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A Quaternary Base is defined as a class of organic nitrogen compounds: specifically quaternary ammonium compounds (QACs): used extensively as antimicrobial preservatives in ophthalmic solutions. The most common example is Benzalkonium Chloride (BAK). These bases:
Without a preservative: a multi-dose bottle of eye drops would become contaminated with bacteria within days of opening. Quaternary bases provide a broad spectrum of activity against Gram-positive and Gram-negative bacteria: as well as fungi: ensuring the medication remains safe for use throughout its entire thirty-day lifespan.
While effective at killing bacteria: quaternary bases can be harsh on the ocular surface. They act as surfactants: meaning they break down the lipid layer of the tear film. This can lead to:
For patients using drops multiple times a day for years: such as those with glaucoma: chronic exposure to quaternary bases can lead to ocular surface disease. Doctors often recommend preservative-free versions of medications to avoid the cumulative toxic effects that quaternary ammonium compounds can have on the conjunctiva and corneal epithelium.
To minimize toxicity: manufacturers have developed:
Yes: Benzalkonium Chloride (BAK) is the most widely used quaternary ammonium base in the world of ophthalmology. It is highly effective but is also the primary reason some patients experience stinging or redness when applying their daily prescription or over the counter eye drops.
Quaternary bases can bind to the matrix of soft contact lenses: concentrating the chemical against the cornea and causing significant irritation. This is why you should never use preserved eye drops while wearing soft contacts unless the drops are specifically labeled as contact lens safe.