An examination of human corneal sensitivity by non-invasive methods.
The aim of this thesis was to design, develop and evaluate a Non-Contact Corneal
Aesthesiometer (NCCA), using a controlled pulse of air, of a pre-determined
First, the system design and alterations are described, then in a series of model
experiments, the standardisation and characteristics of the air-pulse were examined.
These studies revealed that the NCCA could produce a repeatable stimulus of known
volume and rate of air-flow. Furthermore, the air-flow exiting the stimulus air jet
was of a laminar shape, with minimal dispersion. The control of the air-flow
dispersion was further enhanced by using a 0.5mm diameter air jet and a working
distance of lcm. Varying the stimulus duration was not found to influence these
qualities of the air-pulse.
The air-pulse stimulus was shown to possess the ability to produce corneal nerve
stimulation either by surface deformation, temperature change, or both. Using
thermal imaging equipment, a temperature drop in the ocular tear film was
demonstrated that was localised, and limited to the cornea.
A second series of experiments investigated the ability of the NCCA to measure a
corneal sensitivity threshold. These studies indicated that a forced-choice, doublestaircase,
Method of Limits experimental technique produced an accurate threshold,
with low variability in the results, over a minimum time period. Further studies
showed that this threshold measurement was repeatable to within 0.1 mbars.
A database of typical normal sensitivity thresholds, under a number of physiological
variables - corneal location, gender, age and iris colour, was developed. The results
from these studies compared well qualitatively with those using invasive stimuli.
A third series of experiments examined the ability of the NCCA to assess corneal
nerve function when it was under a number of external influences. The first situation
was that of anaesthesia,p roduced by 0.4% benoxinate hydrochloride. Non-contact
corneal sensitivity loss and recovery were shown to return to normal levels 60mins
after instillation of the anaesthetic. The second situation was that of long-term
contact lens wear (i.e. longer than three years). Non-contact sensitivity was shown to
be reduced with both soft and gas-permeable lens wear, although the extent of loss
did not differ between them, nor was it influenced by the length of wear. Thirdly,corneal sensitivity loss and recovery was assessed in subjects following excimer
laser photorefractive keratectomy (PRK). Three laser trials were completed: a
longitudinal myopic study, a transverse myopic study, and a longitudinal hyperopic
study. The results indicated that non-contact corneal sensitivity loss/recovery was
not related to the attempted depth of ablation, and that sensitivity had still not
returned to normal levels one year post-op.