C H A P T E R 2 5
Muscle relaxants
389
be anticipated with their use. See Table 25.1 for usual
indications for each of these agents.
Pharmacokinetics
Baclofen is available in oral and intrathecal forms and
can be administered via a delivery pump for the treat-
ment of central spasticity. Baclofen is not metabolised
but, like the other skeletal muscle relaxants, it is excreted
in the urine.
Contraindications and cautions
Centrally-acting skeletal muscle relaxants are contra
indicated in the presence of any known allergy to any
of these drugs and with skeletal muscle spasms resulting
from rheumatic disorders. In addition, baclofen should
not be used to treat any spasticity that contributes to
locomotion, upright position or increased function.
Blocking this spasticity results in loss of these functions.
All centrally-acting skeletal muscle relaxants should be
used cautiously in the following circumstances: with
a history of epilepsy
because the CNS depression and
imbalance caused by these drugs may exacerbate the
seizure disorder
; with cardiac dysfunction
because
muscle function may be depressed
; with any condition
marked by muscle weakness,
which the drugs could
make much worse
; and with hepatic or renal dysfunc-
tion,
which could interfere with the metabolism and
excretion of the drugs, leading to toxic levels.
No good
studies exist regarding the effects of these agents during
pregnancy and breastfeeding; therefore, use should be
limited to those situations in which the benefit to the
mother clearly outweighs any potential risk to the fetus
or neonate.
Adverse effects
The most frequently seen adverse effects associated with
these drugs relate to the associated CNS depression:
drowsiness, fatigue, weakness, confusion, headache and
insomnia. Gastrointestinal (GI) disturbances, which
may be linked to CNS depression of the parasympa-
thetic reflexes, include nausea, dry mouth, anorexia and
constipation. In addition, hypotension and arrhythmias
may occur, again as a result of depression of normal
reflex arcs. Urinary frequency, enuresis and feelings of
urinary urgency reportedly may occur.
Clinically important drug–drug interactions
If any of the centrally-acting skeletal muscle relaxants
are taken with other CNS depressants or alcohol, CNS
depression may increase. People should be cautioned to
avoid alcohol while taking these muscle relaxants; if
this combination cannot be avoided, they should take
extreme precautions.
TABLE 25.1
DRUGS IN FOCUS Centrally-acting skeletal muscle relaxants
Drug name
Dosage/route
Usual indications
baclofen (Lioresal)
Adults 15 mg PO daily. Via intrathecal infusion
pump, dosage individualised according to
response
Paediatric: intrathecal infusion pump,
24–1199 mcg/day—base dose on child’s
response
Treatment of muscle spasticity associated
with neuromuscular diseases such as
multiple sclerosis, muscle rigidity and
spinal cord injuries
diazepam (Valium)
2–10 mg deep IM or slow IV injection q
3 hours–q 4 hours, 10–60 mg PO daily
Treatment of muscle spasm or spasticity,
including cerebral palsy
orphenadrine (Norflex)
100 mg PO, a.m. and at bedtime, or 60 mg IV
or IM q 12 hours
Relief of discomfort of acute
musculoskeletal conditions in adults;
under investigation for relief of quinidine-
induced leg cramps
Prototype summary: Baclofen
Indications:
Alleviation of signs and symptoms of
spasticity; may be of use in spinal cord injuries or
spinal cord diseases.
Actions:
Gamma-aminobutyric acid (GABA)
analogue; exact mechanism of action is
not understood; inhibits monosynaptic and
polysynaptic spinal reflexes; CNS depressant.
Pharmacokinetics:
Route
Onset
Peak
Duration
Oral
1 hours
2 hours
4–8 hours
Intrathecal 30–60 mins 4 hours
4–8 hours
T
1/2
:
3 to 4 hours; not metabolised; excreted in the
urine.
Adverse effects:
Transient drowsiness, dizziness,
weakness, fatigue, constipation, headache,
insomnia, hypotension, nausea, urinary frequency.
