by Noelle La Croix, DVM, Dip. ACVO
There are 3 major components in all eye drop medications: drug(s), solution (vector), and preservative(s). Eye ointments generally contain fewer preservatives than eye drops as their liquid-free (oil based) vectors greatly inhibit bacterial growth. Eye drops solutions have generally been designed to match the osmolarity and pH of the ocular surface, and to stabilize the drugs to be administered. From a veterinary standpoint, preservatives can be responsible for adverse reactions to ocular medications.
Eye Drops Preservatives
The most common preservative found in eye drops is the bactericidal/microcidal compound benzalkonium chloride (BAK). Benzalkonium chloride disrupts intermolecular interactions within lipid bilayers (cellular membranes) and inhibits numerous intracellular enzymes of bacteria. This compound is also active against some viruses, fungi, and protozoa. Gram-positive bacteria are more susceptible to the adverse affects of BAK than Gram-negative bacteria. Bacterial spores are thought to be resistant to BAK. Unfortunately, BAK is also the most common preservative irritant of eye drops.
Eye drops in single dose packaging do not contain preservatives. GenTeal® (hydroxypropyl methylcellulose) lubricant eye drops contain a preservative (GenAqua,™ sodium perborate) that immediately evaporates (hydrolyzes to hydrogen peroxide and borate) from the cornea during application. These products, and those with similar ‘disappearing preservatives,’ should decrease or eliminate most irritant responses.
Containers of eye drops should be discarded by 6 weeks after their first application. It is common for the tip of an eye drops dispenser to touch the eyelashes, conjunctiva, or cornea while medicating. Bacteria and other foreign matter can be inadvertently drawn back into the container. For this reason, eye drops are generally prescribed in small 2.5 or 5 ml bottles. To prevent cross-contamination eye drops should never be shared by patients. Eye drop preservatives do not generally eliminate bacterial contamination. In one human hospital study, 16% of communal eye drops (406 bottles) were contaminated with bacteria, and 5% were contaminated with severe pathogens.
Chronic eye drops preservative exposure can result in corneal toxicity and other changes of the cornea and conjunctiva. Eye drops preservatives tend to concentrate in the final drops of a bottle that can be particularly irritating. Shaking an eye drops bottle does not effectively eliminate this concentrating, as its solution will evaporate faster than its preservatives.
Eye Drops vs. Ointments
Most veterinary clients report that eye drops are easier to deliver to their pets, but some owners prefer ointments. Ointments release less medication than comparable eye drops, but the duration of drug release is increased. Ocular ointments remain in contact with the ocular surface for longer periods of time than soluble eye drops. Clinically, the resolution of corneal wounds is believed to be more adversely affected by ointments than drops. However, this has not been reproduced experimentally.
The choice of ointment or eye drops is often dependant on owner preference, costs, and/or commercial availability of a drug. However, some drugs are only prescribed in one form. For example, atropine is typically prescribed as an ointment because atropine eye drops can cause excessive drooling in both dogs and cats. About 85% of any eye drop will flow down a pet’s nasolacrimal duct. Atropine eye drops will therefore generate a bitter taste in an animal’s mouth. This can be somewhat prevented by holding the lower nasal lacrimal puncta closed during adminstration. However, most owners find this too difficult a task to perform on their pet. Owners should was their hands after medicating their pet’s eyes, especially in the case of atropine. Even trace amounts of atropine in the human eye can compromise the ability to read or drive.
Eye drops are always preferred over ointments in cases of corneal rupture. Ophthalmic ointments are toxic to exposed intraocular tissue. Ointments applied to corneal ruptures cause endothelial damage, corneal edema, vascularization, and/or scaring. If a corneal rupture is simply suspected, only eye drops are appropriate.
Many pet owners fear placing drops or ointments in their pet’s eyes. Simple instructions and demonstrations will relax owners and increase their compliance. Owners should stand behind their pet, secure its chin with one hand, and use their other hand to medicate the pet’s eyes from above (Figure 1). With multiple ocular medications, owners should be instructed to wait five minutes between individual drug applications. This will prevent one medication from ‘washing out’ another. Drops should always be applied before ointments. Afterwards, any discharge from the eye can be removed with a warm damp cloth.
I hope this brief review helps explain some of the factors involved in dispensing ocular medications. I would also suggest consultation with a veterinary ophthalmologist about specific eye medications (their applications and possible complications) as needed.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: An eye drop is properly administered to “Paul Newman.”