Early-onset progressive encephalopathy with migrant, continuous myoclonus.

Three unusual cases of focal continuous myoclonus with onset during the first months of life, lasting from dozens of minutes to hours, are reported. During disease evolution, prolonged bilateral myoclonic seizures and generalized tonic-clonic seizures occur. Subsequently, a progressive encephalopathy with hypotonia and ataxia appears. A net worsening of the neurological condition is observed after the age of 4-5 years. Cortical atrophy is shown by CCT and MRI. Neurometabolic screening is not contributory. Repeated polygraphic recordings show continuous and segmental myoclonic jerks, localized in different muscles, at frequencies ranging between 0.5-1 c/s and 6-8 c/s. Moreover action myoclonus is recorded. During the first period of disease the EEG does not show any paroxysmal activity. As to the classification, this syndrome corresponds to an early onset progressive encephalopathy of unknown origin, similar in some aspects to Alper's disease. Another problem is the interpretation of the myoclonic phenomena. Some important aspects suggest a cortical origin of the diverse myoclonic phenomena observed in these cases.