Assessing the sensory function of the ocular surface: implications of use of a non-contact air jet aesthesiometer versus the Cochet-Bonnet aesthesiometer.

Assessment of corneal sensory function is commonly carried out using the Cochet-Bonnet aesthesiometer. The limitations of this instrument have lead to development of newer instruments, such as the CRCERT-Belmonte aesthesiometer, which utilise a jet of air as their stimulus. Recent work, however, has demonstrated contradictory effects on ocular surface sensitivity when measured with different types of aesthesiometer. The purpose of the present study was to compare clinical measurements of corneal sensitivity obtained with the CRCERT-Belmonte and the Cochet-Bonnet aesthesiometers and to examine their stimulus characteristics in terms of force exerted. No association was found between central corneal sensitivity measured with the two aesthesiometers in a group of normal subjects. Sensitivity was measured to be lower with the Cochet-Bonnet aesthesiometer than with the CRCERT-Belmonte. Over half of the subjects could not be assessed with the standard Cochet-Bonnet filament and 11% could not be assessed with either filament, as their corneal sensitivity was outside of the range of the instrument. In contrast, all subjects were within the stimulus range of the CRCERT-Belmonte aesthesiometer. Corneal sensitivity measurements made with the non-contact CRCERT-Belmonte aesthesiometer and the Cochet-Bonnet aesthesiometer are not comparable. Due to dissimilarities in the composition of their stimuli, and thus mode of stimulation, it is possible that the two instruments measure different aspects of the neural response. The underestimation of corneal sensitivity by the Cochet-Bonnet aesthesiometer and its inability to measure sensitivity of some subjects at all are important considerations in the assessment of sensitivity loss. Subtle changes are unlikely to be detected with this instrument, particularly at higher sensitivity levels. The Cochet-Bonnet should therefore be used with caution and the 0.08 mm diameter used as the filament of choice. Adoption of a non-contact aesthesiometer as standard for ocular sensitivity measurement should be considered. The non-contact instrument allows superior stimulus reproducibility and better control over stimulus characteristics, in addition to the ability for exploration of the response of all three types of neuro-receptors on the ocular surface.