Accurate Intraocular Pressure Research Articles

Simulation of Air Puff Tonometry Test Using Arbitrary Lagrangian-Eulerian (ALE) Deforming Mesh for Corneal Material Characterisation.

Purpose: To improve numerical simulation of the non-contact tonometry test by using arbitrary Lagrangian-Eulerian deforming mesh in the coupling between computational fluid dynamics model of an air jet and finite element model of the human eye. Methods: Computational fluid dynamics model simulated impingement of the air puff and employed Spallart–Allmaras model to capture turbulence of the air jet. The time span of the jet was 30 ms and maximum Reynolds number was, with jet orifice diameter 2.4 mm and impinging distance 11 mm. The model of the human eye was analysed using finite element method with regional hyperelastic material variation and corneal patient-specific topography starting from stress-free configuration. The cornea was free to deform as a response to the air puff using an adaptive deforming mesh at every time step of the solution. Aqueous and vitreous humours were simulated as a fluid cavity filled with incompressible fluid with a density of 1000 kg/m3. Results: Using the adaptive deforming mesh in numerical simulation of the air puff test improved the traditional understanding of how pressure distribution on cornea changes with time of the test. There was a mean decrease in maximum pressure (at corneal apex) of 6.29 ± 2.2% and a development of negative pressure on a peripheral corneal region 2–4 mm away from cornea centre. Conclusions: The study presented an improvement of numerical simulation of the air puff test, which will lead to more accurate intraocular pressure (IOP) and corneal material behaviour estimation. The parametric study showed that pressure of the air puff is different from one model to another, value-wise and distribution-wise, based on cornea biomechanical parameters.

Assessment of intraocular pressure with an artificial model eye mimicking human eye

AbstractPurposeGiven that clinical significance of intraocular pressure (IOP), accurate IOP measurements are crucial. However, measurement of IOP is affected by corneal properties such as thickness, curvature, and biomechanical properties. To derive the quantitative relationship between corneal variables and IOP, it is valuable to create a theoretical model that represents these various properties of the cornea. We have constructed optically transparent artificial eye phantoms, which have closely similar to a human eye. The purpose of this study is to evaluate the clinical applicability of artificial eye by understanding and quantifying the IOP measurement.MethodThe artificial model eye has made of polydimethylsiloxane (PDMS) and its inside is filled with glycerol solution. We developed three different shapes of model eye to reflect the variation in corneal diameter and radius of curvature. Corneal properties (i.e., elasticity) were varied according to the formulation of PDMS (Young’s modulus: 1.7 MPa, 0.7 Mpa, and 0.4 MPa) and corneal thickness was differed by varying the concentration of ethylene glycol. The effect of each variable on the measurement of IOP was examined quantitatively using Goldmann applanation tonometry.ResultsThe IOP of the model eye showed significant positive correlation with corneal thickness in all shapes of model eye (all p < 0.001). The range of the IOP changed according to Young’s modulus of model eye. (12–80 mmHg, 1–80 mmHg, and 1–32 mmHg in model eyes with 1.7 MPa, 0.7 MPa, and 0.4 MPa, respectively). Relationship between the IOP and thickness differed depending on the Young’s modulus of the model eye.ConclusionsThe IOP of the developed artificial eyeball was successfully measured by Goldmann applanation tonometer in a similar manner to a real human eye. This finding suggests that the artificial model eye may have a value as an independent, reproducible sample that allows comparison of tonometers.

Influence of Ocular Dimensional Change on 24-Hour Intraocular Pressure Measurement With Contact Lens Sensor.

We measured changes in the corneal morphology and intraocular pressure (IOP) before and after the use of contact lens sensor (CLS). Then, we investigated whether the ocular dimensional change affected the measured IOP value with CLS. In this prospective study, we examined 112 eyes of glaucoma patients. We measured the IOP fluctuation using a Sensimed Triggerfish CLS. We measured the corneal morphology before and after making measurements using a CLS in anterior segment optic coherent optical coherence tomography and measured the IOP with a Goldmann applanation tonometer. We investigated the correlation between these ocular dimensional changes and the last IOP value with CLS. Measurement with the CLS significantly increased the IOP (P=0.0305), made the corneal thickness (CT) significantly thicker (P<0.001), and increased the steeper meridian (P=0.0026). The last value of IOP-patterns recorded with CLS was significantly correlated with the IOP immediately after the measurement with Goldmann applanation tonometer (P=0.0134), was not significantly correlated with central and peripheral CT. The difference of steeper and flatter curvature between before and after measurement significantly correlated with the last value of IOP-patterns recorded with CLS. Measurement with a CLS affects the IOP, CT, and corneal steeper meridian. From the last value of IOP-related patterns recorded with CLS, CLS could perceive accurate IOP. The change of corneal curvature might affect CLS output values.

