The goal of this study was to characterize ocular surface discomfort and tear film parameters in an older, male veteran population with normal external anatomy and no confounding factors, such as topical medication use and postsurgical status. Furthermore, a secondary goal was to evaluate whether symptoms were correlated with meibomian versus aqueous abnormalities. We found that a high proportion of men in our population complained of ocular surface symptoms, with almost 50% suffering from severe symptoms. This number suggests a higher burden of symptoms in our hospital-based population compared with previously reported population-based studies in Salisbury, Maryland (where 13% of men had ≥1 symptoms often or all the time)14
and Shihpai, Taiwan (where 30% of men had ≥1 symptoms often or all the time).11
Our numbers were more in line with that of Lu et al.,12
who found that 52% of 1031 Tibetans living in Zeku, China, reported one or more symptoms often or all the time.
Using cutoff definitions for various tear parameters, we also found many objective abnormalities in various tear function measurements, with most patients displaying a lipid tear deficiency or a mixed pattern. These findings have been supported by previous hospital and population-based studies11–14,23–26
Literature Review of Tear Film parameters and the Correlation between DES Signs and Symptoms
When considering the parameters in combination or individually, we could not find good correlation between measured signs and symptoms, despite the inclusion of newer tear film variables, such as osmolarity and meibomian gland parameters. Specifically, neither aqueous nor meibomian tear film parameters were significantly correlated with symptoms. This is supported by previous work that examined the relationship between Schirmer's score and symptoms,27,28
and by a recent article that showed that many patients with objective meibomian gland dysfunction were asymptomatic.29
This finding is problematic given that the main source of morbidity in DES is the symptomatology. DES symptoms, which include irritation, foreign body sensation, and pain, interfere with the ability to work and carry out daily functions.2–4
When treating patients with DES, it is important to understand which objective measures can best predict symptoms, as this can assist in monitoring response to treatment. Furthermore, to have a DES medication approved by the US Food and Drug Administration (FDA), the drug must show that it improves one symptom and one sign of DES over placebo (unpublished FDA mandate). The lack of correlation between currently measured signs and symptoms has limited the ability of innovators to test their products and has therefore hindered new DES therapeutic agents from entering the market.
There are several potential explanations as to why our measured parameters so poorly reflected symptomatology. The first possibility is that measuring tear film parameters at one time point may not be enough to get an overall sense of tear film function. This finding has been suggested by other researchers who also evaluated the presence of symptoms and signs of tear dysfunction on the same day.12,27
Another potential explanation is that our measurement techniques may not have been ideal. For example, with regard to tear osmolarity, studies have theorized that osmolarity levels in the central cornea reach much higher concentrations than those measured in the inferior tear meniscus, and that it is these concentrations that drive discomfort.30,31
Unfortunately, it is very difficult to measure osmolarity levels in the central cornea and there is no commercially available method in which to do so.
Finally, it is possible that there are unmeasured variables that more closely relate to the pathophysiology of ocular discomfort in the setting of tear dysfunction. Corneal nerve activity is one such potential variable. The current data on corneal sensitivity is confusing, however, with a few studies reporting that DES patients have lower sensitivities to mechanical, chemical, and thermal stimuli,32,33
and a few reporting higher mechanical sensitivity.34,35
These studies have limited patient numbers, which may explain the lack of conclusive data. The study of corneal nerve sensitivity is also limited by the lack of a commercially available measurement device.
As with all studies, this work has limitations that need to be considered when interpreting the study results. This study evaluated the symptoms and tear parameters in a population of older US veterans seeking eye care services and, as such, the findings may not be generalizable to other US-based male populations. Moreover, tear film parameters were measured at one time point and stability of measurements cannot be assessed in this study. Our measurements were also obtained using specific scales and techniques and our findings may have been altered if different measures were used. Based on our recruitment criteria, we do not have information on how many patients were screened but declined the invitation to participate in our study. Although this may have affected the frequency of tear film abnormalities in our population, it would not have affected our results on the correlation between symptoms and signs of disease. Furthermore, whereas we collected information on several risk factors associated with DES, we did not have information on others, including occupation and educational attainment.
With these limitations in mind, this study confirms that severe ocular surface symptoms are prevalent in an older, male veteran population. Measurement of standard tear film parameters could not explain the degree of measured symptoms. The study highlights the need for future research regarding the mechanisms behind ocular surface discomfort in patients with tear film disturbances. Only through a better understanding of these mechanisms will we be able to improve treatment outcomes in this chronic, debilitating disease.