Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

MIOTICS
Summary
Feline viral keratitis is a challenging condition to treat.
Intensive treatment in the early stages is more likely to
result in a clinical response. Some cases may require
surgical intervention.
DRUGS AFFECTING PUPIL SIZE
MYDRIATICS
Mydriatics dilate the pupil and can be used diagnostically
to allow fundus examination and therapeutically
to treat uveitis.
Relevant pathophysiology
The pupil can be dilated by activation of the sympathetic
dilator muscle by an adrenergic agonist or by
paralyzing the parasympathetic iris sphincter constrictor
muscle with a parasympatholytic drug.
Inflammation causes production of many inflammatory
mediators, including prostaglandin iridin. This
particular prostaglandin causes an intense pupillary
constriction. This miosis can cause adhesions of the iris
onto the lesion which can result in vision loss. Clinically,
it is important to dilate the pupil with a mydriatic.
Tropicamide
Tropicamide is a short-acting parasympatholytic drug.
Inhibition of the parasympathetic constrictor muscle in
the iris results in dilation of the pupil because the action
of sympathetic dilator muscles is unopposed.
Tropicamide is used at a 1% concentration to dilate
the pupil for diagnostic purposes. The pupil is usually
dilated within 15 min and remains dilated for 4–5 h.
Tropicamide is bound by pigment so in heavily pigmented
eyes, tropicamide takes longer to work and the
pupil stays dilated longer. Tropicamide is not used clinically
to treat miosis because of its short duration of
action.
Phenylephrine
Phenylephrine is a sympathomimetic drug that causes
pupillary dilation by activating the sympathetic dilator
iris muscle. It is usually administered with tropicamide
to maximally dilate the pupil. Maximal pupil dilation
may be useful when attempting to examine the fundus
when a cataract is present or during cataract removal.
It is administered 15–30 min before maximal mydriasis
is required.
Atropine
Atropine is a long-acting parasympatholytic mydriatic
used in management of uveitis-induced miosis. As it can
cause mydriasis for up to a week in a pigmented canine
eye, it is not usually used for diagnostic purposes. Melanocytes
take up excess atropine and then release it as
the intraocular concentration of atropine falls, prolonging
the duration of action of the drug in heavily
pigmented eyes.
Formulations and dose rates
One drop is administered to the affected eye q.6–8 h until the pupil
dilates. This usually takes 1–2 d and is dependent on how much
intraocular infl ammation is present. Once the pupil is dilated, atropine
is used as required to maintain dilation. If the eye is infl amed, atropine
may be required 2–3 times a day to maintain mydriasis. If infl ammation
is controlled, then atropine administration may only be required
once every second day or twice a week.
The degree of pupillary dilation maintained or achieved by 1 drop
of atropine can be used as a guide to how well infl ammation is controlled;
if the pupil stays dilated with 1 drop of atropine, uveitis is well
controlled. Rapid onset of miosis after atropine administration suggests
that infl ammation is not well controlled.
In refractory cases subconjunctival atropine can be administered
as an epibulbar injection. The dose is 0.05 mg (cats and small dogs)
or 0.1 mg (larger dogs). In these cases it is important to increase
anti-infl ammatory therapy by using subconjunctival depot cortisone
injections with the epibulbar atropine injection.
MIOTICS
Clinical applications
Miotics (drugs that cause the pupil to contract) improve
the outflow of aqueous as part of the treatment of glaucoma
and reduce the risk of a posteriorly luxated lens
entering the anterior chamber.
The importance of miotics in veterinary ophthalmology
has decreased substantially over the last few years.
They have been replaced in glaucoma management by
more effective drugs such as topical carbonic anhydrase
inhibitors and topical prostaglandin agonists. The use
of miotics in animals with loose or luxated lenses has
also reduced as it is now recommended that posteriorly
luxated or subluxated lenses should be removed
surgically.
Relevant physiology
Pupillary constriction (miosis) results from stimulating
the parasympathetic nerve which innervates the iris.
This can be achieved with direct-acting drugs (parasympathomimetic)
or indirect-acting drugs that block
hydrolysis of acetylcholine by cholinesterases.
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