|Year : 2016 | Volume
| Issue : 1 | Page : 22-26
Comparison of Goldmann applanation tonometer, Tono-Pen and noncontact tonometer in children
Usha Kaul Raina, Neha Rathie, Anika Gupta, Shantanu Kumar Gupta, Meenakshi Thakar
Department of Ophthalmology, Guru Nanak Eye Centre, Maulana Azad Medical College, New Delhi, India
|Date of Web Publication||10-Feb-2016|
E-187, New Rajendar Nagar, New Delhi - 110 060
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Background: To evaluate the agreement of Goldmann applanation tonometer (GAT) with Tono-Pen and noncontact tonometer (NCT) for measurement of intraocular pressure (IOP) in pediatric age group and to evaluate the correlation between central corneal thickness (CCT) and IOP measured with the tonometers used.
Materials and Methods: IOP was measured in 200 eyes in a group of Indian children, aged between 8 and 18 years using three different tonometers: NCT, the Tono-Pen and GAT. All IOP readings were made in the office settings by the same examiner. Readings obtained were compared between the instruments and with the CCT for each tonometer. Tonometer inter-method agreement was assessed by the Bland-Altmann method. The relations of CCT with absolute IOP values and inter-tonometer differences were analyzed by linear regression.
Results: The mean age was 13.37 3.51 years. The mean IOP values recorded with NCT; Tono-Pen and GAT were 14.38, 15.63, and 12.44 mmHg, respectively. Both Tono-Pen and NCT recorded statistically higher IOP values than the GAT (P = 0.00) regardless of the CCT. The percentage increase of IOP measured over GAT was 15.66% for NCT and 25.70% for Tono-Pen which was also statistically significant. A correlation was found between CCT and IOP values obtained with all the three tonometers.
Conclusion: IOP measurements on children vary significantly between instruments and correlations are affected by the corneal thickness. Further studies on children are needed to determine which instrument is most appropriate and to derive a normative IOP scale for the growing eye.
Keywords: Central corneal thickness, intraocular pressure in children, tonometers
|How to cite this article:|
Raina UK, Rathie N, Gupta A, Gupta SK, Thakar M. Comparison of Goldmann applanation tonometer, Tono-Pen and noncontact tonometer in children. Oman J Ophthalmol 2016;9:22-6
|How to cite this URL:|
Raina UK, Rathie N, Gupta A, Gupta SK, Thakar M. Comparison of Goldmann applanation tonometer, Tono-Pen and noncontact tonometer in children. Oman J Ophthalmol [serial online] 2016 [cited 2020 Feb 29];9:22-6. Available from: http://www.ojoonline.org/text.asp?2016/9/1/22/176096
| Introduction|| |
Comprehensive eye care for children and adolescents requires measurement of intraocular pressure (IOP). The World Glaucoma Congress wrote that "the question whether one tonometer is superior to the others in pediatric patients is unresolved."  The widely accepted international gold standard for IOP measurement in adults is the Goldmann applanation tonometer (GAT).  However, measuring IOP with conventional contact tonometers can be difficult in young patients owing to their lack of co-operation.  In addition, there are various factors such as the central corneal thickness (CCT), subject's seated or recumbent position, eye movements or facial mask expressions, which influence IOP measurement in children. ,,,,,
The Tono-Pen tonometer was developed for use in patients who present with the sort of measurement problems that are often associated with children. The instrument is easy to handle, portable, light weight, and does not require the use of fluorescein.  Tonopen is useful for supine patients and in the presence of corneal pathology.  A significant advantage of noncontact tonometry (NCT) is the elimination of potential hazards associated with all contact tonometers viz., corneal abrasion, reaction to topical anesthetic or flourescein and spread of infection.  However, with NCT, the subject should be able to fixate on the target and hence its use is limited in children with poor fixation, nystagmus, and inability to see the target. Eyes with corneal surface irregularities are also not suitable for use of NCT because the instrument is designed for clear and smooth corneas. 
Although accurate and consistent IOP measurements are difficult to obtain, they are of extreme clinical importance when trying to diagnose and manage pediatric glaucoma.  Moreover, there have been few studies comparing the accuracy of various tonometers conducted in children. ,,, The present study compared IOP readings by GAT, Tono-Pen, and NCT to determine which type is best tolerated in clinical practices with children and which is most reliable. In addition, the influence of CCT on the IOPs measured with each tonometer was evaluated.
| Materials and Methods|| |
An observational study was performed at Guru Nanak Eye Centre, Maulana Azad Medical College, New Delhi from 2012 to 2013. Two hundred eyes of 100 pediatric were included in the study who presented to either the Outpatient Department or Pediatric Ophthalmology Clinic. Children included in the study were between 8 and 18 years of age with the ability to have CCT and IOP measurements taken in the office. Children with any anterior segment dysgenesis, cataract, active inflammation, glaucoma, and those with any gross ocular pathology such as microphthalmos, aniridia, and coloboma were excluded from the study. The study was reviewed and approved by the Institute's Ethics Committee.
