Does air gas aesthesiometry generate a true mechanical stimulus for corneal sensitivity measurement?

Stimulus characteristics of a novel air-based multiple stimulus aesthesiometer

PURPOSE

To identify the stimulus airflow characteristics and confirm the consistency of a novel air jet-based aesthesiometer capable of producing and applying multiple stimuli separated either by time and/or by space.

METHODS

A novel aesthesiometer (Dolphin Aesthesiometer) was designed around a micro-blower under software management. Two nozzle attachments assisted in airflow control (flexible tube 1.6 mm diameter; brass tube 0.5 mm diameter). Four studies that tested the characteristics of the airflow and stimulus consistency were completed: (i) airflow pattern/trajectory, (ii) airflow surface dispersion, (iii) force of airflow across a range of stimulus strengths and (iv) thermal effects on the ocular surface.

RESULTS

Stimulus characteristic studies revealed: (i) airflow is coherent within the expected test distance range for the instrument, and spread rate is constant irrespective of stimulus strength; (ii) airflow dispersion occurs upon encountering a surface and dispersion increases with increasing airflow rate; (iii) a consistent and small force (10-4  N) is applied by the airflow and (iv) repeatable thermal effects occur in relation to the airflow, and the mode of stimulation of the Dolphin aesthesiometer is predominantly thermal in nature.

CONCLUSIONS

These studies confirm the repeatability and consistency of the novel instrument. The device is suitable for measuring corneal sensitivity. The availability of additional air jets allows the application of multiple stimuli to facilitate corneal summation investigations.

Clinical application of the Swiss Liquid Jet Aesthesiometer for corneal sensitivity measurement

CLINICAL RELEVANCE

Corneal sensitivity represents an important indicator for corneal health, its innervation and hence also for ocular disease. It is therefore of great interest from a clinical and research perspective to quantify ocular surface sensation.

BACKGROUND

The aim of this prospective cross-sectional cohort study was to clinically test the within-day and day-to-day repeatability of the new Swiss Liquid Jet Aesthesiometer, employing small droplets of isotonic saline solution for repeatability, and correlate with the Cochet-Bonnet aesthesiometer in a cohort of participants of two different age groups, based on participant feedback (psychophysical method).

METHODS

Participants were recruited from two equally, large age groups: group A (18-30 years) and group B (50-70 years). The inclusion criteria were healthy eyes, Ocular Surface Disease Index (OSDI) ≤ 13, and no contact lens wear. Mechanical corneal sensitivity threshold measurements with means of liquid jet and Cochet-Bonnet methods were carried out twice during two visits (a total of four measurements), with a stimulus temperature equal to or slightly above the ocular surface temperature.

RESULTS

Ninety participants completed the study (n = 45 per age group, average age in group A: 24.2 ± 2.94 years, group B: 58.5 ± 5.71 years). The coefficient of repeatability for the liquid jet method was 2.56 dB within visits and 3.61 between visits. For the Cochet-Bonnet method, it was 2.27 dB within visits and 4.42 dB between visits (Bland Altman with bootstrap analysis). Moderate correlation was observed between the liquid jet and the Cochet-Bonnet method (r = 0.540, p < .001, robust linear regression).

CONCLUSIONS

Swiss liquid jet aesthesiometry offers a new examiner independent method for corneal sensitivity measurement with acceptable repeatability and moderate correlation with the Cochet-Bonnet aesthesiometer. It offers a large stimulus pressure range of 100-1500 mbar and a precision of 1 mbar. Stimulus intensity can be tuned more precisely and much smaller sensitivity fluctuations may be potentially detected.

Age-Related Changes in Corneal Sensitivity

PURPOSE

The aim of this prospective cross-sectional cohort study was to clinically test whether corneal sensation decreases with age, based on subject feedback (psychophysical method), and whether it correlates with general pain perception.

