Date Published: August 24, 2015
Publisher: Public Library of Science
Author(s): Lóránt Dienes, Huba J. Kiss, Kristóf Perényi, Zsuzsanna Szepessy, Zoltán Z. Nagy, Árpád Barsi, M. Carmen Acosta, Juana Gallar, Illés Kovács, Deepak Shukla.
To investigate the characteristics of ocular surface sensations and corneal sensitivity during the interblink interval before and after tear supplementation in dry eye patients.
Twenty subjects (41.88±14.37 years) with dry eye symptoms were included in the dry eye group. Fourteen subjects (39.13±11.27 years) without any clinical signs and/or symptoms of dry eye were included in the control group. Tear film dynamics was assessed by non-invasive tear film breakup time (NI-BUT) in parallel with continuous recordings of ocular sensations during forced blinking. Corneal sensitivity to selective stimulation of corneal mechano-, cold and chemical receptors was assessed using a gas esthesiometer. All the measurements were made before and 5 min after saline and hydroxypropyl-guar (HP-guar) drops.
In dry eye patients the intensity of irritation increased rapidly after the last blink during forced blinking, while in controls there was no alteration in the intensity during the first 10 sec followed by an exponential increase. Irritation scores were significantly higher in dry eye patients throughout the entire interblink interval compared to controls (p<0.004). NI-BUT significantly increased after HP-guar (p = 0.003) but not after saline drops (p = 0.14). In both groups, either after saline or HP-guar the shape of symptom intensity curves remained the same with significantly lower irritation scores (p<0.004), however after HP-guar the decrease was significantly more pronounced (p<0.004). Corneal sensitivity to selective mechanical, cold and chemical stimulation decreased significantly in both groups after HP-guar (p<0.05), but not after saline drops (p>0.05).
Ocular surface irritation responses due to tear film drying are considerably increased in dry eye patients compared to normal subjects. Although tear supplementation improves the protective tear film layer, and thus reduce unpleasant sensory responses, the rapid rise in discomfort is still maintained and might be responsible for the remaining complaints of dry eye patients despite the treatment.
Dry eye with symptoms of ocular discomfort is one of the most commonly reported conditions in eye care with an estimated prevalence ranges from approximately 5% to 35% . Patients who suffer from dry eye have varying levels of symptoms, such as ocular dryness, burning, photophobia, foreign body sensation and redness, and may or may not have clinical signs, such as rapid tear film breakup time, increased osmolarity, and increased ocular surface staining . The cornea has rich innervation of different types of sensory receptors and selective stimulation of these nerve endings evokes distinct sensations . The precorneal tear film protects the ocular surface from external damage but corneal nerve endings are exceedingly close to the surface and can easily react to different types of environmental stimuli, especially when the tear film is abnormal. Current opinion about the origin of the unpleasant sensations that accompany dry eye is that they are primarily due to abnormal activity of cold receptors secondary to ocular surface desiccation and tear hyperosmolarity [4,5]. 10–15% of the corneal nerve fibers are cold sensitive and they appear to be primarily involved in detecting external temperature variations, including those associated with tear evaporation . Although the evaporation rate of the tear film is determined by multiple factors, including protein and aqueous components of the tear film and the mucin coating of the epithelial cells, the status of the lipid layer is crucial in the prevention of evaporation . In particular, the thickness of the lipid layer affects evaporation and a thicker tear film lipid layer found to be correlated with better tear film stability and less symptoms of dry eye [6–8]. In healthy subjects ocular surface desiccation proved to be correlated with increasing ocular discomfort, suggesting that both tear film thinning and tear film breakup stimulate underlying corneal nerves, although tear film breakup produced more rapid stimulation of corneal sensory nerves .
Patients who had been diagnosed as having dry eye symptoms for at least 3 months, with an OSDI score of ≥13 evaluated by the questionnaire of Ocular Surface Disease Index (OSDI)  have been recruited for this study. Subjects who showed significant corneal staining (>Grade 2, Oxford Scale)  were excluded because corneal epitheliopathy could potentially be a confounding factor affecting ocular surface sensitivity [21–23]. Subjects with ophthalmic conditions other than dry eye or systemic disease including blepharitis, meibomitis, lid abnormalities as well as contact lens wearers were also excluded. None of the subjects received any drops at least 6 hours before the measurements. Participants in the control group did not have any clinical signs and/or symptoms of dry eye (OSDI score <10) or significant ocular surface disease and were not using eye drops. Twenty eyes of twenty subjects (12 men, 8 women) were included in the dry eye group with a mean age of 41.88±14.37 years. Fourteen eyes of fourteen subjects (9 men, 5 women) with a mean age of 39.13±11.27 years were included in the control group. There was no statistically significant difference in age or gender distribution between the two study groups. The mean OSDI score was 30.19±15.49 in the dry eye group and 3.45±2.95 in the control group (p<0.001). The non-invasive tear film breakup time in the dry eye group was 4.16±2.44 sec, and in the control group was 13.05±6.01 sec (p<0.001). Dry eye syndrome is a common cause of ocular irritation and the primary management goals are to restore the natural homeostasis of the ocular surface and tear film and thus improve the patient’s ocular comfort and quality of life [26–28]. There are several reports with documented improvement of subjective symptoms and some objective parameters (tear film stability, ocular surface staining) after tear supplementation in the long term [14–18,27], but in most cases frequent administration of eye drops is required to maintain symptom remission. Although appropriate treatment resulting in prompt relief of complaints is the desirable medium of care, the relationship between ocular surface drying and the onset of unpleasant sensations has not yet been clarified in this population. Source: http://doi.org/10.1371/journal.pone.0135629