Abstract
The only known modifiable risk factor for the development and progression of glaucoma is an increased intraocular pressure (IOP). Current follow-up with Goldmann applanation tonometry (GAT) constitutes a suboptimal approach, given out-of-office IOP peaks and fluctuations will not be exposed. Self-tonometry in the home environment could address this unmet need, detecting uncontrolled disease and potentially impacting further therapeutic management. Since recently, iCare HOME was developed as a handheld rebound tonometer for this purpose. The aim of this literature review is twofold. First, the accuracy of iCare HOME compared to GAT will be assessed, including evaluation of the influence of corneal characteristics and the mean IOP. Second, an overview of the reported feasibility and acceptance on the use of iCare HOME will be presented. The literature review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines. Literature was searched on Medline (via Pubmed), Embase, Cochrane Library, and Web of Science (Core Collection) on April 10, 2023. A total of 187 records were identified after deduplication, of which 20 articles were included after full evaluation of their suitability. This study reported that iCare HOME has a moderate to good accuracy compared to GAT and a tendency to underestimate GAT, with a median underestimation of 1 mmHg, ranging from 2.66 mmHg underestimation to 2.08 mmHg overestimation. Success rates ranged between 67.5% and 100% with a median rate of 82.5%. Sufficient acceptance was demonstrated. In conclusion, self-tonometry with iCare HOME constitutes an accurate, feasible, and accepted method for home monitoring of glaucoma, providing additional out-of-office IOP-data, and increasing patient empowerment.
Keywords: Glaucoma, Goldmann applanation tonometry, iCare HOME, rebound tonometry, self-tonometry
INTRODUCTION
Glaucoma is one of the leading causes of preventable blindness worldwide, with an estimated increasing prevalence to 112 million affected patients in 2040.[1] Besides its multifactorial pathogenesis, the only known modifiable risk factor for the development and progression of glaucoma is an elevated intraocular pressure (IOP).[2,3,4,5,6,7] Therefore, the management of the IOP remains the primary objective within the treatment of glaucoma. The current standard follow-up with serial IOP monitoring is conducted with Goldmann applanation tonometry (GAT), accepted as the gold-standard for reliable pressure measurements, although constituting a suboptimal approach given the fact that IOP is a dynamic parameter that varies both in the short term and long term.[8,9,10,11,12,13] Consequently, many of these IOP peaks and fluctuations are prone to be missed, which have been shown to be independent risk factors for glaucoma progression.[14,14,15,16,17] Since a 24-h IOP phasing is an expensive and inconvenient practice to address this problem, self-tonometry could offer a more attractive alternative, possibly affecting preventive and therapeutic clinical decision making as well as increasing patient engagement and access to care.[9,12,18] The new handheld iCare HOME tonometer (TA022, iCare Oy, Vanda, Finland) is commercially available for IOP monitoring by glaucoma patients and suspects in an out-of-office setting, used without the need for topical anesthesia and provided with automatic side recognition (EyeSmart) and positioning assistance for correct ocular alignment (EasyPos).[12,18] Notwithstanding that self-tonometry may be useful for many glaucoma patients, especially in patients with visual field deterioration despite normal in-office pressures, self-tonometry constitutes an added value, as it has been revealed that these patients exhibit a higher mean and peak IOP and a greater IOP fluctuation range, not captured by in-office IOP measurements.[19,20] In recent years, numerous studies have been published on the use and accuracy of iCare HOME in clinical practice. Therefore, the aim of this qualitative literature review is twofold. First, we will assess the accuracy of iCare HOME compared to GAT, including the influence of corneal properties on the measured IOP. Second, we will evaluate the reported feasibility and acceptability of the use of iCare HOME by patients in practice.
METHODS
The literature review was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis recommended guidelines.[21] Literature was searched on Medline (via Pubmed), Embase, Cochrane Library, and Web of Science (Core Collection). Only articles written in English were eligible. No filter based on date of publication was used. A total of 409 references were identified through database searching on April 10, 2023. No additional records were found through other sources. After automatic (136 duplicates) and manual (86 duplicates) deduplication, 187 records remained eligible for inclusion. Based on the initial selection by title and abstract, 130 records were excluded on four grounds, in particular studies with incompatible research questions (48 articles), usage of other tonometry devices (69 articles), and absence of original research (13 articles). For an additional two records, full-text articles were not found and were not included in this study for this reason. Of the 55 remaining records read in full, 20 were included in this study. The other 35 records were excluded for four reasons, notably records written in another language (two articles in German, one article in French and one article in Japanese), studies that used a different method of self-tonometry (four articles), studies with a different research question (13 articles), and finally studies without original research (14 articles). Under the latter category, reviews were not included in this study. The search queries, separately for each database, are found in detail in Appendix 1.
RESULTS
A total of 20 records were included in this review. Of these, 17 studies answered the main research question, namely the accuracy comparison between iCare HOME and GAT regarding IOP measurements. Thirteen studies acted on the second research question, that is patient acceptability and feasibility of using iCare HOME in practice. Table 1 summarizes patient demographics and study characteristics regarding the usage of iCare HOME for all included studies.
Table 1.
