Small‐Amplitude cortical myoclonus in Parkinson's disease: Physiology and clinical observations

Abstract

We studied the occurrence of small-amplitude myo- clonus in 20 idiopathic Parkinson's disease patients who had no evidence of dementia as defined by criteria in the Diagnostic and Statistical Manual of Mental Disorders, fourth edition. Parkinson's disease was diagnosed by United Kingdom Brain Bank criteria, and clinical assessment was performed with the Unified Parkinson's Disease Rating Scale motor score, Hoehn and Yahr staging, and the Mini-Mental State Examination. Clinical assessment showed a range of mild-to-moderate disease severity. All patients underwent polygraphic electro-encephalographic-electromyographic (EMG) recording with back-averaging, somatosensory evoked potential testing, and attempted elicitation of long-latency EMG responses. Multichannel surface EMG recording during muscle activation showed irregular, multifocal, brief (<50 msec) myoclonus EMG discharges. Back-averaging consistently showed a focal, short-latency, electroencephalographic transient prior to the myoclonus EMG discharge. Cortical somatosensory evoked potential waves were not enlarged, and long-latency EMG responses at rest were not present. The small-amplitude myoclonus in such cases arises from an abnormal discharge from the sensorimotor cortex. The mechanism of this cortical myoclonus in Parkinson's disease has differences from the more common "cortical reflex myoclonus" physiology. Advanced parkinsonism is not a requirement for manifestation of this myoclonus type. Although the myoclonus occurred without dementia in these cases, its relationship to the subsequent development of cognitive impairment remains to be defined.

Network dysfunction in α‐synuclein transgenic mice and human Lewy body dementia

“…We searched the charts for myoclonus and seizure disorders, both of which are thought to result from aberrant network excitability 39, 43, 44…”

“…Myoclonus in DLB and PD consists of a sudden brief jerk that is accompanied by a sharp transient in the contralateral sensorimotor cortex on EEG 39, 43. This type of myoclonus, called cortical myoclonus, may be caused by aberrant cortical excitability39, 43, 44 and has been successfully treated with anti‐epileptic drugs 52, 53, 54. In our study, myoclonus was associated with an earlier age of onset of cognitive impairment, raising the possibility that aberrant network excitability may contribute to cognitive dysfunction in DLB.…”

Neurosyphilis presenting as parkinsonism

“…Our case therefore appears unique among those reported in the postantibiotic era in lacking clear additional non-parkinsonian features; the only potential clinical clue was the early onset myoclonus, which subsequently settled after antibiotic treatment, though myoclonus is also described in idiopathic PD 13. Nevertheless, the clear response of all clinical features to penicillin therapy in a patient with neurosyphilitic CSF is strongly suggestive of treponemal parkinsonism.…”

Isolated ring-finger myoclonus

“…(2) Botulinum toxin injection abolished myoclonic jerks of the ring-finger but led to a tonic adduction of his fifth finger. Nevertheless, a further classification was not possible, as MRI did not show a spinal or cortical lesion and the patient did not show classical features of phenotypic similar minipolymyoclonus observed in, for example, multi system atrophy (action or posture dependent, stimulus sensitive) [8] or Parkinson's disease (action dependent, back-averaged EEG-transient prior to myoclonus) [9].…”