Human basic tear fluid osmolality. I. Importance of sample collection strategy

Diurnal variations of tear film osmolarity on the ocular surface

The measurement of tear film osmolarity has been suggested as a gold standard in the diagnosis of dry eye. Many tear film physiological variables oscillate during the day. This review summarises current clinical knowledge regarding diurnal osmolarity variation in the tear film. A critical analysis is presented in respect of of sample size and characteristics, differences in the diurnal osmolarity variation on healthy versus altered tear film conditions or environment, and time of day and number of measurements undertaken. A comparison of 21 studies was made for studies in which one of the main objectives was to analyse the variance of tear film osmolarity at different time-points in a day on human cohorts. Tear film osmolarity appeard to be somewhat influenced by the time of day in healthy subjects and patients with ocular surface disease, or altered by environmental conditions. Both healthy and non-physiological tear film cohorts showed variations in results depending on the study: no variations during the day or statistically different values at some point in the day. These differences could be in the middle of the day or between the beginning and the end of the day, with higher values in the morning than in the afternoon, or even the opposite situation. The possibility of diurnal variations in tear film osmolarity should be considered by the clinician since the time of day when the tear film measurements are made can be critical in making the right diagnosis. Future studies in the diurnal variation field may have to use a well-established range of measurement time-points and a larger group of healthy subjects and and subjects who have a tear film altered by pathological or environmental conditions.

Evaluation of vascular endothelial growth factor (VEGF) level in the tears and serum of age-related macular degeneration patients

Age-related macular degeneration (AMD) is an important cause of irreversible central blindness worldwide. Clinical manifestations range from asymptomatic in early and intermediate AMD to significant vision loss in late AMD. Approximately 10% of cases of early AMD eventually progress to the late advanced stage, influenced by the upregulation of vascular endothelial growth factor (VEGF). In this study, we evaluated VEGF concentration in the tears and serum of AMD patients. Our study revealed a significantly higher level of VEGF in the tears of patients with AMD compared with controls. The tear VEGF level has high sensitivity and specificity, and is significantly related to the severity of AMD, whilst serum VEGF level is non-specific and non-predictive of AMD severity. Thus, VEGF level in the tears may be used as a non-invasive biomarker for AMD progression. A large cohort study is needed for further verification.

Lack of Agreement among Electrical Impedance and Freezing-Point Osmometers

PURPOSE

To assess the interchangeability of tear osmolarity measurements between electrical impedance and freezing-point depression osmometers and to analyze inter-eye tear osmolarity variability measured with these osmometers in healthy subjects.

METHODS

Tear osmolarity was measured using the TearLab osmometer (OcuSense Inc., San Diego, CA) and the Fiske 210 microsample osmometer (Advanced Instruments Inc., Norwood, MA). We randomly selected one eye in 50 subjects (29 women, 21 men; mean age, 33.16 ± 6.11 years) to analyze whether osmolarity measurements by these osmometers were interchangeable. Both eyes of 25 patients (15 women, 10 men; mean age, 34.32 ± 6.37 years) were included to analyze inter-eye osmolarity variability.

RESULTS

The mean tear osmolarity values measured with the TearLab osmometer were higher (305.22 ± 16.06 mOsm/L) than those with the Fiske 210 osmometer (293.40 ± 12.22 mOsm/L), with the intraclass correlation coefficient being 0.23 (p = 0.051). A Bland-Altman plot showed that the systems were not interchangeable because there was a systematic difference, with the limits of agreement being -17.93 to 41.57 mOsm/L. There were no statistically significant differences (p = 0.5006 and p = 0.6533, respectively) between an individual's eyes measured with either osmometer.

CONCLUSIONS

Because the TearLab tear osmolarity measurements were higher than those of the Fiske 210 measurements and the limits of agreement were too wide, the two osmolarity values cannot be used interchangeably. In healthy subjects, there is no difference in tear osmolarity between right and left eyes of the same individual measured with both instruments.

Evaluation of tearing in oculoplastics assisted by tear osmolarity measurement

PURPOSE

To determine the tear osmolarity in patients with tearing secondary to dry eye and other pathologies, and to determine the prevalence of dry eye disease among patients with tearing in an oculoplastics setting.

METHODS

108 eyes of 54 patients with a chief complaint of tearing were prospectively recruited. Subjects were excluded if they used eye drops or contact lenses within 2 hours of assessment, had a history of refractive surgery, an active ocular allergy, or evidence of a systemic disease which affects tear production. A full medical and ocular history was taken with a complete eye exam pertinent to dry eye. Tear osmolarity was measured using the TearLab device. A clinical diagnosis of dry eye was made based on findings, without reference to tear osmolarity.

RESULTS

Among 86 eyes symptomatic for tearing, 32 eyes had dry eye disease (37%). Patients with dry eye had a significantly higher median tear osmolarity compared to that in patients with other diagnoses (308 mOsm/L vs. 294 mOsm/L, p < 0.0001). At a cut-off of 308 mOsm/L, tear osmolarity resulted in a sensitivity of 50% and a specificity of 88% for the diagnosis of dry eye.