Patient demographics and study characteristics of all included studies
| Study | Icare HOME usage protocol | Number of participants* | Type of patients† | Mean/median age (years) | Sex distribution† |
|---|---|---|---|---|---|
| Chen et al., 2016[22]§ | 5/day, 3–6 days Baseline at first and second visits |
Total: 87 Actual: 67 |
POAG or OHT, otherwise unspecified | 64 | 40% female 60% male |
| Dabasia et al., 2016[23] | Individual measurements | Total: 76 Actual: 56 |
65% POAG 18% healthy subjects 12% PCAG 5% OHT |
68 | 55% female 45% male |
| Mudie et al., 2016[35] | Individual measurements | Total: 171 Actual: 127 |
Glaucoma or suspected glaucoma, otherwise unspecified | 62.1 | 50% female 50% male |
| Noguchi et al., 2016[24] | 2 h-interval, 1 day | 43 | Healthy subjects | 28.3 | 56% female 44% male |
| Termühlen et al., 2016[25] | Individual measurements | 154 | 66% glaucoma 34% healthy subjects |
58.7 | 50% female 50% male |
| Pronin et al., 2017[26] | Individual measurements | Total: 100 Actual: 73 |
Glaucoma or OHT, otherwise unspecified | 67.5 | 53% female 47% male |
| Quérat and Chen, 2017[27] | 4–5/day, 3 days Baseline at first and second visits |
Total: 60 Actual: 46 |
Healthy subjects | 48 | 60% female 40% male |
| Takagi et al., 2017[36] | Individual measurements | Total: 130 Actual: 128 |
64% NTG 19% POAG 8% developmental glaucoma 8% suspected glaucoma 1% SOAG |
57.5 | 65% female 35% male |
| Brown et al., 2018[28] | Individual measurements | 100 | 92% POAG 6% PCAG 2% OHT |
67.5 | 53% female 47% male |
| Huang et al., 2018[37] | 4/day, 4–6 weeks | Total: 40 Actual: 27 |
67% suspected glaucoma 33% glaucoma |
61.8 | 67% female 33% male |
| Cvenkel et al., 2020[29] | Individual measurements | Total: 117 Actual: 96 |
69% POAG 18% SOAG 12% OHT |
57.4 | 54% female 46% male |
| Huang et al., 2020[30] ‡,§ | Individual measurements | Total: 500 Actual: 498 |
57% suspected glaucoma 37% healthy subjects 6% POAG or SOAG |
55.8 | 51% female 49% male |
| Hyatt et al., 2020[38] | Individual measurements | Total: 234 Actual: 226 |
Healthy subjects | 34.6 | 58% female 42% male |
| McGarva et al., 2021[10] | 1 h-interval, 1 day 2 h-interval, 2 days |
18 | 78% POAG 17% OHT 5% PACG |
69 | 39% female 61% male |
| McGlumphy et al., 2021[31] | 4/day, 7 days | 61 | 64% POAG 24% SOAG 8% suspected glaucoma 4% PACG |
63.2 | 59% female 41% male |
| Ogle et al., 2021[32] | 2/day, 7 days Baseline at first and second visits |
40 | Glaucoma, otherwise unspecified | 58 | 45% female 55% male |
| Scott et al., 2022[33] | 4/day, 2 weeks with 4–6 weeks interval without measurements | 27 | 63% POAG 19% OHT 11% SOAG 7% NTG |
65.9 | 59% female 41% male |
| Barbour-Hastie and Tatham, 2023[39] | 4/day, 2 days | 12 | Glaucoma, otherwise unspecified | 60.1 | 58% female 42% male |
| Hu et al., 2023[40] | 4/day, 4 days | Total: 20 Actual: 19 |
POAG, PCAG, OHT or suspected glaucoma, otherwise unspecified | 55.4 | 65% female 35% male |
| Quérat and Chen, 2023[34] | Individual measurements | Total: 61 Actual: 54 |
56% healthy subjects 4% glaucoma |
56 | 59% female 41% male |
*Total number of participants indicates the number of participants who met inclusion criteria according to the study protocol. The difference with the actual number of participants, if any, is caused by participants not being able to complete the full study protocol and/or not meeting all conditions in terms of qualified and certified use of the iCare HOME device, †Percentages for type of patients and sex distribution were rounded or calculated without decimals, ‡All studies utilized iCare HOME data performed by inexperienced participants themselves, except Huang et al., §Chen et al. made use of iCare ONE in 35 participants and iCare HOME in 52 participants. Huang et al. made use of GAT in 94 participants and handheld applanation tonometry in 404 participants as reference standard to compare with iCare HOME measurements. POAG: Primary open angle glaucoma, PCAG: Primary closed angle glaucoma, SOAG: Secondary open angle glaucoma, OHT: Ocular hypertension, NTG: Normal tension glaucoma
Accuracy comparison between iCare HOME and Goldmann applanation tonometry
Mean difference between iCare HOME and Goldmann applanation tonometry
A total of 15 studies described the mean difference between IOP measurements of iCare HOME and GAT, accompanied by a 95% limits of agreement (LoA) in ten studies. All mean differences were calculated as iCare HOME minus GAT, unless otherwise described. Both negative (being an underestimation of iCare HOME compared to GAT) and positive (being an overestimation of iCare HOME compared to GAT) mean differences were found. The median IOP difference was 1 mmHg underestimation, ranging from 2.66 mmHg underestimation to 2.08 mmHg overestimation. Thirteen out of 15 studies reported an underestimation of iCare HOME compared to GAT, with a median underestimation of 1.1 mmHg (range, 0.15–2.66 mmHg) within this group.[22,23,24,25,26,27,28,29,30,31,32,33,34] In contrast, four out of 15 studies reported an overestimation of iCare HOME, with a median overestimation of 0.91 mmHg (range, 0.08–2.08 mmHg) within the latter group.[31,32,35,36] All figures with related standard deviations and LoA are found in Table 2.
Table 2.
