Tonometer Intraocular Pressure Research Articles

Effects of alfaxalone, medetomidine, and xylazine on intraocular pressure in domestic pigeons (Columba livia domestica).

To evaluate the effects of alfaxalone, medetomidine, and xylazine on intraocular pressure (IOP) in domestic pigeons (Columba livia domestica). Eight 12-month-old pigeons (16 eyes). The pigeons were randomly assigned to three treatment groups (10 mg/kg of alfaxalone, 0.2 mg/kg of medetomidine, or 10 mg/kg of xylazine) with a 7-day washout period. The IOP was measured using a rebound tonometer and calibrated using the formula y = 0.439x + 2.059, where y is the tonometric IOP and x is the actual IOP. All three drugs significantly reduced IOP. Alfaxalone led to the least reduction at 5.2 mm Hg, medetomidine reduced IOP to 12.5 mm Hg, whereas xylazine resulted in the greatest reduction at 15.3 mm Hg. Alfaxalone achieved its maximum IOP reduction in 6 min, whereas medetomidine and xylazine required 95 and 115 min, respectively. Both alpha-2 agonists, medetomidine, and xylazine, showed a prolonged duration of effect and a greater reduction in IOP than those of alfaxalone. All three medications provided adequate sedation without any discernible adverse effects. The findings revealed the varied effects of these drugs on IOP in pigeons, potentially providing valuable insights that could be useful for broader applications in veterinary medicine.

Analysis of the effect of keratotopographic parameters on tonometric intraocular pressure

BACKGROUND: Applanation tonometers remain popular in most countries of the world; in Russia, the Maklakov tonometer is one of them. When using a Maklakov tonometer, intraocular pressure values are obtained indirectly through the cornea, which has individual characteristics, which certainly introduces its own errors into the accuracy of the measurement results.
 AIM: The aim of this study was to investigate the influence of keratotopographic parameters on tonometric intraocular pressure.
 MATERIALS AND METHODS: A study of 16 keratotopographic indices (using ALLEGRO Oculyzer Wavelight Oculyzer II) and of tonometric intraocular pressure values obtained using Maklakov tonometer (NGm2-“OFT-P”) was carried out in 500 patients (1000 eyes). Among the study population there were patients with both emmetropic refraction — 8 eyes (0.8%), and patients with refractive errors 992 eyes (99.2%), among them: 978 eyes (97.8%) had myopic refraction 14 (1.4%) eyes had isolated refraction with myopic astigmatism. 889 eyes (88.9%) combined myopic refraction with myopic astigmatism.
 RESULTS: Based on the analysis of 16 keratotopographic parameters and tonometric intraocular pressure values, 500 patients (1000 eyes), the greatest influence of 13 studied parameters of the cornea was determined, such as: K1 (keratometry of flatter meridian); K2 (keratometry of steeper meridian); Km (mean keratometry); Rf (radius of the flatter meridian); Rs (radius of the steeper meridian); Rm (mean radius of curvature); Rmin (minimal radius of curvature); Rper (average radius of curvature between the 6 mm and 9 mm zone center); IVA (index of vertical asymmetry); ISV (index of surface variance); CKI (central keratoconus index); Thickness (thickness); IHD (index of height decentration) — on intraocular pressure values.
 CONCLUSIONS: Based on the results obtained, we determined the effect of topographic parameters of the cornea on tonometric intraocular pressure, and created a formula for correcting intraocular pressure values based on the original formula we developed: Pt keratotopographic corrected = Pt + (±ISVp) + (±IVAp) + (±Rmp) + (±Kmp) + (±CKIp) + (±Thicknessp), where Pt keratotopographic corrected is the corrected tonometric intraocular pressure and Pt is the tonometric intraocular pressure.

