Understanding the stimulus of an air-jet aesthesiometer: computerised modelling and subjective interpretation

Assessment of corneal sensory function is more frequently being carried out using novel non-contact methods, which promise better stimulus control and a greater intensity range than more traditional techniques. An examination of the characteristics of the air-jet stimulus of these 'non-contact' aesthesiometers is important to enable an understanding of how these instruments stimulate the cornea. The purpose of this study was to model the air-jet stimulus of the CRCERT-Belmonte Aesthesiometer (CBA) and its interaction with the cornea.

A computerised simulation was used to model the CBA stimulus and this model was then examined in the context of verbal descriptions of the supra-threshold stimulus provided by 27 human subjects with normal corneas. The computational fluid dynamics model and the subjective descriptions considered CBA airflow values between 70 and 400 mL min(-1) .

The computational fluid dynamics results showed that the CBA air-jet stimulus behaves as expected in terms of force exerted and stabilises and subsides quickly during the 1 s duration for which it is turned on/off. The computer modelling demonstrated that the stimulus of the CBA consists of a central core of air flow, similar in diameter to the CBA nozzle, which exerts maximum pressure at the corneal apex. This core is surrounded by a less prominent zone of lower pressure, extending out to the corneal periphery at higher flow rates. The computer modelling results were confirmed by the subjective descriptions of intensity and corneal area stimulated, whereas relation to duration of sensation was more equivocal.

The CBA probably stimulates sensory receptors over a greater area of the ocular surface compared to the traditional nylon filament aesthesiometer and may recruit neurons sensitive to additional stimulus modalities. Future computational models should also incorporate temperature as an important aspect of the corneal response. This study has effectively demonstrated that it is possible to create a virtual model of an air-jet aesthesiometer stimulus using computational methods and that this model is corroborated by subjective descriptions provided by subjects.