A written informed consent was obtained from parents or guardians of all the children. All children underwent a detailed ophthalmologic examination including visual acuity using Snellen's chart, cycloplegic refraction with 2% homatropine, slit lamp examination, fundus examination, keratometry, and axial length measurement. CCT was measured using Pac scan plus digital biometric ruler (Pacscan 300 AP, Sonomed, Lac Success, New York). The pachymeter probe was placed on the center of the cornea, and three consecutive central corneal pachymetry values were recorded. The mean of the readings also was calculated and recorded.
IOP measurements were done using noncontact tonometer (NCT 10, Shin Nippon, Japan), Tono-Pen (Medtronic Ophthalmic Incorporation, Jacksonville, Florida, USA) and GAT (Haag-Streit AG, Bern, Switzerland) in that sequence, at an interval of 15 min, right eye followed by left. For NCT and Tono-Pen readings, an average of three successive measurements was taken. Before the measurement of GAT, the drum was reset to approximately 10 mmHg after each reading, and the biprism was cleaned with 3% hydrogen peroxide. IOP was recorded after instilling topical anesthetic (proparacaine hydrochloride) and fluorescein dye in the eye. All measurements were done in the afternoon between 2 and 3 pm in sitting position and by the same examiner to avoid any inter-observer variation. To avoid any intra-instrument variability, the same tonometers were always used. GAT was calibrated each week, and Tono-Pen calibration was carried out each day prior to IOP recording.
The mean value of three consecutive measurements by each tonometer was used for final analysis. For CCT also, an average of three successful readings was used. Inter-method agreement was assessed, and the influence of CCT on the IOPs measured was evaluated.
The data thus obtained was analyzed using SPSS 17 (SPSS Statistics for Windows, Version 17.0. Chicago: SPSS Inc.). The difference between the two means was observed by the paired t-test, and a P < 0.05 was considered statistically significant. Agreement between the methods was observed by estimating the limits of agreement and assessed using Bland and Altmann plots.  Correlation between the CCT and the IOP measured by various instruments was studied using Pearson's/Spearman's rank correlation coefficient.
| Results|| |
Results of 200 eyes of 100 healthy subjects (47 boys and 53 girls) were included in the study. Mean age of the subjects was 13.37 ± 3.51 (range: 8-18 years).
Mean IOP recorded was 14.38 ± 2.35 mmHg by NCT, 15.63 ± 2.78 mmHg by Tono-Pen and 12.46 ± 1.44 mmHg by GAT. There was statistically significant difference between the IOP measured by the three instruments regardless of the corneal thickness (P = 0.00). The percentage increase over GAT was 15.66% for NCT and 25.70% for Tono-Pen. On an average NCT overestimated IOP readings by GAT by 1.96 ± 4.16 mmHg while Tono-Pen overestimated GAT by 3.2 ± 5.18 mmHg within the IOP range studied. The difference in IOP measurement was more in the higher IOP range when the average IOP was more than 14 mmHg.
There was good inter-method agreement, i.e., between GAT and Tono-Pen and GAT and NCT with 95% limits of agreement falling between −1.98 to 8.38 and −2.2 to 6.12, respectively [Figure 1] and [Figure 2].
|Figure 1: Bland-Altman plot for Goldmann applanation tonometer and Tono-Pen|
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|Figure 2: Bland-Altman plot for Goldmann applanation tonometer and noncontact tonometry|
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Mean CCT was 539.65 ± 30.48 μm (range: 464-604 μm). Thicker corneas were associated with higher IOP with all the three tonometers used. CCT was found to have a significant correlation with IOP measured with Goldmann and NCT while Tono-Pen readings varied less with a change in corneal thickness. For 100 μm change in CCT, IOP changed by 1.03 mmHg with GAT, 2.41 mmHg with NCT and only 0.81 mmHg with Tono-Pen [Figure 3],[Figure 4] and [Figure 5]. Of the other factors studied such as axial length, average keratometry values, and spherical equivalent, none was found to have a significant correlation with IOP measured with the three instruments.
|Figure 3: Correlation of Goldmann applanation tonometer and central corneal thickness|
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|Figure 4: Correlation of noncontact tonometry and central corneal thickness|
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| Discussion|| |
For many years, applanation tonometry was considered to set a standard against which other instruments were measured. Measuring IOP in children is more challenging because it requires certain capabilities from the instrument and some degree of experience from the user. The instrument must provide quick, accurate readings. For these reasons, the standard Goldmann tonometer, which is mounted on a slit lamp, is not suitable for children. However, recent reviews that have compared a number of instruments suggest that depending on the population and the application, different instruments may be preferred. ,,,
Studies in the past have determined the agreement between different tonometers. Minckler et al.  compared GAT with the first generation Tono-Pen (Tonopen-1) in adults and found similar readings in the IOP measured by the two methods. They reported that Tonopen-1 underestimated the IOP in eyes with higher IOP and overestimated the IOP in eyes with lower IOP. Frenkel et al. also compared Tono-Pen and GAT in adults and concluded that the Tonopen measures IOP in a manner that corresponds well to the GAT in the 11-20 mmHg interval, and fairly well in the 4-10 mmHg and 21-30 mmHg interval. 
