A noncontact applanation tonometer. Description and clinical evaluation

Comparison of intraocular pressure measurements with the tono-pen, goldmann applanation tonometer, and noncontact tonometer in nonglaucomatous pseudophakic children

BACKGROUND

To evaluate the agreement between the Goldman applanation tonometer (GAT), Tono-Pen, and noncontact tonometer (NCT) in the measurement of intraocular pressure (IOP) in pseudophakic children.

METHODS

The medical records of nonglaucomatous pseudophakic children between 2009 and 2019 were retrospectively analyzed. A total of 46 eyes of 23 patients operated for bilateral pediatric cataract were included in the study. The patients' mean age was 13.4 ± 4.1 years. Central corneal thickness (CCT) and IOP values measured with the GAT, Tono-Pen, and NCT were recorded. Agreement between the tonometers was evaluated by intraclass correlation coefficients (ICC) and the Bland-Altman method.

RESULTS

The mean IOP of the 46 eyes included in the study was measured as 13.7 ± 2.3 mm Hg with the GAT, 16.0 ± 2.3 mm Hg with NCT, and 16.5 ± 2.3 mm Hg with the Tono-Pen (p < 0.001). There is no statistical difference between NCT and Tono-Pen measurements, while GAT measurements were significantly lower than those of the NCT and Tono-pen. ICC values showed fair agreement between NCT and Tono-Pen (ICC = 0.720), whereas there was poor agreement between GAT and NCT (ICC = 0.501) and Tono-pen (ICC = 0.314).

CONCLUSIONS

With all devices included in the study, thicker corneas were associated with higher IOP measurements. Although there was moderate agreement between the NCT and Tono-Pen, there was a statistically significant difference in the IOP values provided by the three devices. Our results suggest these devices should not be used interchangeably.

Cross-Sectional Study of Differences between Intraocular Pressure Measurements using Goldmann, iCare, and Air-Puff Tonometers and their Correlation with Central Corneal Thickness

Purpose

To investigate the agreement between the Goldmann tonometer (GAT), the air-puff tonometer, and the iCare tonometer in intraocular pressure (IOP) evaluation as well as their association with central corneal thickness (CCT) in normal participants, glaucoma patients, and patients following refractive surgery.

Methods

This is a cross-sectional study conducted on 204 eyes from 102 patients. The study consisted of three equal groups: group I (control group, n = 34), group II (glaucoma patients on medication, n = 34), and group III (refractive surgery patients, n = 34). All patients were subjected to examination (complete ocular examination, refraction, and IOP measurement).

Results

A total of 102 participants were included in the study with both genders distributed equally. The mean ± standard deviation age was 44.12 ± 12.8 years in the control group while it was 46.29 ± 13.24 years in the glaucoma group and 40.68 ± 15.86 years in the refractive surgery group. Overall, there was a high correlation between the three methods. The mean IOP measured by GAT was 14.03 ± 3.43. The mean IOP measured by iCare was 15.16 ± 3.46. The mean IOP measured by air-puff was 16.66 ± 3.6. The iCare showed the most significant agreement with the GAT (intraclass correlation coefficient [ICC] 0.985, P > 0.05) and the mean difference in IOP between GAT and iCare was 1.1 (95% limits of agreement, -0.62-+2.85 mmHg). The mean difference in IOP between iCare and air-puff was 1.5 and it was 2.6 between GAT and air-puff. There were no significant differences in IOP measurements between GAT and iCare tonometer or between iCare tonometer and air-puff in all groups (P > 0.05). However, there were significant differences in IOP measurements between GAT and air-puff in all groups (P < 0.001). The ICC between all studied methods was strong (ICC > 0.92 for all). Regarding CCT, the mean CCT was 517.14 ± 29.82 μm. There were significant positive correlations between increasing CCT and increasing IOP by GAT, iCare, and air-puff tonometer among the three groups (P < 0.001).

Conclusions

In conclusion, the iCare tonometer, specifically the iCare PRO RT model, is a reliable and efficient alternative instrument for assessing IOP. The IOP values obtained with the iCare PRO RT were found to be consistent with those obtained using the air-puff and GAT.

[Tonometry: Review and Perspectives]

Reliable and repeated IOP measurement are essential in the diagnosis and treatment of glaucoma. In this second part, the other contact tonometry and non-contact tonometry are presented. The clinical value of the different methods and the value of multimodality in tonometry will be discussed based on a review of the literature, and the latest innovations with telemetric IOP sensors will be introduced.

Tonometrie: Rückblick und Ausblick (Teil 2)

In 2. Teil des Beitrags werden die sonstigen Kontakttonometer und die Nonkontakttonometrie präsentiert. Es wird anhand einer Revue der Literatur über den klinischen Wert der verschiedenen Methoden und den Wert der Multimodalität in der Tonometrie diskutiert; ferner werden die letzten Innovationen mit den telemetrischen IOD-Sensoren eingeführt.

Accuracy of Intraocular Pressure Measurements of Eyes with Therapeutic Contact Lenses after Vitrectomy

Purpose: To evaluate the accuracy of intraocular pressure (IOP) measurements obtained by a rebound and non-contact tonometer in eyes with a therapeutic contact lens (CL) after vitrectomy.Methods: In 60 eyes of 60 patients who underwent vitrectomy for vitreoretinal disease, IOP was measured using a rebound tonometer (iCare ic200®; IOPRT) and non-contact computerized air puff tonometer (CT-80, IOPNCT), before and after wearing a CL (Purevision2®, +0.0 diopter). The mean IOP of three consecutive measurements were analyzed, and a comparative analysis with IOP measured by a Goldman applanation tonometer (IOPGAT) was performed.Results: The mean IOPRT without and with the CL was 12.55 ± 5.43 and 13.12 ± 5.13 mmHg, respectively, showing a statistically significant difference (p = 0.02) and strong positive correlation (r = 0.90, p < 0.001). The mean IOPNCT with and without the CL was 12.18 ± 3.24 and 12.17 ± 3.14 mmHg, showing no statistically significant difference (p = 0.17). The consistency with IOPGAT (12.57 ± 5.22 mmHg) was highest in IOPRT without the CL, followed by IOPRT with the CL, IOPNCT without the CL, and IOPNCT with the CL (intraclass correlation coefficients = 0.955, 0.945, 0.856, and 0.850, respectively). In addition, the rebound tonometer successfully measured IOP, regardless of whether the CL was worn; however, the non-contact tonometer failed to measure IOP in seven eyes without the CL and nine with the CL. No difference was observed according to intraocular tamponade type.Conclusions: A rebound tonometer can be used as an alternative IOL measuring method in eyes for which it is difficult to use a Goldman applanation tonometer due to the postoperative presence of a therapeutic CL.

Noninvasive Visualization of the Tear Film Microaerosol During Noncontact Tonometry Measurements

PURPOSE

To investigate the aerosol generation by a noninvasive real-time observation device and assess the conditions relating to aerosolization during intraocular pressure (IOP) measurements using a commercial noncontact tonometer (NCT).

STUDY DESIGN

Prospective experimental and healthy eye studies.

METHODS

In an initial experimental study, we devised a model mannequin eye to investigate how air puff pressure and IOP of the eye affected aerosol generation. In the human study including 20 healthy volunteer control subjects, the number of tear aerosol particles generated at 20 and 40 mm Hg air puff pressures with and without eye drop was investigated. The recorded aerosol visualization video was analyzed and the number of aerosol particles generated in 5 seconds after IOP measurement was measured.

RESULTS

The experimental and human studies confirmed the aerosol generation during NCT measurements. In the experimental study, when the air puff pressures were set at 20 and 40 mm Hg, a lower IOP (5 mm Hg) generated significantly more aerosols than a higher IOP (25 mm Hg) (20 mm Hg, P = .0159; 40 mm Hg, P = .0079). There was also a significant positive correlation between the air puff pressure and the number of aerosol particles in both high- and low-IOP eyes (P < .001). At an air puff pressure of 40 mm Hg, the amount of aerosol generated was significantly higher with eye drop than without eye drop (P = .047).

