Actual Intraocular Pressure Research Articles

Intraocular pressure change after myopic laser in situ keratomileusis as measured on the central and peripheral cornea

Background: Laser in situ keratomileusis (LASIK) leads to inaccurate measurement of intraocular pressure (IOP). This study aimed to determine the efficacy and reliability of IOP measurement on the peripheral cornea after LASIK.Methods: The IOP was measured in 52 eyes of 26 subjects before LASIK and one week and one month after LASIK using non‐contact tonometry. The measurements were made on the central and peripheral cornea and the results were subjected to statistical analysis.Results: The mean measured IOP on the central cornea was decreased by 6.425 ± 2.544-mmHg at one week and 5.752 ± 3.863-mmHg at one month post‐operatively. The mean IOP measured on the peripheral cornea was decreased by only 0.921 ± 2.054-mmHg at one week post‐operatively. Most notably, the mean IOP measured on the peripheral cornea was increased by only 0.158 ± 2.979-mmHg at one month post‐operatively with no statistical significance (p > 0.05). Furthermore, a linear regression of ablation depth versus change in IOP measured on the central cornea was demonstrated, but the peripheral IOP did not display such a statistically significant correlation with the depth of ablation at both one week and one month after operation.Conclusion: The IOP measured on the peripheral cornea is closer to the actual IOP. Therefore, it is more accurate and reliable to measure IOP on the peripheral cornea than on the centre after LASIK.

Changes of Intraocular Pressure During Experimental Vitrectomy

Purpose: To compare the infusion pressure shown by a vitrectomy device with the actual intraocular pressure (IOP) observed during pars plana vitrectomy. Furthermore, we evaluated the effects of variable parameters on the actual IOP during vitrectomy surgery.Materials and methods: Porcine eyes were obtained within 24 h of slaughter. Actual IOP was measured by a digital manometer during vitrectomy using the vented gas forced infusion (VGFI) system, as well as the gravity system. We analyzed the actual IOP according to the groups divided by remnant volume of infusion fluid: (500 ml; control group, 250, 125, and 50 ml). Finally, actual IOP was determined after changing variable parameters such as cutting rate, vacuum pressure, and the VGFI setting.Results: Settings for a VGFI system and pressure supplied by a gravity system significantly correlated with actual IOP (r = 0.99, p = 0.0001; r = 0.99, and p = 0.0001). Actual IOP declined with decreasing volume of infusion fluid. If the volume of infusion fluid was <125 ml, actual IOP decreased significantly compared with the control group and the difference in actual IOP reflected a significant difference in the VGFI setting of 30 mmHg. Cutting rate as well as VGFI setting and vacuum pressure affected actual IOP.Conclusions: Infusion pressure shown by the vitrectomy device was similar to actual IOP in porcine eyes. However, volume of infusion fluid and variable parameters could change the actual IOP during pars plana vitrectomy. Our results may help to optimize the ideal parameters such as infusion pressure, vacuum pressure, and cutting rate of vitrectomy systems used to treat vitreoretinal diseases.

Simultaneous measurement of intraocular pressure in the anterior chamber and the vitreous cavity

To simultaneously measure, using manometry, the changes of intraocular pressure (IOP) in the anterior chamber and in the vitreous cavity of a single porcine eye, induced by an external column of water. We prospectively measured IOP simultaneously in seven freshly enucleated porcine eyes in both the anterior chamber and the vitreous cavity. Measurements were obtained through blood-pressure transducers connected to 21-gauge catheters. A third cannula was inserted into the anterior chamber to increase the IOP; when the cannula was connected to the water column, the pressure increased rapidly from 0 to 180 mmHg. Changes were registered every 5 seconds during the first minute. Statistical analysis was performed using the Wilcoxon signed-rank test. P ≤ 0.05 was considered significant. Before the IOP was increased, the median basal IOP value was 18 mmHg in the anterior chamber and 23 mmHg in the vitreous cavity (P > 0.05). Sixty seconds after the end of the experiment, the median IOP value was 135 mmHg (range 122-145) in the anterior chamber and 55 mmHg (range 16-68) in the vitreous cavity (P < 0.01). Cannulation of the anterior chamber measures the actual IOP; however, the IOP measured by cannulation in the vitreous cavity using the same catheter diameter was different, possibly because of the viscosity of the vitreous. The viscosity of the vitreous probably makes the cannulation method of IOP evaluation in the vitreous cavity inaccurate.

