Ocular surface sensitivity plays a role in dry eye and ocular comfort through its probable influence on the neural feedback loop that regulates tear secretion, but little is known specifically about the role of eyelid sensitivity in ocular surface health.1,2 Recently, a number of research groups have focused their interest on characterizing the lid margins3–7 and the lid margin's association with ocular surface diseases such as meibomian gland dysfunction,8 and ocular discomfort.7,9–12 The lid margins consist of two strips of tissue located at the edge of the upper and lower lids, posterior to the eyelashes. Three distinct zones were recently described within the lid margins, being a band of cornified epidermis immediately behind the posterior margin of the meibomian orifices, followed by a mucocutaneous junction zone containing parakeratinized cells, followed by the stratified epithelium of the palpebral conjunctiva.3,5 Reports from the International Workshop on Meibomian Gland Dysfunction have highlighted both the high prevalence of meibomian gland dysfunction in the normal population13 and the paucity of information available on the relationships between meibomian gland dysfunction and ocular surface signs and symptoms, including the relevance of the role of the lid margin.14
Studies linking staining of a region of the upper-lid margin known as the “lid wiper” to symptoms in contact lens wearers and non-wearers have speculated that this area is important in driving symptoms of ocular discomfort,9–11 although the etiology for this has not been fully rationalized. However, given the probable link between symptoms and corneal innervation in dry eye,1,2 it is likely that the innervation of the lid margin similarly plays a role in discomfort. Very few studies have attempted to measure the sensitivity of the eyelids,15–18 and little is known about the sensory innervation of this region.
Studies of the “lid wiper epitheliopathy” hypothesize that it is the movement of the upper eyelid over the corneal surface that is the key influence.9–11 However, little has been published about the differences between the upper- and lower-eyelid margins.19 Marx in his 1924 article originally reported “no noticeable differences between the two (eyelids)” in terms of their anatomy,20 yet it is clear that differences exist between the two, such as the number of meibomian glands each contains (30–40 superiorly vs. 20–30 inferiorly).21
The aim of this study was therefore to characterize the lid margin sensitivity and to explore the relationships between lid margin sensitivity and key markers of ocular surface health, including lid margin staining, meibomian gland dysfunction, tear osmolarity, and ocular symptoms. A secondary aim was to look for differences and associations between lower- and upper-eyelid characteristics.
A prospective pilot study was conducted. All procedures were approved by the University of New South Wales Human Research Ethics Committee, and all subjects signed a statement of informed consent before commencing the study.
Twenty-seven subjects were enrolled (7 men, 20 women), with a mean age of 31 ± 14 (range: 19–60) years. Exclusion criteria included Sjogren syndrome, thyroid and connective tissue disease, all forms of conjunctivitis, ocular infection within the past month, a previous history of refractive or eyelid surgery, gross eyelid or ocular abnormalities, current use of ocular medication, use of artificial tear preparations within the past 2 h or ocular ointment within the past 3 days, or regular contact lens use within the past 6 months. These criteria were chosen based on the fact that Millodot reported a complete recovery of sensitivity to a Cochet-Bonnet stimulus within 4 months after long-term polymethyl methacrylate wear.22
Measurements were carried out in the order of least to most invasive as described later in the text, on the right eye of each subject. Both eyes were deemed to be similar because it has been shown that no contralateral differences in sensitivity exist between eyes.23,24 Self-reported ocular symptoms were measured using the Ocular Surface Disease Index (OSDI)25 and Dry Eye Questionnaire (DEQ).26 For the DEQ, a total score for frequency, a.m. intensity, p.m. intensity, and intrusiveness of symptoms was obtained from the sum of scores of the six symptoms for each respective category. A total intensity score was obtained by averaging the a.m. and p.m. intensity scores. The total DEQ score was calculated by adding the total frequency, intensity, and intrusiveness scores. Tear osmolarity was measured on-eye (TearLab Corporation).
Lid margin staining was characterized using sequential instillation of sodium fluorescein (Bausch & Lomb UK Limited) and lissamine green (Rose Stone Enterprises) and graded according to the categorical scale described by Korb et al.10 Lissamine green dye solution was prepared by soaking a lissamine green strip in 200μL of unpreserved saline solution for 1 min. Briefly, a 25μL drop of 2% fluorescein was instilled into the lower conjunctival sac of the right eye, followed by a 25μL drop of lissamine green dye preparation 1 min after. After 4 min, a second drop of 25μL of 2% fluorescein was instilled, followed by a second 25μL of lissamine green dye 1 min after. One minute after the last instillation, the eye was examined for lid margin staining. Fluorescein staining was graded under cobalt blue light and a Wratten filter, whereas lissamine green staining was graded under white light. Slit lamp settings for both fluorescein and lissamine green staining were set to 10× magnification, beam width was set at the widest setting, and illumination was set at the highest level. The horizontal length of the lid extending from the puncta to lateral canthus was examined and graded for both fluorescein and lissamine green staining. Sagittal height (width) of lid margin staining was assessed as a percentage of the width of the region of the lid margin extending from Marx line to the subtarsal fold for both fluorescein and lissamine green staining. The final fluorescein staining grade was taken as the average of the horizontal and sagittal fluorescein grades. Similarly, the final lissamine green staining grade was taken as the average of the horizontal and sagittal lissamine green grades. The final lid margin staining grade was taken to be the higher of the final fluorescein or lissamine green grades. Thus for each subject, two lid margin staining grades were obtained, one for the upper and one for the lower lid. Care was taken to exclusively assess the region of the lids described as the lid wiper and to exclude Marx line staining, which is visible in the majority of healthy eyes.27 Lid eversion was required for upper-eyelid, but not for lower-eyelid, staining assessment. Lids were handled cautiously, to avoid inducing iatrogenic staining through lid manipulation.6
Grading of meibomian gland dysfunction was conducted in the interval between the two dye instillations. Meibomian gland dysfunction was assessed using a modified system21 that involved grading of the thickening, rounding, irregularity, and telangiectasia of the lid margins; meibomian gland dropout; capping, pouting, obliteration, vascular invasion, and retroplacement of the orifices; and tear film foaming. Individual scores for each of these variables formed the total meibomian gland dysfunction score, with a higher score being indicative of more dysfunction.