CONCLUSIONS

NCT generates significant aerosolization from the tear film, the amount of which is determined by the IOP and the air puff pressure and the presence of eye drop use before the measurements.

Eye orbit effects on eyeball resonant frequencies and acoustic tonometer measurements

The eye orbit has mechanical and acoustic characteristics that determine resonant frequencies and amplify acoustic signals in certain frequency ranges. These characteristics also interfere with the acoustic amplitudes and frequencies of eyeball when measured with an acoustic tonometer. A model in which a porcine eyeball was embedded in ultrasonic conductive gel in the orbit of a model skull was used to simulate an in vivo environment, and the acoustic responses of eyeballs were detected. The triggering source was a low-power acoustic speaker contacting the occipital bone, and the detector was a high-resolution microphone with a dish detecting the acoustic signals without contacting the cornea. Dozens of ex vivo porcine eyeballs were tested at various intraocular pressure levels to detect their resonant frequencies and acoustic amplitudes in their power spectra. We confirmed that the eyeballs' resonant frequencies were proportional to intraocular pressure, but interference from orbit effects decreased the amplitudes in these resonant frequency ranges. However, we observed that the frequency amplitudes of eyeballs were correlated with intraocular pressure in other frequency ranges. We investigated eye orbit effects and demonstrated how they interfere with the eyeball's resonant frequencies and frequency amplitudes. These results are useful for developing advanced acoustic tonometer.

Differential impact of prostaglandin analogues on agreement of intraocular pressure measurements obtained by Goldmann applanation, rebound, and noncontact tonometry

Background

To evaluate the effect of topical prostaglandin analogues on agreement of IOP measurements obtained by Goldmann applanation tonometry (GAT), rebound tonometry (RBT), and noncontact tonometry (NCT) in eyes with primary open- angle glaucoma (POAG).

Methods

Intraocular pressure measurements were obtained using GAT, RBT, and NCT in patients with POAG with or without prostaglandin analogues. The agreement between each tonometry was analysed using Bland-Altman analyses in those with or without prostaglandin analogues. The effect of average IOP on IOP differences was also evaluated.

Results

Among a total of 86 subjects included in the study, 44 patients were using prostaglandin analogues. The difference in IOP measured by GAT and RBT was marginally greater in those with (GAT-RBT: − 0.94 ± 1.63 mmHg) prostaglandin analogues than in those without (− 0.33 ± 1.22 mmHg, P = 0.06). The difference in IOP measured by GAT and NCT was significantly greater in the prostaglandin group (GAT-NCT: 2.40 ± 2.89 mmHg) than in the group without prostaglandin analogues (0.41 ± 1.63 mmHg, P < 0.01). While there was no significant relationship between the average of all tonometries and the difference between tonometries in those without prostaglandin analogues, both RBT and NCT underestimated IOP relative to GAT at higher IOP in those using prostaglandin analogues.

Conclusion

Intraocular pressure measured by RBT and NCT was similar to that measured by GAT in those without prostaglandin analogues. RBT overestimated and NCT underestimated IOP compared to GAT in those using prostaglandin analogues.

Intrasession and Intersession Variabilities of Intraocular Pressure Measured by Noncontact Tonometer in Normal Volunteers

Purpose

Intraocular pressure (IOP) measured using a noncontact tonometer is evaluated by performing multiple measurements because IOP is affected by the ocular pulse. We investigated the relationship between value fluctuations in multiple measurements during noncontact tonometer measurements and cardiac rate.

Materials and Methods

Forty-two healthy subjects were included and IOP was measured using a noncontact tonometer. The measurement was performed three times each for the right eye and the left eye, for a total of six times. Blood pressure and cardiac rate were measured at the same time as the IOP measurement. Using repeated-measures analysis of variance, we examined whether the measured IOP and cardiac rate fluctuate throughout the day over the course of 4 days.

Results

There was a fluctuation in the IOP in a sequence only on day 1 of the four measurement days (P < 0.001). The IOP on day 1 tended to be high for the first and second measurements (P = 0.0111–0.0015). Systolic blood pressure and diastolic blood pressure did not fluctuate over the 4 days (P = 0.6247 and 0.7132), but cardiac rate was high only on day 1 (P = 0.0276).

Conclusion

The IOP on day 1 tended to be high in the first and second measurements. The IOP measured on days 2–4 did not fluctuate during the sequence of measurements. The cardiac rate measured at the same time as the IOP was high only on the first day.

Changes in Intraocular Pressure after Transepithelial Photorefractive Keratectomy and Femtosecond Laser In Situ Keratomileusis

PURPOSE

To investigate the changes in intraocular pressure (IOP) and biomechanically corrected IOP (bIOP) in patients undergoing transepithelial photorefractive keratectomy (TPRK) and femtosecond laser in situ keratomileusis (FS-LASIK) and to determine the effects of preoperative biomechanical factors on IOP and bIOP changes after FS-LASIK and TPRK.

DESIGN

A retrospective comparative study.

METHODS

We retrospectively investigated the IOP and corneal biomechanical changes in 93 eyes undergoing FS-LASIK and 104 eyes undergoing TPRK in a clinical setting. Preoperative and postoperative data on ophthalmic and Corvis ST examinations, in vivo Young's modulus, and noncontact tonometry were analyzed. Marginal linear regression models with generalized estimating equations were used for intragroup and intergroup comparisons of IOP and bIOP changes.

RESULTS

In the univariate model, IOP reduction after FS-LASIK was 2.49 mmHg higher than that after TPRK. In addition, bIOP reduction after FS-LASIK was 1.85 mmHg higher than that after TPRK. In the multiple regression model, we revealed that IOP reduction after FS-LASIK was 1.75 mmHg higher than that after TPRK. Additionally, bIOP reduction after FS-LASIK was 1.64 mmHg higher than that after TPRK. Postoperative changes in bIOP were less than those in IOP. In addition, Young's modulus and CBI had no significant effect on postoperative IOP and bIOP changes. We establish a biomechanically predictive model using the available data to predict postoperative IOP and bIOP changes after TPRK and FS-LASIK.

CONCLUSIONS

Reductions in IOP and bIOP after FS-LASIK were 1.75 mmHg and 1.64 mmHg, respectively, more than those after TPRK, after adjustment for confounders. We revealed that the type of refractive surgery and peak distance (PD) were significant predictors of postoperative IOP and bIOP changes. By contrast, depth of ablation showed a significant effect on only IOP changes.

Intraocular pressure according to different types of tonometry (non-contact and Goldmann applanation) in patients with different degrees of bilateral tearing

Purpose

To investigate intraocular pressure (IOP) readings by non-contact tonometry (NCT) and Goldmann applanation tonometry (GAT) for patients with different degrees of bilateral tearing.

Methods

In this study, we reviewed the medical charts of patients complaining of different degrees of bilateral tearing. The tear meniscus height (TMH) and IOP with NCT and GAT were measured. In each patient, a comparison of IOP readings between the eye with lower TMH and the contralateral eye with higher TMH was evaluated. The TMH was graded as follows: grade 1 (low): TMH < 0.2 mm; grade 2 (moderate): 0.2 mm ≤ TMH < 0.6 mm; grade 3 (high): TMH ≥ 0.6 mm. Subsequently, a comparison of IOP readings among eyes with low, moderate, and high TMH was also performed.