Change in Intraocular Pressure of Normal Healthy Eyes During Sit-up With Various Postures

Purpose: To investigate the change in intraocular pressure (IOP) of normal healthy eyes at different positions while performing sit-ups. Methods: Twenty normal healthy eyes of 20 subjects were studied. IOP was measured while in the 'sitting upright', 'supine', and 'head-down' position to determine postural effect on IOP when executing a sit-up. Measurements were taken after the performance of a sit-up and during the performance of a sit-up, in both the head up and head down positions to test the effect of the exercise on the IOP. In addition, the IOP after an 'isometric exercise with Valsalva's maneuver in the supine or head-down position' was measured to determine the actual IOP during a sit-up. Results: The postural change from sitting upright to tilting the head in a downward position caused the IOP to increase. After the performance of a sit-up, the IOP decreased in both the upright and supine positions, though IOP increased in the head-down position. After the isometric exercise with Valsalva's maneuver, IOP increased significantly regardless of position. Conclusions: IOP increased significantly during sit-ups, but decreased afterward. Future studies should be performed to evaluate the long-term effects of IOP change during and after sit-ups with respect to the prognosis of glaucoma.

Assessing intraocular pressure by rebound tonometer in rats with an air-filled anterior chamber

To compare rebound tonometer and cannulation as methods for measuring intraocular pressure (IOP) in rats. The accuracy of the TonoLab rebound tonometer was determined in eight cannulated rat eyes. IOP was manipulated by changing air pressure from 20 to 100 mmHg at 10-mmHg intervals, and the IOP was measured with the rebound tonometer at each level. The average value of three repeated pressure readings was recorded. Correlation analysis and comparison with the Bland and Altman method were performed. The intraclass correlation coefficient was calculated to assess intraoperator variability. The IOP values measured with the TonoLab rebound tonometer were well correlated with the actual IOP (r (2) = 0.963, P = 0.01). The mean of the difference between the rebound tonometer and actual (cannulation) IOP was 7.41 +/- 7.87%. The intraclass correlation coefficient was 0.9, indicating low intraoperator variability. The rebound tonometer showed high accuracy and reliability for IOP measurement in rat eyes.

Evaluation of intraocular pressure at the end of cataract surgery

To assess the accuracy of estimating intraocular pressure (IOP) at the end of cataract surgery and to suggest a method of verifying this value. University hospital in a suburban area. Sixty-nine eyes having phacoemulsification cataract surgery without complications were studied. In stage 1, the surgeon estimated IOP using digital pressure at the end of cataract surgery while a second investigator checked the actual IOP using a handheld tonometer. The estimated and true values were compared. In a second group of patients, the IOP was measured at the end of surgery in 30 eyes using a specifically designed tonometer, the Ocular Kasaby Barraquer 20/30 (OKBT-20/30) (Ocular Instruments, Inc.). The true values were then measured using the handheld tonometer to verify the accuracy of the OKBT-20/30. In stage 1, 37.7% of the estimates were outside the "acceptable" IOP range of 10 mm Hg or higher to 30 mm Hg or lower. Accuracy of estimates decreased toward the extremes of IOP. This suggests that eyes with IOP values outside the acceptable range are likely to be left as such at the end of surgery. In stage 2, when the IOP was measured with the new instrument and rechecked with the electronic tonometer, 93.3% of eyes had an IOP within the reference range of 20 to 30 mm Hg; 62.3% of eyes had an IOP within this range when estimated. Because the digital estimate can be misleading, the OKBT-20/30 tonometer or a similar device should be used routinely to ensure each eye is left with a desirable IOP at the end of cataract surgery.

