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M. Chen, H. Liu, C. Begley, M. Jansen, A. Bradley; Sensory Response to Hyperosmolar Solutions. Invest. Ophthalmol. Vis. Sci. 2007;48(13):427.
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Tear film hyperosmolarity is often cited as a factor in dry eye etiology. The purpose of this investigation was to determine the threshold for sensation of hyperosmolar solutions and the pain response above threshold.
40 microliter drops of sucrose and NaCl solutions, adjusted to 300-1100mOsm/L, were instilled into the eyes of 5 subjects. Expt. #1: Hyperosmolarity thresholds were determined using a 2AFC method in which subjects had to identify which eye received a drop of 400-575mOsm/L (15 drops of each osmolarity in 25mOsm/L steps) while the other eye had a reference drop of 300mOsm/L. Expt. #2: Supra-threshold pain responses were obtained for drops instilled into one eye of 5 subjects (3 drops of each osmolarity from 300 -1100mOsm/L in 100mOsm/L steps). Subjects scored the discomfort from each drop on a 0-10 scale and marked the level of irritation, stinging, burning, pricking, and cooling on visual analog scales. Pain sensation data were fitted by power functions to obtain the exponent of increase for comparison to traditional pain literature.
Using the 2AFC method, the average threshold for detection of hyperosmolar solutions was 454.4±10.2mOsm/L for sucrose and 448±15.1mOsm/L for NaCl. The discomfort level increased as a power of 4.15±0.49 for sucrose and 3.52±1.50 for NaCl with increasing osmolarity. The increase in sensation level against osmolarity varied dramatically among scales with burning, stinging and irritation exhibiting the highest levels of perceptual saliency. Average exponents of sensation against osmolarity for irritation, stinging and burning were 2.68±0.03 for sucrose and 2.79±0.13 for NaCl, while pricking was 2.4 for both sucrose and NaCl, and less than 2.0 for cooling.
Our results show that stinging and burning are the typical sensory response to hyperosmolar solutions, which is consistent with response of the underlying corneal innervation to a chemical stimulus. The exponent of increase is consistent with other relatively painful stimuli. The threshold for detection of hyperosmolar solutions is well above reports of overall tear film osmolarity in dry eye. Thus, stinging and burning reported by dry eye patients may be due to local, transient spikes of tear film osmolarity over 600mOsm/L
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