Tear osmolarity in the assessment of response to therapy in dry eye

A substantial proportion of normal, healthy individuals also experience DED symptoms under special circumstances. Some work-related environments that involve constant use of the computer, alone or in combination with exposure to air-conditioning are among the most common. With the increase in popularity of devices such as smartphones and tablets, symptoms of dry eye have been reported to increase in new segments of the population.

Currently, the diagnosis, treatment and follow-up of DED is based on the presence of classical symptoms reported by the patient along with the presence of clinical signs, such as punctate keratopathy, a short tear break-up time, reduced tear production (Schirmer test), or ocular redness. In most cases a combination of the aforementioned warrants the diagnosis of DED and initiation of therapy. However, in general, no ancillary tests are routinely used to corroborate or guide the diagnosis of dry eye disease.

In recent years, in-office measurement of tear osmolarity in patients with symptoms of DED has been proposed as an alternative method of diagnosing and following this condition. Tear osmolarity levels have been demonstrated to increase in patients with dry eye disease, due in part to an increased concentration of solutes in a reduced tear volume. High tear osmolarity levels are known to be associated with inflammation on the ocular surface and are considered to be one of the signature features of the disease.¹

In this regard, some groups have studied the performance of the TearLab Osmolarity System (TearLab Corp., San Diego, California, USA), an osmometer with the capability to perform quick, in-office readings of patients' tear osmolarity. Some of these studies report that the TearLab Osmolarity System can identify patients with DED, and a tear osmolarity level above 308–315 mOsm/L has been proposed for the most sensitive detection of patients with DED.³ One of these studies suggested a reduction in tear osmolarity levels in response to therapy.⁴

However, tear osmolarity on the ocular surface is dynamic and can be rapidly affected by many factors, which adds an important source of variability to this test. Amongst the most common factors that can potentially alter tear osmolarity are tear evaporation and dilution (whether through the tear reflex or topical drops). Not surprisingly, there exists a certain degree of variability in the levels of tear osmolarity measured with the TearLab Osmolarity System. Some publications have reported significant differences in tear osmolarity between fellow eyes as well as between consecutive samples from the same patient.^5,6

The clinical signs and symptoms of DED are always in flux, and this constant change is a hallmark of the disease. In fact, to date, it has been nearly impossible to establish strong correlations between some of the various signs and symptoms of DED, as the signs and symptoms that are most evident at one visit may change at the next. For this reason, a comprehensive evaluation of all the clinical signs and symptoms and current treatments are of the utmost importance in the diagnosis, follow-up and treatment strategy of patients with dry eye disease.