Results

A total of 120 eyes of 60 patients were enrolled. When comparing the two eyes of a patient, the eye with higher TMH showed higher NCT readings and larger difference in IOP readings between the two tonometries than the eye with lower TMH (P < 0.001 and P < 0.001, respectively). When TMH was classified into grades according to the degree, the high TMH eyes showed higher NCT readings than did the low and moderate TMH eyes (P < 0.001 and P = 0.001, respectively). In addition, the high TMH eyes showed a larger difference in IOP readings between the two tonometries than did the low and moderate TMH eyes (P < 0.001 and P < 0.001, respectively).

Conclusion

Eyes with higher TMH showed higher NCT readings and a larger difference in IOP between the two tonometries (NCT and GAT) than those with lower TMH. In patients with tearing, the NCT value may be inaccurate, so it is necessary to measure the GAT.

Corneal perforation after noncontact tonometry in patients with active recurrent herpes simplex keratitis: case report

PURPOSE

To report iatrogenic complications and to review the potential complications caused by noncontact tonometry (NCT) in related literature.

METHODS

This case report describes two cases of active recurrent herpes simplex keratitis (HSK) on top of a thin corneal scar. The cornea was perforated by the air pulse from the NCT, resulting in an air bubble in the anterior chamber.

RESULTS

Both patients were diagnosed with active recurrent necrotizing stromal HSK on top of a thinned corneal scar after previous therapeutic treatment involving tissue adhesive glue with a bandage contact lens (BCL) to treat a perforated cornea. During a follow-up visit, both patients reported similar symptoms of acute pain and fluid exuding from their eyes immediately after undergoing NCT. Slit-lamp examination revealed a perforated cornea with an intracameral air bubble. Treatment involved use of tissue adhesive glue and BCL in both cases.

CONCLUSION

NCT may not be sufficiently safe in patients with active infectious keratitis, particularly in cases where the cornea is relatively thin and necrotic.

Factors Affecting Intraocular Pressure Measured by Noncontact Tonometer

Purpose

To investigate the mean values of intraocular pressure (IOP) measured with non-contact tonometer (NCT) and evaluate the factors that may affect IOP.

Methods

A total of 850 subjects who were admitted to our clinic between March 2005 and February 2006 were recruited for the study. Subjects having blepharitis, conjunctivitis, corneal diseases, glaucoma suspicion, or glaucoma were not accepted to the study. All subjects were questioned about systemic diseases. IOP measurement with NCT and central corneal thickness (CCT) with ultrasound pachymetry were performed for each patient between 9 and 11 AM.

Results

The mean ages of 367 (43.2%) male subjects and 483 (56.8%) female subjects were 43.9±18.1 and 40.7±18.0 years ± SD, respectively. Since right and left eye IOP, CCT, and keratometric values were significantly correlated, right eye values were used for statistical purposes. Mean IOP values in males and females were 13.2±3.0 and 13.5±2.9 mmHg, respectively. Mean CCT values were 552.5±34.7 μm for males and 550.1±34.3 μm for females. In multiple regression analysis, IOP was found to be associated with gender, refractive error, CCT, and the presence of diabetes mellitus (DM).

Conclusions

Gender, CCT, the presence of DM, and refractive error may be significantly associated with IOP in this particular population.

The synergistic effect of inflammation and metabolic syndrome on intraocular pressure: A cross-sectional study

Intraocular pressure is associated with metabolic syndrome. C-reactive protein (CRP) is associated with cardiovascular disease, irrespective of the presence of metabolic syndrome. In this study, we examined the synergistic effect of CRP and metabolic syndrome on intraocular pressure.A total of 1041 subjects were included for data analyses in this cross-sectional study. Intraocular pressure was measured using a noncontact tonometer, and serum CRP levels were measured using a commercially available kit.The intraocular pressure was significantly higher in the subjects with metabolic syndrome than in those without (14.1 ± 3.0 vs 13.4 ± 3.0 mm Hg, P = .002). Furthermore, intraocular pressures significantly increased according to CRP tertiles (13.1 ± 3.0, 13.7 ± 3.0, and 13.8 ± 3.0 mm Hg from the lowest to highest tertile of CRP, respectively; P = .002). The highest intraocular pressure was observed in subjects with metabolic syndrome in the highest CRP tertile (P value for trend < .001). Multivariate linear regression analysis revealed that the influence of CRP was independent of metabolic syndrome and that high CRP levels were significantly associated with high intraocular pressure (95% confidence interval: 0.080-1.297, P = .027).In conclusion, systemic inflammation, reflected by serum CRP levels, is associated with high intraocular pressure in subjects with and without metabolic syndrome.

Using CorvisST tonometry to assess glaucoma progression

Purpose

To investigate the utility of the Corneal Visualization Scheimpflug Technology instrument (CST) to assess the progression of visual field (VF) damage in primary open angle glaucoma patients.

Method

A total of 75 eyes from 111 patients with primary open-angle glaucoma were investigated. All patients underwent at least nine VF measurements with the Humphrey Field Analyzer, CST measurements, axial length (AL), central corneal thickness (CCT) and intraocular pressure (IOP) with Goldmann applanation tonometry (GAT). Mean total deviation (mTD) progression rates of the eight VFs, excluding the first VF, were calculated and the association between progression rate and the other listed measurements was analyzed using linear regression, and the optimal to describe mTD progression rate was selected based on the second order bias corrected Akaike Information Criterion (AICc) index.

Results

VF progression was described best in a model that included CST parameters as well as other ocular measurements. The optimal linear model to describe mTD progression rate was given by the equation: -8.9–0.068 x mean GAT + 0.68 x A1 time + 0.31 x A2 time -0.39 x A2 length– 1.26 x highest deformation amplitude.

Conclusion

CST measurements are useful when assessing VF progression in glaucoma patients. In particular, careful consideration should be given to patients where: (i) an eye is observed to be applanated fast in the first and second applanations, (ii) the applanated area is wide in the second applanation and (iii) the indentation is deep at the maximum deformation, since these eyes appear to be at greater risk of VF progression.

Corneal biomechanics - a review

PURPOSE

In recent years, the interest in corneal biomechanics has strongly increased. The material properties of the cornea determine its shape and therefore play an important role in corneal ectasia and related pathologies. This review addresses the molecular origin of biomechanical properties, models for their description, methods for their characterisation, techniques for their modification, and computational simulation approaches.

RECENT FINDINGS

Recent research has focused on developing non-contact techniques to measure the biomechanical properties in vivo, on determining structural and molecular abnormalities in pathological corneas, on developing and optimising techniques to reinforce the corneal tissue and on the computational simulation of surgical interventions.

SUMMARY

A better understanding of corneal biomechanics will help to improve current refractive surgeries, allow an earlier diagnosis of ectatic disorders and a better quantification of treatments aiming at reinforcing the corneal tissue.

The Relationship between Corvis ST Tonometry and Ocular Response Analyzer Measurements in Eyes with Glaucoma

It is important to compare the results of Corneal Visualization Scheimpflug Technology instrument (CST) measurements and Reichert Ocular Response Analyzer (ORA) parameters. The purpose of the study was to investigate the association between CST measurements and ORA parameters in ninety-five patients with primary open-angle glaucoma. Measurements of CST, ORA, axial length (AL), average corneal curvature (CC), central corneal thickness (CCT) and intraocular pressure (IOP) with Goldmann applanation tonometry (GAT) were carried out. The association between CST and ORA parameters was assessed using linear regression analysis, with model selection based on the second order bias corrected Akaike Information Criterion index. Measurements from ORA (corneal hysteresis [CH] and corneal response factor [CRF]) had high intraclass correlation coefficients (ICC) and low coefficients of variation, but some CST parameters showed much lower reproducibility, namely: A1 length, A2 length, highest concavity time and peak distance. Of 12 CST parameters tested, 8 were significantly correlated with CH and 10 were significantly correlated with CRF, however, the magnitude of the correlation coefficients were weak to moderate at best. The optimal model to explain CH using CST measurements was given by: CH = -76.3 + 4.6*A1 time + 1.9*A2 time + 3.1 * highest concavity deformation amplitude + 0.016*CCT (R² = 0.67, p <0.001). Similarly, the optimal model for CRF was given by: CRF = -53.5 + 4.2*A1 time + 1.9*A1 length + 20.8*A1 deformation amplitude + 0.8*A2 time + 0.017*CCT (R² = 0.73, p <0.001). ORA parameters show higher reproducibility than CST measurements. Although many CST parameters are significantly related to ORA parameters, the strengths of these relationships are weak to moderate.

