| Muscle relaxants: dantrolene |
Dantrolene works by a direct action on skeletal muscles in which it reduces muscle tone by depressing calcium-mediated myofibril contractions. It has no effect on neural pathways and has fewer central adverse effects than baclofen and diazepam. Dantrolene and diazepam are equally effective in spasticity caused by various upper motor neurone disorders. However, diazepam causes more drowsiness and dantrolene causes more muscle weakness.
After oral administration, the plasma half-life is approximately 9 hours.
Most patients who experience a reduction in spasticity with dantrolene also have an increase in weakness. Therefore it is mainly useful in bed-bound patients who would not be troubled by a decrease in muscle power. Adverse effects are common and the long-term safety and efficacy of dantrolene has not been established. Serious adverse effects include hepatitis (probably idiosyncratic), seizures and pleural effusions with pericarditis. Baseline liver function tests should be performed before treatment, and periodically during therapy (eg 3-monthly). The most common adverse reaction is muscle weakness, which can result in slurred speech, drooling and enuresis. Other common adverse effects are drowsiness, dizziness, diarrhoea, nausea and fatigue; these are usually transient.
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