Dynamic Scheimpflug Ocular Biomechanical Parameters in Healthy and Medically Controlled Glaucoma Eyes.

To evaluate the relationship between biomechanical parameters measured with a dynamic Scheimpflug analyzer and glaucoma. Cross-sectional observational data of 47 eyes of 47 consecutive subjects with medically controlled primary open-angle glaucoma and 75 eyes of 75 healthy subjects examined with a dynamic Scheimpflug analyzer (Corvis ST) were retrospectively investigated. Eight biomechanical parameters were compared between eyes with and without glaucoma using multivariable models adjusting for intraocular pressure (IOP), central corneal thickness, age, and axial length. In multivariable models, glaucoma was negatively correlated with A1 time (P<0.001, coefficient=-0.5535), A2 time (P=0.008, coefficient=-0.1509), radius (P=0.011, coefficient=-0.4034), and whole eye movement (P<0.001, coefficient=-0.0622). Negative correlation between glaucoma and 3 parameters (A1 time, A2 time, and radius) consistently indicate larger deformability of the cornea and negative correlation between glaucoma and whole eye movement indicate smaller eye movement, in glaucoma eyes. There were significant correlations of many biomechanical parameters with other baseline factors (8 parameters with IOP, 2 with central corneal thickness, 4 with age, and 7 with axial length). Eyes with medically controlled glaucoma were more deformable than healthy eyes, which may increase the risk of optic nerve damage through an underestimation of IOP and biomechanical vulnerability of the globe. Many parameters showed a significant correlation with baseline factors, suggesting the importance of adjustment for these confounding factors when evaluating the correlation between biomechanical parameters and ocular diseases. These results suggest the relevance of measuring biomechanical properties of glaucoma eyes for accurate IOP measurement and risk assessment.

Influence of preoperative astigmatism on corneal biomechanics and accurate intraocular pressure measurement after micro-incision phacoemulsification.

To define the corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg) and corneal compensated intraocular pressure (IOPcc) prior to and following coaxial micro-incision phacoemulsification in patients with corneal astigmatism. Of 97 patients with cataracts were enrolled in the study. Group 1 included patients with corneal astigmatism (K1-K2) values of K1-K2<+1.0 D, and group 2 with values of K1-K2 ≥+1.0 D and ≤+2.25 D. Coaxial micro-incision phacoemulsification of a corneal incision of 2.0 mm with intraocular lens (IOL) implantation was performed. CH, CRF, IOPg, IOPcc, waveform score (WS) were measured preoperatively and one week, one month postoperatively using an Ocular Response Analyzer. Axial length (AXL) was calculated by Tomey Optical Biometer OA 2000. Group 1 consisted of 51 patients with mean corneal astigmatism value of +0.49±0.25 D. Group 2 included 46 patients with astigmatism of +1.43±0.43 D. In group 1, CRF (t=2.68, P<0.05), CH (t=2.64, P<0.05) and WS (t=3.51, P<0.05) were significantly lower one week postoperatively, when compared to the preoperative values. CRF significantly decreased (t=3.61, P<0.05) when measured one month following the surgery. In group 2 CH (t=5.92, P<0.05), and WS (t=3.96, P<0.05) were significantly lowered one week after cataract surgery. Moreover, we observed a significant decrease in IOPg (t=2.24, P<0.05), CRF (t=5.05, P<0.05) and CH (t=2.31, P<0.05) one month after phacoemulsification. There was no statistically significant (t=-0.83, P=0.41) difference in AXL between study groups. CRF, CH and IOPg are reduced in patients with preoperative corneal astigmatism equal or higher than +1.0 D and lower than +2.25 D. Hence, bias of IOPg measurement in these patients may cause underestimation of the real IOP both before and after cataract surgery. The measurement of IOPcc allows the precise assessment of IOP pre- and postoperatively, independently on corneal astigmatism, CH and CRF values.