Comparison of Goldmann applanation tonometer, Tono-Pen and noncontact tonometer in children

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.

Effects of Central Corneal Stromal Thickness and Epithelial Thickness on Intraocular Pressure Using Goldmann Applanation and Non-Contact Tonometers

PURPOSE

To investigate whether corneal thickness parameters measured by optical coherence tomography (OCT), such as central corneal thickness (CCT), central corneal stromal thickness (CCST), and central corneal epithelial thickness (CCET), influence the intraocular pressure (IOP) difference measured by Goldmann applanation tonometry (GAT) and non-contact tonometry (NCT).

METHODS

In total, 50 eyes from 50 subjects without glaucomatous defects were included in this retrospective, cross-sectional study. We measured IOP using GAT and NCT and calculated the difference between the two methods. CCT was measured by a Cirrus HD-OCT device using anterior segment imaging. The basement membrane of the epithelium, which was seen as a high-reflection line in the OCT image, was taken as a reference line to measure CCST and CCET.

RESULTS

The mean IOP measured by GAT and NCT was 16.7 ± 3.0 and 18.1 ± 3.8 mmHg, respectively. The mean IOP difference was 1.5 ± 1.7 mmHg, and the IOP measured by NCT was 8.4% ± 11.3% higher than that measured by GAT. The CCET and CCST were 57.9 ± 5.6 and 501.7 ± 33.8 μm, respectively. CCT showed a positive correlation with both GAT IOP (r = 0.648, P < 0.001) and NCT IOP (r = 0.676, P < 0.001). Although CCST showed a significant correlation with GAT IOP and NCT IOP, CCET did not. The difference between GAT IOP and NCT IOP increased with CCT (r = 0.333, P = 0.018), and CCET was positively correlated with the IOP difference between GAT and NCT (r = 0.435, P = 0.002).

CONCLUSIONS

IOP increased with greater CCT, and CCST seemed to have a more important role than CCET. CCET also increased with greater CCT, and this may be a possible explanation for the increasing difference in IOP between GAT and NCT with increasing CCT.

Comparison of Intraocular Pressure Measurements Obtained by Rebound, Noncontact, and Goldmann Applanation Tonometry in Children

PURPOSE

To compare intraocular pressure (IOP) among rebound, noncontact, and Goldmann applanation tonometry (GAT) and their relationships to central corneal thickness in children.

DESIGN

Diagnostic protocol comparison and evaluation.

METHODS

In right eyes of 419 children, mean IOP, rates of successful measurement with 3 tonometries, and intermethod agreement by Bland-Altman plot were assessed. The influences of central corneal thickness, and of average IOP of 3 tonometries on IOP differences between tonometries, were evaluated.

RESULTS

The mean age was 8.89 ± 3.41 years (3-15 years). There was significant difference in mean IOP of each tonometry; GAT showed the lowest values (P < .05). The IOP was successfully measured by noncontact tonometry in 89%, by rebound tonometry in 75%, and by GAT in 64% of children less than 10 years old, and in 100%, 98%, and 94% of children older than 10 years, respectively. The IOP of each tonometer positively correlated with central corneal thickness (P < .05). The mean differences and limits of agreement were 0.81 ± 6.19 mm Hg (noncontact minus rebound), 2.56 ± 4.62 mm Hg (rebound minus GAT), and 1.81 ± 4.76 mm Hg (noncontact minus GAT). Rebound and noncontact tonometry overestimated IOP relative to GAT for thicker central corneal thicknesses. Rebound tonometry overestimated IOP relative to GAT and noncontact tonometry at higher average IOP of 3 tonometries.

CONCLUSIONS

Rebound, noncontact, and Goldmann applanation tonometries can be considered appropriate methods for children, though noncontact tonometry is the most accessible. Given the risk of false-positive diagnosis of pediatric glaucoma, attention should be devoted to children with IOP within a suspicious range or thicker cornea.

The Relationship between Corvis ST Tonometry Measured Corneal Parameters and Intraocular Pressure, Corneal Thickness and Corneal Curvature

The purpose of the study was to investigate the correlation between Corneal Visualization Scheimpflug Technology (Corvis ST tonometry: CST) parameters and various other ocular parameters, including intraocular pressure (IOP) with Goldmann applanation tonometry. IOP with Goldmann applanation tonometry (IOP-G), central corneal thickness (CCT), axial length (AL), corneal curvature, and CST parameters were measured in 94 eyes of 94 normal subjects. The relationship between ten CST parameters against age, gender, IOP-G, AL, CST-determined CCT and average corneal curvature was investigated using linear modeling. In addition, the relationship between IOP-G versus CST-determined CCT, AL, and other CST parameters was also investigated using linear modeling. Linear modeling showed that the CST measurement ‘A time-1’ is dependent on IOP-G, age, AL, and average corneal curvature; ‘A length-1’ depends on age and average corneal curvature; ‘A velocity-1’ depends on IOP-G and AL; ‘A time-2’ depends on IOP-G, age, and AL; ‘A length-2’ depends on CCT; ‘A velocity-2’ depends on IOP-G, age, AL, CCT, and average corneal curvature; ‘peak distance’ depends on gender; ‘maximum deformation amplitude’ depends on IOP-G, age, and AL. In the optimal model for IOP-G, A time-1, A velocity-1, and highest concavity curvature, but not CCT, were selected as the most important explanatory variables. In conclusion, many CST parameters were not significantly related to CCT, but IOP usually was a significant predictor, suggesting that an adjustment should be made to improve their usefulness for clinical investigations. It was also suggested CST parameters were more influential for IOP-G than CCT and average corneal curvature.

Comparison of three methods of tonometry in normal subjects: Goldmann applanation tonometer, non-contact airpuff tonometer, and Tono-Pen XL

Objective

We aimed to compare intraocular pressure (IOP) measurements via three different tonometers: the Goldmann applanation tonometer (GAT), the Tono-Pen^® XL (TPXL), and a non-contact airpuff tonometer (NCT).

Methods

This was a cross-sectional study of 200 eyes from 200 patients. Right eyes of all patients were included in this study. IOP was measured via GAT, NCT, and TPXL by three physicians. Each physician used one of the tonometers. Measurements via the three devices were compared.

Results

The mean IOP was 15.5±2.2 mmHg (range 10–22) with the GAT, 16.1±3.0 (range 9–25) with the TPXL, and 16.1±2.8 (range 10–26) with the NCT. Bland–Altman analysis showed that the mean difference between measurements from the NCT and the GAT was 0.6±2.3 mmHg. The mean difference between the TPXL and GAT measurements was 0.7±2.5 mmHg. The mean difference between the NCT and TPXL measurements was −0.02±3.0 mmHg. There was no significant difference between the groups according to a one-way analysis of variance (ANOVA) test. P-values were 0.998 for NCT–TPXL, 0.067 for NCT–GAT, and 0.059 for TPXL–GAT.

Conclusion

The NCT and TPXL provide IOP measurements comparable to those of the gold standard GAT in normotensive eyes.

Clinical comparison of Pulsair non-contact tonometer and Goldmann applanation tonometer in Indian population

INTRODUCTION

Goldmann applanation tonometer (GAT) is the gold standard for Intraocular Pressure (IOP) measurement but has disadvantage of being contact device and problems with portability. The aim of the study was to compare the Keeler's Pulsair noncontact tonometer (NCT) with GAT in Indian Population.

