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Myoclonus is a shock-like contraction of muscles which occur irregularly and asymmetrically. Such jerks occur repetitively in the same muscle groups and range from a flicker in a single muscle to contraction in a group of muscles sufficient to displace the affected limb. Pathophysiology

The precise nature of myoclonus remains unclear. Several forms exist, some clearly related to epilepsy; others may be associated with damage to inhibitory mechanisms in the brainstem reticular formation. Myoclonus may result from pathological changes affecting a variety of different sites including the motor cortex, cerebellum and spinal cord. Clinical features

Myoclonic movements when repetitive vary in frequency between 5-60/minute. The muscles of the face, oral cavity and limbs are preferentially affected. The movements may be accentuated or precipitated by visual, auditory or tactile stimulation. Repetitive stimulation may result in a crescendo of myoclonus which resembles a seizure.

Physiological myoclonus occurs in sleep (hypnic jerks), with anxiety and in infants when feeding. Causes

Myoclonus occurs in many rare conditions of the nervous system. Four groups of disorder are recognised:

Progressive myoclonus Familial disorders:

- Lafora body disease

- Tay Sach's disease

- Gaucher's disease

- Ramsay Hunt syndrome

- Benign polymyoclonus

Degenerative disease:

- Subacute sclerosing panencephalitis

- Alzheimer's disease

- Pick's disease

- Diffuse Lewy body disease

- Huntington's disease

- Prion disease

- Creutzfeldt-Jacob disease

Metabolic disease associated with transient myoclonus

- Hyponatraemia

- Hypocalcaemia

- Renal, hypoxic, hepatic encephalopathy

- Non kctotic hyperglycaemia

- Hypoglycaemia

Miscellaneous disorders

- Cerebral anoxia

- Vasculitides

- Sarcoidosis

- Paraneoplastic disease

- Mitochondrial disease

- HIV encephalopathy

- Whipple's disease

Epileptic disorders in which myoclonus occurs Generalised seizures: — associated with petit mal

- during prodrome of grand mal

- photosensitive myoclonus Juvenile myoclonic epilepsy

Lennox Gastaut syndrome (atypical petit mal, drop attacks and mental retardation) West's syndrome

Palatal myoclonus - an unusual myoclonic disorder with rapid regular movements of the soft palate and occasionally of the pharyngeal and facial musculature. Palatal movements occur at a rate of 120-140/minute. This disorder is associated with degenerative changes in the olivary and dentate nuclei. Treatment

Benzodiazepine drugs such as clomazepam may suppress myoclonic movements. Piracetam (G.A.B.A. analogue) and levodopa or dopamine agonists are also used.

An exaggerated startle response can be confused with myoclonus. This is often physiological but can be disabling - hyperecplexia (Startle disease).

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