Descemet membrane endothelial keratoplasty and glaucoma.

The aim of this review is to describe the relationship between Descemet membrane endothelial keratoplasty (DMEK) and glaucoma. Glaucoma after DMEK is a serious complication that may cause permanent visual loss, affect donor endothelial cells and graft survival. The mechanisms of raised intraocular pressure (IOP) after DMEK include reverse pupillary block in the early postoperative period, and steroid response in the late phase. The reduced risk of immunogenic graft rejection after DMEK necessitates a shorter duration of steroids, which may in turn reduce the risk of steroid response. On the other hand, eyes with preexisting glaucoma that undergo DMEK may have a poorer prognosis than those without glaucoma. Early recognition and treatment of raised IOP is important after DMEK. Accurate IOP measurements after DMEK may be difficult to obtain because of the presence of an air bubble, corneal oedema, and corneal irregularities. A prophylactic peripheral iridectomy is recommended to prevent reverse pupillary block. Patients should posture face-up and be evaluated in the early postoperative period, as patients may be asymptomatic despite raised IOP. In order to reduce the risk of steroid response, weaker steroids may be prescribed after 1-3 months without adverse effects on DMEK outcomes.

Assessing the True Intraocular Pressure in the Non-human Primate.

For glaucoma patients, high intraocular pressure (IOP) is a risk factor for progressive neuropathy. Similarly, animal models used to study the disease are based on an experimental elevation of IOP. Thus, accurate IOP measurements are important in characterizing experimental models and resulting effects. The purpose of the present study was to investigate IOP measurements in a non-human primate model of experimental glaucoma by comparing clinical tonometry (Tono-Pen and TonoVet) to the true IOP from intracameral manometry. A total of 17 rhesus macaque eyes from 12 animals were used for this study. Eleven eyes had no previous experimental intervention, whereas six eyes were at varying stages of laser-induced experimental glaucoma. IOPs were adjusted by inserting a needle in the anterior chamber that was attached to a pressure transducer and syringe pump system. The anterior chamber IOP was adjusted to values between 10 and 50 mmHg and corresponding measures with Tono-Pen and TonoVet were taken. The IOPs by TonoVet and Tono-Pen were linearly related over the range of pressures tested (slope = 0.68 normal/healthy and 0.72 experimental glaucoma). For the most, TonoVet measures overestimated IOP at all anterior chamber pressure settings (mean difference of 3.17 mmHg, 95% CI 12.53 to -4.74 normal and 3.90 mmHg, 95% CI 12.90 to -6.53 experimental glaucoma). In contrast, Tono-Pen measures overestimated IOP at lower IOPs and underestimated at higher IOP (slope = -0.26 normal and -0.21 experimental glaucoma). The TonoVet and Tono-Pen tonometers that are often used to assess IOP in both clinical and experimental settings generally reflect the status of IOP, but the results from this study suggest that the instruments need calibration with true anterior chamber pressure for accurate measures in experimental models of glaucoma.

Upcoming Methods and Specifications of Continuous Intraocular Pressure Monitoring Systems for Glaucoma.

Glaucoma is the leading cause of irreversible blindness and vision loss in the world. Although intraocular pressure (IOP) is no longer considered the only risk factor for glaucoma, it is still the most important one. In most cases, high IOP is secondary to trabecular meshwork dysfunction. High IOP leads to compaction of the lamina cribrosa and subsequent damage to retinal ganglion cell axons. Damage to the optic nerve head is evident on funduscopy as posterior bowing of the lamina cribrosa and increased cupping. Currently, the only documented method to slow or halt the progression of this disease is to decrease the IOP; hence, accurate IOP measurement is crucial not only for diagnosis, but also for the management. Due to the dynamic nature and fluctuation of the IOP, a single clinical measurement is not a reliable indicator of diurnal IOP; it requires 24-hour monitoring methods. Technological advances in microelectromechanical systems and microfluidics provide a promising solution for the effective measurement of IOP. This paper provides a broad overview of the upcoming technologies to be used for continuous IOP monitoring.