MATERIALS AND METHODS

Eighty-one subjects were screened from a Glaucoma clinic of a tertiary care centre in North India. The IOP was measured by Pulsair NCT and GAT after explaining the procedure. Central corneal thickness (CCT) was measured to avoid its bias on IOP readings. The data were analyzed using SPSS software.

RESULTS

The mean age of subjects was 49.9±8.8 (mean±SD) years. The mean IOP as taken by Pulsair NCT was 15.79±4.07mmHg and that for GAT was 17.02±4.23mmHg (p=0.062). The mean CCT was 0.536±0.019mm. A positive Pearson's correlation coefficient of 0.909 (p=0.0001) was found between the two instruments. Bland and Altmann analysis showed a fair agreement between the two tonometers at lower IOP range.

CONCLUSION

Pulsair NCT can be used as a screening tool for community practices but is not reliable in the subjects with higher IOP range.

Comparison of optometrist glaucoma referrals against published guidelines

PURPOSE

To examine if community optometrists follow published guidelines for referral of patients with suspect glaucoma to the hospital eye service.

METHODS

A retrospective audit of new optometrist-initiated referrals to the Glaucoma Service at Moorfields Eye Hospital, London was performed. Clinical data from referral letters recorded included evidence of intraocular pressure (IOP) measurement and tonometer used. Referral letter information was compared to 2009 guidelines published jointly by the College of Optometrists and Royal College of Ophthalmologists on referring glaucoma suspect patients.

RESULTS

A total of 289 new patients were seen in the Glaucoma Service over a 6 week period from 4th January 2011; a 100% hospital record retrieval rate was obtained. Of these, 114 (39%) were optometrists initiated referrals. Optometrist letters were available for 105 patients. IOP measurements were recorded in 102 (97%); most practitioners used non-contact tonometry (NCT; n = 69; 68%). Practitioners recorded <4 NCT readings per eye (4 readings: n = 3, 4%; 3 readings: n = 42, 61%; 2 readings: n = 6, 9%; 1 reading: n = 18, 26%). Seventy-seven patients (73%) reported with raised IOP as the main referral reason; of these, 33 (43%) were referred with raised IOP in isolation. NCT was the instrument used in the majority of these raised IOP cases (n = 56; 73%). In cases where raised IOP in isolation was the referral reason, 24 (73%) were recorded using NCT; 10 (30%) provided repeat IOP measurement data in the letter. One hundred and two (97%) referred patients attended the hospital appointment. Thirty (29%) were deemed not to have glaucoma, with the remainder diagnosed with ocular hypertension (n = 25; 25%), glaucoma suspect/glaucoma (n = 35, 34%) or narrow angles requiring intervention (n = 12, 12%). Thirty of the 33 patients referred with raised IOP in isolation attended their hospital visit. Ten (33%) of these patients were subsequently discharged. Six (20%) patients had IOP within normal limits when measured with applanation tonometry; all of these patients were kept within the service.

CONCLUSIONS

Community optometrists using NCT for measurement should be reminded of the guidelines on number of readings to take and also the value of repeating NCT measures when appropriate. This may help to increase the accuracy of glaucoma suspect referrals to hospital eye service.

Influence of central corneal thickness on measured intraocular pressure differentials: Nidek RKT-7700, Topcon CT-80 NCTs and Goldmann Tonometer

PURPOSE

We sought to compare the intraocular pressure (IOP) measured by RKT-7700 and CT-80 noncontact tonometers (NCTs) with that measured by Goldmann applanation tonometry (GAT). We also examined the influence of central corneal thickness (CCT) on the agreement between both NCTs and GAT in a sub-population of healthy, young normals.

METHODS

Triplicate IOP and CCT measurements were obtained twice from one randomized eye of 49 subjects (28 males and 21 females) aged 22.2 ± 1.3 (mean ± S.D.) years. Goldmann tonometry was performed subsequent to assessment with the RKT-7700 and CT-80, to negate the 'ocular massage effect', followed by ultrasound pachymetry. The results from each method were compared and assessed for repeatability and between-observer reproducibility. Analysis was performed to determine the correlation between the differences in IOP measurements and corneal thicknesses.

RESULTS

The mean differences (±S.D.) in sessions 1 and 2 respectively between RKT-7700 and GAT (2.6 ± 2.0 and 2.7 ± 1.4 mmHg), between CT-80 and GAT (2.8 ± 2.0 and 3.2 ± 1.3 mmHg) were statistically significant (p < 0.0001). The repeatability coefficients in sessions 1 and 2 respectively were; ±1.2 and ±1.0 mmHg (GAT), ±1.3  and ±1.6 mmHg (CT-80), ±2.3  and ±1.7 mmHg (RKT-7700) and inter-observer reproducibility was; ±1.9 (RKT-7700), ±2.3 (GAT) and ±2.6 mmHg (CT-80). Between the corneal thickness and the differences in GAT-measured and NCT-measured IOPs, there was a significant negative correlation and a trend for a larger difference in thicker corneas.

CONCLUSION

Neither the RKT-7700 nor the CT-80 can be used interchangeably with the Goldmann tonometer, although all three tonometers give repeatable measurements of IOP, in this sub-population. IOP differences between GAT and the NCTs significantly correlated with CCT, with the possibility of even greater differences in thicker corneas.

Tonometry and Care of Tonometers

Intraocular pressure (IOP) remains the only modifiable risk factor in the management of glaucoma. Hence, IOP and its appropriate measurement deserve our ongoing interest. Over the years, not only has our understanding of glaucoma changed but also has changed our approach to the measurement of the IOP. This review is an attempt to elucidate the commonly techniques of tonometry, and critically evaluate each of them, in current glaucoma practice.

How to cite this article: Maheshwari R, Choudhari NS, Singh MD. Tonometry and Care of Tonometers. J Current Glau Prac 2012;6(3):124-130.

The influence of lens power and center thickness on the intraocular pressure measured through soft lenses: a comparison of two noncontact tonometers

PURPOSE

To quantify the influence of soft contact lens power and thickness on the intraocular pressure (IOP).

METHODS

Thirty-nine young, healthy adult volunteers completed this study. One eye of each subject was randomly assigned either a +6D or a -6D high water content daily disposable lens. The other eye was fitted with the second lens. Triplicate measurements of IOP were taken before, during, and after contact lens wear. Each time, IOP was assessed in a randomized order with two noncontact tonometers. The lenses were swapped between eyes during a second session of measurements, one week later.

RESULTS

In the first session with the +6D lenses, the average IOPs (±SDs) before, with the lenses fitted, and after the lenses were removed, were: 14.3 ± 2.9 mmHg, 17.0 ± 3.3 mmHg and 13.9 ± 3.1 mmHg, respectively, for the CT80 and 13.6 ± 3.1 mmHg, 17.1 ± 4.5 mmHg and 13.3 ± 2.9 mmHg, respectively, for the PT100. The corresponding values for the first session with the -6D lenses were: 14.3 ± 3.1 mmHg, 13.1 ± 3.1 mmHg and 14.1 ± 3.3 mmHg, respectively, for the CT80 and 13.6 ± 3.2 mmHg, 13.0 ± 3.0 mmHg and 13.6 ± 3.2 mmHg, respectively, for the PT100. IOP significantly (P<0.05) increased (+ΔIOP=2.7 ± 0.4 mmHg with the CT80 in the first session) with the +6D lenses, but decreased (P<0.05) when the -6D lenses were fitted (-ΔIOP=0.6 ± 0.2 mmHg with the PT100 in the first session). The soft contact lens-induced changes were consistent between sessions but varied between tonometers.