Relationship Between Travoprost and Central Corneal Thickness in Ocular Hypertension and Open-Angle Glaucoma

To evaluate whether treatment with travoprost, an F2a prostaglandin analog, affects central corneal thickness (CCT) and whether intraocular pressure (IOP) response to the medication is related to baseline CCT. This was a prospective, interventional, nonrandomized, nonconsecutive, clinical trial. In this multicenter study, 379 total patients, 220 with newly or previously diagnosed open-angle glaucoma (OAG), 141 with ocular hypertension (OHT), and 18 unspecified, were recruited from 15 Canadian sites. IOP and CCT assessments were performed at baseline and 6 weeks after treatment with travoprost. Patients on IOP-lowering therapy at the time of enrollment were washed out for 4 weeks before baseline examinations. IOP was measured with Goldmann tonometers and CCT with Accutome IV pachymeters. Statistical analysis was performed with S-PLUS software. Posttherapy mean IOP decreased by 6.31 mm Hg or 24.4% (P < 0.001), and regression analysis indicated relatively greater IOP reduction in patients with higher pretherapy IOP (slope = 0.64; 95% CI, 0.54-0.76). Mean CCT decreased by 6.9 microm (P < 0.001). IOP reduction was not related to CCT reduction (slope = 0.253; 95% CI, -0.232 to 0.739; P = 0.305). Percent IOP decrease was not related to baseline CCT (slope = -0.02; 95% CI, -0.06 to 00.02; P = 0.33) in the total study sample. When OHT and OAG groups were considered separately, the OAG patients had less percent IOP decrease with thicker baseline CCT (slope = -0.067; 95% CI, -0.13 to -0.004; P = 0.037). Treatment with travoprost decreased IOP significantly and was associated with CCT thinning, which had little or no effect on actual IOP decrease. In the OAG group, IOP decrease was found to be statistically smaller in patients with thicker corneas.

Comparison of Dynamic Contour Tonometry and Goldmann Applanation Tonometry in Glaucoma Patients and Healthy Subjects

To investigate the agreement in the measurement of intraocular pressure (IOP) obtained by dynamic contour tonometry PASCAL (DCT-PASCAL) and Goldmann applanation tonometry (GAT) in glaucoma eyes and healthy eyes with different central corneal thickness (CCT). Prospective cross-sectional study. In a randomized order, three consecutive IOP measurements were performed on 197 eyes of 107 subjects by one examiner using both DCT-PASCAL and GAT on all eyes. Furthermore, ultrasonic pachymetry was performed. The Spearman correlation coefficient (r) was determined to compare IOP readings between DCT-PASCAL and GAT. Regression-based Bland and Altman analysis was used to evaluate agreement between the instruments. Mean IOP values obtained by both instruments were significantly correlated in healthy and glaucoma eyes (all healthy eyes [n = 66]: r = 0.8, P < .001, all glaucoma eyes [n = 131]: r = 0.96, P < .001). Neither GAT nor DCT-PASCAL showed a significant correlation with CCT (GAT: all eyes: r = 0.009, P = .9, DCT-PASCAL: all eyes: r = -0.05, P = .5). Bland and Altman analysis revealed the existence of proportional bias. Thus, 95% limits of agreement between the instruments varied with the actual IOP measurement. DCT-PASCAL and GAT revealed a strong correlation in IOP measurements between glaucoma and healthy eyes. However, the analysis of agreement indicated some discrepancies between the instruments. Measurements with both GAT and DCT-PASCAL were not correlated with central corneal thickness.

