Preservatives a Double-Edged Sword

By Dr Sam Kain

The most common reason to use medicated eyedrops on a long-term basis around the world is Glaucoma. Glaucoma is the name we give to a group of eye conditions that affect the optic nerve and pose a significant, long-term risk to vision. The most common treatment for glaucoma is medication in the form of eyedrops. These eyedrops have transformed the way we care for people with glaucoma and made complete blindness from glaucoma a rarity in modern times. For decades now preservatives have been added to these medications in order to prevent bacterial contamination and extend the shelf life of the formulations. This in turn has made them safer and more convenient to use which has benefitted millions of people around the world. As is often the case though these preservatives can be a mixed blessing and in recent years more interest has focussed on the problems they can cause for some people.

There are several different preservatives that are used but the most common and the best studied is benzalkonium chloride (BAK). BAK is a very effective antiseptic and has been used in eyedrops for many years. Pilocarpine was the first effective glaucoma drop and has been used for around 100 years. The addition of BAK not only improved the shelf life of these early drops but also may have helped the medication to be absorbed better. Unfortunately, BAK is also a cause of eye irritation in some people. It can cause redness, soreness and in some people can incite an allergic response resulting in swelling and watering of their eyes and inflammation of their skin. As well as these effects on the surface of the eye there is also evidence that there may also be effects on some tissues inside the eye which in some people may work against the beneficial effects of the medication itself.

Laboratory studies have confirmed that at least some types of cells from deep inside the eye respond to exposure to BAK by activation of inflammation pathways. If other types of cells are also affected, particularly cells in the pressure regulating trabecular meshwork part of the eye, then there may be an overall negative effect on pressure control. This may be one of the reasons why sometimes drops which have been working well for years seem to stop working. In medicine this is known as tachyphylaxis and is a commonly encountered problem when managing glaucoma.

There are a number of well-established treatment pathways when someone appears to be affected by the preservative in their eyedrops. The first step may be to try a different drop with a different type of preservative. Another option is to try a preservative free drop. There are a number of drops on the market which do not have preservatives although these may be more expensive. In general, they are packaged in single dose vials to avoid contamination.

If drops can no longer be used, then laser surgery or incisional surgery become the most common treatment options.

In summary, preservatives in glaucoma eye drops can be a double-edged sword. While they protect against contamination, they can sometimes have adverse effects which begin to limit the therapeutic benefits of the medication itself. Ophthalmologists must therefore tailor treatment regimens to individual patients, balancing efficacy and ocular safety.

Dr Sam Kain is a Consultant Ophthalmologist with a special interest in glaucoma, cataract surgery and oculoplastics. He is a trustee of GNZ and a fellow of the Royal Australian and New Zealand College of Ophthalmologists (FRANZCO).

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