CONCLUSIONS

The measurement of IOP through soft contact lenses resulted in consistent, statistically significant differences in IOP, which were not uniform across tonometers and which did not appear to be solely related to the central thickness of the soft contact lenses.

Comparison between Tonopachy and other tonometric and pachymetric devices

PURPOSE

This study aimed to compare central corneal thickness (CCT) measurement by Tonopachy to that by Pentacam and ultrasound pachymetry, and intraocular pressure (IOP) measurement to that by Goldmann applanation tonometry (GAT). The reproducibility of CCT and IOP measurements by Tonopachy was also evaluated.

METHODS

In 104 eyes of 104 patients, CCT was measured by Tonopachy, Pentacam, and ultrasound pachymetry, and IOP was measured by Tonopachy and GAT. Each CCT and IOP measurement was compared using Pearson correlation, repeated measures analysis of variance, and Bland-Altman plots. In 30 subjects, CCT and IOP measurements by Tonopachy were repeated to evaluate intrasession and intraobserver variability.

RESULTS

Both CCT and IOP measurements were highly correlated among all instruments used in this study. CCT measurements by three pachymeters were statistically different (p<0.0001). CCT measured by ultrasound pachymetry was lowest (541.7 ± 30.6 μm) whereas those by Tonopachy and Pentacam showed no difference (557.3 ± 34.3 and 558.0 ± 33.7 μm, respectively). Tonopachy overestimated CCT by 13.9 μm when compared with ultrasound pachymetry. There was a statistically significant difference between IOP measurements by two tonometers (p<0.0001); IOP measurement was higher by Tonopachy than by GAT (13.9 ± 4.2 and 12.5 ± 3.2 mm Hg, respectively). Tonopachy overestimated IOP measurements by 1.2 mm Hg compared with GAT. Intersession agreements for IOP and CCT measurements by Tonopachy were excellent (intraclass correlation, 0.902 and 0.962, respectively) with 95% limits of agreement ranging from -1.4 to 2.2 mm Hg and from -12.9 to 12.1 μm, respectively.

CONCLUSIONS

Although CCT and IOP measurements obtained by Tonopachy were reproducible and showed close agreement with ultrasound pachymetry and Pentacam, and GAT, careful attention should be paid when comparing Tonopachy CCT measurement to ultrasound pachymetry or its IOP measurement to GAT as the values may not be interchangeable. Tonopachy is a reliable instrument for evaluating CCT and IOP.

Two-position measurement of intraocular pressure by PT100 noncontact tonometry in comparison with Goldmann tonometry

Background

The purpose of this study was to evaluate the precision of intraocular pressure measurements obtained by PT100 noncontact tonometry in a handheld and slit lamp-mounted position in comparison with that of Goldmann applanation tonometry in healthy young adults.

Methods

Sixty eyes from 60 subjects (30 men and 30 women) aged 22 ± 1 (range 20–24) years participated in this study. Triplicate intraocular pressure measurement of a randomly selected eye was obtained by a noncontact tonometer in a handheld and slit lamp-mounted position in a randomized order, with the Goldmann applanation tonometer always performed last. A second measurement session was carried out after one week to assess repeatability.

Results

The mean ± standard deviation of intraocular pressure readings in the first and second session, respectively, with the three techniques were: handheld position, 14.52 ± 3.28 mmHg and 15.26 ± 2.11 mmHg; slit lamp-mounted position, 14.01 ± 2.80 mmHg and 15.16 ± 2.34 mmHg; and Goldmann applanation tonometer, 14.86 ± 3.26 mmHg and 15.16 ± 2.42 mmHg. There were no significant differences (P > 0.05) between the techniques in the intraocular pressure measurements returned (Goldmann applanation tonometer vs handheld and Goldmann applanation tonometer vs slit lamp-mounted). The Goldmann applanation tonometer measured intraocular pressure 0.34 mmHg higher than handheld and 0.85 mmHg higher than slit lamp-mounted in session 1, and in session 2 Goldmann applanation tonometer intraocular pressure measurement was the same as with the slit lamp-mounted method but lower than with the handheld method by 0.11 mmHg. In PT100 handheld vs slit lamp-mounted comparisons, there were no significant differences (P > 0.05) between intraocular pressure measurements returned by both techniques in sessions 1 and 2. Intrasession and intersession repeatability coefficients for Goldmann applanation tonometer intraocular pressure and slit lamp-mounted intraocular pressure were similar, and better in comparison with those for handheld intraocular pressure.

Conclusion

The Goldmann applanation tonometer and PT100 noncontact tonometer in both positions studied here are reliable, consistent techniques for measurement of intraocular pressure, and can be used interchangeably for obtaining intraocular pressure values in young normal subjects. Repositioning of the PT100 tonometer from hand-held to slit lamp-mounted improved its precision and reduced variation with respect to the Goldmann applanation tonometer.

Comparison of intraocular pressure measurements with the portable PT100 noncontact tonometer and goldmann applanation tonometry

Purpose:

Noncontact tonometers are useful when regulations preclude use of contact tonometers by medical students and other nonophthalmologists. Our study compared the measurements by the portable, noncontact tonometer (PT100) with Goldmann applanation tonometry (GAT).

Methods:

This was a prospective study of 98 eyes from 98 patients. Intraocular pressure (IOP) was measured by GAT and the PT100 (Reichert, Buffalo, NY).

Results:

Mean IOP measurements showed no significant differences in measurements performed by the two tonometers (P = 0.64). Measurements by the two tonometers were in agreement by ≤3 mmHg in 92.8% of eyes. Linear regression analysis of PT100 vs GAT measurements revealed a slope of 0.98 with r² = 0.58. Bland–Altman analysis showed a mean difference of measurements by GAT and PT100 of −0.3 mmHg with two standard deviation = 7.1 mmHg.

Conclusion:

The portable noncontact PT100 tonometer provides IOP measurements comparable to GAT within the normal range of IOP.

Influence of lens material and intra-ocular pressure on the outcome of non-contact tonometry over soft contact lenses

PURPOSE

To quantify the influence of soft lens rigidity and power on the difference between intraocular pressure values (DeltaIOP) obtained by non-contact tonometry through soft lenses in situ and over the cornea (IOP).

METHODS

Both eyes of 25 patients, attending for either routine contact lens check up, were fitted with a (I) low water content silicone hydrogel lenses of relatively high modulus of rigidity (modulus of rigidity 1.2MPa, Focus Night and Day, Cibavision) and (II) high water content daily disposable of relatively low modulus of rigidity (0.91MPa, Focus Dailies, Cibavision). IOP measurements were taken over the cornea and repeated over the lens after insertion. Lens powers ranged from -7.50D to +6.00D.

RESULTS

ANOVA revealed DeltaIOP was associated with both lens power and material (p<0.05). Multiple linear regression revealed relationships between DeltaIOP (y) lens power (x(1)) and IOP (x(2)) as follows (I), right eyes, y = 0.899x(1) - 0.172x(2) + 5.659 (F = 15.615, r = 0.766, p < 0.001, n = 25) (I), left eyes, y = 0.993x(1) - 0.101x(2) + 4.694 (F = 23.368, r = 0.825, p < 0.001, n = 25) (II) right eyes, y = 0.399x(1) - 0.370x(2) + 6.595 (F = 11.804, r = 0.719, p < 0.001, n = 25) (II) left eyes, y = 0.561(1) - 0.225x(2) + 4.153 (F = 28.736, r = 0.723, p < 0.001, n = 25)

CONCLUSION

DeltaIOP appears to be related to lens power, material and to a lesser extent IOP. Practitioners should derive their own empirical relationship between DeltaIOP, lens power and IOP for the specific types of soft lenses they commonly use. This would improve efficiency in the screening of IOP in more susceptible soft lens wearers.

