Corneal Thickness Response after Anesthetic Eye Drops: Our Own Results and Meta-Analysis

Comparison of central corneal thickness measured by automatic and manual analysis of optical coherence tomography

PURPOSE

To compare central corneal thickness (CCT) measured with optical coherence tomography (OCT) using an automatic algorithm (A-OCT) vs. manual measurements (M-OCT) with respect to the gold standard ultrasound pachymetry (USP).

METHODS

CCT measurements were performed on both eyes of 28 healthy subjects at four times of the day. A-OCT used an automatic software analysis of the corneal image, M-OCT was performed by two operators by setting a digital calliper on the corneal borders, and USP was performed after corneal anesthesia. Measurements were compared using a three-way repeated measures ANOVA. Bland-Altman plots were used to evaluate the agreement between OCT measurements and USP.

RESULTS

Both A-OCT and M-OCT significantly underestimated the USP measures, with the mean difference, i.e., the systematic error, being larger for A-OCT (- 19.0 µm) than for M-OCT (- 6.5 µm). Good reproducibility between the two operators was observed. Bland-Altman plots showed that both OCT methods suffered from proportional errors, which were not affected by time and eye.

CONCLUSIONS

Measuring CCT with OCT yielded lower values than USP. Therefore, clinicians should be aware that corneal thickness values may be influenced by the measurement method and that the various devices should not be used interchangeably in following up a given patient. Intriguingly, M-OCT had less systematic error than A-OCT, an important outcome that clinicians should consider when deciding to use an OCT device.

Repeatability and agreement of central corneal thickness measurements with a new handheld non-contact pachymeter

PURPOSE

To compare the repeatability of central corneal thickness (CCT) measurements taken with a new handheld pachymeter (Occuity PM1 pachymeter) and to assess its agreement with ultrasound biometry and two commercially available optical biometers in participants with normal eyes.

METHODS

Three consecutive CCT measurements of the right eye of 105 participants with normal corneas were acquired by the PM1 pachymeter, Lenstar LS 900 and Oculus Pentacam HR in a random order. This was followed by three measurements with a handheld ultrasound pachymeter (UP) (Pachmate 2). Repeatability and the repeatability limit were calculated for each device and Bland-Altman limits of agreement (LoA) were determined for the PM1 pachymeter compared to the other devices.

RESULTS

The mean CCT (±SD) was 551.04 ± 33.43, 558.62 ± 31.46, 549.41 ± 31.00 and 539.73 ± 29.50 μm for the PM1 pachymeter, UP, Lenstar and Pentacam, respectively. The repeatability limits (expressed as the within subject SD for repeat measurements) were 14.02, 13.68, 4.99 and 9.90 μm, respectively. The closest agreement was between the PM1 and Lenstar (mean difference = -1.63 μm with LoA 10.72 μm below and 13.97 μm above the readings obtained with the Lenstar). The PM1 underestimated CCT compared to UP (mean difference = 7.58 μm, LoA 24.63 μm below and 9.47 μm above UP). The agreement was lowest between the PM1 and Pentacam (mean difference = -11.30 μm, LoA between 4.29 and 26.89 μm).

CONCLUSIONS

The PM1 pachymeter shows excellent precision for CCT measurements across a range of corneal thicknesses in normal eyes and provides a safe and easy-to-use alternative to ultrasound pachymetry.

Comparison of central corneal thickness in healthy eyes using ultrasound pachymetry, non-contact specular microscopy and a high-resolution Scheimpflug camera

PURPOSE

To compare and analyze the interchangeability in measuring central corneal thickness (CCT) using ultrasonic pachymetry (USP, PACHMATE 2, DGH, Inc, Exton, PA, USA), non-contact specular microscopy (NCSM, CEM-530, Nidek CO, LTD, Gamagori, Japan) and a high-resolution Scheimpflug Camera (Pentacam HR, OCULUS, Wetzlar, Germany).

METHODS

An observational, cross-sectional study was performed recruiting 216 volunteers, for a sample size of 216 eyes with no ocular abnormalities other than refractive errors. All subjects underwent pachymetric measurements obtained by USP, NCSM and Pentacam HR. Examinations were performed by the same examiner with USP always following the noncontact examinations.

RESULTS

The mean CCT (± SD) was 560.30 ± 38.80 μm, 556.76 ± 36.83 μm and 547.31 ± 35.28 μm for USP, NCSM and Pentacam HR, respectively. The Bland-Altman analysis showed that the highest concordance was found between USP and NCSM, with differences between - 13.18 μm and 20.26 μm. For the differences between measurements obtained with Pentacam HR and USP, the differences at Bland-Altman plot were between - 28.25 and 13.57 μm. The lowest concordance was found for the CCT values measured with Pentacam HR and NCSM, with differences between - 25.67 and 6.86 μm. The intraclass correlation coefficient (ICC) between all pairs of measurements was between 0.979 and 0.987, suggesting that the mean measurements were strongly correlated.

CONCLUSION

Measurements obtained with all three devices had high correlation. USP and NCSM were found in good agreement and high concordance, too. The above results indicate that these two devices are interchangeable in clinical practice. Pentacam HR may be a useful alternative for measuring CCT; however, it significantly underestimates CCT and cannot be used interchangeably with the other devices that we studied.

In Vivo Determination of the Human Corneal Elastic Modulus Using Vibrational Optical Coherence Tomography

Purpose

To determine the in vivo elastic modulus of the human cornea using vibrational optical coherence tomography (VOCT).

Methods

Vibrational analysis coupled with optical coherence tomography (OCT) was used to obtain the resonant frequency (RF) and elastic modulus of corneal structural components. VOCT corneal thickness values were measured using OCT images and correlated with corneal thickness determined with Pentacam (Oculus, Wetzlar, Germany). Moduli were obtained at two locations: central cornea (CC) and inferior cornea (IC). Measurements were obtained with and without anesthetic eye drops to assess their effect on the modulus measurements.