Measurements of lid margin sensitivity were made at the center of the occlusal surface of the upper-and lower-lid margins using a Cochet-Bonnet esthesiometer with a 0.08 mm-diameter filament (Luneau Ophtalmologie, France; Fig. 1) and the ascending method of limits to determine threshold to stimulation.28 Cochet-Bonnet measurements were performed free hand, without the need for magnification by a pre-presbyopic investigator, using a calibrated instrument. In preliminary experiments, attempts were made to use a Cochet-Bonnet esthesiometer mounted on a slit lamp; however, this proved very difficult to use for the purpose of lid margin measurements, and we found our hand-held method more able to ensure consistent correct perpendicular placement of stimuli. Commencing with the longest filament length, four stimulus presentations were made at each intensity level; threshold was recorded as the level where two or more positive responses to the stimulus were recorded. Subjects were instructed to look down, with their upper lid gently elevated to expose the lid margin, for threshold determination of the upper-lid margin, and to look up, with their lower lid gently depressed, for measurements of the lower-lid margin. Investigators ensured that the precise occlusal region of the lid margin was assessed by using the anatomical features of the eyelid (e.g., meibomian gland openings, Marx line, eyelashes, etc.) as a guide. Measurements were conducted alternately between the upper- and lower-lid margins. An interval of 30 s between stimuli presentation at one location was allowed to minimize the effect of adaptation due to multiple exposures.29 All measurements were conducted between 9 a.m. and 5 p.m. at normal room temperature (range: 21.3–26.1°C) and humidity (range: 38–63%). Each specific measurement was performed by a single observer.
Spearman ρ was used to study associations, and differences between variables were examined using Wilcoxon signed rank test. Significance was determined at a confidence level of 95%.
The lid margin characteristics of the subjects in this study were within normal expected ranges for staining, meibomian gland dysfunction, and sensitivity. Group mean values for these characteristics are presented in Table 1. All subjects were categorized as normal or mildly symptomatic, with a median OSDI score of 12.5 (range: 0–48). The median DEQ score obtained was 42 (range: 22–76). Mean tear osmolarity was 296 ± 12 (range: 279–327) mOsmol/L.
Table 2 examines the inter-relationships between lid margin sensitivity, lid margin staining, meibomian gland dysfunction, tear osmolarity, and ocular symptoms. A positive correlation was observed between staining of the upper lid and tear osmolarity (r = 0.41, p = 0.04; Fig. 2). Lower-lid sensitivity was positively correlated with tear osmolarity (r = 0.46, p = 0.02) but negatively correlated with meibomian gland dysfunction (r = −0.51, p = 0.01; Fig. 3). There was no association between tear osmolarity and OSDI score (r = −0.07, p = 0.72) or the total score for the DEQ (r = −0.05, p = 0.80).
The upper-lid margin was less sensitive and displayed less staining, but showed more evidence of meibomian gland dysfunction, than the lower-lid margin (Table 1). The sensitivity of the upper and lower eyelids was strongly correlated (r = 0.71, p < 0.001), as was the meibomian gland dysfunction score (r = 0.79, p < 0.001; Fig. 4). The staining of the lower- and upper-eyelid margins did not correlate (r = 0.31, p = 0.12). The majority (21 of 27, 78%) of subjects displayed some staining of the lower-lid margin, but surprisingly, only four of 27 (15%) subjects displayed staining of the upper-lid margin.