Effect of Corneal Curvature and Corneal Thickness on the Assessment of Intraocular Pressure Using Noncontact Tonometry in Patients After Myopic LASIK Surgery

To evaluate the effect of corneal curvature and corneal thickness on the assessment of intraocular pressure (IOP) using noncontact tonometry (NCT) in patients after myopic LASIK surgery. All patients who had myopic LASIK in a university-based eye clinic between February 2002 and May 2002 were retrospectively analyzed. Preoperative NCT was compared with postoperative NCT, postoperative corneal thickness, and postoperative corneal curvature. The difference between the mean preoperative NCT (15.46 +/- 2.50 mm Hg) and postoperative NCT (6.30 +/- 1.57 mm Hg) was significant (9.16 +/- 1.96 mm Hg, P < 0.010). Preoperative NCT significantly correlated with postoperative NCT (P < 0.001), postoperative corneal thickness (P = 0.006), and postoperative anterior corneal curvature (P < 0.010). Both corneal thickness and anterior corneal curvature affect IOP assessment in patients with myopic LASIK. Although correction formulas can be used to estimate the actual IOP, alternative methods should be investigated to assess IOP independent of corneal thickness and curvature.

Comparison of the use of new handheld tonometers and established applanation tonometers in dogs

To examine the practical aspects, accuracy, and reproducibility of 2 new automatic handheld tonometers in dogs and compare them with results for 2 established applanation tonometers. 15 freshly enucleated canine eyes for manometric evaluation and 20 conscious research dogs, 20 client-owned dogs, and 12 dogs with acute glaucoma for clinical tonometry. Calibration curves were determined for all 4 tonometers on 15 enucleated canine eyes. Intraocular pressure (IOP) was measured with each tonometer consecutively in conscious dogs, with the MacKay-Marg applanation tonometer as the reference device. Measurements were repeated in 20 sedated dogs. An induction-impact tonometer was evaluated clinically on dogs with acute glaucoma. Additionally, measurements obtained by an experienced and an inexperienced examiner and with or without use of topical anesthesia were compared. The portable pneumatonometer was cumbersome and time-consuming. Compared with results for the reference applanation tonometer, and confirmed by manometry, the portable pneumatonometer increasingly underestimated actual IOP values with increasing IOP. The induction-impact tonometer provided accurate and reproducible measurement values. There was a significant strong correlation between the IOP values obtained by the 2 examiners (r2, 0.82) and also with or without topical anesthesia (r2, 0.86). In dogs with glaucoma, the fitted line comparing values for the reference applanation tonometer and induction-impact tonometer closely resembled an ideal 1:1 relationship. Use of the portable pneumatonometer in dogs appears to have disadvantages. The induction-impact tonometer appears to provide a promising alternative to the use of applanation tonometers in dogs.

Ex vivo Evaluation of Tono-Pen and Pneumotonometry in Cat Eyes

Purpose: To evaluate the validity and intraobserver reliability of intraocular pressure (IOP) measurements with both pneumotonometry and the Tono-Pen in a closed ex vivo system in cat eyes. Methods: IOP was increased step by step in 5 enucleated cat eyes, while taking IOP measurements with the Tono-Pen and pneumotonometry. The outcomes were compared to readings of a digital manometer simultaneously measuring the actual pressure in the anterior chamber. Results: Pneumotonometry overestimated IOP below 15 mm Hg and underestimated pressures above 20 mm Hg. Tono-Pen tonometry considerably underestimated IOP over the whole spectrum in all of the eyes tested. The pneumotonometer was identified as the more valid and reliable instrument for cat eyes. Conclusion: Both tonometers are clinically useful tools to assess IOP for glaucoma studies using a cat animal model. However, one has to consider underestimation of IOP in the upper ranges. A correction formula can be used to calculate the actual IOP.

Underestimate of Tonometric Readings after Photorefractive Keratectomy Increases at Higher Intraocular Pressure Levels