The method for determining intraocular pressure according to keratotopographic indicators

Relevance. In most countries of the world, applanation tonometers remain popular, such as the Maklakov tonometer and the Goldman tonometer. When measuring ophthalmotonus, such tonometers receive IOP values indirectly through the cornea, which certainly introduces its own errors into the measurement results. The creation of methods for measuring IOP different from the currently existing ones is relevant. Purpose: to create a new method for determining intraocular pressure without tonometry, based on the indicators of the individual profi le of the cornea with its parameters, indices and autorefractometry data. Materials and methods. Statistical analysis of 16 parameters of a keratotopograph (ALLEGRO Oculyzer, WaveLight Oculyzer II), data of an autorefractometer (TONOREF Nidek device) and data of tonometric intraocular pressure was carried out using a Maklakov tonometer (НГм2-«ОФТ-П») in 500 patients (1000 eyes). Among the sample population there were patients with both emmetropic refraction – 8 eyes (0.8%), and patients with refractive errors 992 eyes (99.2%), among them: 978 eyes (97.8%) had myopic refraction, 14 (1.4%) eyes had isolated refraction with myopic astigmatism. 889 eyes (88.9%) combined myopic refraction with myopic astigmatism. Results. Based on the analysis of keratotopographic parameters, autorefractometry values and tonometric IOP of 500 patients (1000 eyes), we have created a new method for determining intraocular pressure without tonometry, presented in the form of a mathematical model: Pt keratotopographic = 61.9 – 0.06 × SPH – 2.39 × Rf + 0.64 × Rmin – 0.15 × log2 (IVA) – 31.9 × CKI – 0.006 × Thickness. Conclusions. Based on the results obtained for determining IOP without physical interaction with the eye, the mathematical model created by us can be used in cases where the use of any tonometer is impossible.

Comparative analysis of Maklakov and automated ocular tonometry methods of intraocular pressure measurement

PURPOSE. To study the factors affecting the results of intraocular pressure (IOP) measurement by different ocular tonometry methods.MATERIAL AND METHODS. The study involved 72 patients (128 eyes) aged 22 to 86 years. IOP was measured in all patients using Corvis ST tonometer, Topcon ST-800 pneumotonometer, iCare IC 100 portable tonometer, and Maklakov applanation tonometer. The results of IOP measurements were analyzed depending on the following factors: age, gender, presence of glaucoma or suspicion for glaucoma, presence of refractive errors, axial length of the eye, thickness of the cornea.RESULTS. The greatest difference in IOP values was observed between the Maklakov method (tonometric IOP) and iCare IC 100 device (true IOP). In some cases (9–22%), tonometric IOP obtained using the Maklakov method was equal or lower than the values obtained with other methods.Results of the correlation analysis confirmed the high correlation of the IOP values obtained with Topcon CT-800, iCare IC 100 and Corvis. On the other hand, IOP values obtained with the Maklakov method had lower correlation dependence with IOP values obtained using other tono metry methods. The strongest correlation of IOP and corneal thickness was observed when using Topcon CT-800. The IOP value obtained with the Maklakov method had the lowest correlation dependence with corneal thickness.CONCLUSION. The lowest correlation dependence of Mak lakov IOP measurements with IOP values obtained using Topcon CT-800, iCare IC 100 and Corvis indicates that the reliability of the Maklakov method significantly depends on the compliance with the examination technique.

Optical and biometric indicators of the eye in children with juvenile glaucoma combined with myopia

Purpose. To analyze optical and biometric indicators of children's eyes with various stages of congenital juvenile glaucoma (CJG) combined with myopia.Material and methods. We examined 17 patients (31 eyes) aged 11 to 17 (averagely 14.0 ± 0.2 years) diagnosed with CYG who underwent, prior to surgical or conservative treatment, a regular ophthalmological examination supplemented with the measurement of the central corneal thickness (on an automatic contactless tonometer-pachymeter by NIDEK, USA), and the index of corneal deformation (ICD) by the Shkrebets technique.Results. The analysis showed a possible correlation between 1) tonometric intraocular pressure (Pt) and the axial length of the eye, 2) Pt and the ratio of excavation to optic disk diameter (E/ON), 3) axial length of the eye and the central corneal thickness at the terminal CYG stage, and 4) the axial length and the refraction at the initial stage of CYG.Conclusion. As the glaucomatous process progresses, children with CYG combined with myopia show an increase of myopic refraction, a decrease in fibrous membrane rigidity, pretrabecular and trabecular changes, axial elongation, increased IOP due to an imbalance between the production of intraocular fluid and its outflow, an expansion of the excavation of the optic disk and a decrease in the central corneal thickness.