CONCLUSIONS

A significant proportion of patients with tearing in an oculoplastics practice had dry eye disease. The high specificity of tear osmolarity may render it a useful tool to rule in dry eye disease and may assist the oculoplastic surgeon in more accurately determining the cause of tearing.

Daytime variations of tear osmolarity and tear meniscus volume

OBJECTIVES

The aim of this study was to determine the pattern of variations in tear osmolarity and tear meniscus volume in patients with dry eyes and in healthy control subjects over an 8-hr daytime period.

METHODS

Ten normal subjects (5 men and 5 women with a mean age of 27±7 years) and 10 dry eye patients (4 men and 6 women with a mean age of 36±12 years) who had been diagnosed on the basis of having an ocular surface discomfort index >12 and a tear breakup time of <10 sec or Schirmer test score of <5 mm were included. The tear meniscus volumes of the participants were measured using ultrahigh resolution optical coherence tomography (OCT), and tear osmolarity was measured using the TearLab Osmolarity System. Both measurements protocols were conducted on the right eye of each participant every 2 hrs beginning at 8:30 AM and ending at 4:30 PM. The OCT imaging was performed first and was followed by osmolarity testing.

RESULTS

The mean tear osmolarity of the dry eye patients was 304.0±10.8 mOsm/L, and the mean tear osmolarity of the normal subjects was 298.0±14.2 mOsm/L (P>0.05). Over the course of 8 hrs, the average measured osmolarities of the dry eye group varied by approximately 21.9±13.5 mOsm/L (range, 6-43 mOsm/L), and the average measured tear osmolarities of the normal group varied by approximately 21.0±9.2 mOsm/L (range, 8-35 mOsm/L). At 2:30 PM, the average volume of the tear menisci in the dry eye group was significantly lower than that of the subjects in the normal group (P<0.05). No correlations between the tear meniscus volumes and tear osmolarities of either group were observed.

CONCLUSIONS

Variations in the tear osmolarities of individual dry eye patients and healthy normal control subjects were documented over the course of 8 daytime hours. No relationships between tear osmolarities and tear meniscus volumes were observed.

Increased tear osmolarity in patients with severe cases of conjunctivochalasis

PURPOSE

To evaluate and compare tear osmolarity in different grades of conjunctivochalasis.

PATIENTS AND METHODS

Forty subjects divided into four groups based on grading of conjunctivochalasis by lid-paralell conjunctival fold (LIPCOF) test and tear osmolarity evaluated by TearLab osmometer. Schirmer test, tear film break up time (BUT) and vital staining were also performed.

RESULTS

Patients with mild and moderate conjunctivochalasis showed similar tear osmolarity to normal subjects. A significant increase (P < 0.008) of osmolarity was found in patients with severe conjunctivochalasis. A significant increase of osmolarity was found in patients with abnormal BUT (P = 0.018) and in patients with positive lissamine-green staining (P = 0.023). No association could be elicited when results of osmolarity were compared to those of Schirmer test or when compared those of fluorescein staining to osmolarity, respectively.

CONCLUSION

Hyperosmolarity of tear was reported as a fact of ocular surface inflammation. While in mild and moderate conjunctivochalasis tear osmolarity was similar to normal controls, increased osmolarity can refer to the presence of ocular surface inflammation in these patients with severe conjunctivochalasis.

Tear osmolality and ferning patterns in postmenopausal women

PURPOSE

To compare tear osmolality and ferning patterns in postmenopausal women (PMW) with and without dry eye symptoms.

METHODS

Thirty-seven healthy PMW (>50 years of age), not on hormone replacement therapy, were categorized as being symptomatic or asymptomatic of dry eye based on their responses to an Allergan "Single-Item Score Dry Eye Questionnaire" (SIDEQ). They subsequently completed the Allergan "Ocular Surface Disease Index" (OSDI) questionnaire. Tear samples were collected from participants to evaluate osmolality and ferning patterns. A novel freezing point depression osmometer (Advanced Instruments Inc., Model 3100 Tear Osmometer), was used to measure the osmolality of the tear film. The tear ferning test was performed and evaluated for the quality of ferning based on the Rolando grading system.

RESULTS

SIDEQ responses revealed 21 symptomatic and 16 asymptomatic participants. The OSDI total score was 6.5 +/- 5.9 for the non-dry-eyed (NDE) group and 25.7 +/- 12.4 for the dry-eyed (DE) group. The subscores for the DE group were significantly greater than the NDE group (p < 0.001). Osmolality values in DE individuals were significantly different from NDE (328.1 +/- 20.8 vs. 315.1 +/- 11.3 mOsm/kg; p = 0.02). Fifty percent of the DE participants showed type II ferning patterns and 29% of the DE participants showed type III ferning patterns, whereas the NDE participants showed either type I (44%) or II (66%) ferning patterns. There was a significant difference between the DE and NDE participants for the ferning patterns (p = 0.019). There was no significant correlation between tear osmolality and tear ferning (DE: r = 0.12; p > 0.05, NDE: r = -0.17; p > 0.05).

CONCLUSIONS

Osmolality in mild and moderately DE PMW is higher than in NDE PMW and tear ferning is a rapid, simple, noninvasive laboratory procedure that indicates altered tear quality in PMW with symptoms of dry eye.

