The roles of neural adaptation and sensitization in contact lens discomfort

Purpose: To investigate the roles of neural adaptation and sensitization in contact lens discomfort (CLD). Methods: Cooling stimuli (20 degrees C) were applied to the cornea in a group comprising 24 symptomatic and 25 asymptomatic contact lens (CL) wearers as well as 15 non-CL wearing controls, using a computerized Belmonte esthesiometer. The adaptation paradigm consisted of 20 repetitive stimuli at threshold, sub- and supra-threshold levels. The sensitization paradigm involved five levels of suprathreshold stimuli ranging between 1x to 2x threshold. Following each stimulus, participants rated the sensation magnitude regarding intensity, coolness and irritation. Measurements were taken with habitual CL (BL_CL), after 2 weeks of no-CL (No_CL) and after restarting habitual CL wear (ReSt_CL). Results: The symptomatic subjects exhibited a lower threshold but reported enhanced sensations during the adaptation and sensitization paradigm, compared to the asymptomatic and control groups (all p <= 0.021). At the BL_CL and ReSt_CL visits, they showed increased ratings to repeated subthreshold stimuli (p = 0.025) and greater irritation during the sensitization paradigm (p <= 0.032). Ratings in asymptomatic and control groups were relatively unchanged over time (p >= 0.181). Logistic regression revealed a link between the augmented sensory responses and increased likelihood with CLD. Conclusion: The maladaptive sensory responses seen in CLD subjects, with reduced adaptation and heightened sensitization to ocular surface stimulation, suggest an imbalance between sensitization and adaptation in CLD. As CLD may represent a reversible subcategory of dry eye, it can serve as a human dry eye model for studying the neurosensory effect of ocular surface stimulation.