Glaucoma can be defined in the public health context as a progressive optic neuropathy with characteristic optic nerve head structural changes and functional visual dysfunction often accompanied by typical visual field defects. It is a multifactorial disease and is most often, though not always, associated with increased intraocular pressure (IOP). It is the second leading cause of preventable global blindness among those who are 50 years and older and the third leading cause of blindness globally. The global prevalence of glaucoma in individuals with age between 40 and 80 years is 3.54%. The number of people affected by glaucoma has increased from 64.3 million in 2013 to 76 million in 2020, and it is projected to increase to 111.8 million by 2040. The global distribution of glaucoma is disproportionate, and Asia houses 60% of all patients with glaucoma and 76.7% of those with primary angle-closure glaucoma (PACG).
Elevated IOP is recognized as the most important risk factor for the development and progression of glaucoma. Even though age and family history are considered risk factors, IOP continues to be the only disease-modifying factor that is tractable and predisposed to modulation. A reliable and authentic IOP measurement is imperative since its assessment constitutes the principal screening, diagnosis, and management strategy for patients with glaucoma. Furthermore, there is a 10%–18% increased risk of developing glaucoma for a 1 mmHg increase in IOP, and similarly, an IOP decrease of 1 mmHg curtails the advancement of glaucoma by 10%. This singularly accentuates the importance of accurate IOP measurement for the successful management of glaucoma.
Diverse and distinctive methods of tonometry are in practice for measuring IOP. Since its inception in 1948, Goldmann applanation tonometer (GAT) acquired recognition for its accuracy and reliability in measuring a wide range of IOPs. GAT is still regarded as the “gold standard” that employs a noninvasive measurement technique operating on the Imbert–Fick principle (P = F/A), which states that “the pressure (P) inside an ideal thin-walled sphere is directly related to the force (F) applied to applanate its surface, divided by the flattened area (A).” The precision and repeatability of the device are contingent, assuming a standard central corneal thickness (CCT) of 520 mm (changed to ~ 545 mm for ultrasound optical pachymetry) and a regular corneal composition. GAT measurement is susceptible to discrepancies in CCT and pathological conditions such as corneal curvature, axial length, irregular corneal surface, corneal edema, abnormal corneal curvature, scleral thickness, and rigidity. Besides these deficits, GAT requires anesthesia and is not ideal for resistive patients who are bedridden, in the pediatric setting, as well as for patients who have undergone refractive surgery. Perkins applanation tonometer (PAT) is a portable handheld adaptation of GAT and was proven to provide clinically accepted comparability with that of GAT.[16,17]
Therefore new, portable, noninvasive transpalpebral/eyelid/digital tonometers such as TGDc-01, Diaton, and EASYTON (ET) were introduced to mitigate the measurement error induced by the corneal thickness and other biomechanical properties.[6,18] Numerous research publications have assessed the extent of reliability and applicability of transpalpebral tonometers (TTs) such as TGDc-01, Diaton,[20,21] and EASYTON in clinical practice with variable results. Several publications[15,23] discuss various tonometers in general, and Chakraborty et al., in their review which compared TT with GAT, concluded that the former overestimated IOP in the lower ranges and underestimated in the higher IOP ranges. In 2012, Cook et al. conducted a meta-analysis which compared the agreement of various tonometers that are in routine use in clinical practice with that of GAT. The study determined a pooled estimate with a mean difference (MD) of − 0.5 mmHg and a 95% limits of agreement (LoA) from − 6.9 to 5.9 mmHg for TTs and 46% of the measurements were estimated to be within the range of 2 mmHg. However, to the best of our knowledge, no meta-analysis has been published recently comparing the reliability of TTs with GAT. Therefore, this systematic review and meta-analysis aims to update the meta-analysis published by Cook et al. by focusing primarily on comparing the reliability and agreement of TTs against the gold standard GAT among individuals presenting for ophthalmic examinations. The current paper describes the methodology employed for conducting the systematic review and meta-analysis.
