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

Abstract

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.

Age-Related Changes in Corneal Sensitivity

“…Stimulus intensity is controlled with variable pressure levels, and its working principle and relevant physical properties, as well as its reliability for clinical application, were recently described elsewhere. 51,52 Corneal sensitivity measurements with means of SLACS were compared with those obtained with the only commercially available CB esthesiometer, which applies a mechanical stimulus in the form of a fine nylon filament to the cornea with varying pressures by adjusting its length to produce different stimulus intensities to the ocular surface. Although the SLACS stimulus differs in nature from the nylon filament, it was heated to match the OST to eliminate any thermal components and to generate a stimulus of mechanical nature.…”

“…The working principle and relevant physical properties of this new prototype as well as its reliability for clinical application have been recently described elsewhere. 51,52 The aim of this study was 1) to clinically explore whether corneal sensation thresholds differ between 2 different age groups, for each of the 2 stimulus types applied, and 2) to correlate corneal sensitivity thresholds and general pain perception.…”

Corneal Nerve Assessment by Aesthesiometry: History, Advancements, and Future Directions

“…The Swiss Liquid Jet Aesthesiometer for Corneal Sensation (SLACS) employs droplets of isotonic saline solution from a microvalve placed on a slit lamp with a temperature sensor, which allows this tool to match the ocular surface temperature to elicit a mechanoreceptor response [ 49 ]. The isotonic saline is not irritative to the cornea, in contrast to the CO 2 gas employed by the Belmonte NCCA.…”

“…In early variations of the device, developed by Ehrmann et al, the frequency of microvalve opening at a fixed pressure of 300 mbar controlled the stimulus strength, and was correlated with a patient’s degree of corneal anesthesia [ 36 ]. A modified version of this liquid jet aesthesiometer has since been adapted to apply differing pressure levels at a constant duration of 40 ms to control stimulus intensity [ 49 ]. This method of testing offers a large pressure range of 100–1500 mbar and a precision of 1 mbar.…”

“…The intensity of the stimulus can be controlled to elicit a more precise response, allowing it to detect smaller changes in sensitivity relative to the Cochet–Bonnet. The liquid jet can also be controlled to match the surface temperature and chemical composition of the cornea, or differ in temperature and pH to assess isolated thermal and chemical sensitivity [ 49 ]. Compared to the Belmonte NCCA, the Swiss liquid jet does not cause secondary evaporative cooling, limiting confounding variables in measurement.…”