Two cases of essential myoclonus, epilepsy, mental retardation and anxiety disorders.

We examined an 18-year-old female and an 18-year-old male with mild mental retardation who suffered from the oscillatory form of sporadic essential myoclonus from an age of 3 years. Although the generalized oscillatory myoclonus resembled severe essential tremor, surface electromyography revealed small myoclonic jerks with frequencies of 6-8 Hz. As concomitant symptoms, the female case exhibited overanxious irritability from early childhood and generalized epileptic seizures occurred from the age of 4 years. In the male case, an obsessive-compulsive disorder and photosensitive convulsive seizures were persistently noted from early childhood. All their symptoms had been stable for at least the last 10 years. Thus, although non-progressive tremulous movements are rare in early childhood, sporadic essential myoclonus is causative. In contrast to hereditary essential myoclonus, sporadic essential myoclonus is considered to be more heterogeneous, especially in the various associated symptoms.