Myoclonus: A Practical Guide to Drug Therapy

Several different types of myoclonus can be distinguished on physiological grounds. Cortical myoclonus arises from an abnormal discharge in the sensori-motor cortex and corticospinal pathways. Brainstem reticular reflex myoclonus and hyperekplexia are forms of generalised myoclonus arising in the brainstem, and palatal myoclonus is a segmental form of brainstem myoclonus. Ballistic overflow myoclonus occurs in hereditary essential myoclonus. Propriospinal myoclonus consists of axial jerks of spinal origin, while segmental spinal myoclonus is thought to arise as a result of the isolation of spinal motoneurons from inhibitory influences or from direct cellular injury. Treatments of first choice for cortical myoclonus are valproic acid (sodium valproate) and clonazepam. Primidone and phenobarbital (phenobarbitone) may also be useful. However, most patients require polypharmacy for adequate symptomatic improvement. Piracetam has advantages in these circumstances, as its addition to existing treatments is rarely accompanied by sedation. 5-Hydroxytryptophan in combination with carbidopa is now rarely used because of gastrointestinal adverse effects. In patients with brainstem reticular reflex myoclonus, valproic acid and clonazepam are the most useful agents. In hyperekplexia, treatment is directed against the disabling tonic spasms, rather than jerks. Carbamazepine, phenytoin and clonazepam are useful agents in this respect. Ballistic overflow myoclonus may improve with anticholinergic drugs, such as benzatropine (benztropine) or trihexyphenidyl (benzhexol). Antiepileptic drugs are disappointingly ineffective in this condition. Treatment of palatal myoclonus is often unsuccessful, but phenytoin, carbamazepine, clonazepam, trihexyphenidyl and baclofen have been effective in some patients. Clonazepam is effective in over half of patients with propriospinal myoclonus, but other anticonvulsants are usually unhelpful. Segmental spinal myoclonus is often resistant to drug treatment, but diazepam, carbamazepine, tetrabenazine and, particularly, clonazepam are sometimes effective. © 1995, Adis International Limited. All rights reserved.