Accuracy comparison between iCare HOME and Goldmann applanation tonometry
| Study | Actual number of participants | Mean difference iCare HOME-GAT* | ICC | Additional findings |
|---|---|---|---|---|
| Chen et al., 2016[22] | 67 | Mean difference (GAT-iCare HOME): 1 mmHg±3 mmHg OD, 0 mmHg±3 mmHg (visit 1) and 1 mmHg±4 mmHg (visit 2) OS | Not reported | Success rate: 77% 78%, 64% and 39% of iCare HOME measurements fell within 3, 2 and 1 mmHg of these of GAT, respectively |
| Dabasia et al., 2016[23] | 56 | Mean difference (GAT-iCare HOME): 0.3 mmHg by patient (95% LoA−4.6–5.2 mmHg), 1.2 mmHg by trainer (95% LoA−3.9–6.3 mmHg) and 1.1 mmHg by participant’s partner (95% LoA−3.2–5.3 mmHg) | Not reported | None |
| Mudie et al., 2016[35] | 127 | Mean difference (GAT-iCare HOME): −0.33 mmHg±3.11 mmHg | ICC: 0.91 (95% CI 0.89–0.94) | Correlation coefficient for association between iCare HOME and GAT: 0.87 91.3% of iCare HOME measurements fell within 5 mmHg of these of GAT |
| Noguchi et al., 2016[24] | 43 | Mean difference (GAT-iCare HOME): 1.03 mmHg±2.52 mmHg (95% LoA−3.91–5.98 mmHg) | ICC: 0.812 (95% CI 0.713–0.886) | Correlation coefficient for association between iCare HOME and GAT: 0.641 |
| Termühlen et al., 2016[25] | 154 | Mean difference: −0.8 mmHg (95% LoA−7.2–5.6 mmHg) | Not reported | |
| Pronin et al., 2017[26] | 73 | Mean difference (GAT-iCare HOME): 2.66 mmHg (95% LoA−3.48–8.80 mmHg) by investigator, 2.58 mmHg (LoA−4.71–9.88 mmHg) by patient | ICC’: 0.903 (95% CI 0.867–0.928) | None |
| Quérat and Chen, 2017[27] | 46 | Mean difference (GAT-iCare HOME): 0.15 mmHg±3.31 mmHg | Not reported | Success rate: 76.6% 68% (73% at second visit) and 71% (75% at second visit) of iCare HOME measurements by patient and investigator, respectively, fell within 3 mmHg of these of GAT |
| Takagi et al., 2017[36] | 128 | Mean difference: 0.70 mmHg (95% LoA−3.07–4.46 mmHg) by patient, 0.91 mmHg (95% LoA−2.75–4.56 mmHg) by investigator | Not reported | Success rate: 98% 90.6% and 97.7% of iCare HOME measurements fell within 3 and 5 mmHg of these of GAT, respectively |
| Brown et al., 2018[28] | 100 | Mean difference: −2.66 mmHg±3.13 mmHg (95% LoA−8.80–3.48 mmHg) | ICC’: 0.903 (95% CI 0.867–0.928) | Success rate: 73% |
| Cvenkel et al., 2020[29] | 96 | Mean difference (GAT-iCare HOME): 1.2 mmHg±2.4 mmHg (95% LoA−3.4–5.9 mmHg) | Not reported | 93.8%, 83.3% and 67.7% of iCare HOME measurements fell within 5, 3 and 2 mmHg of these GAT, respectively |
| Huang et al., 2020[30] | 498 | Mean difference: −1.7 mmHg±2.7 mmHg (95% LoA−7.0–3.6) | Not reported | 92% and 71.5% of iCare HOME measurements fell within 5 and 3 mmHg of these of GAT, respectively |
| McGlumphy et al., 2021[31] | 61 | Mean difference: −0.33 mmHg OD and 0.08 mmHg OS | Not reported | None |
| Ogle et al., 2021[32] | 40 | Mean difference: preloan 1.38 mmHg OD (95% LoA−7.27–4.52) and 1.48 mmHg OS (95% LoA−8.89–5.94), postloan 2.08 mmHg OD (95% LoA−8.48–4.33) and−1.25 mmHg OS (95% LoA−7.05–4.55) | ICC’: preloan 0.748 OD (95% CI 0.567–0.859) and 0.840 OS (95% CI 0.719–0.912) by nurse and patient, respectively, postloan 0.796 OD (95% CI 0.648–0.886) and 0.908 OS (95% CI 0.834–0.950) | 78% preloan and 73% postloan OD, 63% preloan and 78% postloan OS of iCare HOME measurements fell within 3 mmHg of these of GAT |
| Scott et al., 2022[33] | 27 | Mean difference: −1.09 mmHg±3.45 mmHg | Not reported | None |
| Quérat and Chen, 2023[34] | 54 | Mean difference (GAT-iCare HOME): 2.17 mmHg (95% LoA−0.70–5.04) by patient, 2.22 mmHg (95% LoA−0.80–5.24) by investigator | ICC’: 0.930 (95% CI 0.880–0.959) ICC: 0.975 (95% CI 0.960–0.985) |
Success rate: 88.5% 72% of iCare HOME measurements fell within 3 mmHg of these of GAT |
*Each mean difference represents the mean difference between iCare HOME and GAT, unless otherwise stated. LoA: Lower limits of agreement, CI: Confidence interval, ICC: Intraclass correlation coefficient (ICC: Intra-user repeatability, ICC’: Inter-user repeatability), GAT: Goldmann applanation tonometry, OD: Oculus dexter, OS: Oculus sinister, IOP: Intraocular pressure
Repeatability and intraclass correlation coefficient
The repeatability of the iCare HOME IOP measurements was evaluated in six studies using the intraclass correlation coefficient (ICC). Here, values below 0.5 represent poor reliability, between 0.5 and 0.75 moderate reliability, between 0.75 and 0.90 good reliability and above 0.90 excellent reliability. Four studies assessed the conformity of IOP measurements made by multiple individuals (i.e., inter-user repeatability), with a median ICC of 0.903 (range, 0.748–0.930).[26,28,32,34] By contrast, three studies assessed the conformity of multiple IOP measurements made by the patient (i. e. intra-user repeatability), with a median ICC of 0.91 (range, 0.812–0.975).[24,34,35] In addition, two other studies evaluated the inter-user repeatability without calculating an ICC value. Dabasia et al. described a significant difference between iCare HOME IOP measurement error by the patient and a third party executer.[23] In contrast, Takagi et al. found no significant difference between the mean IOP of iCare HOME by the patient and investigator, yet with a difference of more than 3 mmHg in 2.3% and 5 mmHg in 0.8% of the cases.[36] All values with 95% confidence intervals are summarized in Table 2.
