Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 25 OCULAR CLINICAL PHARMACOLOGY
TOPICAL ANESTHETICS
Topical anesthetics provide local anesthesia and allow
minor procedures to be performed, e.g. eversion of the
third eyelid or removal of a superficial corneal foreign
body. Topical anesthetics may also facilitate eye examination
when there is considerable ocular pain. In such
cases it may also be necessary to use sedatives and/or
systemic analgesics to facilitate eye examination.
Topical anesthetics are toxic to corneal epithelium
and therefore can only be used once for diagnostic procedures.
If their use is continued, e.g. to relieve ocular
pain, corneal ulceration will develop. This is the result
of reduced blinking producing exposure of the cornea
and inhibition of the normal neurogenic function needed
for normal corneal health.
Formulations and dose rates
Apply 1 drop to the eye q.30 s for 3–4 applications. Corneal anesthesia
develops quickly but to maintain conjunctival anesthesia, repeated
applications are required. If the eye is severely infl amed or painful,
additional applications will be required.
Proparacaine
Proparacaine is the most commonly used topical ophthalmic
anesthetic. It causes roughening of the corneal
surface, which may make intraocular examination difficult.
It may be necessary to use artificial tears to
smooth the corneal surface to permit fundus examination
after its use.
Proparacaine should be refrigerated to delay degeneration
of the preparation, which results in brownish
discoloration. If this occurs the preparation should be
discarded.
Lidocaine (lignocaine)
Injectable preparations of lidocaine (lignocaine) can be
used topically to achieve local anesthesia. Compared
with other local anesthetics, they can be irritating when
first applied and may result in conjunctival hyperemia.
CORNEAL DEHYDRATING AGENTS
The normal cornea is kept relatively dehydrated by the
metabolic activity of corneal endothelial cells. Corneal
edema can develop for a variety of reasons. In these
cases corneal dehydrating agents (topical hyperosmotics)
can be used to reduce corneal edema for diagnostic
purposes to allow intraocular examination. Longerterm
therapy can also be used.
Topical glycerin can be used to temporarily clear the
cornea to allow intraocular examination. Because it can
be quite irritating, pretreatment with topical anesthesia
is recommended.
Sodium chloride (5%) has been used as either a drop
or an ointment to treat corneal edema and corneal
bullous keratopathy. It must be administered at least
four times daily.
DRY EYE THERAPY
Keratoconjunctivitis sicca is a common condition in
dogs but less common in cats. Diagnosis is made with
a Schirmer tear test strip. The normal Schirmer tear test
is 15 mm wetting/min or greater for the dog and 10 mm/
min or greater for the cat.
Clinically dry eye in dogs can manifest as corneal and
conjunctival disease and is characterized by a copious
mucopurulent discharge. The aim of dry eye therapy is
to improve the precorneal tear film.
Ciclosporin
In over 80% of early dry eye cases, tear production can
be restored to normal with ciclosporin. Ciclosporin is
primarily used in humans to prevent organ transplant
rejection. It is believed that dry eye in the dog may be
the result of an immune-mediated reaction against the
lacrimal glands. Ciclosporin may have efficacy in dry
eye because it reduces this reaction. It is also believed
that ciclosporin may have a direct lacrimogenic effect,
possibly via prolactin receptors.
Ciclosporin is considered the treatment of choice for
dry eye. It has also been used to treat pannus in German
shepherds, punctate keratitis, vascular keratitis and
other inflammatory corneal and conjunctival disease.
Usually, a minimum of 3 months’ treatment is necessary
to reduce corneal pigmentation and lipidosis in dogs.
Formulations and dose rates
A commercial preparation of 0.2% ciclosporin ointment (Optimmune®,
Schering-Plough) is available in many countries. If this
product is not available, systemic ciclosporin can be diluted to a 1%
or 2% concentration in white mineral oil.
Apply a small amount of ointment or 1 drop of solution to the affected
eye q.12 h for at least 3–4 weeks. In some cases maximal response is
not seen for up to 8 weeks. Initially, when the eye is dry, it is important
to also treat the eyes with ocular lubricants such as artifi cial tears and
protective ointments. To maximize effectiveness the eye should be
clear of all discharge before ciclosporin is applied.
If a good clinical response is seen, ciclosporin treatment can be
reduced to q.24 h or once every second day. Even if the Schirmer tear
test readings are not increased by ciclosporin therapy, clinical signs
are often relieved as ciclosporin inhibits the mediators that cause
clinical signs of dry eye.
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