Ocular surface studies have focused primarily on meibomian gland health, with little interest given to the rest of the lid margin appearance.30,31 A small number of clinical groups focusing their attention on lid margin staining have recently highlighted the potential for a significant role of the lid margin in ocular surface disease.4,9–11 This study provides, for the first time, simultaneous measurements of a number of clinical characteristics of both lower- and upper-lid margins, including lid sensitivity, in a group of healthy subjects. It has been the authors' experience that ophthalmic practitioners are prone to assume that the lower and upper eyelids are similar20 and consequently are predisposed to examine the lower eyelid much more closely or solely, owing to ease of viewing. Our results clearly demonstrate such assumptions to be erroneous. We confirmed for the first time in a single study that the lower lids tend to display more staining19 and less meibomian gland dysfunction,21 but are more sensitive,15 than the upper lids. We observed lower-lid margin staining much more frequently than expected, with more than a third of our population displaying some level of lower-lid staining. Upper-lid staining was much less frequent. Sheering stress generated by inadequate lubrication between the moving eyelid and cornea has previously been suggested to explain the relationship between upper-lid staining and discomfort. We postulate that lower- and upper-lid staining are caused by different mechanisms, with the pooling of tears in the lower tear lake resulting in chemical injury to the lower-lid margin from prolonged exposure to proinflammatory normal tear components such as tear cytokines. This pooling may also lead to a longer interaction time with the stains at the lower meniscus, resulting in larger amounts of lower-lid staining. This is supported by the lack of correlation between the staining of the upper vs. the lower lids. The meibomian glands of the lower lids also showed less signs of dysfunction compared with those of the upper lids. This may, in part, explain why the lower lids have better sensitivity than the upper lids. It is possible that the greater levels of gland obstructions detected in the upper lids and the associated changes to the gland secretions may impede the ability of the sensory fibers innervating the lid to detect sensation.
Our study was unable to detect a relationship between lid staining and symptomatology; however, the study population consisted of largely asymptomatic subjects with little to no staining of the upper lid, and this may have restricted our ability to confirm this relationship. Upper-lid staining was associated with tear osmolarity; however, only four of 27 subjects displayed upper-lid staining. Confirmation of these findings in a larger study population is therefore required. Lower-lid staining was not associated with tear osmolarity. In this study, tear osmolarity was measured in vivo at a single location in the temporal lower-lid tear meniscus. Transient but large amounts of variation in the osmolarity of the tear film across the ocular surface have been previously proposed as an explanation for localized tear instability and the associated ocular sensations of burning and stinging.32 Such undetectable localized changes in tear osmolarity may also be responsible for the fact that a relationship between tear osmolarity and staining can be demonstrated at the upper lid but not the lower lid.
In this study, increased sensitivity of the lower-lid margins was associated with hyperosmolarity of the tear film. A correlation between corneal sensitivity and tear osmolarity has previously been demonstrated in a mixed population of healthy and dry-eye subjects, although lid sensitivity was unfortunately not assessed.33 Our findings suggest that hyperosmolarity-induced changes in sensitivity potentially occur at all ocular surfaces, perhaps linked to an increased exposure of nerve endings to proinflammatory cytokines present in hyperosmolar tears. The association between corneal and conjunctival sensitivity is well established29; however, the association between lid sensitivity and that of other regions of the ocular surface has yet to be evaluated and was not the subject of this study. Such an association may be expected based on the overlapping nature of the receptive fields innervating the ocular surface, which has been shown in animal models.34 An association between tear osmolarity and upper-lid sensitivity, in contrast, could not be demonstrated in this study. This is perhaps related to the lower volume of tears present in the upper-lid meniscus,35 making it more problematic to demonstrate an association. In our study, the meibomian glands of the upper lids showed more signs of dysfunction than those of the lower eyelids (Table 1); such obstructive changes to the meibomian glands could also adversely impact the ability of the upper lids to detect and respond to changes in tear osmolarity.
Although carefully done, manipulation of the eyelids could have influenced sensitivity and increased variability in our measurements. For practical reasons and to minimize the need for manipulation, lid sensitivity was measured at a location roughly level with the opening of the meibomian glands, slightly anterior to the lid wiper and Marx line, and this is a limitation of our study. In contrast to our previous experience performing Cochet-Bonnet corneal and conjunctival measurements, we found when assessing the lid margin that it was very easy to visualize when the contact is made because the lid is not a transparent tissue. It is possible that the sensitivity of other regions of the lid margin, for example, regions displaying significant levels of epithelial staining, may be quite different to that measured in our study. Given the existence on the lid margins of three subzones with clear anatomical differences,3,5 it is likely that regional variations in sensitivity will exist. However, such investigations remain very problematic because of the difficulties associated with sampling these regions. Women have been shown to have higher and more variable conjunctival and corneal sensitivity measurements than men.29 This effect is thought to be primarily driven by hormonal changes manifesting during and after menopause. This is unlikely to have significantly influenced the results of our study, as the population included only three women and one man aged >50 years out of 27 subjects. The inclusion of both genders in our study sample size does not negate the value of the relationships demonstrated by our study.
In summary, our study confirms that there are clear clinical differences between the lower and upper lids and demonstrates, for the first time, significant relationships between tear osmolarity and lid characteristics, including lid sensitivity.
School of Optometry and Vision Science
The University of New South Wales
Sydney, New South Wales 2052
This work was presented at the American Academy of Optometry Annual Meeting; October 12–15, 2011; Boston, MA.
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eyelid margin; ocular surface sensitivity; vital staining; ocular symptoms; esthesiometer; meibomian gland dysfunction