Design and registration
A systematic review and meta-analysis study design will be employed to summarize the methods-comparison studies published, which compared TTs with GAT. The development, conduct, design, and reporting of this study protocol are in congruence with the Preferred Reporting Items for Systematic Reviews and Meta-analyses Protocol (PRISMA-P)[24,25] statement and the Guidelines for Reporting Reliability and Agreement Studies (GRRAS). Supplement 1 show the PRISMA-P checklist. This protocol is registered with the International Prospective Register of Systematic Reviews (registration number - CRD42022321693). Even though the current study will use retrospective data, as mandated by our institutional policy, the study with the project code RET202200390 was presented to our Institutional Ethics Committee and was approved on May 30, 2022.
Studies that compare the accuracy of TTs with that of GAT or its equivalent will be considered. A customized search will be conducted to include studies done on human subjects and original research articles published (full text and abstract) in English. When an article is deemed relevant based on the review of its abstract and if the full text is in another language, an attempt will be made to translate the same into English before incorporating it into the review. Studies that used adult participants with normal eyes or with ocular conditions such as glaucoma, ocular hypertension (OHT), and keratoconus will only be included. We will exclude studies that involve children and adolescents (below 18 years). Review articles, case reports, conference abstracts, guidelines, editorials, commentaries, and opinion articles will be excluded. Those studies that compare TTs with other types of tonometers and studies that have used TTs alone without a GAT or PAT comparison will also be excluded.
The data will be gathered employing an electronic search using PubMed, Cochrane Library, and Google Scholar. The search will include a group of terms related to transpalpebral tonometry and IOP measurement. The literature search will include articles published from January 2000 to September 2022. Keywords that will be used in all search engines are (“Tonometer” OR “tonometry” OR “Goldmann Applanation tonometer” OR “contact tonometer” AND “Transpalpebral Tonometer” OR “Digital Eyelid Tonometer” OR “TGDc-01” OR “Diaton” OR “EASYTON”) AND (“Diagnostic accuracy”). Further, in order to ensure the most recent studies are not overlooked, the exact search will be repeated before the final data analysis.
The citations ascertained through the search strategy will be exported to Zotero; bibliographic management software and the duplicates will be removed. Two reviewers (NB and JS) will scrutinize the title and abstracts based on predetermined selection criteria, and an inventory of all the selected articles will be created. Any disagreement will be resolved through deliberation and mutual consensus. Full text of all the selected articles will be retrieved and will be read thoroughly to authenticate their appropriateness to be considered for data extraction. The reference lists of all the studies that will be included for analysis will be reviewed to search for any articles that may have been missed during the search. The process of study selection procedure is presented through the PRISMA flow chart [Fig. 1].
Data extraction will be performed by two reviewers (JS and VS) using a standard data extraction form that will incorporate essential variables that need to be synthesized from the primary studies. Inconsistency in data extraction between JS and VS will be finalized by the third reviewer BS. To establish whether the agreement between two autonomous reviewers is at random or by chance, Kappa statistics will be used to identify the degree of agreement between the two reviewers.
The following available data will be extracted from the eligible studies using an Excel sheet: the name of the first author and the year of publication, the country in which the study was carried out, study design (retrospective or prospective), the number of eyes included in the study (sample size), mean and standard deviation (SD) of age, mean and SD of IOP measurements for TT and GAT, MD (m diff), the SD of MD (SD m diff), LoA (low and high), posture used for obtaining IOP using TT (sitting or supine), and patient preference between the two instruments.
In studies where a single outcome is not reported and where different groups of patients are involved, each group will be considered a separate study. Similarly, in studies that used pre- and post-procedure, each will be considered an independent study. Data will be extracted separately for each of the aforesaid scenarios. Also, if a study has two measurements taken with the same device using two separate enumerators, the measurement taken by the first enumerator will be considered for data extraction.
The statistical analysis will be conducted using ReviewManager (RevMan) 5.4 (The Cochrane Collaboration) and StataBE 17 (StataCorp). The risk of bias analysis will be performed using RevMan and the rest of the meta-analysis will be carried out using StataBE 17. The forest plot will be generated to show the individual and pooled SD and 95% LoA, along with the author name, year of publication, study weights, and mean percentage error. Earlier publications on meta-analysis using methods-comparison studies[29,30] have used the framework suggested by Williamson et al. But the current review will attempt to apply a recent framework published by Tipton and Shuster for the meta-analysis of Bland–Altman studies based on the LoA approach.
