Dry eye signs and symptoms often inconsistent

Issue: November 2008

ByStephen M. Cohen, OD

Dry eye is commonly characterized as a self-perpetuating cycle of tear film instability and ocular surface desiccation. Biological or environmental factors may lead to destabilization of one or more of the tear film’s primary components — lipids, aqueous, mucins — or the thin glycocalyx layer. The result is a more-rapid-than-normal tear film break-up time (TFBUT); areas of the tear film may evaporate before the replenishing blink, leaving the surface of the eye unprotected.

Through prolonged contact with air and environmental particulate matter, ocular surface cells may become desiccated and die. The glycocalyx, which is a natural meshwork of mucins and other particles that is essentially the glue between the hydrophobic corneal epithelium and the tear film, cannot do its job if surface cells are damaged. Ultimately, the ability of the tear film to adhere to the corneal epithelium is compromised, increasing the rate of TFBUT and intensifying surface exposure.

Diagnostic, treatment challenge

Dry eye poses a diagnostic and therapeutic challenge. Numerous causes of dry eye coincide with numerous degrees of severity, variable clinical manifestations and particular tear film deficiencies.

Common dry eye symptoms include sensations of burning, stinging, general dryness, photophobia and grittiness, along with foreign body sensation and blurring of the vision. The severity of these symptoms can range from an irritating awareness, such as one might experience in the dry air in an airplane cabin, to a debilitating decrease in visual function and quality of life.

Clinical signs of dry eye are evaluated by the clinician with a battery of tests. The standard diagnostic regimen includes at least one measure of tear film stability, such as TFBUT, a determination of ocular surface desiccation, as can be observed via fluorescein or lissamine green staining and potentially a tear volume or flow reading, which can be estimated with Schirmer’s strips or fluorophotometry. The ability of the tear film to protect the ocular epithelium can also be assessed using a diagnostic ratio called the Ocular Protection Index (OPI).

The blinking action naturally replenishes the tear film, spreading it across the ocular surface. However, if the tear film breaks up before a blink occurs, some part of the eye will be exposed to desiccating elements — this is the principle of OPI.

Due to intermittent ocular surface exposure, patients in the earlier stages of the dry eye cycle may complain of dry eye symptoms without exhibiting substantial clinical signs. Clinical observations have suggested that ocular surface damage can lead to desensitization over time, and thus patients in the later stages of the dry eye cycle may exhibit severe staining and shorter TFBUT values without complaining of standard dry eye symptoms.

It is important for clinicians to understand the potential discrepancy between the signs and the symptoms reported by dry eye patients; many have either clinical signs or subjective symptoms, but not necessarily both. This is evident in dry eye patients’ reactions to the Controlled Adverse Environment (ORA Clinical Research and Development): While 63% experience worsening of signs and symptoms under such conditions, 19% have symptomatic responses only, and the other 18% only see changes in clinical signs, according to Casavant and colleagues.

It is, therefore, important that dry eye therapies be intended to control both signs and symptoms of dry eye.

Duration affects findings

The duration of dry eye may significantly affect what clinicians observe in their patients. Clinical findings in a study of 289 patients by Casavant and colleagues support this theory. The median duration of diagnosis was shown to be 3 years in symptom-only responders, 5 years in patients displaying both symptoms and signs and 6.5 years in those with clinical signs alone.

A determination of ocular surface desiccation, as observed here via fluorescein staining, is important when evaluating a patient’s dry eye.

Images: Cohen S

One would, therefore, expect patients who complain of moderate to severe dry eye symptoms but exhibit few or no clinical signs to be younger, with more epithelial neural sensitivity. This patient profile may include contact lens wearers or smokers, or possibly misdiagnosed ocular allergy sufferers.

On the other hand, those with significant clinical signs but few or no symptoms have often had dry eye a long time, and in theory their ocular surfaces are somewhat desensitized. Often this is more common in patients who have had chronic dry eye stemming from systemic disease or hormonal changes.

It is important to understand this because successful treatment for these sign-only dry eye patients restores health to the ocular surface, including neural sensation, and it may result in a temporary manifestation of symptoms. To effectively treat this population, be careful not to desert the treatment plan before considering that this phenomenon might indicate improvement.

To help slow or stop the dry eye cycle, it is important that an artificial tear not simply bulk up the aqueous layer of the tear film, but provide an opportunity for the ocular surface to self-repair. A tear substitute capable of approximating the glycocalyx could give the epithelium the chance to rebuild the microvilli that help generate and hold the natural glycocalyx. One that hinders the dry eye cycle by allowing the eye’s natural defenses to kick in is valuable for any type of dry eye patient.

For more information:

Stephen M. Cohen, OD, can be reached at 10900 N. Scottsdale Road, #301, Scottsdale, AZ 85254; (480) 513-3937; e-mail: stephen.cohen@doctormyeyes.net. He has served as principal investigator in numerous U.S. Food and Drug Administration studies on dry eyes and contact lens solutions.

Reference:

Casavant J, Ousler GW III, Wilcox Hagberg K, et al. A correlation between the signs and symptoms of dry eye and the duration of dry eye diagnosis. Invest Ophthalmol Vis Sci. 2005;46:E-abstract 4455.