The eye drops were supposed to help his vision. Why did he feel worse?
She immediately emailed Bicket, telling the doctor that she was going to stop this drug and just use the others. Perhaps it was this drug that was causing the photophobia, dry eye and now the burn.
âI’m okay with any short-term IOP drop experiment you want to run,â Bicket replied. But the patient’s symptoms did not match the usual side effect profile of the drugs she was using. There was another possibility, Bicket added: It might not be just one drop, but all of them. They all contain a preservative called benzalkonium chloride (BAK). “Yes this what you don’t tolerate, “Bicket wrote,” arresting one agent against another won’t help.
The patient decided to stop them all, she wrote to Bicket. It was a risky action, as the drops were important in maintaining his pressure and preventing further damage. But the pain and sensitivity to light were unbearable.
The gap between the 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. Photophobia too. It must have been the BAK. The patient turned to PubMed for more information. There were a lot of them there. Preservatives were essential in preventing the growth of bacteria in drug vials containing more than a single dose, and BAK was the most commonly used preservative in over-the-counter and prescription eye drops.
The patient’s discomfort, she found, was not due to an allergy to the preservative, but rather a response to the mode of action of BAK. This compound kills germs by dissolving the layer of lipids that make up their outer protective layer. Here’s the problem: the eyes are kept from drying out by a similar protective layer – of tears. Tears are made up of a thin layer of fluid from the lacrimal gland (tear), which in turn is covered with a layer of oil made by the meibomian glands. BAK breaks down this outer protective lipid layer, exposing the salty fluid to air. In many who have dry eyes, the unprotected liquid evaporates and the patient’s eyes become even drier. Eye drop users who shed enough tears will not be affected, but many are not. Aging will also reduce this protective layer, putting older users of medicines containing BAK at a higher risk of dry eyes. Dryness can eventually lead to permanent damage to the cornea, the clear outer layer of the eye.
The patient immediately switched to single-dose bottles of drops; these do not need preservatives at all. With this change, her eyes began to heal. It’s been five years, and she still can’t see well in her left eye, and she now also has glaucoma in her right eye. But she has figured out how to work with her vision and her glaucoma is well controlled.
Bicket, who is now at the University of Michigan, was fascinated by the difference between visual acuity measured in the office and patients’ perception of their ability to see. Recent research that she and her colleagues published shows that it can take weeks or even months compared to the acuity tested. The first question anyone facing eye surgery will ask, Bicket told me, is how long will it take them to recover enough to return to work, read, or drive. âThe simple answer,â she says, âis that we just don’t know. But Bicket is working hard to find out.