Baclofen

GABA-B agonist for the treatment of spasticity and muscle hypertonus

Baclofen is a gamma-aminobutyric acid B receptor agonist (GABA-B agonist) and is one of the most commonly used muscle relaxants for spastic movement disorders. As a structural analogue of the inhibitory amino acid GABA (gamma-aminobutyric acid), baclofen inhibits the transmission of nerve impulses in the spinal cord, thereby reducing pathologically increased muscle tension (spasticity). In Germany, baclofen is available under the brand name Lioresal and as a generic, both as tablets for oral ingestion and as a solution for intrathecal pump use in severe spasticity.

Spasticity is a complex symptom that can occur in various neurological conditions, including multiple sclerosis, spinal cord injuries, cerebral palsy, and stroke. Left untreated, it can lead to pain, contractures, sleep disorders, and a considerable impairment of quality of life. Baclofen represents one of the most important and best-studied medications in the therapy of these conditions.

Mechanism of Action

Baclofen acts as a selective agonist at GABA-B receptors, localised pre- and postsynaptically in the spinal cord and brain. GABA-B receptors are G protein-coupled receptors (Gi/o protein) whose activation leads to inhibition of the cAMP signalling pathway, opening of potassium channels (hyperpolarisation), and inhibition of voltage-dependent calcium channels.

Presynaptically, baclofen inhibits the release of excitatory neurotransmitters (glutamate, aspartate, substance P) from primary afferent neurons by activating GABA-B receptors. This reduces afferent sensory signal transmission to spinal motor neurons. Postsynaptically, hyperpolarisation of motor neurons leads to inhibition of their firing threshold and thus a reduction in muscle tone and reflex activity.

Compared to GABA-A agonists (e.g. benzodiazepines), baclofen has a lower sedation potential with targeted action on spasticity; however, complete freedom from sedation is not to be expected, as GABA-B receptors are also present supraspinally.

With intrathecal application (directly into the cerebrospinal fluid space), considerably lower doses than with oral use are required, as the active ingredient is immediately available at the spinal site of action and does not need to cross the blood-brain barrier.

Indications

• Spasticity in multiple sclerosis: Most common indication; baclofen reduces pathological muscle stiffness, pain from spasms, and improves mobility
• Spasticity after spinal cord injuries (paraplegia, tetraplegia): Both orally and intrathecally for severe spasticity
• Cerebral palsy: Intrathecal baclofen in children and adults with severe generalised spasticity
• Spasticity after stroke or traumatic brain injury: As a supplement to physiotherapeutic measures
• Off-label applications: Alcohol dependence (approved in France, off-label in Germany), hiccup, neuropathic pain, gastro-oesophageal reflux through reduction of transient sphincter relaxations

Dosage and Administration

Oral therapy (adults): Start gradually with 5 mg three times daily; weekly increase by 5 mg per dose depending on efficacy and tolerability. Usual maintenance dose: 30 to 75 mg daily, divided into 3 to 4 individual doses. Maximum daily dose: 100 mg (in exceptional cases up to 120 mg under medical supervision).

Children (oral): Possible from 1 year of age; starting dose 2.5 mg per dose three times daily, gradual increase according to clinical response. Target dose: 0.75 to 2 mg/kg body weight/day, max. 40 mg/day in children under 8 years.

Intrathecal use: Via implanted pump; after test dose (25 to 50 micrograms intrathecally) and positive response, pump implantation. Daily doses of 12 to 2000 micrograms as needed.

Always take baclofen with meals or milk to minimise gastrointestinal side effects. Abrupt discontinuation must absolutely be avoided; dose reduction must occur slowly over several weeks.

Side Effects

Very common and common: Drowsiness (somnolence) and sedation are the most common side effects, particularly at the start of therapy. Muscle weakness and muscle hypotonia (with excessive dosing may lead to worsened walking difficulties), dizziness, nausea, dry mouth, headache.

Occasional: Confusion, hallucinations, depressive mood, euphoria, sleep disorders, ataxia, tremor, visual disturbances, bladder dysfunction (urinary retention or incontinence), constipation, diarrhoea, elevated liver values.

Rare and serious: Seizures (particularly with abrupt discontinuation), paradoxical increase in spasticity, cardiovascular effects (bradycardia, hypotension). With intrathecal use, there is a risk of overdose due to pump malfunction, which can lead to severe coma and respiratory arrest (emergency situation).

Important: Abrupt discontinuation of baclofen can trigger a severe withdrawal syndrome: hyperthermia, severe increase in spasticity, confusion, hallucinations, seizures, and in rare cases rhabdomyolysis and multi-organ failure. This applies particularly to intrathecal therapy in the event of pump failure.

Interactions

CNS-depressant substances: Alcohol, benzodiazepines, opioids, antihistamines, tricyclic antidepressants, and other sedatives considerably enhance the sedating effects of baclofen. The combination can lead to severe drowsiness, respiratory depression, and coordination disorders.

Antihypertensives: Baclofen can enhance the blood pressure-lowering effect; blood pressure monitoring is recommended in combination.

Lithium: Combination can lead to enhanced symptoms of hyperkinesia; close neurological monitoring required.

Tricyclic antidepressants (amitriptyline): Can enhance the muscle-relaxing effect of baclofen; dose adjustment should be considered with concurrent use.

Ibuprofen and other NSAIDs: Possible deterioration of renal function with concurrent ingestion; baclofen is predominantly excreted renally, so impaired renal function can lead to elevated plasma levels and enhanced side effects.

Special Notes

Renal insufficiency: Baclofen is excreted 70 to 80 percent unchanged via the kidneys. With impaired renal function, the active ingredient accumulates considerably and can cause serious signs of toxicity. The dose must be adjusted to renal function; extreme caution is required in severe renal insufficiency.

Pregnancy: Baclofen crosses the placenta. Teratogenic effects were observed in animal studies at high doses. No adequate controlled studies are available in humans. Baclofen should be used during pregnancy only if the benefit clearly outweighs the risk.

Epilepsy: Particular caution is required in patients with epilepsy, as baclofen can lower the seizure threshold. Regular EEG monitoring is advisable.

Psychiatric conditions: In patients with pre-existing psychiatric disorders (depression, psychoses), baclofen can exacerbate or trigger psychiatric symptoms. Careful monitoring is necessary.

Driving ability: Drowsiness and dizziness impair reaction capacity. Patients should refrain from driving vehicles at the start of therapy or with dose adjustments.

Frequently Asked Questions

How long does it take for baclofen to work in spasticity?

The effect of oral baclofen begins gradually after several days to weeks, as the dosage must be increased slowly. Significant relief of spasticity is frequently noticeable after 2 to 4 weeks. Intrathecal baclofen acts more rapidly, as it is immediately available at the site of action.

Does baclofen cause dependence?

Baclofen has no classical addiction potential in terms of abuse or craving. However, physical dependence develops with prolonged use, which requires gradual tapering of therapy. Abrupt discontinuation can trigger dangerous withdrawal symptoms.

Can baclofen be used for alcohol dependence?

In France, baclofen is approved for this indication. In Germany, use for alcohol dependence is off-label and requires a strict individual benefit-risk assessment and medical supervision. The evidence is mixed; some patients benefit, others show insufficient response.

References

• Product information Lioresal (Novartis), as of 2024
• Guideline on spasticity therapy of the German Society for Neurology (DGN), 2023
• European Medicines Agency (EMA): Baclofen EPAR
• Dario A, Tomei G: A benefit-risk assessment of baclofen in severe spinal spasticity. Drug Safety, 2004
• Federal Institute for Drugs and Medical Devices (BfArM): Product monograph Baclofen