Influence of corneal characteristics
Eleven studies discussed the role of corneal characteristics regarding the accuracy of IOP measurements by iCare HOME and its influence on the IOP difference between iCare HOME and GAT, of which eight studies were able to demonstrate a significant association. It is important to note that the manufacturer of iCare HOME only recommends reliable usage of iCare HOME at a CCT between 500 and 600 µm and corneal astigmatism below 3 diopters (D). Lower CCT measurements were associated with increasing underestimation of iCare HOME and thus larger mean absolute IOP differences in six studies.[23,25,26,27,29,30] Conversely, lower CCT measurements were associated with decreasing underestimation of iCare HOME and thus smaller mean absolute IOP differences in one study.[28] Higher CCT measurements were associated with increasing overestimation of iCare HOME in three studies and, in contrast, increasing underestimation of iCare HOME in two studies, and, similarly for both associations, larger mean absolute IOP differences.[23,25,27,28,36] An overview of these results with demonstrating figures is given in Table 3. In addition, Brown et al. found that both corneal hysteresis (CH) and corneal resistance factor (CRF) were associated with IOP measurements from iCare HOME and GAT, where CCT was not. Furthermore, CCT, CH and CRF were significantly associated with the IOP difference between iCare HOME and GAT, with smaller differences seen at lower CCT, higher CH and lower CRF.[28]
Table 3.
Influence of corneal characteristics on the accuracy of iCare HOME
| Study | Actual number of participants | Association with CCT* | Direction of association† | |
|---|---|---|---|---|
| Dabasia et al., 2016[23] | 56 | Significant association with mean IOP difference | More underestimation of IOP with CCT <500 μm or >600 μm | Mean difference of−0.1 mmHg with CCT 500–600 μm, 1.9 mmHg with CCT <500 μm and 1 mmHg with CCT >600 μm |
| Mudie et al., 2016[35] | 127 | No significant assocation with mean IOP difference | None | |
| Termühlen et al., 2016[25] | 154 | Significant association with mean IOP difference (however, only for left eyes and both eyes together) | More underestimation of IOP with CCT <617.3 μm More overestimation of IOP with CCT >617.3 μm |
|
| Pronin et al., 2017[26] | 73 | Significant association with IOP measurements of GAT and iCare HOME | More underestimation of IOP with lower CCT | For every 10 μm thinner CCT, 0.29 mmHg lower IOP for GAT and 0.48 mmHg lower IOP for iCare HOME |
| Quérat and Chen, 2017[27] | 46 | Significant association with mean IOP difference | More underestimation of IOP with CCT <500 μm More overestimation of IOP with CCT >600 μm |
Mean difference of 0 mmHg with CCT 500–600 μm, 1 mmHg with CCT 500 μm and−2 mmHg with CCT >600 μm |
| Takagi et al., 2017[36] | 128 | Significant association with mean IOP difference | More overestimation of IOP with higher CCT and corneal astigmatism | For every 10% thicker CCT, 1.2% increase in mean difference |
| Brown et al., 2018[28] | 100 | Significant association with mean IOP difference | Less underestimation with lower CCT | |
| Huang et al., 2018[37] | 27 | No significant association with iCare HOME accuracy | None | |
| Cvenkel et al., 2020[29] | 96 | Significant association with mean IOP difference and IOP measurements of GAT and iCare HOME | More underestimation of IOP with lower CCT | For every 10 μm thicker CCT, 0.16 mmHg decrease in mean difference |
| Huang et al., 2020[30] | 498 | Significant association with mean IOP difference | More underestimation of IOP with lower CCT | Mean difference of−3 mmHg with CCT 500 μm, −2 mmHg with CCT 540 μm and 0 mmHg with CCT 630 μm |
| Ogle et al., 2021[32] | 40 | No significant association with mean IOP difference | None | |
*By default, the mean difference is calculated between the IOP measurements of iCare HOME minus GAT, unless otherwise specified in Table 2, †Over- or underestimation of the IOP refers to the IOP measured with iCare HOME in comparison to GAT. CCT: Central corneal thicknessGAT: Goldmann applanation tonometry, IOP: Intraocular pressure
Additional findings
A total of 13 studies examined whether there was proportional bias, more specifically, whether more under-or overestimation by iCare HOME compared to GAT was found depending on the mean IOP, of which ten studies were able to demonstrate a significant association. Lower mean IOP ranges were associated with more underestimation of iCare HOME in seven out of ten studies.[22,24,25,26,27,28,30] Higher mean IOP ranges were associated with more overestimation of the measured IOP by iCare HOME in six out of ten studies.[10,22,24,25,27,34] Thus, four out of 11 studies were able to demonstrate both associations above.[22,24,25,27] All results, including the different IOP cutoff values, of the studies concerned are summarized in Table 4. Although systemic proportional bias could not be demonstrated by Mudie et al., this study detected that at lower and higher IOP ranges, high degrees of under-and overestimation, respectively, could be found, which, however, could not be found at an intermediate IOP of 16–23 mmHg.[35]
Table 4.
