She immediately sent an email to Bicket, telling the doctor that she was going to stop that medication and just use the others. Maybe it was this medicine that was causing the photophobia, the eye dryness and now the burning.
“I’m fine with any short-term IOP-drop experiment you’d like to run,” Bicket wrote back. But the symptoms the patient was having didn’t match the usual side-effect profile of any of the medications she was using. There was another possibility, Bicket added: Maybe it’s not any single drop, but all of them. They all contain a preservative called benzalkonium chloride (BAK). “If that is what you don’t tolerate,” Bicket wrote, “stopping one agent vs. another won’t help.”
The patient decided to stop them all, she wrote to Bicket. It was a risky action, because the drops were important for keeping her pressure down and avoiding further damage. But the pain and light sensitivity were unbearable.
The Lag Between Test and Reality
Three days later, the patient had her answer. Her eyes felt so much better without the drops. The gritty feeling when she blinked was gone. So was the photophobia. It had to be the BAK. The patient turned to PubMed to read up on it. There was a lot there. Preservatives were essential to prevent the growth of bacteria in bottles of medications that contained more than a single dose, and BAK was the most commonly used preservative in both over-the-counter and prescription eye drops.
The patient’s discomfort, she discovered, was not due to an allergy to the preservative but rather was a response to the way BAK works. This compound kills germs by dissolving the layer of lipids that make up their outer protective coat. Here’s the problem: Eyes are kept from drying out by a similar protective coat — of tears. Tears are composed of a thin sheet of liquid from the lacrimal (tear) gland, which is in turn covered by a layer of oil made by the meibomian glands. BAK breaks down this outer protective lipid layer, exposing the salty fluid to the air. In many of those who have dry eyes, the unprotected fluid evaporates, and the patient’s eyes become even dryer. Eye-drop users who make enough tears won’t be affected, but many do not. Aging will also reduce this protective layer, putting older users of medications containing BAK at higher risk of drying eyes. The dryness can eventually lead to permanent damage to the cornea, the clear outermost layer of the eye.
The patient immediately switched to single-dose bottles of the drops; these don’t need any preservatives at all. With that change, her eyes began to heal. It has been five years, and she still can’t see well out of the left eye, and she now has glaucoma in the right one as well. But she has figured out how to work with the vision she has, and her glaucoma is well controlled.
Bicket, who is now at the University of Michigan, has been fascinated by the difference between the visual acuity measured in the office and patients’ own sense of how well they can see. Research she and colleagues published recently shows that this can lag behind the tested acuity by weeks or sometimes months. The first question anyone facing eye surgery will ask, Bicket told me, is how long will it take for them to recover enough to go back to work, or to read or drive. “The simple answer,” she says, “is we just don’t know.” But Bicket is working hard to find out.