A Non-Contact Corneal Aesthesiometer (NCCA) Validation Study in Normal Eyes

Abstract

Purpose : Validation of a modified version of an NCCA (NCCAm) and comparison with the Cochet Bonnet Aesthesiometer (CBA) in subjects with normal corneas.

Methods : The NCCA device described by Murphy et al (1996, 1998) to measure corneal sensitivity was modified as follows: The air reservoir was placed outside the enclosure and the fan inside, and a 3-way solenoid valve was used to save space. An adapter for the Goldmann tonometer was designed to hold the nozzle to ease accessibility at a standard slit lamp. A 250 µm filter was added to the patient line as a precaution against dust and contaminants. A user-friendly calibration feature was implemented while a manual valve was added to the exhaust line to ensure equalization of pressure in the exhaust and patient lines. The device allows for fine pressure adjustments down to 0.02 mbars. The configuration chosen was a 0.5 mm nozzle diameter, 0.9 second puff duration and 1 cm distance to cornea.

The left eye central corneal sensitivity of 21 subjects (12 females) was tested using the NCCAm and the CBA gold standard was used for comparison. The NCCAm pressure started at 0.01 mbar (subthreshold) and increased in 0.02 mbar intervals until the participant felt the stimulus (ascending threshold). Then the pressure was decreased until the patient no longer felt a stimulus and the last felt pressure was recorded (descending threshold). The mean of the two measurements was their NCCAm result. This was repeated for 3 rounds at 5 minute intervals.

Results : All CBA measurements were normal (6.0 cm). The NCCAm ascending threshold was repeatedly higher (mean 0.06 mbars; p<0.001) than the descending threshold, but this difference was stable (linear mixed-effect model: F=0.5, df=2, p=0.62). Furthermore, an intra-class analysis (ICC) showed an excellent agreement between the three rounds (ρ_ICC = 0.90 [95% CI: 0.83-0.95]). NCCAm mean (±SD) threshold values of 0.177±0.045 mbar ranging from 0.085 to 0.532 mbar were obtained. Consistent with Murhpy 1996, the NCCAm allows corneal sensistivity measurement below the detection threshold of CBA.

Conclusions : The NCCAm is a compact user-friendly device enabling reliable corneal sensitivity measurements. It also allows the detection of inter-subject differences that are not detected with the traditional CBA.

This is a 2021 ARVO Annual Meeting abstract.