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dry
eye dx and tx
Diagnosing
Dry Eye: 100+ Years Post-Schirmer
BY
KELLY K. NICHOLS, OD, MPH, PHD
If you could design a new diagnostic
test for dry eye, what characteristics would it have? Some of our existing tests
have shown the test of time (Schirmer, circa 1903) while others were ahead of
their time (Touch tear microassay).
Ideally, a new test would be
sensitive enough to detect small yet meaningful differences between dry eye and
non-dry eye patients. It would be valid and repeatable and wouldn't result in false
positives or incorrect diagnoses. Of course, the test would be inexpensive, foolproof,
fast and billable to any insurance plan. Sounds perfect, right? How close are we
to this ideal test?
Gold Standard Diagnostic
Tests
No single clinical test
is used to diagnose dry eye. In general, dry eye is a gestalt diagnosis based largely
on patient-reported symptoms. Clinicians often perform a battery of diagnostic tests
as part of an ocular surface disease work-up, including symptom assessment, measurement
of aqueous production (Schirmer or phenol red thread test), assessment of corneal/conjunctival
integrity (fluorescein and lissamine vital dye staining) and evaluation of tear
stability (tear breakup time [TBUT]).
How do we interpret the results?
Most clinicians weigh the burden of evidence: More abnormal tests indicate a greater
likelihood of disease or more severe disease. I believe clinicians give symptoms
more weight when clinical tests are normal or borderline (no staining, borderline
TBUT, normal Schirmer). If the tests are obviously abnormal, treatment is easier
to monitor.
Screening Tests
Clinicians perform tests
to screen for and diagnose disease. Any new test is compared to truth — a new
test to diagnose cancer is evaluated on patients known to have cancer by biopsy.
Ideally, you want the screening test (faster, easier, less invasive) to reproduce
the result of the more difficult and costly gold standard test.
A screening test with high sensitivity
can correctly identify those with known disease. High specificity results if a test
correctly classifies patients who don't have the disease. Sensitivity and
specificity are trade-offs — increasing sensitivity by selecting a different
abnormal cutpoint results in reduced specificity. With most dry eye tests, specificity
is higher than sensitivity for the cutpoints generally reported in the literature.
In other words, most dry eye tests are better at ruling out dry eye.
Are the Tests Worth the
Effort?
So why do we perform these
tests? Patient-reported symptoms tell us what's wrong, in the patient's own words.
Improving or removing symptoms is important to both patients and clinicians.
Corneal and conjunctival staining
is an indicator of overall ocular surface health — so if the ocular surface
is damaged, the proverbial Pandora's Box of possible corneal infections is open.
While the Schirmer test can be wildly
unrepeatable, it is so only for those who have mild or no dry eye disease. It's
highly sensitive and repeatable for diagnosing aqueous deficient dry eye.
What Will the Future Bring?
Ultimately, research will
identify the underlying biochemical and physiological etiology of specific symptoms,
resulting in clinical screening tests that are microassay-based. We'll develop foolproof
methodology of collecting tiny volume tear samples and the technology to easily
evaluate critical biochemical tear biomarkers in the office.
In the future we'll need to keep
up with emerging technology related to ocular surface disease as well as other ocular
conditions. Are you ready?
Dr. Nichols is
an associate professor at The Ohio State University College of Optometry in the
area of dry eye research.
Contact Lens Spectrum, Issue: February 2007