METHODS

Subjects were recruited from 2 equally large age groups: group A (18-30 years) and group B (50-70 years; n = 45 per group). The inclusion criteria were healthy eyes, Ocular Surface Disease Index ≤13, and no contact lens wear. Corneal sensitivity threshold (CST) measurements were performed twice during each of the 2 visits, with the aid of the new Swiss liquid jet esthesiometer for corneal sensitivity (SLACS) and Cochet-Bonnet (CB) esthesiometer. A general pain sensitivity score was obtained from all participants.

RESULTS

Ninety subjects completed the study (n = 45 per age group, average age in group A: 24.2 ± 2.94 years, group B: 58.5 ± 5.71 years). Statistically higher CSTs for age group B were only observed for SLACS (mean difference: 1.58 dB, P < 0.001). No correlation was observed between the pain score and the CSTs obtained with either esthesiometry method (r = 0.11, P = 0.25 for liquid jet and r=-0.076, P = 0.61 CB).

CONCLUSIONS

A statistically significant decrease in corneal sensitivity was observed for the older age group with SLACS in this study, with CB however only a trend in the same direction was noted. General pain perception was not found to correlate with ocular surface sensation.

Corneal sensitivity in silicone hydrogel and rigid gas permeable contact lens wear

PURPOSE

The aim of this prospective cross-sectional cohort study was to test the effect of silicone hydrogel (SH) and rigid gas permeable (RGP) contact lens (CL) wear on corneal sensitivity, applying the new Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) and the Cochet-Bonnet (CB) aesthesiometer, based on subject feedback (psychophysical method).

METHODS

Participants were recruited for three equally large groups: Group A (SH CL), Group B (RGP CL) and Group C (non-CL wearers). Inclusion criteria were healthy eyes and OSDI ≤ 13. Corneal sensory thresholds were determined twice during two visits, with aid of SLACS and CB.

RESULTS

96 participants completed the study (n = 33 in groups A and C, n = 30 in group B); average age in group A: 27.42 ± 6.83 years, group B: 36.90 ± 9.68 years and group C: 26.06 ± 6.19 years. No statistically significant difference in corneal sensitivity was observed between the three groups for either method (p = 0.302 for SLACS, p = 0.266 for CB; Kruskal-Wallis rank sum test). Higher CSTs were obtained for males than for females in both CL groups with SLACS, and with CB only in the RGP CL group (p = 0.041 in Group A, p = 0.006 in Group B with SLACS; p = 0.041 in Group B with CB; bootstrap analysis with age correction and gender balancing). No correlation was observed between CL comfort and corneal sensitivity for neither method applied (for SLACS r = 0.097 and p = 0.51, for CB r = 0.17 and p = 0.15; robust linear mixed model).

CONCLUSIONS

No difference in corneal sensitivity with CL compared to non-CL wear was noted in this study. However, lower levels of corneal sensitivity were observed in the male CL groups, warranting further investigation.

Association between mask-associated dry eye (MADE) and corneal sensations

To determine the risk of mask-associated dry eye (MADE), we investigated the fluorescein tear break-up time (FBUT), ocular surface temperature and blood flow, along with corneal sensitivity, in mask wearers. We enrolled 60 mask wearers (mean age, 27.1 ± 5.2 years) and then measured FBUT, corneal temperature and conjunctival blood flow without wearing masks (no mask), with masks, and with taped masks. We defined MADE as the condition in which dry eye symptoms appeared and the FBUT with mask was less than 5 s. The FBUT with a mask was significantly shorter compared to the no mask and taped mask groups (P < 0.01 and P < 0.05). The corneal temperature difference and conjunctival blood flow difference were significantly higher after wearing a mask than after wearing a taped mask (P < 0.01). Of the 60 subjects, 13 were diagnosed with MADE. Pain sensitivity and the Ocular Surface Disease Index (P < 0.05 and P < 0.01) were significantly higher in the MADE group, with the FBUT without masks (P < 0.05) significantly shorter than in the non-MADE group. MADE may be associated with corneal hypersensitivity. Wearing masks decreased FBUT and increased ocular surface temperature and blood flow. Taping the top edge of masks prevented these changes. Fitting masks properly may reduce MADE risk.

