Tear osmolarity correlates with dry eye signs and symptoms

Changes in tear osmolarity may precede clinical findings of dry eye, and patients with symptomatic dry eye that has not reached clinical significance seem to have higher and more variable osmolarity measurements, according a study published in the November issue of Cornea. As a result, researchers have recommended that osmolarity measurements be included in every dry eye evaluation.

“We dry eye clinicians have been looking for ways to diagnose dry eye at an earlier stage than we are able to now. Currently, the diagnosis is made mostly based on the clinical signs such as decreased tear film break up time or decreased Schirmer levels or ocular surface staining, which are all late findings,” said senior author Esen K. Akpek, MD, The Bendann Family Professor of Ophthalmology and Rheumatology Director, Ocular Surface Disease and Dry Eye Clinic associate director, Johns Hopkins Jerome L. Greene Sjögren's Syndrome Center Director, Cornea and External Disease Fellowship, The Wilmer Eye Institute, Johns Hopkins University, Baltimore, MD.

“I have the clinical impression that if dry eye is diagnosed and hence treated early, it can be cured, meaning before permanent damage occurs to the tear secreting tissues, if the dryness is treated, the tear homeostasis can be re-instituted. Therefore early diagnosis might be relevant. In clinic, a lot of patients complain of eye discomfort but we cannot see any tear film of ocular surface abnormality. I think that it is important to diagnose those patients timely so that they can treated appropriately. Based on previous publications and my clinical experience, I thought that perhaps high osmolarity could be a finding of dry eye. So we designed the study,” he explained.

Dr. Akpek and colleagues conducted this study to assess the distribution of tear film osmolarity in patients with dry eye and any associations it may have with other ocular surface parameters.

They included 131 patients with clinically significant symptoms and staining for dry eye, 52 patients with symptoms-only dry eye, and 42 controls with no significant symptoms or staining, and found that tear osmolarity was significantly different across all groups (P=0.01).

Patients with clinically significant dry eye demonstrated the highest tear osmolarity (312.0 mOsm/L) and control patients the lowest (305.6 mOsm/L). Tear osmolarity measurements for patients with symptoms-only dry eye fell somewhere in between (307.4 mOsm/L).

These researchers also found that the patients with clinically significant dry eye had a tendency to have a greater inter-eye osmolarity difference compared with those with symptoms-only dry eye and controls (12.0 vs 9.1 vs 9.0, respectively; P=0.06).

Upon multivariable regression modelling, they found an association between higher tear osmolarity and higher Ocular Surface Disease Index (OSDI) discomfort subscore (P=0.02) and higher corneal and conjunctival staining scores (P < 0.01 for both). Finally, they observed no correlation between tear osmolarity in the worse eye and corresponding tear film break-up time (TFBUT) or Schirmer test (P > 0.05 for both).

“Somehow, patients with dry eye are unable to regulate the composition of their tears. [It] could be due to too much evaporation, for example due to poor oil layer (meibomian gland dysfunction) or inflammation of the lacrimal gland (Sjögren’s syndrome),” said Dr. Akpek.

In the future, measuring osmolarity may become a way to better and more quickly diagnose dry eye, according to Dr. Akpek, who offered some practical tips for clinicians.

“Multiple measurements should be taken (like blood sugar or urine test) and each time both eyes should be normal. If not, this probably means dry eye. One normal value does not rule out dry eye. And again, absence of staining or reduced Schirmer levels but high osmolarity indicates early stage dry eye. Those patients should be treated carefully to make sure the disease does not become a progressive inflammatory condition,” he said.

This study was supported in part by a research grant from Allergan, Inc. and Jerome L. Greene Sjögren's Center, Johns Hopkins University, and an unrestricted grant to the Flaum Eye Institute at the University of Rochester from Research to Prevent Blindness.
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