To determine whether tonometric readings of increases in intraocular pressure (IOP) during the water-drinking test (WDT) are affected by variations in central corneal thickness (CCT) induced by photorefractive keratectomy (PRK). Data from 30 randomly selected eyes of 30 patients (18 men and 12 women; mean age, +/- SD: 33.9 +/- 7.6 years) undergoing bilateral PRK for myopia (-6.57 +/- 2.39 D) were obtained. Objective refraction, anterior radius of corneal curvature (R), CCT, and IOP measurements at baseline and at different time intervals after ingestion of 1 L of water within 5 minutes, were performed before and 6 months after PRK. All measured IOPs were recalculated by a correction factor for R and CCT and expressed as corrected intraocular pressure (IOPC) measurements. The mean R +/- SD was 7.84 +/- 0.20 and 8.76 +/- 0.34 mm, and the mean CCT was 544.83 +/- 19.69 and 453.97 +/- 29.95 microm, before and after PRK, respectively. The mean IOP at baseline was 15.05 +/- 2.78 and 9.83 +/- 2.56 mm Hg, and during WDT was 18.32 +/- 3.42 and 11.42 +/- 3.10 mm Hg at 10 minutes, 18.59 +/- 2.99 and 11.54 +/- 2.54 mm Hg at 20 minutes, 17.80 +/- 2.85 and 10.87 +/- 2.22 mm Hg at 30 minutes, 16.35 +/- 3.02 and 10.26 +/- 2.21 mm Hg at 45 minutes, and 14.90 +/- 2.52 and 9.81 +/- 2.32 mm Hg at 60 minutes, before and after PRK, respectively. The mean IOPC at baseline was 13.64 +/- 2.33 and 13.05 +/- 2.98 mm Hg, and during WDT was 16.61 +/- 2.77 and 15.08 +/- 3.59 mm Hg at 10 minutes, 16.96 +/- 2.69 and 15.33 +/- 2.96 mm Hg at 20 minutes, 16.10 +/- 2.50 and 14.42 +/- 2.60 mm Hg at 30 minutes, 14.92 +/- 2.72 and 13.62 +/- 2.65 mm Hg at 45 minutes, 13.82 +/- 2.27 and 13.05 +/- 2.55 mm Hg at 60 minutes, before and after excimer laser treatment, respectively. Pre- and postoperative IOPs and percentages of IOP increase differed significantly (P < 0.05), in particular at the peak, as did IOPCs but not the percentages of increase in IOPC, apart from the highest values. Corneal changes after PRK for myopia may induce an uneven underestimate of the IOP increases. The inadequacy of a correction factor to compensate for CCT and R at high IOP levels indicates that other biomechanical factors may play a role when the cornea is subjected to dynamic actual IOP variation. Such increase of the well-known underestimate of IOP after PRK at higher actual IOPs may have significant clinical implications in tonometric assessment of subjects at risk of glaucomatous damage.

Accuracy of Intraocular Pressure Measurements in New Zealand White Rabbits

Rabbits are commonly used for the evaluation of drugs and surgery to lower intraocular pressure (IOP). The accuracy of intraocular pressure measurement is therefore critical in the analysis of data and subsequent extrapolation to humans. The purpose of this study was to establish the most reliable technique, from several currently available methods, of measuring IOP in New Zealand White rabbits. The IOPs of 11 healthy New Zealand White rabbits were measured with a fluid-filled pressure transducer system that was connected to the anterior chamber of each animal so that the IOP could be varied by altering the height of a bottle of balanced salt solution. Intraocular pressures were recorded over a range of 50 to 0 mm Hg by the transducer system, and comparative measurements at the same pressures were performed with the Tonopen XL (Mentor, Norwell, MA), the Perkins handheld applanation tonometer (Clement Clarke, Harlow, UK), and the Ocular Blood Flow (OBF) pneumatonometer (OBF Laboratories, Wiltshire, UK). All three tonometers underestimated the true IOP, especially at higher pressures. Although the mean difference in actual and tonometric IOP was least in the Tonopen XL, there was a high degree of variability through the entire range of IOPs measured. The Perkins tonometer, although slightly less accurate than the Tonopen XL, was more consistent in measurement. The OBF pneumatonometer was the least-reliable estimate of IOP. All tonometers underestimate IOP, with increasing inaccuracy at higher pressures. The Perkins applanation tonometer was the most reliable for measuring IOP in the range of 0 to 50 mm Hg in New Zealand White rabbits.

Can patients guess their intraocular pressure?

