Ring chromosome 20 and epilepsy: diversity of seizures presented by the patients as studied in video-EEG study. Is a subcortical dysfunctionning mainly involved in the epileptogenesis?

We report our observations in four patients with a chromosome 20 ring mosaicism who developed severe drug-resistant epileptic seizures starting at the age of 4 to 8 years. We detailed the clinical features using video-electroencephalographic recordings (EEG) and single photon emission computed tomography (SPECT). We compared the characteristic clinical features in our patients with the rather disparate collection of clinical cases reported in the literature. None of the patients had any dysmorphism and only one, with a majority chromosome 20 ring clone (77%), had mental retardation. The interictal EEGs could be normal or showed diffuse theta waves lasting half an hour and more with no identifiable clinical manifestations. The attacks themselves could be short or very long. The characteristic features seemed to be electrical seizures without apparent clinical manifestations, seizures with isolated obscuration of consciousness as well as seizures suggesting clinically focal involvement while EEG shows diffuse abnormalities without electroclinical parallelism. Those video-EEG findings, combined to SPECT results argue for a mainly subcortical mechanism for epilepsy. Taking account the very long duration of ictal abnormalities, we hypothesize that the distal part(s) of the short and/or long arms of chromosome 20 carry genes implicated in the subcortical mechanisms involved in the termination of the seizures.