Proportional bias
| Study | Actual number of participants | Significant association between iCare HOME accuracy and mean IOP‡,§ | Direction of association*,† |
|---|---|---|---|
| Chen et al., 2016[22] | 67 | + | Underestimation at mean IOP <18 mmHg (visit 1) and 20 mmHg (visit 2) Overestimation at mean IOP >18 mmHg (visit 1) and 20 mmHg (visit 2) |
| Mudie et al., 2016[35] | 127 | − | Underestimation of >7.5 mmHg at mean IOP 6–16 mmHg in 1.7% of cases Overestimation of >7.5 mmHg at mean IOP >23 mmHg in 4.6% of cases |
| Noguchi et al., 2016[24] | 43 | + | Underestimation at mean IOP <18 mmHg Overestimation at mean IOP >18 mmHg |
| Termühlen et al., 2016[25] | 154 | + | Underestimation at lower mean IOP ranges Overestimation at higher mean IOP ranges |
| Pronin et al., 2017[26] | 73 | + | Underestimation at lower mean IOP ranges |
| Quérat and Chen, 2017[27] | 46 | + | Underestimation at mean IOP <15 mmHg Overestimation at mean IOP >15 mmHg |
| Takagi et al., 2017[36] | 128 | + | Not otherwise specified |
| Brown et al., 2018[28] | 100 | + | Underestimation at lower mean IOP ranges |
| Cvenkel et al., 2020[29] | 96 | − | None |
| Huang et al., 2020[30] | 498 | + | Underestimation at lower mean IOP |
| McGarva et al., 2021[10] | 18 | + | Progressive overestimation at mean IOP >15 mmHg Good accuracy at mean IOP <15 mmHg |
| McGlumphy et al., 2021[31] | 61 | − | None |
| Quérat and Chen, 2023[34] | 54 | + | Progressive overestimation at mean IOP >15 mmHg Good accuracy at mean IOP <15 mmHg |
*By default, the mean difference is calculated between the IOP measurements of iCare HOME minus GAT, unless otherwise specified in Table 2, †Over- or underestimation of the IOP refers to the IOP measured with iCare HOME in comparison to GAT, ‡+ indicates the presence of significant association between iCare HOME IOP measurements and mean IOP,§− indicates the absence of significant association between iCare HOME IOP measurements and mean IOP. IOP: Intraocular pressure
Five studies evaluated potential factors, excluding corneal characteristics, that may influence the mean difference between IOP measurements of iCare HOME and GAT. First, higher age was associated with a larger underestimated mean IOP difference in one out of four studies, of which the other three studies could not demonstrate a significant association.[23,30,32,36] Second, astigmatism of more than 3D was associated with a larger underestimated mean IOP difference of 2 mmHg in one study.[23] Third, other investigated variables, notably spherical equivalent, vitreopapillary adhesions, K-values, gender and glaucoma severity, had no significant influence on the mean IOP difference between iCare HOME and GAT.[23,32,35]
Eight studies calculated the percentage of iCare HOME measurements that fell within a given number of mmHg of GAT measurements, of which a median percentage of 92.9% (range, 91.3%–97.7%) of measurements fell within 5 mmHg, 73% (range, 63%–90.6%) of measurements fell within 3 mmHg and 65.85% (range, 64%–67.7%) of measurements fell within 2 mmHg of these of GAT.[22,27,29,30,32,34,35,36] Only one study reported that 39% of iCare HOME measurements fell within 1 mmHg of these of GAT.[22] These figures are presented in Table 2 as additional findings.
Feasibility and acceptance of iCare HOME
Success rates in iCare HOME usage
Fifteen studies reported their success rate, notably the number of participants who were able to use the iCare HOME device correctly according to study design, training protocol, and certification criteria. Success rates ranged between 67.5% and 100%, with a median success rate of 82.5%.[10,22,23,24,26,27,28,29,34,35,36,37,38,39,40] Furthermore, Dabasia et al. made a supplementary distinction between success rates of healthy and glaucoma patients, with healthy patients achieving a success rate of 83% and glaucoma patients achieving 71%.[23] An overview of all separate rates is given in Table 5. Five of the included studies mentioned their success rate in using the device, but without further discussion of given education, grounds for failure and patient satisfaction.[22,27,28,34,36] These figures are listed in Table 2 as additional findings. Ten studies reported the reasons why patients were unable to certify and use iCare HOME correctly according to the study protocol, particularly explaining the difference between the total and actual patient numbers. These are listed in Appendix 2.
Table 5.
Feasibility and acceptance of iCare HOME
| Study | Actual number of participants | Success rate of correct use iCare HOME* | Reported easiness of use iCare HOME | Reported comfort of use iCare HOME | Average time to learn to use iCare HOME correctly and other unique findings |
|---|---|---|---|---|---|
| Dabasia et al., 2016[23] | 56 | 74% (56/76) | 84% agreed | 95% agreed | Average learning time: 21 min No association between ability to perform self-tonometry and gender, contact lens wear, hand dexterity, educational level, VPA or refractive error was found |
| Mudie et al., 2016[35] | 127 | 74.3% (127/171) | Not reported | Mean subjective discomfort score: 6.2 (baseline), 7.1 (after certification) and 5.8 (after last measurement) | No assocation between ability to perform self-tonometry and age, gender, race, handedness, spherical equivalent and K-value was found |
| Noguchi et al., 2016[24] | 43 | 100% (43/43) | 39.5% agreed (46.5% were neutral) | Not reported | None |
| Termühlen et al., 2016[25] | 154 | Not reported | Not reported | VAS comfort score 1.7/5† | None |
| Pronin et al., 2017[26] | 73 | 73% (73/100) | 71% agreed (successful patients) 24% agreed (unsuccessful patients) |
92% agreed (successful patients) 88% agreed (unsuccessful patients) |
Average learning time: 20.3 min No significant difference between successful and unsuccessful participants Association between ability to perform self-tonometry and higher age and worse visual acuity was found, whereas no association with handedness and sex was detected |
| Huang et al., 2018[37] | 27 | 67.5% (27/40) | Not reported | Not reported | No association between ability to perform self-tonometry and age, gender, ethnicity or handedness was found |
| Cvenkel et al., 2020[29] | 96 | 82% (96/117) | 78.5% agreed | 77.4% agreed | Average learning time: 14.2 min Positive correlation with age, but not with gender of visual acuity No correlation between acceptability and age or gender |
| Hyatt et al., 2020[38] | 226 | 97% (226/234) | 69.7% agreed | 90.4% agreed | Association between time to successful measurement and perceived ease of use No association between time to successful measurement and age or ocular comfort |
| McGarva et al., 2021[10] | 18 | 100% (18/18) (training protocol) 94% (17/18) (study protocol) |
Not reported | Not reported | None |
| Ogle et al., 2021[32] | 40 | Not reported | 65% agreed | 95% agreed | None |
| Scott et al., 2022[33] | 27 | Not reported | 92% agreed | Not reported | None |
| Barbour-Hastie and Tatham, 2023[39] | 12 | 83% (10/12) | 100% agreed | 100% agreed | Average learning time: 15.2 min |
| Hu et al., 2023[40] | 19 | 95% (19/20) | 73.7% agreed | Not reported | None |
*Success rates are expressed as percentages, specifically the number of patients who were able to use iCare HOME correctly according to the study protocol and design. Absolute numbers of patients can be found in brackets, †The VAS comfortscore used by Termühlen et al. ranges from 1 (excellent) to 5 (poor). VPA: Vitreopapillary adhesions, VAS: Visual Analog Scale
Education, certification, feasibility and acceptance of iCare HOME usage
All 13 studies mentioned the education participants received to master the use of the device, more or less elaborated in detail. In addition, ten studies specified the certification criteria that were applied for valid IOP measurements with iCare HOME. The standard education and certification procedure proposed by the manufacturer can be found in Appendix 3, in addition to the modified training protocols and certification criteria used for the various studies concerned. Twelve studies utilized a questionnaire to assess patient satisfaction in the usage of iCare HOME, primarily the reported easiness and comfort of iCare HOME usage, calculated as the proportion of participants who agreed that the device was easy and comfortable to use, respectively. Easiness of use was reported by nine studies, with a median rate of 72.35% (range, 24%–100%).[23,24,26,29,32,33,38,39,40] Comfort of use was stated by six studies, with a median rate of 92% (range, 77.4%–100%).[23,26,29,32,38,39] Finally, four studies calculated the average time it took participants to learn to use the device correctly, with a median timeframe of 17.75 min (range, 14.2–21 min).[23,26,29,39] A summary of all individual results in terms of reported easiness and comfort, as well as other unique findings, are found in Table 5.
