dry eye dx and tx

The New Dry Eye, Part 2: Tear Osmolarity

BY KELLY K. NICHOLS, OD, MPH, PHD

In continuation of my August column, I will carry on the discussion of the Report of the International Dry Eye Workshop (DEWS) and the new definition of dry eye (Table 1), which published in The Ocular Surface in April. The evidence of progress documented in the report will continue to provide a blueprint for future research.

While my August column focused on the patient symptoms aspect of the new definition, in this column I'll discuss tear film osmolarity. Assessing tear film osmolarity has been touted as the gold standard diagnostic test as far back as the mid-1980s. Unfortunately, an instrument to accurately measure small-volume tear samples rapidly, accurately and reliably hasn't been available. So why is osmolarity part of the dry eye definition? Does the evidence support this addition?

New Instruments

The truth is that there isn't a commercially available instrument capable of measuring osmolarity of small tear samples either in the scientific laboratory or in clinical practice. Two companies, Advanced Instruments and Ocu-Sense, are developing a nanoliter tear osmometer. Each company is following a different approach to measuring osmolarity.

Advanced Instruments has produced many osmometers used in a variety of industries. The company is incorporating the freezing-point technology used in its larger-volume instruments in the small-volume instrument. For comparison, the tear osmometer uses approximately 0.3μl to 0.5μl of tears (the volume of tears on the ocular surface including the tear prism at any given time is approximately 9μl). The OcuSense instrument uses electric potential to assess osmolarity, and it also uses a similarly small sample. This instrument is intended to be very user-friendly and targets the clinical practice as the primary user. Both companies are expecting to have instruments available within the next year.

Old Issues

The promise of new instrumentation doesn't resolve the issue that we've had difficulty measuring osmolarity over the years because of limitations in the needed tools. Given this, some relatively small-sample instruments (Wescor Osmometer, Clifton Osmometer) have provided osmolarity data over the years. Khanal et al (2005) published a meta-analysis of this data that suggested a referent value of 316 mOsm/kg as a cut-point at which higher values would indicate ocular surface disease.

Osmolarity as a Global Indicator

Evidence of increased tear osmolarity in dry eye has led to the inclusion of tear osmolarity into the dry eye definition. Many clinicians and researchers support the hypothesis that increased osmolarity can occur as a function of the dry eye disease process and is involved in initiating symptoms of ocular irritation. What remains to be seen is whether the cut-point of 316 mOsm/kg will hold up once the newer instrumentation has been incorporated into clinical practice for the assessment of dry eye. CLS

To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #143.

Dr. Nichols is an associate professor at The Ohio State University College of Optometry in the area of dry eye research.