The effect of repeated applanation on subsequent IOP measurements

BACKGROUND

In studies aimed at assessing the accuracy and repeatability of non-contact tonometers, the order in which these tonometers and the Goldmann tonometer are used is usually randomised despite studies in the literature that demonstrate an ocular massage effect that occurs post-applanation but not after non-contact tonometry. The purpose of this study was to investigate the effect of repeated corneal applanation on subsequent assessments of IOP.

METHODS

Data were obtained from 65 left eyes of 65 young, oculovisual normals. Three sets of IOP measurements were obtained, one set with the Goldmann applanation tonometer and two with the Topcon CT80 non-contact tonometer (one set each before and after applanation with the Goldmann tonometer), in each one of two separate measurement sessions, one week apart.

RESULTS

The average (and SD) IOP measured with the Goldmann tonometer in the first session (14.8+/-2.9 mmHg) did not vary significantly from the IOP measured with the non-contact tonometer (pre-applanation) in both sessions or with the average Goldmann IOP in the second session. The bias (mean difference +/- SD) between methods was 0.3+/-1.4 mmHg and 0.4+/-1.4 mmHg, respectively, for the first and second sessions, with the CT80 (pre-applanation) recording the higher IOP in both sessions. The within-session repeatability coefficients were +/-2.3 mmHg, +/-2.6 mmHg, +/-2.1 mmHg and +/-2.0 mmHg for the CT80 (pre-applanation) in the first and second sessions, and the Goldmann tonometer in the first and second sessions, respectively. Test-retest repeatability coefficients were +/-2.8 mmHg and +/-2.5 mmHg for the CT80 (pre-applanation) and the Goldmann tonometer respectively. Post-applanation with the Goldmann tonometer, there was a statistically significant (p<0.05) reduction (1.5+/-1.2 mmHg in session 1) in the IOP measured with the non-contact tonometer in both sessions.

CONCLUSION

These results suggest that repeated corneal applanation leads to a statistically significant reduction in IOP on subsequent measurements.

Assessment of the accuracy and reliability of the Topcon CT80 non-contact tonometer

BACKGROUND

The reliability of non-contact tonometers has been reported extensively in the literature. This study was designed to assess reliability of the new Topcon CT80 non-contact tonometer in normotensive subjects, using the Goldmann tonometer as the standard.

METHODS

The accuracy of the Topcon CT 80 non-contact tonometer was assessed by comparing its IOP assessments with those of the Goldmann applanation tonometer, on 60 right eyes of young healthy subjects with normal intraocular pressures. Each subject's intraocular pressure was assessed with each technique on two separate occasions, one week apart. The reliability of each technique was determined by the assessment of its inter-session repeatability using the Bland-Altman method. The 95 per cent limits of agreement for the two methods were also determined.

RESULTS

No statistically significant difference was found between the average intraocular pressures measured with the two techniques (p > 0.05). The inter-session repeatability indices for the two techniques did not differ significantly (p > 0.05). The mean difference in intraocular measurements between the two techniques was 0.2 +/- 1.5 mmHg (mean +/- SD) and the 95 per cent limits of agreement were -3.14 and +2.74 mmHg, with the non-contact tonometer returning higher readings than the Goldmann tonometer.

CONCLUSION

In this sample of normotensive subjects, the Topcon CT80 non-contact tonometer proved to be accurate and as reliable as the Goldmann tonometer in the assessment of intraocular pressure. Thus, it can be used as an objective clinical method for the assessment of normal intraocular pressure.

[Tonometry in change. From indentation and applanation to contour adaptation]

The dynamic contour tonometer (DCT) is the first and only noninvasive contact tonometer that is capable of measuring intraocular pressure (IOP) directly and continuously. The touch of the pressure-sensitive tonometer tip induces the cornea to gently assume a shape (contour) which it will naturally assume when pressure on both sides is equal. A force field establishes between tonometer tip and cornea, that corresponds exactly to IOP. A piezoresistive pressure sensor, integrated into the surface of the tonometer tip, precisely measures IOP continuously and therefore also records time-dependent modulations of IOP as "ocular pulse amplitude" (OPA). Dependence of the DCT on biomechanical properties of the cornea is substantially smaller than with traditional tonometers that applanate or indent the cornea.

Dynamic Contour Tonometry

PURPOSE

With tonometers currently in use intraocular pressure is indirectly determined by measuring a physical quantity related to a specified deformation of the cornea. We present a new principle of direct, continuous, and transcorneal intraocular pressure measurement, describe its theoretical foundation, and evaluate its application on the basis of an in vitro model.

METHODS

On a living human eye an optimized pressure-sensitive contact surface was determined by performing pressure measurements with differently shaped tonometer heads. Based on these results and on the theoretical model, a Dynamic Contour Tonometer was constructed and validated on eye bank bulbi against a manometric reference pressure.

RESULTS

A concave contact surface with a radius of curvature of 10.5 mm creates a distribution of forces between the central contour matching area of the tip and the cornea that equals the forces generated by the internal pressure of the eye. A sensor integrated into the surface having the same contour measures the intraocular pressure closely to the manometric reference pressure in human cadaver eyes. The accuracy of the tonometer appears to be unaffected by variations in corneal properties.

CONCLUSION

Dynamic Contour Tonometry eliminates most of the systematic errors arising from individual changes of corneal properties that adversely influence all types of applanation tonometers. The advantage of measuring the true pressure in combination with the capability of registering dynamic pressure fluctuations discloses new tonometric opportunities to diagnose and classify different types of glaucoma.

Varying effects of corneal thickness on intraocular pressure measurements with different tonometers

AIMS

To compare the intraocular pressure (IOP) measurements using the Goldmann applanation tonometer (GAT), noncontact tonometer (NCT), and ocular blood flow tonometer (OBFT), and to evaluate the effects of varying central corneal thickness (CCT) on the readings.

METHODS

Ultrasound pachymetry and tonometry were performed on 170 eyes. Using the corrected GAT values that took CCT into account as the standard, we calculated the NCT and OBFT measurement errors related to the CCT variable by a linear regression model. Group comparisons were performed with the chi2 test and one-way ANOVA test. The correlation between the various tonometer measurements was analysed by Pearson's correlation method.

RESULTS

Both the NCT (r=0.872, P<0.001) and OBFT measurements (r=0.861, P<0.001) were highly correlated with the GAT measurements. IOP measurements using the three tonometers were all correlated with CCT (all P<0.001), with the NCT measurements showing the greatest regression coefficient (beta=0.063, r=0.650) and the GAT measurements the least (beta=0.037, r=0.496). A linear regression model indicated that a 10 microm change in CCT resulted in a NCT measurement deviation of 0.47-0.98 mmHg and an OBFT measurement deviation of 0.29-0.81 mmHg.

CONCLUSION

Pressure readings with the GAT, NCT, and OBFT are all affected by CCT, with the NCT being the one most affected and the GAT the least. Our findings suggest CCT an essential variable to consider in interpreting IOP readings, especially for the NCT measurements.

How to control measurement variation in estimating the mean intraocular pressure reducing effect at a specific point in time

OBJECTIVE

To clarify how to control measurement variation in estimating the mean intraocular pressure reducing effect at a specific point in time.

METHODS

A set of 629 open-angle glaucoma or ocular hypertensive patients, eligible for treatment in both eyes, previously untreated, were reanalyzed at 3 or 6 months of intraocular pressure (IOP) reducing treatment. The trial design had parallel treatment groups; one group receives treatment A and the other group receives treatment B. The standard deviation (SD) of the IOP response variable was calculated based on a single eye per patient, mean of right and left eye (bilaterally identical treatment within each group) or difference between right and left eye (paired-eye difference within each group; one eye receives active treatment and the other eye receives placebo). The effect of using mean of replicated recordings on the SD of the patient group mean IOP was also studied. The results were generalized by applying a measurement variation model.