DBA/2J mouse model for experimental glaucoma: pitfalls and problems.

The DBA/2J mouse has been described as a model for congenital experimental glaucoma. It develops anterior segment anomalies with synechiae and pigment dispersion leading to raised intraocular pressure and glaucomatous damage. However, there are serious practical considerations when using this model in longitudinal studies. We followed 118 mice from 12-48 weeks of age in a pharmaceutical trial. Here we report on the findings in control animals (n = 37). Intraocular pressure was measured weekly, electrophysiology and optical coherence tomography every 6 weeks. A subset also had invasive intraocular pressure measurements performed prior to euthanasia. Although intraocular pressure eventually rose by 9 months in most animals, tonometry was complicated by corneal calcification in the majority of animals rendering intraocular pressure measurement unreliable. Invasive intraocular pressure did not correlate with non-invasive measures. Loss of scotopic threshold response and thinning of inner retinal layers on optical coherence tomography was observed over time, suggesting glaucomatous damage, but this occurred in some animals without raised intraocular pressure. Poor pupil dilation significantly affected electrophysiology, optical coherence tomography and fundus imaging; 22% of animals developed major systemic complications leading to high dropout rate. The DBA/2J experimental glaucoma model shows variability in expression, and its pathological changes cause major difficulties in assessing disease progression. From our experience, the model presents significant challenges for drug studies in glaucoma, as there are many confounding factors: difficulty with accurate intraocular pressure measurement, in vivo imaging, and electrophysiology recording and a high dropout rate. In addition, there may be an underlying neurodegenerative process independent of intraocular pressure.

Assessment of intraocular pressure sensing using an implanted reflective flexible membrane

Glaucoma is a neurodegenerative condition that is the leading cause of irreversible blindness worldwide. Elevated intraocular pressure (IOP) is the main cause for the development of the disease. The symptoms of this form, such as deterioration of vision and scotomas (loss of visual fields), appear in the latter stages of the disease. Therefore, an IOP monitoring device is needed for better, simpler, and faster diagnosis, and to enable a fast treatment response. We present a theoretical assessment as well as preliminary experimental results of a simple approach for easy, optical, IOP self-monitoring. It is based on a polydimethylsiloxane membrane coated with a reflective layer and a Hartmann–Shack wavefront sensor. Nearly linear correlation is found between membrane deformation and Zernike coefficients representing defocus primary spherical aberration, with high sensitivity and negligible dependence on the measurement distance. The proposed device is expected to provide an accurate IOP measurement resolution of less than ± 0.2 ?? mm ? Hg with a pressure dependence on working distances < 0.7 ?? mm ? Hg / cm for a thick membrane; the corresponding values for a thin membrane are ± 0.45 ?? mm ? Hg and < 0.6 ?? mm ? Hg / cm , respectively, at typical IOP values—up to 40 mm Hg.

Tonometer validation and intraocular pressure reference values in the normal chinchilla (Chinchilla lanigera).

To determine accuracy and precision of three commonly used tonometers (TonoVet® and TonoLab® (ICare Oy, Finland)-rebound tonometers, and Tono-Pen VET™ (Reichert, NY)-applanation tonometer) in normal chinchillas, and to establish a normal intraocular pressure (IOP) reference range in this species. The anterior chambers of three chinchilla eyes were cannulated ex vivo and readings obtained at manometric IOPs from 5 to 80 mmHg, using each of the three tonometers in random order. Data were analyzed by linear regression, ANOVA, and Bland-Altman plots. Tonometry was performed in both eyes of 60 chinchillas (age 8 weeks-16.2 years) using the TonoVet® and relationship between age and IOP analyzed using linear regression. For all statistical tests, P < 0.05 was significant. Intraocular pressure values obtained using the Tono-Pen VET™ and TonoVet® (in dog calibration mode;'d') showed strong linear correlation with manometry within the physiologic and clinically relevant range of IOP (0-50 mmHg). The TonoVet® 'd' setting displayed significantly greater precision over the full range of IOP evaluated than the Tono-Pen VET™, and both TonoVet and Tono-Pen VET™ were significantly more accurate than the TonoLab® tonometer. Mean ± SD IOP (TonoVet® 'd') in chinchillas was 9.7 ± 2.5 mmHg, and the 95% reference interval was 4.7-14.7 mmHg. Both the Tono-Pen VET™ and TonoVet® provided clinically acceptable estimates of IOP in chinchillas. The TonoVet® provides accurate and precise IOP values, while Tono-Pen VET™ derived measurements showed greater variability. Values obtained either with the TonoLab® or TonoVet® used in the 'unspecified' calibration setting were inaccurate in this species.