Prospective controlled study of vapor pressure tear osmolality and tear meniscus height in nasolacrimal duct obstruction

PURPOSE

To determine tear osmolality (TO) and tear meniscus height (TMH) in patients with functional (FNLDO) and primary acquired (PANDO) nasolacrimal duct obstruction. Additionally, to determine the effect of successful dacryocystorhinostomy surgery on these tear parameters.

DESIGN

Prospective case-controlled interventional case series.

METHODS

Up to 20 microl of basal tears were collected from 20 age- and gender-matched control subjects, 33 patients with FNLDO, 28 patients with PANDO, and 31 patients after successful dacryocystorhinostomy. TMH was measured with videoreflective dacryomeniscometry, and TO was measured with vapor pressure osmometry.

RESULTS

Thirty-nine percent of the patients with FNLDO (13/33) elected to undergo dacryocystorhinostomy surgery, compared with 64% of the patients with PANDO (18/28; P = .09). TMH was significantly higher in the disease groups (FNLDO median, 630 +/- 187 microm; PANDO median, 620 +/- 210 microm) compared with the control subjects (median 262 +/- 50 microm; P < .01). In those patients who underwent surgery, TMH was slightly higher in FNLDO than PANDO (P = .08), and TMH reduced significantly after surgery (P < .05). Mean TO in the control subjects was 313 +/- 17 mmol/kg, in FNLDO was 309 +/- 19 mmol/kg, and in PANDO was 315 +/- 24 mmol/kg (P > .05). TO was similar in those patients who progressed to surgery compared with the total group and was unaffected by surgery.

CONCLUSION

TO was similar in normal control subjects and in the watery eye group and was unchanged after dacryocystorhinostomy surgery. Despite the slightly higher TMH in patients with FNLDO, a higher proportion of patients PANDO elected to have surgery. Relative to FNLDO, patients with PANDO who progressed to surgery had lower TMH and normal TO, which suggests that a complete lacrimal drainage obstruction induces negative feedback that results in no reduction in TO.

Assessment of Tear Film Dynamics: Quantification Approach

The dynamics of the tear film are reviewed with specific reference to the biophysical aspects: distribution, turnover and elimination through evaporation, drainage, and absorption. The review concentrates on quantitative assessments and is confined to aspects of the dynamics that can be fully and directly measured. The techniques of fluorophotometry, fluorescein clearance, lacrimal scintigraphy, evaporimetry and osmometry are described. Reports in the literature for values of tear turnover (flow), evaporation and osmolarity for normal and dry eyes are collated. Indices of tear film dynamics based on these measurements, including tear function index, total tear flow, and osmolarity, are discussed in relation to their potential in the differential diagnosis of dry eye and new referent values for the disease suggested. The limitations of derivation and application of these indices are discussed.

Diagnosis of dry eye

Dry eve disease is characterized by symptoms, ocular surface damage, reduced tear film stability, and tear hyperosmolarity. There are also inflammatory components. These features can be identified by various kinds of diagnostic tests (symptom questionnaires, ocular surface staining, tear break-up time, and osmometry), although there may not be a direct correlation between the number or severity of symptoms and the degree of ocular surface damage or tear deficiency. Once the diagnosis of dry eye disease has been established, further tests can be used to classify the condition into tear-deficient or evaporative dry eve. The two forms of dry eye are not mutually exclusive and often co-exist. The optimal diagnosis of dry eye disease, therefore, depends on the results of several tests, and this article suggests an appropriate order for performing these tests at a single clinic visit.

Human basic tear fluid osmolality. II. Importance of processing strategy

Osmolalities of 100 human inferior tear prism fluid samples collected from two subjects and 100 standard solution samples (290 mOsm/Kg) were determined from their melting-point temperatures with the Clifton Nanoliter Osmometer. Accuracy and reliability comparisons were made for endpoints obtained using a single-sample simultaneous-recalibration (SS/SR) strategy, vs a lowest-of-six periodic-recalibration (LS/PR) strategy. Tear fluid osmolality values based on the LS/PR strategy were significantly lower (hypotonic) than for the SS/SR strategy (overall mean = 302.4 and 307.8 mOsm/Kg, respectively; p < 0.0012). The mean difference (5.4 mOsm/Kg) resulted from the combined effects of machine drift and reduced reliability of endpoint determination for tear fluid in comparison to that of a homogenous standard solution. LS/PR osmolality was linearly correlated to SS/SR osmolality with a slope of 0.433 and the two endpoint strategies resulted in equivalent values at 298.4 mOsm/Kg. Eighty-four percent (84%) of individual osmolality readings were greater than 298.4 mOsm/kg, and an 'averaging-type' effect caused LS/PR osmolalities to be increasingly less than SS/SR values above this value. These outcomes from two subjects were approximated by a parallel statistical model. We suggest that physiological representation of basic human tear fluid is more accurate using the single-sample simultaneous-recalibration strategy. Basic human tear prism fluid is more hypertonic and has a greater within-subject range in normal (non-dry eye) humans than previously realized.