Non-Contact Corneal Aesthesiometer Research Articles

Neurodegeneration of the cornea and retina in patients with type 1 diabetes without clinical evidence of diabetic retinopathy.

AimDiabetic retinopathy (DR) is widely considered the earliest and most common microvascular complication of diabetes. However, recent studies have shown that retinal nerve fiber layer and corneal nerve abnormalities may be present in diabetic patients without retinopathy. This preliminary study aimed to establish if structural and functional changes in the nerve fiber layer of the retina and cornea occur in patients with type 1 diabetes (T1DM) without retinopathy.MethodsTwenty patients with T1DM, without clinical evidence of retinopathy (Age: 47.0 ± 2.5 years; Duration diabetes: 27.0 ± 3 years) and 15 age-matched healthy control subjects underwent detailed medical neurological examinations. Ophthalmic examinations using Spectral Domain Optical coherence tomography (SD-OCT), Standard Automated Perimetry (SAP), Flicker Defined Form High Edge Perimetry (FDF), Corneal Confocal Microscopy (CCM) and Non-contact corneal Aesthesiometry (NCCA) were performed to quantify the structure and function of the nerves in the retina and cornea, respectively.ResultsAt the structural level, retinal nerve fiber layer thickness (RNFL) was significantly reduced in the superior nasal (p=0.001) and inferior temporal (p=0.004) sectors, in diabetic patients. Retinal ganglion layer function was reduced in the patient group when assessed using Flicker Defined Form Perimetry (FDF), but this was not significant. The function of the cornea assessed by corneal sensitivity, using a non-contact corneal aesthesiometer (NCCA), was significantly reduced (p=0.001). Structural assessment of corneal nerves using corneal confocal microscopy (CCM) showed reduction at corneal nerve fiber density (CNFD) (p=0.01), branch density (CNBD) (p=0.006) and length (CNFL) (p=0.01) in patients with diabetes. Compared to control subjects, the percentage of abnormality in patients with T1DM for RNFL was 32% while the FDF was abnormal in 61% of patients. Corneal abnormality was observed in 47% for NCCA, 28% for CNFD, and 17% for CNFL. There was no correlation between neuronal damage in the retina and cornea.ConclusionsNeuronal abnormalities were observed in both the retina and cornea of diabetic patients without evidence of retinopathy. The prevalence of structural and functional changes was higher in the retina compared to the cornea. This preliminary study suggests that structural neuronal changes may occur in parallel and correlate with functional changes. The assessment of corneal and retinal nerve structure may be clinically useful for detecting and monitoring the earliest stages of diabetic microvascular abnormalities.

Corneal Sensitivity with Contact Lenses of Different Mechanical Properties

To examine the changes in corneal sensitivity after overnight wear of contact lenses with different mechanical properties. Twenty young-adult subjects wore a silicone hydrogel, rigid gas-permeable, or orthokeratology (OK) contact lens in randomized order for a single night of wear in the right eye only. All lenses were matched in Dk/t (∼46 ISO Fatt). Changes in corneal apical radius r(o), asphericity Q, and corneal refractive power (Medmont E300) were measured. Changes in central corneal sensitivity were also measured by a masked investigator using two instruments: Cochet-Bonnet (COBO) aesthesiometer and Non-Contact Corneal Aesthesiometer (NCCA). There were significant differences in corneal topographic change from baseline between the lens types for r(o), Q, and corneal refractive power. There were also significant differences in the change from baseline (mean ± SD) in corneal sensitivity between lens types using the COBO (silicone hydrogel, 0.02 ± 0.17 g/mm(2); rigid gas-permeable, 0.03 ± 0.20 g/mm; OK, 0.22 ± 0.33 g/mm(2)). A significant increase in threshold from baseline was only seen in the OK lenses (p = 0.006). There was no change in sensitivity thresholds from baseline for any lens type using the NCCA (p > 0.05). Central corneal sensitivity is reduced after a single overnight wear of OK lenses, as measured using the COBO aesthesiometer. This suggests that the mechanical force exerted by contact lenses may influence corneal sensitivity.