RESULTS

The "paired-eye difference within each group" design was clearly most effective; the SD of the IOP response variable was 2.2 mm Hg. Using mean of the right and left eye in a "design with bilaterally identical treatment in each group" was the next best solution; the SD of the IOP was 3.0 mm Hg. The SD of the IOP was 3.2 to 3.4 mm Hg when based on a single eye per patient. The minimization of the SD gave important reduction of number of patients required to enroll in a new trial. Using a single IOP recording per patient produced similar SD of the patient group mean IOP as using mean of replicated IOP recordings when measured with Goldmann tonometry.

CONCLUSION

Calculating the mean IOP by using measurements from both eyes of the patient gives important reduction of the variability.

Measurement of Intraocular Pressure Using the NT-4000: A New Non-Contact Tonometer Equipped with Pulse Synchronous Measurement Function

PURPOSE

NT-4000 (Nidek Co. Ltd., Gamagori, Japan) is a new non-contact tonometer (NCT) equipped with pulse synchronous measurement function that can measure intraocular pressure (IOP) synchronized with the ocular pulse. The purpose of this study was to evaluate the usefulness of NT-4000 in normal subjects and in patients with glaucoma and ocular hypertension.

METHODS

This study included 175 eyes of 175 subjects. Firstly, the IOP was measured using NT-4000 without the pulse synchronous measurement function (NTn). Secondly, the IOP at peak, middle, and trough phases of the pulse signal were measured using NT-4000 with the pulse synchronous measurement function (NTp, NTm, NTt, respectively). Additionally, the IOP was measured with Goldmann applanation tonometer (GT). The coefficient of variation (CV) of three readings in the NCT measurements was used to evaluate the intra-session reproducibility. Statistical comparisons were performed using Wilcoxon signed rank test and one-way analysis of variance with Scheffe's test. Linear regression analysis was used to calculate correlation coefficients. P values less than 0.05 were accepted as statistically significant.

RESULTS

The CV of NTn, NTp, NTm, and NTt were 6.4%, 5.5%, 4.9%, and 5.2%, respectively. The CV of NTp, NTm, and NTt were significantly smaller than that of NTn (P = 0.007, P < 0.001, P < 0.001, respectively). NTp was significantly higher than NTt (P = 0.038). GT was significantly correlated with NTn, NTp, NTm, and NTt (r = 0.898, P < 0.001; r = 0.912, P < 0.001; r = 0.908, P < 0.001; r = 0.900, P < 0.001, respectively).

CONCLUSION

NT-4000 can detect the fluctuation of IOP associated with the ocular pulse.

Relation Between Intraocular Pressure and Systemic Health Parameters in Taiwan

OBJECTIVE

The aim of the study was to evaluate the association between intraocular pressure (IOP) and several systemic health parameters in Taiwan.

METHODS

We retrospectively reviewed the medical records from all participants who had been examined in China Medical University Hospital. A total of 1,271 subjects (mean age, 50.0 +/- 12.0 years; age range, 21 to 85) who had not undergone any medical treatment for ocular hypertension or glaucoma were included in this study. Systemic factors (age, sex, blood pressure, total cholesterol, triglycerides, fasting glucose, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, body mass index, waist circumference) were measured. One-way analysis of variance, Pearson correlation coefficients, and multivariate linear regression were used.

RESULTS

There were 674 (53.0%) males and 597 (47.0%) females in this study. After controlling for the other covariates, the multivariate linear regression analysis showed that in males and females, systolic blood pressure was positively associated with IOP. That is, a change in systolic blood pressure by 1 mm Hg would alter the measured IOP by 0.024 mm Hg in males (95% CI, 0.002, 0.046; P = 0.031) and 0.030 mm Hg in females (95% CI, 0.006, 0.053; P = 0.014). In males, age was inversely associated with IOP. That is, a change in age by 1 year would alter the measured IOP by -0.039 mm Hg (95% CI, -0.059, -0.020; P = 0.0001).

CONCLUSIONS

IOP was found to have a strong positive association with systolic blood pressure in males and females. IOP was inversely associated with age in males. This study may provide basic information for further studies on IOP in Taiwan.

The relationship between intraocular pressure and refractive error adjusting for age and central corneal thickness

PURPOSE

To investigate the relationship between intraocular pressure (IOP) and refractive errors after adjusting for age, central corneal thickness (CCT), and other related factors.

METHODS

IOP, CCT and refractive errors were measured in the right eyes of 1855 subjects, aged 40-82 years, in a cross-sectional study design. Subjects were divided into groups by refractive status: hyperopia, emmetropia, mild myopia, moderate myopia, or high myopia. With adjustments for age, CCT, blood pressure, obesity, education, hypertension, diabetes, and smoking status, IOP was estimated for each refractive status using a general linear model.

RESULTS

IOP increased with advancing degrees of myopia, even after adjustment for age, CCT, and other related factors (p = 0.011). Estimated IOP of moderate myopia was significantly higher than that of emmetropia (p = 0.022).

CONCLUSIONS

Our results confirm the positive association between IOP and increasing degrees of myopia. This finding would support the hypothesis that the relationship between glaucoma and myopia might be pressure mediated.

Age-related changes in intraocular pressure in a large Japanese population: a cross-sectional and longitudinal study

OBJECTIVE

To assess the influence of aging, blood pressure, and body mass index (BMI) on intraocular pressure (IOP) in a large Japanese population.

DESIGN

Cross-sectional and longitudinal study.

PARTICIPANTS

The participants in this study were 69,643 Japanese men and women 20 to 79 years of age. They were office workers and their family members who had received annual health examinations from 1989 to 1997. The records of the participants who received health examinations were reviewed retrospectively. Each participant was examined according to a standard protocol, including tonometry with a noncontact tonometer, anthropometric measurements, and blood pressure measurements. The data from the subjects' most recent visit were analyzed cross-sectionally. The data from the 68,998 men and women of the total participants who were born between the 1920s and 1960s were used in longitudinal analysis.

TESTING

Tonometric and anthropometric measurements.

MAIN OUTCOME MEASURES

Mean values of IOP, systolic blood pressure, diastolic blood pressure, and BMI were determined. The relationship among IOP, age, systolic blood pressure, diastolic blood pressure, and BMI was studied using the multiple linear regression model with cross-sectional analysis. In longitudinal analysis, regression coefficients of IOP against age, systolic blood pressure, diastolic blood pressure, and BMI were calculated using the mixed effect model.

RESULTS

The mean (+/-standard deviation) IOP values for men and women were 11.9+/-2.5 and 11.5+/-2.4 mmHg, respectively. In cross-sectional analysis, IOP decreased significantly with age (P < 0.001). However, longitudinal analysis showed that IOP increased significantly with age in both men and women (P < 0.001). Systolic blood pressure and BMI were positively correlated to IOP in both the cross-sectional and longitudinal studies.

CONCLUSION

The authors found an inconsistency in the change in IOP against age between cross-sectional and longitudinal analysis. It is suspected that birth cohort differences in ocular characteristics influence IOP in the Japanese population.

A new electromechanical method for measuring intraocular pressure

A new electromechanical method to measure intraocular pressure is presented. A light probe was made to collide with an eye (cornea or sclera) and to bounce back. The impact was registered by an acceleration sensor or the duration of contact with the eye was measured using the phenomenon of the electrical conductivity of biological objects. Laboratory experiments were carried out using pig eyes pressurized with water leveled to different heights. Preliminary clinical trials were carried out comparing the values obtained with those of the Goldmann applanation tonometer. The time of contact and the deceleration time decreases and the deceleration maximum increases in proportion with IOP. A new kind of tonometer can be constructed based on this principle. Measurement can be performed to cornea or sclera without an anesthetic drop. Further study is required to evaluate the accuracy and usefulness of this method.