Early diagnosis of pigmentary glaucoma with pseudo-normal pressure in a patient after radial keratotomy (case study)

The article describes a clinical case of diagnostics and diagnosis of pigmentary glaucoma with pseudo-normal pressure in a patient who had previously underwent radial keratotomy (RKT).This case is interesting in that the detection of pigmentary glaucoma can be significantly complicated by a falsely low level of intraocular pressure (IOP) in the myopic eye after previous RKT and secondary hypermetropic shift, combined with the difficulty of an in-depth ophthalmoscopic assessment of the state of the optic nerve head in myopia, as well as perimetric study due to keratotomy scars. Tonometric IOP readings in such eyes should be considered carefully and supplemented with examination of other characteristic clinical signs of pigmentary glaucoma. This would help establish the diagnosis early and timely prescribe pathogenetic treatment.

Effects of postural changes on measured intraocular pressure and repeatability of PT-100 tonometer and agreement with applanation and indentation tonometry

Background: Repeatability and validity are important components of precision in any measurement system.Aim: This study aimed to determine the effect of change in head and neck position and body posture on the repeatability of intraocular pressure (IOP) measurements with PT-100 non-contact tonometer and compare with Goldmann, PT-100 and Schiøtz tonometer readings.Setting: Optometry clinic, Saudi Arabia.Methods: The IOP was measured in one selected eye of 84 healthy participants (mean aged 21.9 ± 2.0 years) using PT-100 in three head and body positions, Goldmann tonometer and Schiøtz on two separate visits, in a randomised fashion. Central corneal thickness (CCT) was measured using an ultrasound pachymeter. The repeatability, agreement and correlations between CCT and IOP differentials were assessed.Results: The IOP measured in three head and body positions with the PT-100 were similar and comparable to Goldmann IOPs in sessions one and two with 74% and 86% of PT-100 measurements within ±3 mmHg of the Goldmann tonometry, respectively, for sessions one and two. The Schiøtz tonometer-measured IOP was higher than the Goldmann IOPs (p 0.05) with 60% and 44% of Schiøtz IOPs within ±3 mmHg of the Goldmann tonometer IOP in sessions one and two, respectively. The limits of repeatability and reproducibility were best with the PT-100 and worst with the Schiøtz tonometer. The mean CCT (552±36 µm) was negatively correlated with differences between Goldmann and both PT-100 and Schiøtz-measured IOP.Conclusion: Postural changes did not affect the validity and repeatability of PT-100 readings. PT-100 measurements were interchangeable with Goldmann tonometer. Schiøtz overestimated Goldmann IOP in thicker corneas more than the PT-100.

Клинический случай диагностики первичной открытоугольной глаукомы у пациентки после ранее выполненной передней радиальной кератотомии

The article describes a clinical case of diagnosis and detection of primary open-angle glaucoma (POAG) in a patient after previously performed anterior dosed radial keratotomy (ADRK). The reasons that hampered the primary diagnosis of POAG in the presented patient at an outpatient appointment with an ophthalmologist were: inadequate assessment of the ratio of the state of the lenses and low vision; a falsely underestimated level of tonometric intraocular pressure (IOP), as well as the absence of an in-depth ophthalmoscopic assessment of the condition of the optic nerve head, failure to comply with the mandatory perimetric examination in such cases. It should be especially remembered that there is a high likelihood of underestimation of applanation tonometry indices after performing ADRK, therefore, the assessment of its results should be treated with caution, identifying other clinical signs characteristic of glaucoma. This will make it possible to diagnose glaucoma in time and prescribe pathogenetic treatment in a timely manner. Key words: anterior dosed radial keratotomy, primary open-angle glaucoma, intraocular pressure.