PurposeTo determine whether patients can guess their intraocular pressure (IOP). DesignPatient survey. MethodsWe asked consecutive patients to guess their IOP and then indicate the IOP and the symptoms that allowed them to guess. ResultsOf 132 patients, 22 (17%) believed they could guess their IOP, usually based on a periocular symptom (n = 20, 91%). Nine of these patients (45%) correctly identified whether they were below or above the pressure indicated by the symptom. In two patients who required no symptoms to guess and in nine whose IOP was above their symptom threshold, the mean difference of the guessed IOP from the actual IOP did not differ (± 3.3 mm Hg) from that of control patients (± 2.1 mm Hg, n = 50; r = .19). ConclusionsPatients who believe they can guess their IOP appear as often inaccurate as accurate in assessing their IOP related to a symptom threshold and as accurate as patients who claim they cannot guess their IOP.

Perfusion of the Juxtapapillary Retina and the Neuroretinal Rim Area in Primary Open Angle Glaucoma

The objective of this study is to evaluate capillary blood flow of the juxtapapillary retina and neuroretinal rim area in primary open angle glaucoma (POAG) by a new noninvasive method performing a high-definition topography of perfused vessels of the retina and the optic nerve head with simultaneous evaluation of blood flow. Juxtapapillary retinal and neuroretinal rim area blood flow were measured by scanning laser Doppler flowmetry (SLDF). This new technique is a combination of a laser Doppler flowmeter with a scanning laser system by which the retinal perfusion is simultaneously quantified in 16,000 sites of a retinal area of 2.7 x 0.7 mm. In study I, retinal and optic nerve head blood flow were evaluated by SLDF in 43 patients with POAG and 43 healthy individuals. The mean age of the POAG group was 56 +/- 12 years and of the control group 42 +/- 15 years. In study II, age-matched normals (n = 21) were compared with glaucoma eyes with topical therapy (n = 30) and with glaucoma eyes without topical therapy (n = 16). In study I, the eyes with POAG had an average cup/disc ratio (C/D) of 0.75 +/- 0.20, with an average mean defect of the visual field of 5.3 +/- 5.4 dB. The actual intraocular pressure was 17.8 +/- 4.18 mm Hg in the POAG group and 15.45 +/- 1.82 mm Hg in the control group. For statistical analysis, two age-matched groups of 28 normal eyes of 28 persons with 27 glaucomatous eyes of 27 patients were compared. In normals the blood flow of the neuroretinal rim area was significantly higher than that of the juxtapapillary retinal area (+7.73%, p < 0.01). In POAG both juxtapapillary retinal blood flow and neuroretinal rim area blood flow were significantly decreased compared to an age-matched control group: neuroretinal rim area flow -71% and juxtapapillary retina flow -49%. The decrease of neuroretinal rim area blood flow did significantly correlate with C/D. We found no correlation between reduction of juxtapapillary retinal blood flow and C/D or mean defect. Both glaucoma eyes with and glaucoma eyes without topical therapy showed significant decreased juxtapapillary blood flow compared to normals. We found no significant difference in the juxtapapillary blood flow between glaucoma eyes with and without topical therapy. In POAG, optic nerve head blood flow and juxtapapillary blood flow were significantly decreased.

Scanning laser Doppler flowmetry in glaucoma.

Scanning laser Doppler flowmetry is a new means for the measurement of capillary perfusion. We studied the retinal and neuroretinal rim capillary perfusion with the Heidelberg Retina Flowmeter in one randomly selected eye of 31 healthy volunteers, 42 primary open angle glaucoma (POAG) patients and 17 normal pressure glaucoma (NPG) patients. The reproducibility of the measurements on the temporal and nasal retina and on the disc rim was 19%, 26% and 28% in the unselected control group, 12% in the POAG and 12%, 13% and 10% in the NPG group, respectively. Optic disc flow was significantly higher than retinal flow (p < 0.000001). Differences in corresponding flow values between healthy volunteers and glaucoma patients as well as between POAG and NPG patients were not statistically significant. There was no correlation between the temporal and nasal flow values of the same eyes. The average variation in flow between adjacent frame positions was 18.5%. Actual intraocular pressure and the type of treatment had no influence on the retinal flow. The results suggest that the position of the test-frame is very important for the final result. We could not find any significant difference in the perfusion between glaucomatous and control eyes with the Heidelberg Retina Flowmeter.