Tear production and corneal sensitivity in diabetes

Diabetic patients are at significant risk of developing corneal lesions such as superficial punctate keratitis, recurrent corneal erosions, persistent epithelial defects, and microbial keratitis. The aim of this study was to investigate whether diabetes mellitus is correlated with both reduced corneal sensation and reduced tear production. In 25 type II diabetic patients with a history of retinopathy only and in 25 nondiabetic control subjects (age and sex matched), we performed noncontact corneal aesthesiometry and assessed basal tear production using Schirmer's test with topical anesthesia. The noncontact corneal aesthesiometer (NCCA) is a new noninvasive device for quantifying threshold corneal sensitivity. The diabetic patients demonstrated a significantly reduced Schirmer's test result (P<.001) and significantly reduced corneal sensitivity (P<.01). Our study supports previous reports of reduced basal tear production, lending more support to the theory of a peripheral neuropathy affecting lacrimal gland function in diabetes. We also confirmed reduced threshold corneal sensitivity in diabetic patients using the NCCA.

Corneal Sensitivity Is Reduced and Relates to the Severity of Neuropathy in Patients With Diabetes

The focus in relation to the consequences of nerve damage in diabetes has been the loss of sensation in the feet predisposing to the development of foot ulceration and lower extremity amputation (1). However, the cornea is 300–600 times more sensitive than skin (2). In addition to serving a protective function, corneal nerves regulate corneal epithelial integrity, proliferation, and wound healing (3). In diabetic patients, corneal sensitivity is reduced (4), due to a loss of corneal nerve fibers (5), which leads to corneal keratopathy (6,7) and a susceptibility to injury, with recurrent erosions and ulcers (8). Thus, these changes are analogous to the diabetic foot, but because the cornea is not exposed to high pressures, ulceration infrequently occurs. Corneal sensation is mediated via myelinated A-δ and C-nerve fibers (9,10), which can be evaluated using the Cochet-Bonnet aesthesiometer (C-BA) (2) or the noncontact corneal aesthesiometer (NCCA) (11), respectively. In diabetic patients, the C-BA has been used to show a reduction in corneal sensitivity in some (12) but not other (13) studies. Furthermore, loss of corneal sensation has been related to the severity of retinopathy (12) and neuropathy (14). We have assessed corneal sensitivity in a large group of diabetic patients using both C-BA and NCCA to assess concordance between the two methods and also to define whether C-nerve fibers are the earliest to undergo damage (15). Using corneal confocal microscopy, we …

Tear‐film lipid layer morphology and corneal sensation in the development of blinking in neonates and infants

The aim of the study was to evaluate the role of lipid layer thickness and corneal sensation in the development of blinking in neonates. The study group comprised sixty-four neonates and infants (mean age 27.5 +/- 15 (sd) weeks, range 3.4-52) whose mothers were attending a general practice healthy baby clinic. Spontaneous eye-blink activity was determined from digital videographic recordings; tear film lipid layer morphology wasexamined using interference patterns produced by the Keeler Tearscopetrade mark Plus over a five-point grading scale (higher grades are associated with thick and stable lipid films); corneal sensation threshold was assessed with the Non-Contact Corneal Aesthesiometer (NCCA), using the eye-blink response as an objective indication that the cooling stimulus had been felt; palpebral aperture dimensions were measured using calibrated digital still images of the eye in the primary position. The overall mean spontaneous blink-rate was found to be 3.6 (+/- 0.3) blinks min(-1), and the mean interblink time was 21.6 (+/- 2.8) s. The lowest blink-rates were observed in the 0-17-week age group (average 2 blinks min(-1)). The blink-rate showed a highly significant correlation with age (r = 0.46, P < 0.01). The overall mean lipid layer grading was 3.6 (+/- 0.2 SE) arbitrary units. Higher grades were found in the newborn and the mean grading score reduced with age (P < 0.01). The mean sensation threshold to blink (TTB) was 0.69 (0.04 SE) mbar, which did not differ from a control group of older subjects (P > 0.05). There was a rapid increase in palpebral aperture length and width from birth to 1 year old, with surface area increasing by 50% over the same period. We concluded that the low rate of spontaneous eye blink activity in neonates is associated with a thick stable lipid layer that may be a function of a small palpebral aperture. Furthermore, neonates appear to have the capacity to detect ocular surface cooling, which is a major trigger for spontaneous blinking.

