Myoclonus and Startle Syndromes
The term “myoclonus” is used to identify brief and jerky involuntary movements, originating from brief active contractions of muscles (positive myoclonus) or brief interruptions of ongoing electromyographic activities (negative myoclonus). Myoclonic movem
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Roongroj Bhidayasiri and Onanong Jitkritsadakul
Myoclonus presents as the most rapid and brief movements of all hyperkinetic disorders, characterized by sudden, brief, shock-like involuntary movements [1]. The duration of the movement, or jerk, is usually between 10 and 50 milliseconds (ms) and rarely lasts longer than 100 ms. Positive myoclonus is a term used for these shock-like movements that are caused by active muscle contractions. Negative myoclonus, conversely, occurs when the movements are Electronic supplementary material The online version of this chapter (doi:10.1007/978-3-7091-1628-9_30) contains supplementary material, which is available to authorized users. R. Bhidayasiri, MD, FRCP, FRCPI (*) Professor of Neurology and Director, Chulalongkorn Centre of Excellence for Parkinson’s Disease and Related Disorders, Chulalongkorn University and King Chulalongkorn Memorial Hospital, Thai Red Cross Society, 1873 Rama 4 Road, Bangkok 10330, Thailand Department of Rehabilitation Medicine, Faculty of Medicine, Juntendo University, Tokyo, Japan e-mail: [email protected]; [email protected] O. Jitkritsadakul, MD Department of Medicine, Faculty of Medicine, Chulalongkorn Center of Excellence for Parkinson’s Disease & Related Disorders, Chulalongkorn University and King Chulalongkorn Memorial Hospital, Thai Red Cross Society, 1873 Rama 4 Road, Bangkok 10330, Thailand
produced by a sudden brief interruption of muscle activity [2]. “Asterixis” is a type of negative myoclonus that generally occurs in toxic- metabolic encephalopathies, often associated with reduced alertness. Considered as a separate form of negative myoclonus, “postural lapses” are characterized by interruptions of tonic activity of postural muscles, including the neck, trunk, and proximal muscles, lasting between 200 and 500 ms [2]. Both positive and negative myoclonus may be identified in the same patient, such as in posthypoxic m yoclonus, and can affect the same muscle group, appearing superficially identical. However, negative myoclonus usually involves muscles, which are tonically recruited, and the interruptions of muscle activity generally result in a loss of antigravitational activity and downward joint displacement. In contrast, positive m yoclonus usually involves both flexor and extensor muscles, and the abnormal activation of muscles results in both upward and downward joint displacement [3]. In this chapter, we focus on the differentiation of myoclonus from other movement disorders, clinical approaches, and treatment strategies in patients who present primarily with myoclonus. Selected reviews are provided at the end of this chapter for readers who wish to expand their knowledge on specific aspects of myoclonus.
© Springer-Verlag Wien 2017 C. Falup-Pecurariu et al. (eds.), Movement Disorders Curricula, DOI 10.1007/978-3-7091-1628-9_30
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30.1 Distinguishing Myoclonus from Other Movement Disorders Myoclonus is usually distinguishable from other hyperkinesias by its dis
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