The profile of patients with primary open-angle glaucoma in the Russian Federation (preliminary results of a multicenter population-based study). Part 2

PURPOSE. To determine individual clinical and epidemiological characteristics of the disease in patients with primary openangle glaucoma (POAG) and to establish its relationship with adherence to treatment.METHODS. The analytical, multicenter, combined study involved 1 092 subjects (2 184 eyes) with different forms of glaucoma (general group) were selected using a survey, followed by their clinical and epidemiological analysis.RESULTS. When verifying the diagnosis, eyes with POAG prevailed in the main group (from 89.6 to 92% of all eyes). The average age in the main group at the final examination was 66 (63; 69) years, the duration of the disease was on average 3 years. The number of eyes with the initial stage of glaucoma decreased in 3 years by 17.5-19.1%, and the number of eyes with advanced and severe stages of the disease increased by 10.4-12.9% and 5.1-5.2%, respectively. The hypotensive regimen was changed at the average tonometric intraocular pressure of 24 (23; 26) mm Hg (regardless of the disease stage). When estimating the comorbidity of the main group, the most common concomitant somatic pathology was hypertension. Today, prostaglandin analogues are prescribed in 34.6-36.3% of all cases for all stages of glaucoma. The leading complaint of patients according to the survey was the cost of treatment. In addition, the influence of the organization's pharmacist on the choice of the purchased drug was revealed. However, attending physician remains the main source of information about the disease for patients (92.2%).CONCLUSION. It was found that POAG predictably prevails in Russia, and its diagnosis still comes with a delay. The causes of treatment failures should be recognized as late diagnosis, incorrect treatment of patients with newly diagnosed condition, and the same treatment regimens for paired eyes with different stages of POAG, as well as social and behavioral factors of patient compliance.

Smart Contact Lenses with a Transparent Silver Nanowire Strain Sensor for Continuous Intraocular Pressure Monitoring.

Continuous intraocular pressure (IOP) monitoring can provide a paradigm shift in the management of patients with glaucoma as a facile alternative to conventional diagnostic methods. However, the low sensitivity and functional instability of current IOP sensors have limited their clinical utility in the management of glaucoma. Here, we have developed a smart contact lens integrated with a transparent silver nanowire IOP strain sensor and wireless circuits for noninvasive, continuous IOP monitoring. After confirming the robust stability of the IOP sensor within the smart contact lens in the presence of tears and repeated eyelid blink model cycles, we were able to monitor IOP changes on polydimethylsiloxane model eyes in vitro. In vivo tests demonstrated that our fully integrated wireless smart contact lens could successfully monitor the change in IOP in living rabbit eyes, which was clearly validated by the conventional invasive tonometer IOP test. Taken together, we could confirm the feasibility of our smart contact lens as a noninvasive platform for continuous IOP monitoring of glaucoma patients.

Evaluation of the applanation tonometer Tono-Pen Avia® Vet™ for the measurement of the canine and feline intraocular pressure

Using tonometers, intraocular pressure (IOP) is merely measured indirectly leading to measurement inaccuracies. All available technical devices are designed for humans, hence a calibration via comparison to manometric measurements is necessary for their use in veterinary medicine. In this study, the applanation tonometer Tono-Pen Avia® Vet™ (TPA) was to be calibrated for use in dogs and cats resulting in the calculation of a correction factor. Additional objectives were the determination of reference values for TPA-derived IOP and the evaluation of possible influence of patient characteristics on measurements results. For the manometric study, 10 enucleated eyes from cats and dogs were subject to IOP measurement with a manometer and the TPA covering the range of 5-60 mmHg. The mean percentage of difference between manometer and TPA was used to calculate correction factors. Subsequently, the TPA was employed to measure the IOP in healthy animals without signs of ocular disease for the determination of reference values. The effect of the patient characteristics age, body weight, and gender on the IOP measurement results was examined using statistical mixed models. With rising intraocular pressure, the TPA underestimated IOP to an increasing degree. Multiplication of the TPA-derived IOP with the factor of 1.5 for dogs and cats resulted in values well in accordance with the manometrically-derived, actual IOP. In the second part of the study, a total of 94 dogs (188 eyes) and 64 cats (128 eyes) were examined. Reference values for the IOP measured with the TPA are 9-18 mmHg for dogs and 9-20 mmHg for cats. For both species, age exhibited a significant influence (dog, p = 0.001 and cat, p = 0.008) on the IOP measurement results in that the IOP-values displayed a decrease with increasing age. The TPA underestimated the actual IOP with increasing intraocular pressure. The calculated correction factor of 1.5 is suitable for the conversion of tonometric IOP results into actual pressure values. The TPA is suitable for the measurement of the IOP in dogs and cats.

