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Artificial Tears: the Electrolyte Factor
Drops have differences besides preservatives that can affect your patients' ocular health.
By Michael Goldstein, M.D.

Dry eye is one of the most common conditions encountered by ophthalmologists. The literature suggests that about 14% of adults older than 40 have dry eye, and the prevalence among patients seen in a practice is certainly higher, given that the typical patient is older.

One of the mainstays of dry eye treatment is tear replacement using topical lubricants. However, the marketplace is crowded with tear replacement products. So, the question arises: Aside from viscosity and the presence or absence of a preservative, are all artificial tears similar? And if not, what are the differences?

Here, I'll discuss one difference that appears to have significant bearing on ocular health: the amount and ratio of electrolytes in artificial tears.

The ocular surface epithelium is unique in that it doesn't have a blood supply. It derives its electrolytes and oxygen from the tear film. The tear film, in other words, is a vital fluid.

In every system in the body that depends on a vital fluid, the electrolyte balance of that fluid is crucial for biological function. If serum electrolytes are out of balance, for example, the consequences can be deadly, so the electrolyte balance in Lactated Ringers (used as a serum substitute) must match that of real serum. Similarly, the electrolytes in balanced salt solution must match those of the aqueous humor.

The same reasoning applies to the tear film: The electrolytes in tear substitutes need to match those of the tear film.

Gilbard and co-workers found that unless an eye drop had an electrolyte balance that precisely matched that of the human tear film, there was a loss of conjunctival goblet cells. (Conjunctival goblet-cell density is a very sensitive indicator of ocular surface health.)

Published studies have shown that electrolytes such as potassium are critical to the maintenance of normal conjunctival goblet-cell density. Other important electrolytes in the tear film include: sodium, bicarbonate, calcium, magnesium, and bicarbonate. (The concentrations of important electrolytes found in the tears of normal, healthy patients are shown on the next page.)

Imbalance and Toxicity

Conversely, an imbalance of electrolytes in the tear film is associated with what can be called "electrolyte imbalance toxicity." For example, toxicity caused by preservative-free sodium chloride solution has been demonstrated in rabbit eyes and in tissue culture.

It's worth noting that the toxicity caused by an electrolyte imbalance causes the same changes that are seen in dry-eye disease: increased corneal epithelial cell desquamation and loss of conjunctival goblet-cell density. The result is that the signs and symptoms of electrolyte imbalance toxicity are indistinguishable from the signs and symptoms of dry-eye disease.

The good news is that using better-balanced artificial tears helps to restore the ocular surface. In the early 1980s, O'Leary and Bachman found they could decrease the corneal desquamation caused by preservative-free sodium chloride by adding certain electrolytes to the solution.

Electrolytes as a Buzzword

Given the importance of electrolytes to the health of the ocular surface, it's not surprising that companies now compete by touting the electrolyte composition of their tear replacement products. But all artificial tears currently available on the market are different, especially in terms of their electrolyte composition.

Some have little or no electrolyte content, and even those companies that make a serious attempt to duplicate the electrolyte balance of healthy tears don't list the concentrations on the package.

My Choice

Because peer-reviewed studies and other clinical studies have shown TheraTears (Advanced Vision Research) to match the electrolyte balance of the human tear film and to increase conjunctival goblet-cell density, I recommend them to my patients.

I've also recently begun to recommend the new TheraTears Liquid Gel for my patients to use at nighttime. Electrolyte balance is even more crucial with gels because gels stay on the eye longer than artificial tears.

If your patients aren't getting satisfactory results from their artificial tears, electrolyte imbalance could be part of the problem.

Dr. Goldstein is director of ophthalmology residency training and assistant professor of ophthalmology at the New England Eye Center at Tufts University School of Medicine in Boston. He has no financial interest in TheraTears.