Non-contact determination of intra-ocular pressure in an ex vivo porcine model.

People suffering from glaucoma often endure high intra-ocular pressure (IOP). Methods for determining IOP either contact the eye or are unpleasant to some patients. There is therefore a need for a rapid and patient friendly non-contacting method to determine IOP. To address this need, we developed a tonometer prototype that employs spark-gap induced shock waves and a laser Doppler vibrometer (LDV) that reads the amplitude of membrane waves. The IOP was first identified from the membrane wave propagation velocity first in a custom-made ocular phantom and was then verified in ex vivo porcine eyes. The time-of-flight (TOF) of the membrane wave travelling on a hemispherical membrane was compared to reference IOP values in the sample obtained with an iCare TA01 tonometer. The shock front was characterized by high speed photography. Within one eye, the method achieved an agreement of 5 mmHg (1.96 standard deviation between the shock wave tonometer and the commercial manometer) and high method-to-method association (Pearson correlation, R2 = 0.98). The results indicate that the presented method could potentially be developed into a non-contacting technique for measuring IOP in vivo.

Measuring intraocular pressure with the Pulsair 3000 and Rebound tonometers in elderly patients without an anesthetic.

PURPOSE: To evaluate the utility of the new Rebound tonometer for measuring intraocular pressure (IOP) in an unanesthetized eye; to test patient tolerance, measurement time, and accuracy compared with the Pulsair 3000 tonometer. METHODS: IOP was measured with the Rebound tonometer and the Pulsair 3000 tonometer without an anesthetic in 131 residents of two Finnish nursing homes. The measurement time and possible pain or discomfort experienced by the inhabitants was recorded. RESULTS: The mean differences in IOP readings between the two tonometers were 0.31 mmHg, SD 2.45 mmHg for the right eyes and 0.36 mmHg, SD 2.17 mmHg for the left eyes (P=0.28, multivariate analysis). The correlation constants between the tonometers were 0.84 (right eyes) and 0.80 (left eyes). The Pulsair 3000 caused more discomfort than the Rebound tonometer (36% vs 15%, P=0.01). With the Pulsair, 85%, and with the Rebound tonometer, 95% of the patients felt no pain ( P= 0.14). Measurement of both eyes with the Rebound tonometer took less time (55+/-22 s vs 138+/-55 s, P<0.001). The mean difference was 82 s and the 95% confidence interval of the difference was 66-98 s. CONCLUSION: Measurement of IOP with the Rebound tonometer without an anesthetic is a rapid and well-tolerated procedure. IOP readings of the two tonometers were within +/-1 mmHg in 52.5% of the measurements and within +/-2 mmHg in 71.7% of the measurements.

Lack of type XVIII collagen results in anterior ocular defects.

Mice lacking type XVIII collagen have defects in the posterior part of the eye, including delayed regression of the hyaloid vasculature and poor outgrowth of the retinal vessels. We report here that these mice also have a fragile iris and develop atrophy of the ciliary body. The irises of Col18a1-/- mice can be seen to adhere to the lens and cornea. After the pupils begin to function, the double layer of epithelial cells separates at the apical cell contacts, leading to defoliation of its posterior pigment epithelial cell layer, and extracellular material begins to accumulate in the basement membrane zones of the iris. In contrast to the iris epithelia, where no clear signs of cellular atrophy were detected, the lack of type XVIII collagen resulted in atrophy of the pigmented epithelial cells of the ciliary body, and there were also ultrastructural abnormalities in the basement membrane zones. These changes did not lead to chronically elevated intraocular pressures, however. Our results indicate that type XVIII collagen is needed for the integrity of the epithelial basement membranes of the iris and the ciliary body and that its gene should therefore be taken into account as a new potential cause of anterior segment disorders in the eye.

Method for the noninvasive measurement of intraocular pressure in mice.

PURPOSE: To evaluate the applicability of rebound tonometry for measurement of IOP in the mouse eye. METHODS: An induction-impact (I/I) tonometer, which operates on the rebound principle, was scaled down and adapted to determine IOP of the mouse eye. IOP measurement using this concept is based on contacting the eye with a probe and detecting the motion as the probe collides with the eye and bounces back. The motion parameters of the probe vary according to eye pressure and are used to calculate IOP. A prototype instrument was constructed for measurement of IOP in mouse eyes, and its ability to accurately and reliably measure IOP was tested by comparing the measurements against the manometric (true) IOP determined in cannulated mouse eyes ex vivo. The I/I tonometer was also used to measure IOP in vivo in anesthetized adult C57BL/6 mice. RESULTS: A strong correlation between the true IOP and the I/I measurements (R(2) = 0.95) was found for IOPs in the range of 3.7 to 44.1 mm Hg in cannulated mouse eyes. Repeat determinations in individual eyes showed a low degree of variability in the relationship of the measured IOP with the true IOP. In anesthetized mice, mean IOP +/- SD as determined by rebound tonometry was 9.8 +/- 3.9 mm Hg when the animals were anesthetized with ketamine alone and 7.6 +/- 1.9 mm Hg when a mixture of ketamine, acepromazine, and xylazine was used. Contralateral eyes differed by 0.9 +/- 2.5 and 0.1 +/- 2.7 mm Hg, respectively, for the two anesthetic regimens. CONCLUSIONS: The I/I tonometer can be used for noninvasive, in vivo IOP measurement in mouse eyes. The availability of an easy-to-use, reliable tonometer for IOP measurements in mice will allow more extensive use of the mouse as a model for glaucoma.

Non-invasive determination of intraocular pressure in the rat eye. Comparison of an electronic tonometer (TonoPen), and a rebound (impact probe) tonometer.

BACKGROUND: The reproducibility and accuracy of the induction/impact (I/I) probe device (rebound tonometer) and the TonoPen XL electronic tonometer were compared through the measurement of intraocular pressure (IOP) differences between contralateral rat eyes. METHODS: IOP was measured in 18 female Wistar rats with variable, modest elevations of IOP in one eye. Mean IOP from five measurements on each of the 36 eyes was determined using the rebound tonometer, followed by the TonoPen XL. Following cannulation, true (manometrically determined) IOP was recorded with a calibrated pressure transducer. Differences between the operated and normal eye of each animal for each tonometric method of measurement were correlated with the true difference in IOP. RESULTS: IOP differences between the operated and contralateral normal eye ranged from +5.9 to -1.7 mmHg (mean +1.7 mmHg) when measured by cannulation and from +7.2 to -1.4 mmHg (mean +2.4 mmHg) and +9.8 to -3.2 mmHg (mean +3.6 mmHg) when measured with the rebound tonometer and TonoPen XL respectively. Differences between eyes recorded by rebound tonometer ( y) correlated with those determined by cannulation ( x) in a linear fashion ( y=0.8243 x+1.0721; R(2)=0.6603). Correlation for the TonoPen XL was much weaker ( y=0.8675 x+2.1672; R(2)=0.2077). IOP measurements using the rebound tonometer did not differ significantly from true IOP, whereas TonoPen XL increasingly underestimated IOP with increasing IOP (9-20 mmHg). CONCLUSION: The rebound tonometer displayed greater accuracy and less variability than TonoPen XL in measuring the IOP of the rat eye (range 9-20 mmHg).