Using 24-hr ocular dimensional profile recorded with a sensing contact lens to identify primary open-angle glaucoma patients with intraocular pressure constantly below the diagnostic threshold.

To evaluate the use of a 24-hr ocular dimensional profile recorded with a contact lens sensor (CLS) combined with a single tonometric intraocular pressure (IOP) reading to indicate the potential for exceeding the diagnostic threshold for normal tension glaucoma (NTG) in Japanese patients. Intraocular pressure (IOP) of untreated Japanese NTG patients was measured with tonometry every 3hr from 9:00 to 24:00. The subsequent day a 24-hr CLS (SENSIMED Triggerfish, Sensimed AG, Lausanne, Switzerland) profile was recorded on the same eye. Patients initially diagnosed as NTG were reclassified as NTG when the measured IOP values were consistently <20mmHg or as primary open-angle glaucoma (POAG) when IOP was ≥20mmHg in at least 1 measurement. For each patient and each IOP time-point, IOP values were combined with information provided by the CLS profile ('classifier') to indicate the potential for exceeding the diagnostic threshold value. Statistical analyses were performed for each classifier. A total of 65 patients, 28 males and 37 females (mean ages: 50.8±14.6years and 52.6±10.2years, respectively), were analysed. Following IOP diurnal measurement, five patients (7.7%) were reclassified as POAG. Two classifiers (15:00 CLS and 18:00 CLS) showed high sensitivity and negative predictive value (100%), identifying all the POAG patients. Contact lens sensor information can be used in conjunction with a single tonometric reading to determine patients' potential of having IOP levels exceeding the diagnostic threshold within a 24-hr period, without the need to perform a 24-hr tonometric curve. This would allow clinicians to identify POAG patients who may otherwise be eventually misclassified as NTG.

Elastotonometry for evaluation of biomechanical characteristics of the eye after surgical hyperopia correction

To build a mathematical model that would use elastotonometry to assess biomechanical properties of the eye in treatment of hyperopia with LASIK and FemtoLASIK surgeries. The study included 64 operations: 34 FemtoLASIK surgeries, and 31 LASIK surgeries. All patients before and after surgery underwent standard examination necessary for keratorefractive surgery, including elastotonometry. The next step was the analysis of elastotonometry curves and eye finite element model in the software package Ansys (Ansys, Inc.; U.S.A.). During the analysis, the fibrous membrane was modeled by two spherical segments, which before elastotonometry were influenced only by true intraocular pressure (IOP). There was a problem of determining tonometric IOP when the cornea is pressured with flat bottomed loads during elastotonometry. It was taken into account that at the first stage of loading, the IOP was applied, and then the force corresponding to the weight of the tonometer. After tonometry, IOP increased so that the volume inside the composite membrane corresponded to the value before loading. Mathematical modeling has shown that after surgical treatment of hyperopia, flexural stiffness of the cornea decreases, elevation of the elastotonometry curve increases, and the higher the true IOP, the greater the elevation. The greatest increase in lift due to increased IOP is observed after LASIK operation, while after FemtoLASIK this change is not significant, in which case the dependence of tonometric IOP on the weight of the load is almost linear. The obtained results allow for elastotonometry to be recommend for further medical research as a promising method for assessing biomechanical characteristics of the fibrous membrane of the eye.

Nowoczesna tonometria – więcej niż pomiar ciśnienia

Goldmann tonometer intraocular pressure measurement was described in 1957, and is still the standard method in use. However, its limitations were the basis of numerous studies aimed to develop a new device whose dependence on factors such as the thickness and other characteristics of the cornea would be kept to a minimum. Currently, different devices are tested that do not only measure the intraocular pressurebut also define other parameters that may be useful in the daily practice of ophthalmology. These include non-contact applanation tonometer (ORA, Ocular Response Analyzer, Reichert), which performs the analysis of the viscoelastic properties of the cornea too. The principle of operation is based on a bidirectional applanation using a single stream of air, which enables assessment of the corneal resistance factor and corneal hysteresis

Effect of pathological myopia on biomechanical properties: a study by ocular response analyzer.