Ocular hypotensive mechanism of topical isopropyl unoprostone, a novel prostaglandin metabolite-related drug, in rabbits.

This study was performed to clarify the ocular hypotensive mechanism of topical isopropyl unoprostone (unoprostone), a novel prostaglandin (PG) metabolite-related drug, in the rabbit eye. The intraperitoneal administration of indomethacin (50 mg/kg) 1 hour before administration of topical 0.12% unoprostone partially diminished the intraocular pressure (IOP) reduction, and completely blocked the increase in aqueous PGE2 concentration caused by unoprostone. Aqueous humor dynamics measurements in the unoprostone- and the vehicle-treated contralateral eyes with indomethacin pretreatment revealed that aqueous flow determined by fluorophotometry was not significantly different, 2.3 +/- 0.3 and 2.4 +/- 0.2 microliters/min, respectively; the total outflow facility measurements determined by the two-level constant pressure perfusion method were 0.20 +/- 0.01 and 0.14 +/- 0.01 microliters/min/mmHg, respectively (p < 0.05); the uveoscleral outflow measurements determined by the fluorescein isothiocyanate-dextran perfusion method were 0.49 +/- 0.02 and 0.46 +/- 0.02 microliters/min, respectively (p < 0.05). The magnitude of the IOP reduction induced by unoprostone was estimated to be 5.2 mmHg, which agrees well with the actual IOP reduction. In conclusion, unoprostone lowers the IOP by affecting aqueous outflow pathways, primarily the pressure-dependent conventional pathway and the secondary uveoscleral outflow pathway in rabbits. Endogenous PGs induced by topical unoprostone are also involved in lowering the IOP in rabbits.

Accuracy of IOP Measurement in Keratoconus

The literature has reported that tonometry done on keratoconic eyes shows lower intraocular pressure (IOP) when compared to normal control groups. This implies that keratoconus is a low-pressure eye phenomena. Both noncontact tonometry (NCT) and Goldmann tonometry were performed on 14 keratoconic eyes. In all cases, the IOP measured by NCT was significantly lower than that indicated by Goldmann tonometry when measured on the same cornea. Classical tonometry calibration was determined originally by applying a deforming force to a cornea with normal ocular rigidity. Since keratoconus corneas do not have normal tissue rigidity, the low IOP readings by NCT and Goldmann tonometry do not seem to be a true measure of actual IOP in keratoconuc eyes.

The IOP-IOL. A probe into the eye.

The intraocular lens (IOL) implant can be looked upon as a probe into the space of the eye. Adapting that view, it is logical to furnish the IOL-probe with biomedical sensors that would explore its environment. A sensor is presented for continuous monitoring of intraocular pressure (IOP), incorporated in the haptics of an IOL. The sensor consists of a capacitative spiral circuit, needing no energy, correlating its resonance frequency to the actual IOP. This resonance frequency is remotely and non-invasively detected by an external device located in a spectacle frame.

Oculopression tonometry after argon laser trabeculoplasty

A group of 26 patients (14 female and 12 male ranging in age from 49 to 84 years) who had primary open angle glaucoma underwent argon laser trabeculoplasty in one eye each. Shortly before the laser treatment, and one and twelve (7-14) weeks after the treatment suction cup oculopression tonometry was performed with an increase of intraocular pressure according to 1.8 x actual intraocular pressure. A therapeutically significant decrease of intraocular pressure was observed already one week thereafter, but findings become more significant after a longer follow up. The decrease of intraocular pressure and results of oculopression tonometry indicate that the measured resistance to aqueous outflow decreased significantly after argon laser trabeculotomy.