Ocular Allergy as a Risk Factor for Dry Eye in Adults and Children

Edoardo Villani; Giovanni Rabbiolo; Paolo Nucci

Disclosures

Curr Opin Allergy Clin Immunol. 2018;18(5):398-403. 

In This Article

Ocular Allergy and Neurosensory Abnormalities

The crucial role of neurosensory abnormalities in DED has been recently highlighted including them in the definition of the disease.[9] Tear film hyperosmolarity and instability and ocular surface inflammation are able to change the behavior of the different classes of corneal sensory receptors, inducing peripheral sensitization and, in the long term, inducing nerves damage. The peripheral ocular surface neuropathy can impact on several components of the morpho-functional unit, including tear secrection, blink rate, epithelial and goblet cells trophism, and the behavior of the corneal immune cells. Perpetuation of the vicious cycle can ultimately lead to central sensitization.[18,39]

In DED patients, several clinical studies, using mechanical esthesiometry, showed a reduction of corneal sensitivity. Moreover, imaging studies using in-vivo confocal microscopy to assess the corneal subbasal nerves plexus, demonstrated a DED-related reduction of nerves density and increase of nerves tortuosity and subbasal immune cells density.[39,40]

Similar researches reported similar results, with a few qualitative differences, also in AKC and VKC patients.[40,41] Interesting findings in severe ocular allergy included a good correlation between corneal sensitivity and subbasal nerves quantitative and morphological changes, the correlation between corneal sensitivity and conjunctival goblet cells density, and the confocal demonstration of the presence of stromal nerves morphological abnormalities.[15,23,40,41]

Ocular allergy and DED symptoms include a wide variety of unpleasant sensations, mainly mediated by the corneal sensory innervation. If itching and dryness are historically considered as pathognomonic of the two diseases, symptoms are often overlapping and include tearing, burning, foreign body sensation, and other types of discomfort.[3,8,9,39]

Molecular and functional characteristics of corneal sensory innervation are not yet fully understood. However, ocular sensory neurons can be broadly classified as polymodal nociceptor neurons, cold thermoreceptor neurons, and selective mechano-nociceptor neurons.[39,42] Experimental evidences suggest that ocular allergy can evoke direct chemical activation and sensitization of polymodal nociceptors, discrete sensitization of mechano-nociceptors, and reduction of cold thermoreceptor activity,[43] whereas in DED, the sensitization of polymodal and mechano-nociceptor nerve endings is combined with an abnormal increase in cold thermoreceptor activity.[39]

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