Noninvasive assessment of corneal sensitivity in young and elderly diabetic and nondiabetic subjects.

To investigate the effect of age and diabetes on corneal sensitivity using the noncontact corneal aesthesiometer (NCCA). One hundred sixteen nondiabetic subjects and 111 diabetic subjects (33 type I and 78 type II) were recruited and divided into three age groups: Young (<or=29 years), Middle (30-59 years) and Older (>or=60 years). The exclusion criteria included patients with severe retinopathy requiring treatment, a history of invasive ocular surgery, or a history of conditions known to affect corneal sensitivity. The corneal cooling sensation threshold, for the right eye of each subject, was assessed with a double-staircase method-of-limits technique with the NCCA. This instrument uses a controlled pulse of air to produce a small, localized reduction in the surface temperature of the eye, which is detected by the nerves in the corneal epithelium. Analysis of the scatterplot of each subject's central cooling sensation threshold revealed a gradual loss of sensitivity with increasing age (nondiabetic, r(2) = 0.349; diabetic, r(2) = 0.131). Within the nondiabetic group, inter-age-group comparisons found significant differences between the central corneal cooling sensation thresholds for the three age groups (t-test, P < 0.01). Within the diabetic group, a significant difference was found between the Middle and Older categories only (t-test, P < 0.05). In summary, the Young group was more sensitive than the Middle group, which was more sensitive than the Older group. Within both type I and type II diabetic subjects, there was neither a significant relationship between duration of the disease and corneal sensitivity (t-test, P > 0.05) nor a gender-based difference (t-test, P > 0.05). There is a gradual reduction in corneal sensitivity with increasing age in both nondiabetic subjects and diabetic subjects, along with an increasing variation in the measured threshold. There is no relationship between corneal sensitivity and the time since diagnosis of diabetes for a thermally cooling stimulus, suggesting that the A partial differential and C fibers of the corneal innervation are affected differently by abnormal glucose metabolism in the diabetic cornea.

The minimum stimulus energy required to produce a cooling sensation in the human cornea.

To estimate the minimum stimulus energy required to stimulate the corneal nerves, using a thermally cooling stimulus. The localised temperature change produced in the pre-corneal tear film by the air-pulse stimulus of the Non-Contact Corneal Aesthesiometer (NCCA) was analysed using a thermal imaging camera. Assuming that the cornea behaves as a near perfect black-body, the threshold for energy loss was calculated by combining Stefan-Boltzmann's law and Kirchoff's law for imperfections in black-body radiation, taking into account the likely thermal conductivity of the human cornea. The average normal subject has a threshold for detecting a cooling effect when the local corneal tear film surface temperature drops from a mean of 33.2-32.9 degrees C (i.e. 0.3 degrees C), over a 1 mm diameter circular area (0.785 mm2), and a stimulus time duration of 0.9 s. This gives rise to a sensation threshold for perceiving a loss of heat energy of 7.0 ergs, or 7.0 x 10(-7) joules. The cornea possesses stimulus specific receptors, which have an exquisite sensitivity to a rapid, thermally cooling stimulus.

The minimum stimulus energy required to produce a cooling sensation in the human cornea

Purpose: To estimate the minimum stimulus energy required to stimulate the corneal nerves, using a thermally cooling stimulus.Methods: The localised temperature change produced in the pre-corneal tear film by the air-pulse stimulus of the Non-Contact Corneal Aesthesiometer (NCCA) was analysed using a thermal imaging camera. Assuming that the cornea behaves as a near perfect black-body, the threshold for energy loss was calculated by combining Stefan–Boltzmann's law and Kirchoff's law for imperfections in black-body radiation, taking into account the likely thermal conductivity of the human cornea.Results: The average normal subject has a threshold for detecting a cooling effect when the local corneal tear film surface temperature drops from a mean of 33.2–32.9°C (i.e. 0.3°C), over a 1mm diameter circular area (0.785mm2), and a stimulus time duration of 0.9 s. This gives rise to a sensation threshold for perceiving a loss of heat energy of 7.0ergs, or 7.0×10−7 joules.Conclusion: The cornea possesses stimulus specific receptors, which have an exquisite sensitivity to a rapid, thermally cooling stimulus.