Results

VOCT thickness values correlated positively (R² = 0.97) and linearly (y = 1.039x–16.89) with those of Pentacam. Five RF peaks (1–5) were present, although their presence was variable across eyes. The RF for peaks 1 to 5 in the CC and IC ranged from 73.5 ± 4.9 to 239 ± 3 Hz and 72.1 ± 6.3 to 238 ± 4 Hz, respectively. CC and IC moduli for peaks 1 to 5 ranged from 1.023 ± 0.104 to 6.87 ± 0.33 MPa and 0.98 ± 0.15 to 6.52 ± 0.79 MPa, respectively. Topical anesthesia did not significantly alter the modulus (P > 0.05 for all), except for peak 2 in the CC (P < 0.05).

Conclusions

This pilot study demonstrates the utility of VOCT as an in vivo, noninvasive technology to measure the elastic modulus in human corneas. The structural origin of these moduli is hypothesized based on previous reports, and further analyses are necessary for confirmation.

Translational Relevance

This work presents VOCT as a novel approach to assess the in vivo elastic modulus of the cornea, an indicator of corneal structural integrity and health.

Effect of topical 0.5% tetracaine hydrochloride on intraocular pressure in ophthalmologically normal cats

OBJECTIVES

The aim of this study was to assess the effect of topical tetracaine hydrochloride 0.5% on intraocular pressure (IOP) in ophthalmologically normal cats.

METHODS

Twenty domestic shorthair cats (40 eyes) were used in this study. Each cat was randomly allocated to one of two groups (treatment or control). Baseline IOP (T0) was measured in each cat, and then one drop of tetracaine hydrochloride 0.5% or artificial tears was administered into a randomly chosen eye of each cat in the treatment and control groups, respectively. Repeat IOP measurements were performed at 2 mins (T2), 5 mins (T5), 15 mins (T15) and 30 mins (T30) with a rebound tonometer.

RESULTS

Mean baseline IOP in all eyes was 20.6 ± 2.5 mmHg. After the unilateral administration of tetracaine, mean IOP decreased significantly in the treated eye at T2 (P = 0.01). Mean IOP returned to baseline values at T15. The mean IOPs in the treated eyes at T0, T2, T5, T15 and T30 were 20.6 ± 3.3 mmHg, 18.2 ± 2.5 mmHg, 18.2 ± 3.4 mmHg, 20.2 ± 3.2 mmHg and 19.8 ± 2.7 mmHg, respectively. A significant difference in IOP was found at all time points between the tetracaine and control groups (P <0.03).

CONCLUSIONS AND RELEVANCE

The results of the present study showed a statistically significant reduction in mean IOP 2 mins after the administration of tetracaine hydrochloride 0.5% in the treated eyes of the cats.

Effect of Topical 1% Tetracaine Hydrochloride on Intraocular Pressure in Ophthalmologically Normal Horses; a Pilot Study

The aim of this study was to evaluate the effect of topical 1% tetracaine hydrochloride on the intraocular pressure (IOP) in ophthalmologically normal horses. Thirty eyes of 15 clinically normal horses were used for this study. The animals were randomly assigned to two groups (treatment and control). Prior to the instillation of 1% tetracaine or placebo, the baseline IOPs (T0) of each animal were recorded in both groups. Then one drop of tetracaine was instilled randomly into one eye of each horse in the treatment group (8 horses). In the control group (7 horses), one drop of artificial tear was instilled in one randomly selected eye. The measurements were repeated at 2 minutes (T2), 5 minutes (T5), 15 minutes (T15), and 30 minutes (T30) post instillation via a rebound tonometer. There was no significant difference in the treatment group (P = .3). The peak IOP measured at T2 returned to the baseline value at T30. No significant difference was found in the mean IOP values between the treatment and the control groups, or between the males and females on any of the occasions (P > .05). The Results of this study revealed a nonsignificant increase of the IOP 2 minutes post instillation of 1% tetracaine in horses.

Comparison of central corneal thickness measurements in corneal edema using ultrasound pachymetry, Visante anterior-segment optical coherence tomography, Cirrus optical coherence tomography, and Pentacam Scheimpflug camera tomography

Purpose

To compare the central corneal thickness (CCT) measurements in subjects with corneal edema using ultrasound pachymetry, Visante anterior-segment optical coherence tomography (OCT), Cirrus OCT, and Pentacam Scheimpflug camera tomography.

Methods

This prospective cross-sectional study included 46 eyes of 33 patients with corneal edema and a CCT exceeding 550 μm evaluated by ultrasound pachymetry, Visante OCT, Cirrus OCT, and Pentacam. Two observers measured each eye twice. Intraobserver and interobserver reproducibility were determined and agreement among the devices calculated.

Results

CCT was measured in 40 eyes of 29 patients. Regardless of the CCT, the measurements obtained using Visante OCT, Cirrus CCT, and ultrasound pachymetry were well correlated. Interobserver and intraobserver reproducibility were high among the three devices. Pentacam overestimated the results compared with the other devices, and ultrasound pachymetry was unmeasurable in six (13%) eyes with very thick and opaque corneas. In eyes with mild corneal edema (CCT 551–650 μm), measurements from the four devices were comparable.

Conclusion

All devices reliably measured the CCT <650 μm. In eyes with edema exceeding 650 μm, CCT measurements from the Visante OCT, Cirrus OCT, and ultrasound pachymetry devices showed good reproducibility and were well correlated, while the Pentacam overestimated the values compared to the other devices. Pentacam and ultrasound pachymetry should not be used in eyes with extreme corneal edema and opacity.