Mathematical Discrepancies of the Tono-Pen Applanation Tonometer.

The purpose of the study was to determine if Tono-Pen tonometers use simple average and coefficient of variation (CV) algorithms to calculate intraocular pressure (IOP). IOPs were measured as part of routine ocular examination in 152 client-owned dogs. Using 11 Tono-Pen's, a total of 778 averaged readings were collected. Individual IOP readings, and average IOP and CV displayed by the instrument, were recorded. Average IOP and CV were then manually calculated from individual readings and compared with those displayed by the instrument. The mean absolute difference between the calculated and displayed average IOP was 1.37±2.01 mm Hg (P<0.001). In 6% of cases, the calculated average IOP was 5 to 15 mm Hg different from the displayed average IOP. The difference between the displayed and calculated average IOP was significantly higher in hypertensive eyes with displayed IOP≥25 mm Hg. Calculated CV was equal to, lower than, or greater than displayed CV in 28.6%, 1.5%, and 69.7% of cases, respectively. In 17.6% of cases, calculated CV was >20%, but displayed CV was <5%. Receiver operating characteristic analysis could not correlate number of individual IOP readings with magnitude of difference in average IOP. Calculated average IOP and CV differ significantly from values displayed by the instrument, especially at higher IOPs. A difference of ≥5 mm Hg between calculated and displayed average IOP seen in 6% of cases may impact clinical judgement. Displayed CV<5% does not correlate with accurate IOP measurement based on individual results.

Is the central corneal thickness of diabetic patients thicker than that of non-diabetics’ eyes?

The purpose of the study was to evaluate central corneal thickness in diabetic patients and to compare the results with controls without diabetes mellitus. Sixty-five diabetic patients (65 eyes) constituted the study group, and 50 eyes were from the healthy control group (50 non-diabetic patients). The study group was subdivided into group 1 (no diabetic retinopathy, n = 35), group 2 (mild to moderate nonproliferative diabetic retinopathy, n = 20), and group 3 (proliferative diabetic retinopathy, n = 10). Central corneal thickness measurements in microns were determined using ultrasound pachymetry. The mean central corneal thickness was significantly greater in the study group (567.14 μm ± 14.63 μm) than in the control group (531.14 μm ± 5 μm). In addition, the mean central corneal thickness was found to be greater in group 3 (577 μm ± 12 μm) than in groups 1 (562 μm ± 13 μm) and 2 (566.86 μm ± 15 μm), but the difference did not reach statistical significance. We found that the mean central corneal thickness for diabetic patients was thicker than that of the healthy controls. Thicker central corneas associated with diabetes mellitus should be taken into consideration when obtaining accurate intraocular pressure measurements in diabetics.

An update on postrefractive surgery intraocular pressure determination.

Due to alterations in corneal structure and biomechanics, intraocular pressure (IOP) measurement after refractive surgery has questionable accuracy. Numerous devices and techniques have been evaluated in the past decade. Whereas some methods clearly underestimate postoperative IOPs, other promising leads have emerged that are less affected by laser ablative procedures. It is well established that Goldmann applanation tonometry (GAT) underestimates IOP after refractive surgery. The discussion on the accuracy of different techniques of IOP measurement after refractive surgery remains ongoing. Evidence suggests that techniques that are less reliant on corneal structure and biomechanics may provide less altered IOPs postoperatively. Therefore, devices such as Dynamic Contour Tonometry and tonopen may have great utility after laser ablative surgery. GAT may not be the best method of measuring IOP after refractive surgery. Other devices may be superior to the GAT in providing more accurate IOP postoperatively. It is imperative that clinicians are aware of the possible inaccuracy of IOP acquisition after refractive surgery, in order to provide optimal monitoring for the development and progression of glaucoma.

