Differences of tear film osmolarity between two time-points of the day in healthy subjects

Abstract

Purpose: Tear film hyperosmolarity is considered one the core mechanism of the dry eye along with the tear film stability. Many tear physiological variables oscillate during the day. This study was designed to assess the differences in tear film osmolarity between morning and afternoon in a group of healthy subjects.
 Materials and methods: A total of 25 healthy subjects who fulfilled the study’s inclusion criteria were enrolled for the study. Tear osmolarity was measured using the TearLab™ system in two separated sessions, at 9.30 am and 6.30 pm. A paired t-test and a Bland–Altman test were used to assess the differences between sessions.
 Results: Tear osmolarity (mean ± SD) was 309.96 ± 9.00 and 296.48 ± 12.98 mOsm/l at 9.30 am and 6.30 pm, respectively, being significantly lower at 6.30 pm than at 9.30 am (mean difference ± SD = 13.48 ± 8.69 mOsm/l; paired t-test; p < 0.001).
 Conclusions: Tear film osmolarity does appear to have some influence by the time of day in healthy patients.

Diurnal variations of tear film osmolarity on the ocular surface

“…There is no consensus in the time-point for the measurement session (from 6:00 am 37 to 10:00 pm), 33 or also the interval between them (from 10-minute intervals 34 to four/sixhour intervals) 49 (Table 1). In addition, some studies performed all the measurements in the same day, 35,39,40,42,43,45,47,48,51,52,56 while others followed the progression over two days, 37,46,50 or they repeated the protocol two consecutive days, 38,41 three nonconsecutive days with 2-to 5-day intervals, 49 or over a six-day period 36 (Table 1).…”

“…6 In the assessment of tear film osmolarity diurnal variation, the most common osmometer used was the TearLab (TearLab Corporation, San Diego, CA, USA), [58][59][60][61] an impedance electric osmometer specific for tear film osmolarity measurement (Table 1). [39][40][41][42][43][44][45]47,48,51,52,56 Nowadays, there are only two tear osmolarity assessment specific osmometers available on the market, the TearLab osmometer and the i-Pen osmometer, both based on the electric impedance principle. Electrical impedance measurement has been shown as an accurate and repeatable technique for dry eye diagnosis; 25,30,62,63 moreover, electrical impedance osmometers are more clinically accessible because do not take up as much space and do not require a large volume of tears on each measurement.…”

“…72 It has been established that healthy and pathological subjects show different tear osmolarity values. 6,[21][22][23][24]27,46,73 In diurnal osmolarity variations assessment, most of the studies were focused on healthy subjects (healthy or non-contact lens wearers [non-CLW]), [33][34][35][36][37][38]41,44,46,49,52,56 and less in subjects with altered tear film conditions or environment (pathological, contact lens wearers [CLW] or in biological stress situation subjects) (Table 1). 39,40,42,[45][46][47][48][50][51][52] Another important characteristic to consider when a study sample group is designed on any kind of tear film research study should be avoided to the gender-or age-related changes.…”

“…39,40,42,[45][46][47][48][50][51][52] Another important characteristic to consider when a study sample group is designed on any kind of tear film research study should be avoided to the gender-or age-related changes. 74,75 Whereas most of the studies are focused on subjects between 20 and 40 years old (young adults and adults), [33][34][35][36][37]40,42,44,46,47,49,50,56 not all of them made a point if the studied subjects were young, adults or older participants. 38,39,48,51 It has been reported that tear film function declines throughout life, and test results change with the advancing of age, especially those related to tear film stability (the other core mechanism in DED together with tear osmolarity), osmolarity and evaporation.…”

“…Some studies where healthy subjects were enrolled found show variation in the diurnal osmolarity pattern, where half of them were performed after 2005 or without an specific designed to tear film assessment osmometer (diurnal values between 296.48 ±12.9 and 318 ±31 mOsm, Table 2); 34,36,38,42,46,56 when differences between some measurement points were found, these could be in the middle of the day; 42 or between the beginning and the end of the day, with higher values in the morning than in the afternoon, 38 or even the opposite situation (Table 2). 34,38,46 Similar to healthy subjects, different values at some point in the day were reported in pathological or in biological stress situation subjects (diurnal values between 295.5 ±6.2 and 331 ±3.7mOsm, Table 3).…”