Functional jerks, tics, and paroxysmal movement disorders.

Functional jerks are among the most common functional movement disorders. The diagnosis of functional jerks is mainly based on neurologic examination revealing specific positive clinical signs. Differentiation from other jerky movements, such as tics, organic myoclonus, and primary paroxysmal dyskinesias, can be difficult. In support of a functional jerk are: acute onset in adulthood, precipitation by a physical event, variable, complex, and inconsistent phenomenology, suggestibility, distractibility, entrainment and a Bereitschaftspotential preceding the movement. Although functional jerks and tics share many similarities, characteristics differentiating tics from functional jerks are: urge preceding the tic, childhood onset, rostrocaudal development of the symptoms, a positive family history of tics, attention-deficit hyperactivity disorder or obsessive-compulsive symptoms, and response to dopamine antagonist medication. To differentiate functional jerks from organic myoclonus, localization of the movements can give direction. Further features in support of organic myoclonus include: insidious onset, simple and consistent phenomenology, and response to benzodiazepines or antiepileptic medication. Primary paroxysmal dyskinesias and functional jerks share a paroxysmal nature. Leading in the differentiation between the two are: a positive family history, in combination with video recordings revealing a consistent symptom pattern in primary paroxysmal dyskinesias. In this chapter functional jerks and their differential diagnoses will be discussed in terms of epidemiology, symptom characteristics, disease course, psychopathology, and supportive neurophysiologic tests.