Dynamics, Alterations, and Consequences of Minimally Invasive Intraocular Pressure Elevation in Rats

An important, yet not exclusive, aspect of primary open angle glaucoma is elevated intraocular pressure (IOP) profiles within fluctuations and pressure peaks. The study aimed at establishing minimally invasive methods for recurrent IOP elevation in rats to investigate the impact of IOP dynamics and pathomorphologic retinal alterations during and after IOP elevation. Intraocular pressure was elevated unilaterally in Long Evans rats to a level of ≈35 mm Hg for 1 hour in a total of 30 manipulations within 6 weeks, by using two methods: (1) suction-cup oculopression and (2) loop-adjusted oculopression. Retinal thickness (RT) was measured via optical coherence tomography (OCT), and neuronal survival was analyzed. Additional experiments were performed for "in situ" OCT investigations during exposures to different IOP levels. A mean IOP exposure of +737.3 ± 9.6 ΔIOP mm Hg for loop adjustment and +188.9 ± 16 ΔIOP mm Hg for suction cup was achieved. Optical coherence tomography examination revealed notable changes of RT between controls, untreated, and treated eyes, and evaluation of neuronal loss showed a significant decrease of retinal ganglion cell (RGC) density in both groups. In situ OCT investigation showed paradoxical retinal distortion and deformation of the optic nerve head toward the eye background. After accurate IOP elevation with minimally invasive methods, it was possible to detect RGC loss and retinal thinning. While suction cup is capable of simulating accurate arbitrary IOP profiles, loop adjustment enables the detection of pressure-dependent retinal alterations. For the first time, it was feasible to investigate consequences of variable IOP elevation profiles, including pressure peaks, by using real-time live imaging in vivo.

Red-Free Light for Measurement of Intraocular Pressure Using Goldmann Applanation Tonometer without Fluorescein

To evaluate the use of red-free light for the measurement of intraocular pressure (IOP) using a Goldmann applanation tonometer without fluorescein. This cross-sectional study was carried out on 500 eyes in 250 patients attending the Ophthalmology Outpatient Clinic at Suez Canal University Hospital. The IOP was measured using a Goldmann applanation tonometer mounted on a Haag-Streit slit-lamp. The measurements were performed first using red-free light without fluorescein. Then the measurements were repeated with cobalt blue light and topical fluorescein on the same eyes. The mean IOP was 15.23 ± 3.3 (SD) mm Hg using the red-free light without fluorescein, whereas it was 15.78 ± 3.7 (SD) mm Hg when measured using cobalt blue light after the application of fluorescein to the conjunctival sac. This difference was not statistically significant. Measurement of IOP with a Goldmann applanationtonometer with red-free light and without the use of fluorescein is simple, saves time, and gives an accurate IOP measurement relative to the traditional measurement technique with cobalt blue light and topical fluorescein.

Trans-scleral tactile tonometry: An instrumented approach

This article describes a feasibility study of a novel trans-scleral tonometer based on the use of an instrumented form of digital palpation tonometry. Similar to manual digital palpation tonometery, trans-scleral tonometer utilizes two force probes offset by a fixed distance. Force indentation data from these probes have been shown to correlate with the intraocular pressure (IOP) of the eye. Enucleated porcine eyes were used to experimentally validate the approach. The observed hysteresis in the force data was analyzed using an analytical model that accounts for the outflow of the aqueous humor. The predictions of the model indicate that the primary reason behind the observed hysteresis is stress relaxation (accommodation) in the visco-elastic corneo-scleral shell. Experimental data from eye distention and indentation tests were then used to infer the conditions under which the novel tonometer would be expected to have an accuracy of ±1mmHg. Analysis of the data shows that indentation rates should be kept below 0.5mm/s for a pressure range of 10–35mmHg. Two commonly used pressure control protocols were tested in an effort to ensure accurate IOP values during the palpation tests. Due to the large increase of IOP during digital palpation, the trans-scleral (intra-vitreous) pressurization was found to be inadequate, leading to clogging of the line by the displaced vitreous. No such problems were identified when the eye was pressurized through the cornea and into the anterior chamber. Force data from multiple palpation experiments are used to generate calibration curves for a two-probe conceptual tonometer. The calibration showed that a 10mN of force variation corresponds to 1mmHg of IOP change. A possible implementation using a contoured facial mask is also presented.

