time consuming and needs greater cooperation when compared with those of the Tono-Pen and the RBT. We have seen cooperation problems during examinations, especially in children 7 to 9 years old. Some of the children did not want topical anesthetics to be instilled into their eyes before IOP and CCT measurements and this prolonged the time needed for measurements. However, these difficulties that we have experienced during the measurements were not severe as one could expect. Another limitation of our study is that we did not use the tonometers in a random order. We always used the RBT first. This may have induced some systematic error, although the fact that the mean IOP readings were higher than that of the RBT would argue against a tonography effect.
In conclusion, IOP readings by the Tono-Pen and the RBT are affected by CCT to differing degrees in schoolchildren, like in adults. Larger population-based studies in children are needed to find out correction factors for adjustment of IOP measured with these devices according to CCT.
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