Glaucoma is a leading cause of visual impairment. It has been estimated that 76.0 million of world population will suffer from glaucoma by year 2020.1 Intraocular pressure (IOP) is the crucial modifiable risk factor in the management of glaucoma, but measuring the true IOP clinically can be challenging as it requires direct contact with the eye. Hence, a variety of tonometry devices have been developed to measure IOP. In clinic setting, Goldmann applanation tonometry is widely accepted as the gold standard for IOP measurement. However, the Goldmann applanation tonometer (GAT) must be mounted on a slit lamp, making it less portable to be used in a community setting. Besides, the device requires a trained ophthalmologist/clinician to operate. The Perkins applanation tonometer (PAT) is a handheld device that works on the same principle as the GAT. Established for over 50 years, Perkins is synonymous to the ‘gold standard’ in handheld applanation tonometers. The PAT yields IOP measurements that are closely comparable to GAT.2
The Icare tonometry uses rebound technology, in which a light-weight probe is used to make a momentary contact with the central cornea. The IOP is measured using deceleration rate and contact time of the probe touching the central cornea. The Icare rebound tonometer is used widely in the pediatric and community care practices as a screening tool for IOP measurements because anesthetic is not needed and the probe touch is barely noticed by the patient.
In this study, we aimed to compare the Icare rebound tonometer with the reference handheld method (PAT) in a community eye screening setting and to assess the agreement between these 2 instruments.
This was a cross-sectional, non-interventional study. All subjects who attended the community eye screening program organized by the Eye Department of Faculty of Medicine and Health Sciences, Universiti Malaysia Sarawak, Malaysia during January 1, 2017 to December 31, 2017 were recruited. Those with corneal scar, corneal pathology, active ocular infective disease, recent intraocular surgery, and allergy to topical anesthetic drop were excluded. This study was approved by the Medical Ethics Committee of Faculty of Medicine and Health Sciences, Universiti Malaysia Sarawak (03/2017). Informed consent was obtained from all recruited subjects.
Relevant demographic data such as age and sex were recorded. The IOP measurement by handheld Icare TA01i tonometer (Finland) was first performed by a primary care physician in a sitting position. Six measurements were done on the right eye followed by the left eye without topical anesthesia drop. The device discarded the highest and lowest IOP reading and displayed the mean of the remaining 4 readings. Disposable probe tip was changed for each subject and used in both eyes of the same person if there was no active ocular infective condition.
After half an hour, the IOP of the same subjects was measured using Perkins Mk3 applanation tonometer (Haag-Streit, UK) by a single ophthalmologist (S.L.T.) who was masked to previous readings from the Icare rebound tonometer. Each eye was instilled with an anesthetic agent (proparacaine 5%) and application of fluorescein 1%. The applanation prism tip was cleaned for each subject. The subject was positioned with head inclined backward and slight neck extension. One reading was taken for each eye. The same tonometer was used and calibrated before each session to minimize the intra-instrument variability.
Statistical analyses were performed with the SPSS software (Windows version 22). The mean IOP measured by each tonometer was compared. Pearson correlation coefficient was used to explore the correlation between the IOP measurements of the 2 instruments. The level of agreement between them was assessed using the Bland-Altman analysis. The bias was the mean of the differences between pairs of measurements. The limits of agreement were defined as the mean difference ± 1.96 SD of the differences between the 2 instruments. A P value less than 0.05 was considered statistically significant.
A total of 420 eyes were examined during the study period. The mean (± SD) age of the subjects was 38.6 ± 18.2 years (range, 13-82 years). Among them 67% of the subjects were female. The interclass correlation coefficients between the right and the left eyes were 0.899 for Icare and 0.952 for Perkins, indicating that the 2 eyes in each study group behaved similarly. Therefore, only 1 eye (left eye) was used for analysis.
The Icare significantly overestimated IOP in 52 eyes compared with PAT which recorded IOP within normal range. There was no correlation between IOP overestimation with patient's age (Pearson r = -0.28, P = 0.84). Those with high Icare reading also documented with raised PAT reading. After removing the outliers, the paired sample t-test revealed a statistically significant mean difference between the 2 instruments (t = 17.068, df = 419; P < 0.001). The mean IOP was high in Icare rebound tonometer (16.3 ± 4.0 mm Hg) compared with Perkins applanation (13.4 ± 2.3 mm Hg), with a mean difference of 2.90 ± 3.5 mm Hg (Table 1). In the younger age group, we found a difference in IOP measured by the 2 tonometers, with Icare overestimated IOP in the pediatric group than in the adult group (Fig. 1).