To evaluate the ocular response analyzer (ORA) measurements of patients with pathological myopia in comparison with those of emmetropic control subjects, and to investigate the correlation between these ORA measurements and spherical equivalent (SE). Measurements of 53 eyes of 53 subjects with pathological myopia (SE>-6.00 D) were compared with those of 60 eyes of 60 emmetropic controls. Corneal hysteresis (CH), corneal resistance factor (CRF), noncontact tonometer intraocular pressure (IOPg), and corneal-compensated IOP (IOPcc) were obtained for each subject. The refractive error value was determined as SE via a cycloplegic refraction test. The mean age was 54.1±18.9y (ranging from 5 to 88) in the pathological myopic group and 56.2±19.0y (ranging from 6 to 89) in the control group. There were no significant differences between the groups concerning age and sex. CH and CRF were significantly lower in the pathological myopic group than in the control group (P<0.001, P=0.005, respectively). IOPcc and IOPg were significantly higher in the pathological myopic group than in the control group (P<0.001, P=0.009, respectively). There were significantly positive correlations between CH and SE (r=0.565, P<0.001) and between CRF and SE (r=0.364, P=0.007). There were significantly negative correlations between IOPcc and SE (r=-0.432, P=0.001) and between IOPg and SE (r=-0.401, P=0.003). The present study displayed that pathological myopia affected biomechanical properties measured by ORA. The results of corneal biomechanical properties measured by ORA may need to be appreciated by taking refraction into account. Further, pathological myopia might be related with the increased IOP.

Diurnal Variation in Intraocular Pressure Measured by Ocular Response Analyzer in Korean Patients with Normal Tension Glaucoma

Purpose: To analyze the diurnal change in intraocular pressure (IOP) and corneal biomechanical properties measured using the Ocular Response Analyzer (ORA; Reichert Inc., Depew, NY, USA) in Korean patients with normal tension glaucoma (NTG) patients. Methods: Intraocular pressure (Goldmann applanation tonometer IOP [GAT IOP], Goldmann-correlated IOP [IOPg], corneal-compensated IOP [IOPcc]) and corneal hysteresis (CH), corneal resistance factor (CRF) and central corneal thickness (CCT) were measured in 21 eye of NTG patients (12 males, 9 female) at 3 hour intervals for 48 hours using ORA. We recorded the time of each parameter that showed the lowest and the highest values of during the 48 hour testing period (Day 1 and Day 2) and evaluated the change of diurnal variation using Repeated measures analysis of variance (Re-ANOVA). Results: Peak IOP measured with GAT and ORA occurred at 6 AM-9 AM, 3 PM-6 PM and the trough IOP at 9 PM-12 AM during the 48 hour period. CCT, GAT IOP, IOPcc and IOPg measurements showed statistically significant variations (p 0.05). Conclusions: In Korean NTG patients, IOP exhibits significant diurnal variation, with higher values during the dawn and afternoon and lower values before retiring. Clinically, measurements of IOP performed in the afternoon could aid in the detection of relatively elevated IOP. J Korean Ophthalmol Soc 2015;56(12):1913-1920

Hourly Awakening vs Continuous Contact Lens Sensor Measurements of 24-Hour Intraocular Pressure