Bradfield et al.  in their study in children and adolescents found that in an office setting, Tono-Pen measurements tended to be slightly lower than GAT when IOP was <11 mmHg and slightly higher than GAT when IOP was >11 mmHg. In normal children, average differences between IOP measured by Tono-Pen and GAT were small, although there was substantial test-retest variability. Bordon et al.  compared the accuracy of IOP measurement in pediatric patients afflicted with retinopathy of prematurity using three tonometers: Perkins, Schiotz, and Tono-Pen. A good correlation was found between Tono-Pen and Perkins tonometers, whereas the Schiotz measurements were significantly higher. Hence, they suggested that Tono-Pen with a higher correlation coefficient can be used reliably to assess IOP in pediatric patients. Kageyama et al.  compared the ICare rebound tonometer with NCT in healthy children and concluded that IOP measurements performed using ICare are better tolerated in the pediatric population, as compared with measurements using NCT, especially in children below the age of 6 years. A recent study by Feng et al. compared rebound tonometry, NCT, and GAT and their relationships to CCT in children. They suggested that all three tonometers would be clinically acceptable for pediatric patients with IOP within the normal range of values, but children with IOPs within a suspicious range should be re-evaluated carefully. 
In our prospective study, we compared GAT, Tono-Pen and NCT in pediatric eyes. Findings from this study show that both Tono-Pen and NCT provided statistically higher IOP values (15.63 ± 2.78 mmHg and 14.38 ± 2.35 mmHg, respectively) than GAT (12.46 ± 1.44 mmHg) regardless of the corneal thickness. The difference in the IOP recordings with different tonometers was more for higher IOP range. This is probably because a lesser amount of force is required to applanate a softer eye and vice versa. This difference is probably exaggerated in the case of Tono-Pen as its principle combines both indentation and applanation.
IOP measurement is influenced by the CCT.  IOP measurements obtained with GAT tended to be overestimated in eyes with thick corneas and underestimated in eyes with thin corneas in children.  Bradfield et al. found that the GAT measured IOP was 1.9 mmHg higher for every 100 um increase in CCT in children.  This accords with the results of a prior small pediatric study by Muir et al.  They suggested that this small difference in the effect of CCT may be secondary to the large corneal surface area contacted by the GAT device. Whereas, Tono-Pen measurements have been found to be less dependent than GAT on the biochemical properties of cornea.  Recently, Feng et al.  also found the rebound and noncontact tonometry to overestimate IOP relative to GAT for thicker CCT.
In this study also, thicker corneas were associated with higher IOP recordings with all three tonometers used. CCT was found to have a significant correlation with IOP measured with GAT and NCT while Tono-Pen readings varied less with a change in corneal thickness. These findings are probably because Tono-Pen applanates a smaller area of cornea compared to GAT and hence, its recordings are less influenced by the CCT.
To the best of our knowledge, this is the first study of IOP measurements on healthy Indian children. It contributes to the literature on IOP in the young eye and provides a comparative study to compare the commonly used tonometers in children along with the correlation between CCT and IOP measurements. However, our study has a few limitations. First, we enrolled only normal subjects and hence it might not be appropriate to extrapolate this data in children with or suspected of glaucoma. Second, in this study, IOP was measured sequentially with NCT, Tono-Pen, and GAT. As noted by Feng et al., this might contribute to the higher mean IOPs recorded with NCT and Tono-Pen because after repeated IOP measurements the child may feel more comfortable with the procedure. The application of topial anesthetic for GAT may also contribute to less anxiety and increased cooperation by the child.
| Conclusion|| |
Each of the three tonometers evaluated in this study can be routinely used in the clinical setting for IOP measurement in children. However, eyes with high IOP measurements must be carefully evaluated for the effect of CCT and other factors which might influence IOP readings in children. Tonopen may be used for children in the presence of corneal pathology, as it is least affected by variations in corneal thickness and NCT may be used in the presence of corneal or other ocular infection when a contact procedure is contraindicated. However, further studies for IOP measurements on large populations of children, with and without eye disease or conditions that may affect IOP, from a variety of countries and ethnic groups is required, to find an ideal and accurate tonometer applicable for every age group.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]