Working principle and relevant physical properties of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) evaluation

Purpose

To describe and evaluate relevant physical properties of the Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity (SLACS) for ocular surface sensitivity measurement.

Methods

Characteristics of Liquid Jet (LJ) droplets (consisting of isotonic saline solution) were analysed: vertical and horizontal displacement and speed of LJ droplets were recorded with the aid of the High Speed Photron FASTCAM NOVA S6 camera (stimulus duration: 40 ms). Stimulus mass was assessed for 20 sets of 10 LJs with aid of a microbalance (pressure range of 100–1500 mbar).

Results

Because continuous flow LJ disintegrated into droplets in the lower pressure range (<700 mbar), pulsed stimuli were applied in order to obtain similar stimulus characteristics across the applied pressure range. For all measurements, very little variability was observed. Vertical and horizontal displacement did not exceed 0.13 mm in either direction. The mass per shot showed an unexpected cubic dependency on pressure. Up to approximately 700 mbar, LJ speed showed an almost linear relationship. For the pressure range of >700–1500 mbar, variability increased and speed decreased compared to the expected in a linear manner. However, this may be caused by the difficulty of identifying pattern changes of LJ droplets from one high speed image frame to the next with increasing stimulus speed, when determining LJ speed via pixel count.

Conclusions

Swiss Liquid Jet Aesthesiometer for Corneal Sensitivity was shown to deliver fine droplets with a pulsed stimulus mode, in a repeatable manner with precise localisation to the ocular surface. Very little variability was observed in LJ speed and mass for the typical pressure range required for clinical sensitivity measurements.

Detectability and Bias Indices of Pneumatic Corneal Stimuli Using Signal Detection Theory

Purpose

To evaluate the feasibility of using signal detection theory (SDT) in estimating criterion and detectability indices for corneal pneumatic stimuli and test corneal psychophysical data against linking hypotheses from nonprimate physiology using Bayesian analysis.

Methods

Corneal pneumatic stimuli were delivered using the Waterloo Belmonte esthesiometer. Corneal thresholds were estimated in 30 asymptomatic participants and 1.5× threshold stimuli were used as signals (with 0.4 probability). There were 100-trial mechanical and cold stimulus experiments and 50-trial chemical experiments. Trials were demarcated auditorily and “yes” or “no” recorded after each trial. Cold stimulus experiments were conducted with 0.6 signal probability. Criterion (c), likelihood ratio (lnβ), and d′ were calculated from the yes-no responses.

Results

Average d′ was 0.59 ± 0.1, 1.65 ± 0.37, and 1.14 ± 0.3 units for cold, mechanical, and chemical stimuli, respectively. Bayes factors obtained using Bayesian analysis of variance mildly favored (BF₁₀ = 1.55) differences between d′s of the stimulus types, with no support for differences in criteria between stimulus types. Multiple comparisons of d′ supported linking hypotheses based on nociception and nerve conductance theories.

Conclusions

Our experiments are the first to demonstrate the feasibility of estimating SDT indices and test different hypotheses. The conservative strategy (reporting “no” more often) chosen by participants was anticipated due to relatively large proportion of catch trials.

Translational Relevance

SDT when using pneumatic esthesiometry is vital to evaluate bias in responses of participants. Considering the varied forms of inherent noise in the corneal sensory system, SDT is critical to understand the sensory and decisional characteristics.

Correlation between Blood Flow and Temperature of the Ocular Anterior Segment in Normal Subjects

Purpose: To determine a correlation between temperature and blood flow in the ocular anterior segment, and their effects on corneal temperature. Methods: In experiment 1, we recruited 40 eyes and measured the temperature and blood flow in the ocular anterior-segment (upper/lower eyelid skin, palpebral and bulbar conjunctiva, and cornea) before and after application of warm compresses. In experiment 2, we recruited 20 eyes and measured the same tissues before and during stimulation using water and capsaicin solution in the oral cavity. Results: In experiment 1, the temperatures of the upper/lower eyelid skin and cornea increased significantly until 15 min after the application of the warm compress; the temperatures of the palpebral and bulbar conjunctiva increased significantly until 10 min. The blood flow in the upper/lower eyelid skin and bulbar conjunctiva increased significantly until 10 min, and that of the palpebral conjunctiva increased significantly until 15 min. In experiment 2, the temperatures were correlated significantly with the blood flow in the upper and lower eyelid skin and palpebral and bulbar conjunctiva. The temperature of all locations and palpebral conjunctival blood flow contributed independently to the corneal temperature. Conclusions: In the ocular anterior segment, the temperature and blood flow were correlated significantly, and contributed to the corneal temperature.