All studies of 24-hour intraocular pressure (IOP) rhythm conducted to date have used repeated IOP measurements requiring nocturnal awakenings, potentially disturbing sleep macrostructure. To evaluate the effects on sleep architecture and IOP rhythm of hourly awakening vs a contact lens sensor (CLS) to continuously monitor IOP without awakening. Cross-sectional study at a referral center of chronobiology among 12 young healthy volunteers, with a mean (SD) age of 22.3 (2.3) years. Volunteers underwent two 24-hour IOP measurement sessions during a 2-month period. The eye order and session order were randomized. During one session, the IOP of the first eye was continuously monitored using a CLS, and the IOP of the fellow eye was measured hourly using a portable noncontact tonometer (session with nocturnal hourly awakening). During the other session, the IOP of the first eye was continuously monitored using a CLS, and the IOP of the fellow eye was not measured (session without nocturnal awakening). Overnight polysomnography was performed during the 2 sessions. A nonlinear least squares, dual-harmonic regression analysis was used to model the 24-hour IOP rhythm from the CLS data. Comparisons of acrophase, bathyphase, amplitude, and the midline estimating statistic of rhythm were used to evaluate the effect of hourly awakening on IOP rhythm. To evaluate the effects of hourly awakening on sleep architecture, comparisons of sleep structure were used, including total sleep period, rapid eye movement, wake after sleep onset, absolute and relative total sleep time, and non-rapid eye movement sleep (N1, N2, and N3). A 24-hour IOP rhythm was found in all individuals for the sessions with and without awakening (P < .05). Hourly awakening for nocturnal IOP measurements increased wake after sleep onset (P = .04) but did not seem to change total sleep time, total sleep period, sleep efficiency, or slow-wave and rapid eye movement sleep stage duration (P > .30). Hourly awakening during noncontact tonometer IOP measurements did not seem to alter the mean variables of the 24-hour IOP pattern evaluated using CLS, including signal, maximum signal, minimum signal, acrophase, and bathyphase (P > .15). The 24-hour IOP rhythms seem to be unaffected by hourly nocturnal awakening for IOP measurements in young healthy individuals.

Intraocular Pressure

There exists a concern about the general accuracy of intraocular pressure (IOP) measurements using tonometry, and especially Goldmann applanation tonometer (GAT) because it considers the cornea as an infinite thin shell. In this study, the relationship between the true IOP and tonometric IOP, external curvature radius (ECR), and central corneal thickness (CCT) is explored. In this study, the calibration of the IOP measurements through GAT for different values of CCT, ECR, and E (Young modulus), is done through computational simulations of the mechanical behavior of the cornea subjected to the applanation process using the finite element method (FEM). Previous to this simulations, experimentations on rabbits were performed to confirm that inaccurate readings are obtained with GAT in certain conditions. This methodology is also followed to establish the range of corneal parameters of patients for which the GAT measure of pressure is reliable. The calibration equation for GAT measurements is developed from a statistical multiple linear regression analysis. Based on a statistical variable analysis of the computational modeling results, a calibration equation is established for the GAT that relates the true IOP with the ECR, CCT, and GAT measurements. Our results show that GAT measures are linearly dependent on the modulus of elasticity of the cornea; nevertheless, if we consider a healthy cornea with a specific modulus of elasticity, it is possible to correct the measure with a linear equation involving CCT and ECR.

Assessment of Corneal Biomechanical Behavior Under Posterior and Anterior Pressure

To determine the biomechanical response of the rabbit cornea to inflation under posterior and anterior pressure. Twelve Japanese white rabbits were included in the study. A randomly selected eye from each animal was subjected to posterior pressure in an inflation test rig, and the other eye was subjected to anterior pressure after manually reversing its curvature. Specimens were loaded by cycles of pressure up to 40 mmHg, and the experimentally obtained pressure-deformation data were used to derive the stress-strain behavior of each eye using an inverse modeling procedure. The differences between the two groups in corneal thickness, diameter, and intraocular pressure (IOP) were not statistically significant (P=.935, .879 and .368, respectively). Corneas tested under posterior pressure displayed significantly higher stiffness (as measured by the tangent modulus) than those inflated by anterior pressure (P<.001). Cornea is a nonlinear viscoelastic tissue that presents different mechanical properties when tested under posterior and anterior pressure. The determination of the behavior under both forms of pressure could contribute to the construction of accurate finite element simulations of corneal behavior and the correction of tonometric IOP measurements. The difference in mechanical behavior between anteriorly and posteriorly loaded corneas in the study, although significant, could have been partly affected by the changes in microstructure possibly caused by changing corneal form to enable anterior loading.

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

BackgroundThe 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.MethodsSixty 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.ResultsThe 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.ConclusionThe 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.