DISCUSSION
Accuracy comparison between iCare HOME and Goldmann applanation tonometry
Given the potential advantages of self-tonometry, it is critical to demonstrate the accuracy of iCare HOME compared to GAT, as well as identify factors that might compromise reliability. A total of 15 studies were found describing a mean difference between the two instruments, with a median underestimated IOP difference of 1 mmHg. An underestimation of IOP by iCare HOME was found in 13 studies, ranging from 0.15 mmHg to 2.66 mmHg with a mean IOP underestimate of 1.1 mmHg.[22,23,24,25,26,27,28,29,30,31,32,33,34] In contrast, four studies found an overestimation of IOP by iCare HOME, ranging from 0.08 mmHg to 2.08 mmHg with a mean IOP overestimate of 0.91 mmHg.[31,32,35,36] Notwithstanding the large heterogeneity between studies, both in terms of patient population as in study design, we believe a moderate to good correlation between iCare HOME and GAT can be confirmed, with generally small, but statistically and potentially clinically significant, differences between the measurement methods and with an overall tendency to underestimate the actual IOP. In addition, seven studies calculated the percentage of iCare HOME measurements that fell within 3 mmHg of the IOP measured with GAT. These figures ranged from a minimum of 63% to a maximum of 90.6% with a median percentage of 73%, indicating that the vast majority of IOP differences between iCare HOME and GAT were in fact minor, with even smaller differences in glaucoma patients compared to healthy subjects.[22,27,29,30,32,34,36] The finding that iCare HOME has a tendency to underestimate GAT is in clear contrast to the results of the iCare HOME device’s predecessor, iCare ONE, which systematically measured higher IOP values compared to GAT.[41,42,43] Several reasons have already been proposed as contributing factors for this underestimation, including accommodation, by looking at the green signal light, and incorrect placement of the probe on the corneal periphery, with a reported difference of up to 4 mmHg.[24,44,45]
Regarding the inter-user repeatability, ICC values ranged between 0.748 and 0.930 with a median value of 0.903, therefore indicating a moderate to excellent repeatability of iCare HOME measurements of the participant and trained professional.[26,28,32,34] Notwithstanding, Dabasia et al. did indicate a significant difference between iCare HOME measurements between the patient and other performers, although Takagi et al. could not confirm this finding.[23,36] In addition, Ogle et al. demonstrated an increasement of the inter-user repeatability after patients measured their IOP during 7 days, indicating a short-learning curve with increasing familiarity with the device.[32] Regarding the intra-user repeatability, figures ranged between 0.812 and 0.975 with a median value of 0.91, indicative of good to excellent repeatability for multiple iCare HOME measurements made by the same participant, even after only brief education sessions.[24,34,35] Accordingly, we can conclude that there is sufficient repeatability of iCare HOME measurements, described as an essential condition for the tonometer to be used in practice.
As regards the influence of CCT on the accuracy of iCare HOME compared to GAT, mixed results were found, although it has already been extensively demonstrated that lower IOP values are measured with a thinner CCT, both by rebound tonometry and GAT.[25,28] Eight out of eleven studies were able to demonstrate a significant association between CCT and the mean difference in IOP measured by iCare HOME compared to GAT. Both the associations for lower and higher CCT measurements with the mean IOP difference were contradictory. First, lower CCT measurements were associated with increasing underestimation and thus larger absolute mean IOP differences in six studies, while, in contrast, lower CCT measurements were associated with decreasing underestimation and thus smaller absolute mean IOP differences in one study.[23,25,26,27,28,29,30] Second, higher CCT measurements were associated with increasing overestimation in three studies and increasing underestimation in two studies, and thus larger absolute mean IOP differences in all studies.[23,25,27,28,36] In addition, four of the above studies found a combination of these associations. Here, Termühlen et al. and Quérat et al. could demonstrate the combination of increasing underestimation at lower CCT measurements and increasing overestimation at higher CCT measurements.[25,27] In contrast, Dabasia et al. reported the combination of increasing underestimation at lower and higher CCT values outside the recommended CCT range between 500 and 600 µm.[23] Finally, Brown et al. described the combination of decreasing underestimation at lower CCT measurements and increasing underestimation at higher CCT measurements.[28] Despite these heterogeneous results, it is relevant to emphasize that adherence of the manufacturer’s guidelines remains important, in particular the approval to use iCare HOME exclusively within a CCT range of 500–600 µm, in which IOP differences appear to be minimal. Especially for more extreme CCT values, larger IOP differences between iCare HOME and GAT have to be considered.
