C H A P T E R 3 2
Cholinergic agonists
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Therapeutic actions and indications
The direct-acting cholinergic agonists act at choliner-
gic receptors in the peripheral nervous system to mimic
the effects of ACh and parasympathetic stimulation.
These parasympathetic effects include slowed heart rate
and decreased myocardial contractility, vasodilation,
bronchoconstriction and increased bronchial mucus
secretion, increased GI activity and secretions, increased
bladder tone, relaxation of GI and bladder sphincters,
and pupil constriction (see Figure 32.1).
The agent bethanechol, which has an affinity for
the cholinergic receptors in the urinary bladder, is
available for use orally and subcutaneously to treat non-
obstructive postoperative and postpartum urinary reten-
tion and to treat neurogenic bladder atony. It directly
increases detrusor muscle tone and relaxes the sphinc-
ters to improve bladder emptying. Because this drug is
not destroyed by acetylcholinesterase, the effects on the
receptor site are longer lasting than with stimulation by
ACh. See Table 32.1 for additional indications.
BOX 32.1
Drug therapy across the lifespan
Cholinergic agonists
CHILDREN
Children may be more susceptible to the adverse effects
associated with the cholinergic agonists, including
gastrointestinal (GI) upset, diarrhoea, increased salivation
that could lead to choking, and loss of bowel and bladder
control, a problem that could cause stress in the child.
Children should be monitored closely if these agents are
used and should receive appropriate supportive care.
Bethanechol is approved for the treatment of
neurogenic bladder in children older than 8 years of age.
Neostigmine and pyridostigmine are used in the control
of myasthenia gravis and for reversal of neuromuscular
junction blocker effects in children. Care should be taken
in determining the appropriate dose based on weight.
Edrophonium is used for diagnosis of myasthenia gravis
only.
ADULTS
Adults should be cautioned about the many adverse
effects that can be anticipated when using a cholinergic
agonist. Flushing, increased sweating, increased salivation
and GI upset, and urinary urgency often occur.The person
also needs to be aware that dizziness, drowsiness and
blurred vision may occur, and that driving and operating
dangerous machinery should be avoided.
PREGNANCY AND BREASTFEEDING
In general, there are no adequate studies about the effects
of these drugs during pregnancy and breastfeeding.
Therefore, the cholinergic agonist should be used only
in those situations in which the benefit to the mother is
greater than the risk to the fetus or neonate. Breastfeeding
women who require one of these drugs should find
another way to feed the baby.
OLDER ADULTS
Older people are more likely to experience the adverse
effects associated with these drugs—central nervous
system, cardiovascular, GI, respiratory and urinary
effects. Because older people often have renal or hepatic
impairment, they are also more likely to have toxic
levels of the drug related to changes in metabolism and
excretion.
The older person should be started on lower doses
of the drugs and should be monitored very closely for
potentially serious arrhythmias or hypotension. Safety
precautions should be established if the drug causes
dizziness or drowsiness. Special efforts may also be
needed to help the person maintain fluid intake and
nutrition if the GI effects become uncomfortable.Taking the
drug with food and eating several small meals throughout
the day may alleviate some of these problems.
TABLE 32.1
DRUGS IN FOCUS Direct-acting cholinergic agonists
Drug name
Dosage/route
Usual indications
bethanechol (Urocarb)
10–30 mg PO or SL t.d.s. or q.i.d.
Treatment of non-obstructive
postoperative and postpartum urinary
retention, neurogenic bladder atony in
adults and children >8 years; diagnosis
and treatment of reflux oesophagitis
in adults, and used orally in infants and
children for treatment of oesophageal
reflux
carbachol (Isopto
Carbachol, Miostat)
1–2 drops (gtt) in affected eye(s) as needed,
up to three times a day
Induction of miosis to relieve increased
intraocular pressure of glaucoma; allows
surgeons to perform certain surgical
procedures
pilocarpine (Isopto
Carpine)
1–2 gtt in affected eye(s) as needed, up to six
times per day
Induction of miosis to relieve increased
intraocular pressure of glaucoma; allows
surgeons to perform certain surgical
procedures