Intraocular Pressure and its Relationship to Ocular and Systemic Factors in a Healthy Chinese Rural Population: The Handan Eye Study

Purpose: To describe the distribution of and factors related to intraocular pressure (IOP) in a healthy adult rural Chinese population in northern China.Methods: The Handan Eye Study is a cross-sectional, population-based study of eye diseases among 6,830 (90.4% response rate) rural Chinese persons aged 30+ years. Participants underwent an interviewer-administered questionnaire and a complete ocular examination, including standardized measurement of IOP with Perkins applanation tonometry.Results: After excluding persons with glaucoma, or those with diseases that have a marked effect on IOP or that preclude accurate IOP measurement, 6,101 persons were included in the current analysis. Mean IOP (mean ± standard deviation) of the study population was 15.0 ± 2.8 mmHg. Men had lower IOP than women (14.6 ± 2.8 mmHg versus 15.4 ± 2.7 mmHg, t = −8.37, P < 0.001). In the multivariate analysis younger age, female sex, presence of diabetes mellitus, higher blood pressure, higher body mass index, thicker central cornea and higher myopia were associated with higher IOP.Conclusions: Mean IOP among persons living in rural northern China was similar to that recently reported in southern China. IOP appeared to be lower with increasing age in Chinese subjects, which contrasts with results in white populations.

Modern Monitoring Intraocular Pressure Sensing Devices Based on Application Specific Integrated Circuits

Glaucoma is a neurodegenerative condition that is the leading cause of irreversible blindness worldwide. Elevated intraocular pressure (IOP) is the main risk factor for the development and progression of the disease. Methods to lower IOP remain the first line treatments for the condition. Current methods of IOP measurement do not permit temporary noninvasive monitoring 24-hour IOP on a periodic basis. Ongoing research will in time provide a means of developing a device that will enable continuous or temporary monitoring of IOP. At present a device suitable for clinical use is not yet available.This review contains a description of different devices currently in development for measuring IOP: soft contact lens, LC resonant circuits and on-chip sensing devices. All of them use application-specific integrated circuits (ASICS) to process the measured signals and send them to recording devices. Soft contact lens devices are based on an embedded strain gauge, LC circuits vary their resonance frequency depending on the intraocular pressure (IOP) and, finally, on-chip sensing devices include an integrated microelectromechanical sensor (MEMS). MEMS are capacitors whose capacity varies with IOP. These devices allow for an accurate IOP measurement (up to +/– 0.2 mm Hg) with high sampling rates (up to 1 sample/min) and storing 1 week of raw data. All of them operate in an autonomous way and even some of them are energetically independent.

Accuracy of intraocular pressure measurements in dogs using two different tonometers and plano therapeutic soft contact lenses

To compare and evaluate the accuracy of intraocular pressure (IOP) measured through a therapeutic contact lens, using applanation (TonoPen XL(®)) and rebound (TonoVet(®)) tonometers in enucleated dog eyes. A total of 30 enucleated eyes from 15 beagle dogs. To measure accurate IOP, the anterior chamber of each enucleated eye was cannulated with two 26-gauge needles and two polyethylene tubes were connected vertically to an adjustable reservoir bag of normal saline and a pressure transducer. IOP was measured by the TonoPen XL(®) followed by the TonoVet(®) without a contact lens. After a contact lens was applied to the cornea, IOP was re-measured in the same order. Three consecutive IOP measurements were performed using both tonometers. Without the contact lens, the IOP values obtained by both tonometers correlated well according to the regression analysis (TonoVet(®): γ(2) = 0.98, TonoPen XL(®): γ(2) = 0.97, P < 0.001). The TonoPen XL(®) consistently underestimated values as transducer IOP increased; however, IOP values measured with the TonoPen XL(®) were in close agreement and were less variable than those determined with the TonoVet(®) when a contact lens was applied to the cornea. Bland-Altman analysis was used to determine the lower and upper limits of agreement (TonoVet(®): -29.7 and +21.1 mmHg, TonoPen XL(®): -3.9 and +3.6 mmHg) between the two devices. This study suggests that the TonoPen XL(®) is a useful tonometer for dogs wearing therapeutic contact lenses, and importantly, contact lenses would not need to be removed prior to IOP measurement.