In the past, there have been many studies indicating that rebound tonometry diverges more from GAT at IOPs outside the normal range.[35] Indeed, there does seem to be a tendency to for the difference between iCare HOME and GAT to be minimal at average IOP, with increasing differences at higher and lower mean IOPs. Two patterns of proportional bias across different IOP ranges could be distinguished based on the results of the included studies. First, seven studies reported more underestimation of IOP by iCare HOME compared to GAT at lower mean IOPs.[22,24,25,26,27,28,30] Second, six studies described more overestimation at higher mean IOPs.[10,22,24,25,27,34] Four studies were able to demonstrate the combination of both associations simultaneously. IOP cutoff values were established at 15, 18, and 20 mmHg, from where underestimation (below these values) or overestimation (above these values) was seen.[22,24,27,34]
Feasibility and patient satisfaction of iCare HOME
The reported success rates ranged from a minimum of 67.5% to a maximum of 100%, with a median rate of 82.5%.[10,24,37] Among these, five studies were able to present success rates above 95%.[10,24,36,38,40] The minimal success rate of 67.5% could, at least partially, be explained by the fact that Huang et al. conducted a study over a longer period, namely 4–6 weeks, and many participants failed to complete the required set of measurements.[37] Several hypotheses to explain this relatively high reach of success rates can be considered, including differences in certification criteria in teaching the use of iCare HOME, differences in study duration and iCare HOME usage protocol and the large heterogeneity in nature and size of the included patient population. Therefore, attention should be kept to the fact that the minimal and maximal success rates were reported by two studies with a small number of participants, namely Huang et al. with 40 participants and McGarva et al. with only 18 participants.[10,37]
Numerous studies investigated the acceptance and satisfaction of participants in using iCare HOME in the home environment. Predominantly, the reported easiness and comfort of using iCare HOME were examined for this purpose. First, results regarding the reported easiness varied widely with a broad range from 24% to 100%, although most studies could present a percentage at the higher end of this range with a median rate of 72.35%.[26,39] In addition, the lowest outlier of 24% should be nuanced, as Pronin et al. calculated this score for patients who were unable to meet certification criteria, with a higher score of 71% in those who could.[26] Not surprisingly, for those studies where the distinction was made, all figures were higher for participants who managed to use the device correctly, however, with less pronounced differences than described by Pronin et al.[23] Second, concerning the described comfort of using the device, figures ranged from 77.4% to 100% with a median rate of 92%. These figures were only marginally affected by whether or not the device could be used correctly.[29,39] Given these generally high rates, we can conclude a good to excellent acceptance in the use of iCare HOME, as demonstrated in heterogeneous patient populations. This contrasts with the reported overall experiences of iCare ONE, where to 60% of patients indicated that measuring the IOP was difficult and rather uncomfortable.[25,46,47] Nevertheless, it remains crucial to investigate reasons for failure and perceived difficulties in self-tonometry, as a small, but significant, proportion of participants was unable to handle the device correctly.
Self-tonometry in clinical practice, how do we proceed?
As McGlumphy et al. pointed out, there is no clinical benefit in using iCare HOME in the home setting unless three conditions are met. First, there should be a proven difference from in-clinic IOP measurements for at least a subset of patients. Second, these differences must be predictive of future disease worsening and, thirdly, there must be a change in treatment behavior, based on the information given by home tonometry.[31] Previous research has extensively demonstrated that higher diurnal IOP fluctuations and peak values can be detected by home tonometry outside office hours in comparison to in-office IOP measurements.[10,22,48,49,50] Furthermore, it has been shown that these IOP fluctuations are independent risk factors for glaucoma progression.[14,15,16,17] Finally, to satisfy the third condition, Quérat et al. described a significant difference between the clinicians’ judgment to validate current treatment or to (de) escalate glaucoma therapy, based on the mean and maximum IOPs measured at home, therefore avoiding undertreatment in almost half of the included patients.[48] This finding was confirmed by other authors using self-tonometry.[31,49,51,52,53] Consequently, self-tonometry with iCare HOME can provide essential information, complimentary to in-office IOP measurements.[24] However, whether this new step in glaucoma monitoring leads to an improved prognosis of glaucoma patients, although plausible, could not be demonstrated in this review.
CONCLUSION
The present literature review described a moderate-to-good accuracy of iCare HOME compared to GAT, with a mean IOP difference of 1 mmHg underestimation, ranging from 2.66 mmHg underestimation to 2.08 mmHg overestimation. Furthermore, high success rates and patient satisfaction scores in heterogeneous patient populations were reported, making iCare HOME an accurate, repeatable and to most patients, feasible tonometer. Providing patient-generated out-of-office data and encouraging patient empowerment, self-tonometry with iCare HOME has the potential to address an unmet need by enabling more IOP measurements and eventually guiding further treatment decisions. Further research should focus on whether this potential new dimension to glaucoma care leads to an improvement in disease prognosis.
Limitations
The present literature review does not include quantitative analyses. Given the recurrent heterogeneous results, it would be relevant to calculate overall effects, permitting to draw final conclusions regarding accuracy, feasibility, and acceptance of iCare HOME with more certainty.
Conflicts of interest
There are no conflicts of interest.
