She immediately emailed Bicket informing the doctor that she would stop the medication and just take the others. Perhaps it was this drug that caused the photophobia, the dry eyes, and now the burning sensation.
“I am fine with any short-term IOP dropping experiment you choose to conduct,” Bicket wrote back. But the symptoms the patient was experiencing did not match the usual side effect profile of the medications she was taking. There is another possibility, added Bicket: Maybe it’s not a single drop, but all of them. They all contain a preservative called benzalkonium chloride (BAK). “If you don’t tolerate this,” Bicket wrote, “stopping one agent against another won’t help.”
The patient decided to stop them all, she wrote to Bicket. It was a risky move because the drops were important to keep pressure down and avoid further damage. But the pain and sensitivity to light were unbearable.
The gap between test and reality
The patient had her answer three days later. Her eyes felt so much better without the drops. The gloomy feeling when she blinked was gone. Likewise the photophobia. It had to be the BAK. The patient turned to PubMed for information. There was a lot. Preservatives were essential in preventing bacteria from growing in medicine bottles that contained more than a single dose, and BAK was the most commonly used preservative in both over-the-counter and prescription eye drops.
She found that the patient’s complaints were not due to an allergy to the preservative, but to the way BAK worked. This compound kills germs by dissolving the lipid layer that forms their outer protective covering. Here’s the problem: the eyes are kept from drying out by a similar protective coat – from tears. Tears consist of a thin layer of fluid from the lacrimal gland (lacrimal gland), which in turn is covered by a layer of oil formed by the meibomian glands. BAK breaks down this outer protective lipid layer and exposes the salty liquid to the air. For many people with dry eyes, the unprotected fluid evaporates and the patient’s eyes become even drier. Eye drop users who produce enough tears are not affected, but many are not. Aging also reduces this protective layer, which puts older users of BAC-containing drugs at greater risk of eye drying. Eventually, the dryness can 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 do not need any preservatives at all. With this change, her eyes began to heal. It’s been five years and she still can’t see well with her left eye, and she now has glaucoma in her right eye too. But she has figured out how to work with her vision and her glaucoma is well under control.
Bicket, now at the University of Michigan, was intrigued by the difference between real-world visual acuity and the patient’s own eyesight. Research she and her colleagues recently published shows that this can lag behind the visual acuity tested by weeks or sometimes months. The first question anyone undergoing eye surgery will ask themselves, Bicket told me, is how long it will take them to recover enough to go back to work, read, or drive. “The simple answer,” she says, “is, we just don’t know.” But Billet is working hard to find out.