A Method to Calibrate the Carbon Dioxide (Chemical) Stimuli of Pneumatic Esthesiometer Externally

Purpose

To determine the feasibility of using a portable carbon dioxide (CO₂) sensor to calibrate a pneumatic esthesiometer and then to calibrate the chemical stimuli.

Methods

The chemical stimuli in ocular surface experiments are combinations of medical air and added CO₂ (%CO₂). These stimuli were calibrated using a portable CO₂ sensor (COZIR CM-0041) and data logger, delivered for 100 seconds by using the Waterloo Belmonte esthesiometer. The distances between the sensor and esthesiometer tip were 0 (to measure feasibility), 3, 5, and 10 mm. In experiment I, 100% CO₂ was tested using four different flow rates (50, 100, 150, and 200 mL/min) at three working distances. In experiment II, flow rates of 20 to 100 mL/min and concentrations of 20% to 100% CO₂ were tested in 20 steps at 3 working distances.

Results

The CO₂ sensor correctly reported the esthesiometer extremes of 0% and 100% CO₂ when placed at the esthesiometer tip. There were progressive, systematic increases in concentrations reaching/reported by the sensor with increasing flow rates and nominal concentrations and progressive decreases in measurements with increases in working distance.

Conclusions

CO₂ concentrations in pneumatic esthesiometers can be calibrated and, as expected, vary with flow rate and distance, highlighting the importance of calibration and standardization of CO₂ stimuli in these instruments.

Translational Relevance

Calibrated CO₂, a chemical sensory stimulus in humans, may be used in testing the surface of the eye as well as other membranes within which the CO₂ can be dissolved (e.g., mucous) to produce an acidic stimulus.

Relationship Between the Degree of Iris Pigmentation and Corneal Sensitivity to a Cooling Stimulus

PURPOSE

To explore the relationship between the degree of iris pigmentation and corneal sensitivity threshold (CST) on a variety of different ethnicities, using the air-jet noncontact corneal aesthesiometer and by applying a consistent method of subject iris pigmentation classification.

METHODS

A total of 200 subjects (mean age 23.7 ± 3.1 years, 127 women) participated in this clinical cross-sectional study: 100 whites, 40 Asians, 40 Chinese, and 20 Afro-Caribbeans. CST was assessed within the central cornea using a noncontact corneal aesthesiometer, and the degree of iris pigmentation of each subject was noted according to the Seddon method using a set of graded photographs of iris pigmentation (grades 1-5). Inclusion criteria were absence of ocular disease including dry eye, no contact lens wear, and no use of artificial tears. Statistical testing between ethnicities was made by the pairwise t test with Holm adjustment, and a linear model was set up to analyze the effects of ethnicity and iris grade.

RESULTS

A moderate trend for increasing CST with increasing iris pigmentation grade for all ethnicities was observed (R = 0.46; P < 0.0001), with CST changing from 0.66 ± 0.16 mbars for grade 1, 0.74 ± 0.18 mbars for grade 2, 0.86 ± 0.31 mbars for grade 3, 0.85 ± 0.32 mbars for grade 4, and 1.08 ± 0.40 mbars for grade 5. This correlation was stronger within the white group, representing the only ethnicity with all iris pigmentation grades (R = 0.50; P < 0.0001).

CONCLUSIONS

There is a moderate relationship between corneal sensitivity and the degree of iris pigmentation, with sensitivity increasing as iris pigmentation decreases. This relationship is stronger within whites.