Appendix 1: Search query
| Database | Search query |
|---|---|
| Medline (via Pubmed) | (“iCare HOME”[tiab] OR “self tonomet*”[tiab] OR “home tonomet*”[tiab]) AND (“Glaucoma”[Mesh] OR “Glaucoma*”[tiab] OR “Glaucomatous optic neuropath*”[tiab]) |
| Embase | (‘iCare HOME’:ti, ab, kw OR ‘self tonomet*’:ti, ab, kw OR ‘home tonomet*’:ti, ab, kw) AND (‘Glaucoma’/exp OR ‘Glaucoma*’:ti, ab, kw OR ‘Glaucomatous optic neuropath*’:ti, ab, kw) |
| Web of Science (Core Collection) | (TS=(“iCare HOME” OR “self tonomet*” OR “home tonomet*”)) AND (TS=(“Glaucoma*” OR “Glaucomatous optic neuropath*”)) |
| Cochrane Library | ((“iCare HOME” OR (self NEXT tonomet*) OR (home NEXT tonomet*)):ti, ab, kw) AND ([mh “Glaucoma”] OR ((“Glaucoma*” OR (“Glaucomatous optic” NEXT neuropath*)):ti, ab, kw) |
Appendix 2: Reasons for failure to use iCare HOME
| Study | Reasons for failure to use iCare HOME |
|---|---|
| Chen et al., 2016[22] | Noncompliance with measurement schedule (14/20 patients) Difficulties in tonometer handling or withdrawal (6/20 patients) |
| Dabasia et al., 2016[23] | Incorrect alignment of probe with central cornea (12/20 patients) Incorrect handling of the device (4/20 patients) Unable to meet certification criteria (4/20 patients) |
| Mudie et al., 2016[35] | Unable to perform any IOP measurements during training session (10/44 patients) Unable to meet certification criteria, notably measurements not within 5 mmHg of that of GAT (27/44 patients) Withdrawal due to time constraints (7/44 patients) |
| Huang et al., 2018[37] | Unable to perform any IOP measurements during training session (21/27 patients) Unable to meet certification criteria (6/27 patients) |
| Pronin et al., 2017[26] | Unable to meet certification criteria, notably measurements not within 5 mmHg of that of trainer (8/13 patients) Insufficient number of measurement during study period (3/13 patients) Withdrawal before minimal study period of 4 weeks (2/13 patients) |
| Quérat and Chen, 2017[27] | Noncompliance with time schedule, obtaining several erroneous values over 3 days or missing more than one daily measurement (8/14 patients) Withdrawal after the first visit (4/14 patients) Unable to meet certification criteria (2/14 patients) |
| Takagi et al., 2017[36] | Unable to take IOP measurements within 30 min due to exotropia (2/2 patients) |
| Cvenkel et al., 2020[29] | Strong blinking reflex (5/21 patients) Incorrect alignment of probe with central cornea (5/21 patients) Unable to take IOP measurements within 30 min (4/21 patients) Unable to meet certification criteria (7/21 patients) |
| Barbour-Hastie and Tatham, 2023[39] | Unable to perform any IOP measurements during training session (1/2 patients) Low quality IOP measurements during training session (1/2 patients) |
| Quérat and Chen, 2023[34] | Unable to meet certification criteria, notably measurements IOP difference between trainer and participant >7 mmHg (5/7 patients) and iCare HOME range >7 mmHg (2/7 patients) |
IOP: Intraocular pressure, GAT: Goldmann applanation tonometry
Appendix 3: Certification criteria for iCare HOME usage
| Study | Training protocol and certification criteria*,† |
|---|---|
| Dabasia et al., 2016[23] | Manufacturer standardized training protocol Manufacturer IOP validation criteria, with the following modifications: Median of three IOP measurements of patient must be within 5 mmHg of that of trainer, measured with GAT, a maximal range of the 3 readings by patient is 5 mmHg or less (if first reading is 7–23 mmHg) or 7 mmHg or less (if first reading is >23 mmHg) and maximally 30 min from start of training to obtain 3 reliable IOP measurements |
| Mudie et al., 2016[35] | Manufacturer standardized training protocol Manufacturer IOP validation criteria |
| Noguchi et al., 2016[24] | Training in usage of iCare HOME, not otherwise specified Certification criteria unspecified |
| Termühlen et al., 2016[25] | Training in usage of iCare HOME, not otherwise specified Certification criteria unspecified |
| Pronin et al., 2017[26] | Manufacturer standardized training protocol Manufacturer IOP validation criteria, with the following modification: IOP of patient must be within 5 mmHg of that of trainer, also measured with iCare HOME and maximally 30 min from start of training to obtain 3 reliable IOP measurements |
| Huang et al., 2018[37] | Manufacturer standardized training protocol Correct positioning of device and able to obtain 3 reliable iCare HOME measurements after minimally 10 min of demonstration |
| Cvenkel et al., 2020[29] | Manufacturer standardized training protocol Manufacturer IOP validation criteria, with the following modifications: First IOP measurement of patient must be within 5 mmHg of that of trainer, also measured with iCare HOME, and maximally 30 min from start of training to obtain 3 reliable IOP measurements |
| Hyatt et al., 2020[38] | Demonstration of the device by trainer, after which the trainer measured the participant’s IOP and the participant was allowed to ask questions. After this, the IOP was measured once by the participant himself No certification criteria applied |
| McGarva et al., 2021[10] | Manufacturer standardized training protocol Manufacturer IOP validation criteria, with the following modifications: IOP of patient must be within 5 mmHg of that of trainer, also measured with iCare HOME, and maximal range of first 2 readings by patient is 5 mmHg or less (if first reading is 7–23 mmHg) or 7 mmHg or less (if first reading is >23 mmHg) |
| Ogle et al., 2021[32] | Manufacturer standardized training protocol Manufacturer IOP validation criteria |
| Scott et al., 2022[33] | Training in usage of iCare HOME, not otherwise specified Subject is able to complete a set of IOP readings without trainer intervention |
| Barbour-Hastie and Tatham, 2023[39] | Live video training session for up to 30 min with each patient via a video consulting service. Participants had to measure their IOP afterwards, observed by an optometrist through the video link Following criteria were applied: correct positioning of the device during video training, reliable measurements at the end of training and a minimum of 7–8 measurements during 48 h monitoring with good quality |
| Hu et al., 2023[40] | Manufacturer standardized training protocol Manufacturer IOP validation criteria |
*Manufacturer standardized training protocol: training consists of familiarizing the participant with the iCare HOME tonometer with subsequent demonstration for both the left and right eye by the trainer, a certified health care professional. After this, the participant is given the opportunity to practice with the device, both left and right, with optimization by the trainer if necessary. Once mastered, the participant should take three reliable IOP measurements of each eye, allowing a short time interval between each measurement. This last step is observed, but not intervened, by the trainer. Afterwards, the trainer measures the patient’s IOP once with GAT tonometry, †Manufacturer IOP validation criteria: Three certification criteria are used to consider patients competent in the use of iCare HOME: (a) the first of three iCare HOME readings measured by the patient and GAT reading measured by the trainer differ by 5 mmHg or less, (b) a maximal range of three iCare HOME readings by the patient of 7 mmHg or less and (c) correct positioning of the tonometer during self-use, evaluated by the trainer.[10] IOP: Intraocular pressure, GAT: Goldmann applanation tonometry
Funding Statement
Nil.
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