Myoclonus
Myoclonus refers to brief, isolated, random, non-purposeful jerks of muscle groups in the limbs. Myoclonic jerks occur normally at the onset of sleep (hypnic jerks).Similarly, a myoclonic jerk is a component of the normalstartle response which may be exaggerated in some rare (mostly genetic) disorders. Unlike the movement n disorders discussed so far, myoclonus may occur in disorders of the cerebral cortex, when groups of pyramidal cells fire spontaneously. Such myoclonus occurs in some forms of epilepsy in which the jerks are fragments of seizure activity. Alternatively, myoclonus can arise from subcortical structures or, more rarely, from diseased segments of the spinal cord. Myoclonus, especially of cortical origin, often responds to clonazepam, sodium valproate or piracetam.
Neuropathic pain
Neuropathic pain is caused by dysfunction of the pain perception apparatus itself, in contrast to nociceptive
pain, which arises from a pathological process such as inflammation. Neuropathic pain has distinctive features and is typically described as a very unpleasant, persistent burning sensation. There is often increased sensitivity to touch, so that light brushing of the affected area causes exquisite pain (hyperpathia). Painful stimuli are felt as though they arise from a larger area than that touched and spontaneous bursts of pain may also occur. Pain may be elicited by other modalities such as loud sounds (allodynia) and this is considerably affected by emotional influences. The most common syndromes of neuropathic pain are seen where there is partial damage to a peripheral nerve (‘causalgia’), to the trigeminal nerve (post-herpetic neuralgia) or to the thalamus. Treatment of these syndromes is very difficult. Drugs which modulate various parts of the nociceptive system, such as carbamazepine, tricyclics or phenothiazines, may help but usually only do so partially. Neurosurgical attempts to interrupt various pain pathways sometimes succeed but often increase the sensory deficit and may worsen the situation. Implantation of electrical stimulators has occasionally proved successful.