The effect of long-term, daily contact lens wear on corneal sensitivity.

To assess the effect of long-term, daily-wear soft contact lenses and rigid gas permeable (RGP) contact lenses on corneal sensitivity using a noninvasive, air-pulse stimulus. The central and peripheral (temporal, medial, inferior) corneal sensation thresholds of 40 non-lens wearers, 40 soft lens wearers, and 40 RGP lens wearers were assessed using the Non-Contact Corneal Aesthesiometer (NCCA). The individuals who wore contact lenses were grouped according to the number of years of lens wear (10 years or less, 11-20 years, and 21 years or more). Although a significant reduction in corneal sensitivity was found between the contact lens wearers and non-lens wearers (p = 0.000), no difference was found between the two lens-type subgroups (p = 0.939). This pattern of significance was repeated at each of the peripheral test locations. No relationship between corneal sensitivity and years of lens wear was found centrally (r2 = 0.004) or at any of the peripheral test locations. No significant difference was found between the central corneal sensation thresholds for the different subgroups of lens wear duration (p = 0.469) or for any of the peripheral test locations. Both soft and RGP lens wear produce a similar type of corneal sensitivity loss, although the mechanism for this loss is different for the two lens types. The extent of sensitivity loss is not related to the duration of lens wear and appears to plateau after the first few months of wear. No topographical variation in sensitivity loss was found with lens type or with the duration of lens wear.

Corneal sensitivity and some properties of the tear film after laser in situ keratomileusis.

To investigate central corneal sensitivity, lipid layer structure of the precorneal tear film, and tear volume after laser in situ keratomileusis (LASIK). Central corneal sensitivity was measured using the Non-Contact Corneal Aesthesiometer. The aesthesiometer was mounted on a slit lamp and an airpulse of controlled pressure was directed onto the cornea. When central corneal sensitivity was reduced, a higher air pulse pressure was required to stimulate the cornea. The final central corneal sensitivity threshold measured was recorded in millibars. Tear lipid layer structure was assessed by optical interferometry and classified according to appearance using the Keeler Tearscope. Tear volume was measured using the phenol red cotton thread test. Subjects were recruited from a group of patients after LASIK who had experienced no complications (n=22). The average postoperative time was 14 weeks and measurements were taken on one eye. In bilateral cases, measurements were recorded from the right eye only. Average attempted correction was -6.30 D (range, -2 to -11 D). Age-matched controls were later recruited for central corneal sensitivity threshold (n=24). A second group of age-matched controls were recruited for tear volume and lipid layer structure (n=24). The median (range) was 1.1 mbars (0.2 to 4.3 mbars) after LASIK and 0.58 mbars (0.20 to 1.3 mbars) in the controls; the difference was statistically significant (P = .043). The lipid layer of the tear film tended to be thinner in eyes after LASIK compared with controls (P = .032). The mean (+/- SD) tear volume was 16.9 +/- 8.3 mm after LASIK and 19.8 +/- 7.1 mm in controls. This difference was not statistically significant (P = .492). At 14 weeks postoperatively, central corneal sensitivity was below normal levels and the tear lipid layer was thinner. The poorer quality lipid layer may predispose to symptoms of dry eye after LASIK.

Corneal surface temperature change as the mode of stimulation of the non-contact corneal aesthesiometer.