Pearson correlation coefficient showed a moderate positive correlation between the 2 methods of IOP measurement (r = +0.524, P < 0.001). A linear regression analysis of PAT versus Icare measurements revealed a slope of 0.28 with R2 of 0.255 and y-intercept was 8.77 (Fig. 2).
The Bland-Altman plots demonstrated a fair agreement between the 2 methods of IOP reading (Fig. 3). The 95% limits of agreement between the 2 methods were between -9.73 and 3.93 mm Hg. After removing the multivariate outliers using significant Mahalanobis distance and studentized residuals (>1.96), the linear regression analysis revealed that the correlation coefficient was significantly different from 0 (P < 0.001).
Applanation tonometry is the gold standard for IOP measurement. In community eye screening, a large number of population are to be screened within a limited time frame. Therefore, portability, reliability, and cost are crucial factors influencing tonometry choices used in mass screening. Because of manpower constraint, paramedic personnel and optometrists are usually required to perform a quick IOP reading. Hence a device that is reliable and easy to operate is paramount.
The PAT is handy, portable, and reliably reads IOP in all positions. However, using the device needs a long learning curve, hence it is usually operated by ophthalmic trained personnel. In addition, infection transmission,3,4 risk of corneal abrasion, the need to use topical anesthetic agent and fluorescein dye, as well as the possibility of causing stinging, creating eye heaviness and reducing IOP due to the use of anesthetic drop makes PAT a not-so-ideal applanation tonometer to be used in community eye screening.5
The Icare tonometer is a handheld, battery-operated device that uses small, lightweight, single-use probe negating anesthetic agents or stain. The possibility of cross-infection is lower compared with PAT. Using Icare does not require highly trained personnel as the instrument is easy to handle and use. Furthermore, repeated measurements by Icare rebound tonometer do not reduce IOP caused by the ocular massage effect.6
In our study, the IOP reading began with Icare tonometer followed by PAT in order to reduce the morphological alteration and ocular massage effect7,8 that can be caused by Perkins applanation. Contact applanation has the disadvantage of causing expression of aqueous humor from anterior chamber that subsequently causes artificially lower IOP reading.
Few studies independently concluded positive correlation of Icare rebound tonometer with the reference standard handheld technique, PAT9-11 (Table 2). The Icare rebound tonometer was found to overestimate IOP reading when compared with PAT in both healthy subjects and glaucoma patients (Table 2).9-14 Our study showed that the Icare rebound tonometer read the IOP on average 2.9 ± 3.5 mm Hg higher than the PAT in our Asian population. After removing the multivariate outliers, a linear regression analysis with enter method showed a correlation coefficient that is statistically significantly different from 0 (P < 0.001). This might be due to proportional bias which aroused from measurement errors either by subject or observer factors. In our study, the Icare tonometer overestimated IOP in the pediatric group compared with the adult group. We postulated that cornea rigidity in pediatric population played a role in this finding. More detailed studies into this postulation are required for a definitive conclusion.
This study had several limitations. First, the operating manual of PAT recommends an average of 3 readings, but this study took 1 measurement only. However, this is a community eye screening and time is a constraint for the measurement. Besides there is a possibility of ocular massage effect by repeated reading, so we decided that 1 reading was taken for each participant. Second, central corneal thickness should be measured and correction factors applied in the IOP measurements by the 2 types of tonometers. These were not studied in our study and therefore might affect the mean difference of IOP measurements between the 2 instruments. Third, corneal curvature might affect IOP measurements by Icare tonometer, but this was not measured in this study. Lastly, the IOP measured in our patients usually fell within the normal range of 10 to 21 mm Hg. Further studies are warranted to investigate the reliability of Icare rebound tonometer in measuring higher IOP.
In conclusion, the Icare rebound tonometer is an acceptable screening tool to measure IOP in community practice as it is easier and quicker to use compared with PAT. In this study of Asian subjects, however, the Icare tonometer overestimated IOP with a mean of 2.90 mm Hg higher than the PAT. We therefore suggest using GAT as a confirmatory measurement tool for IOP.
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