Hereditary essential myoclonus in a large Norwegian family.

We examined 56 members of a large Norwegian family with hereditary essential myoclonus, affecting mainly the neck and upper parts of the body, and inherited in an autosomal-dominant pattern. We observed definite myoclonus in nine individuals, probable myoclonus in one, and possible myoclonus in one. There were two other living members who had a history compatible with myoclonus but who had developed a permanent remission, so we did not observe the movements, and two who had involuntary movements only with stress. Writing usually increased the myoclonus in the neck and trunk, but did not produce myoclonus in the arm used for writing. Having a conversation with an individual who was aware of being watched would also usually increase the myoclonus. Alcohol ameliorated the myoclonus in many, but not all, affected members. Activities such as walking and concentrating during reading would usually reduce the myoclonus. Three living members with definite myoclonus also had features of mild focal dystonia, either spasmodic torticollis or blepharospasm, indicating that focal dystonia may exist as part of the clinical spectrum in hereditary essential myoclonus. In addition to examining the members of the family, we videotaped them and obtained blood samples for molecular genetic analysis.