Assessment of risk of bias
Quality Assessment of Diagnostic Accuracy Studies (QUADAS-2) tool will be used for the assessment of the risk of bias. Quality assessment of bias and applicability has become an indispensable component of the systematic review process which ascertains whether the results of individual studies are valid. The focus is to evaluate the introduction of possible systematic error (i.e., bias) based on the methods adopted, not by the comprehensive assessment of methodological quality. The QUADAS-2 risk of bias evaluation is effected in four major domains, namely, participant selection, index test, reference standard, and flow and timing, which incorporates signaling questions to facilitate risk of bias judgment. Assessment will also be performed to identify applicability concerns that pertain to the first three domains. For each domain and concerns regarding the applicability, studies are rated as “low,” “high,” or “unclear.”
Due to the lack of established guidelines for the quality assessment of method-comparison studies, the assessment question in the QUADAS-2 guidelines will be modified analogously to the approach elaborated by Joosten et al. JS and BS will tailor the quality assessment form to achieve relevance and to make it compatible with the current review comparing the agreement and reliability between TT and GAT. A pilot will be administered to ensure both reviewers achieve consistency in their assessment.
The general principles recommended by the Cochrane collaboration will be followed in dealing with the missing data. The corresponding authors of the selected articles will be contacted to request for missing outcome data or, where applicable, for the entire data set to perform further statistical analysis. Pursuant to nonresponse or rejection of data request, data will be imputed based on standardized statistical models. Further, sensitivity analysis will be performed to investigate the robustness and stability of the results, and the potential effect of missing data on review findings will also be discussed.
Testing for heterogeneity
Meta-analysis reporting on heterogeneity has become a standard, and this test explores to ascertain whether the differences between the study findings are due to genuine underlying factors (clinical, methodological or statistical) or due to chance alone. Investigating the existence of coherence among study effects across the included studies in the meta-analysis is imperative since it directly affects the precision with which we can generalize the results of the meta-analysis. The current meta-analysis will use Cochran’s Q test and the I2 statistic to assess heterogeneity among individual studies. It will be considered to have statistically significant heterogeneity when the probability value is less than 0.1 (P < 0.1). A low, moderate, or high heterogeneity will be concluded when the value is less than 25%, between 25% and 75%, and more than 75%, respectively.
Assessment of publication bias
The evidence generated by systematic review and meta-analysis, which explains the best available risks and benefits of medical interventions, has the possibility to facilitate better decision-making in the domain of clinical medicine and public health. However, caution must be applied in interpreting the findings which may be influenced by publication bias that can influence the results drastically. Publication bias refers to the tendency or inclination toward the preparation, submission, and publication of study results that are positive or statistically significant findings. The risk of publication bias will be investigated using a funnel plot, which plots bias against the standard error, and further analysis will be performed using Begg’s rank test and Egger’s weighted regression test.
Subgroup analysis and meta-regression will be used to explain possible sources of statistical heterogeneity when there were differences. The following subgroup analyses will be performed based on the availability of relevant data: (i) devices type, (ii) CCT, (iii) time taken between IOP measurement of TT and GAT, (iv) posture used for the IOP measurement with TT, and (v) patient preference between IOP measured using TT and GAT. The MD will be compared with the listed subgroups to draw appropriate conclusions.
Ethics and dissemination
Systematic review deals with secondary data, and therefore, no primary data will be collected directly from human subject participants. Hence, obtaining informed consent is not necessary. The findings of the review will be promulgated in preeminent peer-reviewed journals and will be disseminated in pertinent forums such as conferences and journal clubs.
The review team has commenced searching for relevant literature in databases based on the inclusion and exclusion criteria mentioned above. We anticipate to complete the review by January 2023.
The available results on the agreement of transpalpebral tonometers compared with GAT show mixed results. No recently published meta-analysis has exclusively compared the agreement and reliability of transpalpebral tonometers with GAT. Therefore, we envisage that our review results might provide additional evidence on the reliability of transpalpebral tonometers. This, in turn, could potentially assist practitioners in making an informed decision about using it as a screening or diagnostic device for clinical practice, outreach camps, or home-based screening. This, we believe, will contribute towards the efforts to enhance early detection and management of glaucoma so that vision loss due to the condition can be prevented to a great extent.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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