The non-contact corneal aesthesiometer (NCCA) assesses corneal sensitivity by using a controlled pulse of air, directed at the corneal surface. The purpose of this paper was to investigate whether corneal surface temperature change was a component in the mode of stimulation. Thermocouple experiment: A simple model corneal surface was developed that was composed of a moistened circle of filter paper placed on a thermocouple and mounted on a glass slide. The temperature change produced by different stimulus pressures was measured for five different ambient temperatures. Thermal camera experiment: Using a thermal camera, the corneal surface temperature change was measured in nine young, healthy subjects after exposure to different stimulus air pulses. Pulse duration was set at 0.9 s but was varied in pressure from 0.5 to 3.5 millibars. Thermocouple experiment: An immediate drop in temperature was detected by the thermocouple as soon as the air flow was incident on the filter paper. A greater temperature change was produced by increasing the pressure of the incident air flow. A relationship was found and a calibration curve plotted. Thermal camera experiment: For each subject, a drop in surface temperature was detected at each stimulus pressure. Furthermore, as the stimulus pressure increased, the induced reduction in temperature also increased. A relationship was found and a calibration curve plotted. The NCCA air-pulse stimulus was capable of producing a localized temperature change on the corneal surface. The principal mode of corneal nerve stimulation, by the NCCA air pulse, was the rate of temperature change of the corneal surface.

Reliability of the Non-Contact Corneal Aesthesiometer and its comparison with the Cochet–Bonnet aesthesiometer

Aims-: To determine the repeatability of measurement of the corneal sensation threshold using the Non-Contact Corneal Aesthesiometer (NCCA); to evaluate the effect of the air-jet diameter and stimulus duration on the threshold; and to compare the sensation thresholds found with the NCCA and with the Cochet–Bonnet Aesthesiometer (C–BA), on the same group of normal, human subjects.Methods-: For all experiments, only the central corneal sensation thresholds were measured. (1) NCCA Repeatability; 17 subjects tested at the same time on five separate days. (2) NCCA Air-Jet Diameter; 15 subjects tested using four different air-jets: 0.35, 0.5, 1 and 2mm. (3) NCCA Stimulus Duration; 15 subjects tested using three stimulus durations: 0.5, 0.9 and 1.5 sec. (4) NCCA and C–BA; 43 subjects tested using each instrument in turn.Results-: (1) No significant differences in mean sensation threshold were detected across the five days of testing, or between individual days. (2) 0.5 and 1mm air-jets produce least variability in mean sensation threshold. (3) No significant difference in mean sensation thresholds was found between the three test durations. (4) For each subject, there was no correlation between each instrument’s mean sensation threshold.Conclusions-: (1) The NCCA was able to assess the corneal sensation threshold in an accurate and repeatable manner. (2) The optimum test parameters were found using the 0.5mm air-jet, over a stimulus duration of 0.9 sec. (3) The C–BA has serious deficiencies in its design which limit its ability to accurately measure corneal sensitivity at low stimulus thresholds.

Reliability of the Non‐Contact Corneal Aesthesiometer and its comparison with the Cochet–Bonnet aesthesiometer

To determine the repeatability of measurement of the corneal sensation threshold using the Non-Contact Corneal Aesthesiometer (NCCA); to evaluate the effect of the air-jet diameter and stimulus duration on the threshold; and to compare the sensation thresholds found with the NCCA and with the Cochet-Bonnet Aesthesiometer (C-BA), on the same group of normal, human subjects. For all experiments, only the central corneal sensation thresholds were measured. (1) NCCA Repeatability; 17 subjects tested at the same time on five separate days. (2) NCCA Air-Jet Diameter; 15 subjects tested using four different air-jets: 0.35, 0.5, 1 and 2 mm. (3) NCCA Stimulus Duration; 15 subjects tested using three stimulus durations: 0.5, 0.9 and 1.5 sec. (4) NCCA and C-BA; 43 subjects tested using each instrument in turn. (1) No significant differences in mean sensation threshold were detected across the five days of testing, or between individual days. (2) 0.5 and 1 mm air-jets produce least variability in mean sensation threshold. (3) No significant difference in mean sensation thresholds was found between the three test durations. (4) For each subject, there was no correlation between each instrument's mean sensation threshold. (1) The NCCA was able to assess the corneal sensation threshold in an accurate and repeatable manner. (2) The optimum test parameters were found using the 0.5 mm air-jet, over a stimulus duration of 0.9 sec. (3) The C-BA has serious deficiencies in its design which limit its ability to accurately measure corneal sensitivity at low stimulus thresholds.