Benzalkonium chloride is
known to have significant toxic effects on the corneal
epithelium even when compared with other preserva-
tives. Such effects could be demonstrated for the corneal
epithelium of the rabbit in numerous scanning electron
microscopic studies. Even in concentrations
of 0.001% to 0.1% benzalkonium chloride leads to loss
of microvilli, disruption of intercellular connections and
finally even to complete desquamation of the superficial
cell layers.
Benzalkonium chloride affects the diffusion barrier
of the corneal epithelium in two ways: (1) benzalkonium
chloride leads to disruption of the zonulae occludentes,
which seal off the superficial epithelial cells, thereby al-
lowing aqueous substances into the intercellular space
of the epithelium; (2) the benzalkonium chloride mole-
cules are incorporated into the cellular membranes of
the epithelial cells by their lipophilic chains, thus provid-
ing gates for ionic, aqueous substances to penetrate
through the lipophilic membranes into the intraceUular
space. An increase in corneal epithe-
lial permeability due to topically applied benzalkonium
chloride has been demonstrated both in vitro and in
vivo in rabbits, as well as in human subjects.
The benzalkonium chloride molecules are bound onto
the corneal surface immediately after instillation, wher-
eby the preservative escapes rapid washout by the tear
film. Even 9 days after instillation of a single drop
containing 0.01% benzalkonium chloride residues of the
preservative have been detected in the rabbit epithelium
by a radiocarbon technique. The half-time life of
benzalkonium chloride in the corneal epithelium of the
rabbit is about 20 h, so application several times
daily leads to an accumulation of the preservative. Al-
though preservatives seem to damage the corneal epithelium
of rabbits more than human beings,
it is conceiv-
able that when benzalkonium chloride is used as a pre-
servative in artificial tears and is applied several times
per day over long periods, it is accumulated in the corne-
al epithelium, thus leading to further destabilization of
the compromised dry corneal surface. In addition, as
benzalkonium chloride is a detergent, it emulsifies the
lipid layer of the tear film, further compromising the
already insufficient tear film in dry eyes.
The noxious effect of benzalkonium chloride as a pre-
servative in artificial tears was demonstrated both objec-
tively and quantitatively in the present clinical study.
In contrast to the other two tear substitutes tested in
this study, artificial tears preserved with benzalkonium
chloride failed to improve the disturbed corneal epitheli-
al barrier function in dry eyes.
Preservation of artificial
tears with benzalkonium chloride should therefore be
abandoned.