MYOCLONIC ABSENCE-LIKE SEIZURES AND CHROMOSOME ABNORMALITY SYNDROMES

Purpose: We explored the relationship between myoclonic absence seizur es (MAS) and underlying chromosome disorders. Methods: Among 14 patien ts with MAS observed in three centers, 5 had typical cryptogenic myocl onic absence epilepsy (MAE), 2 had MAS associated with other seizure t ypes (1 with signs of a neuronal migration abnormality and 1 with sign s of a metabolic disorder), and 7 had MAS, with or without other seizu re types, complicating a chromosome abnormality syndrome-2 with trisom y 12p, 4 with Angelman syndrome, and 1 with inv dup (15). Results: In the 7 patients with chromosomopathy, MAS appeared at a mean age of 2.9 years (range 4 months to 6 years 6 months), had a duration of 4-20 s, and were accompanied by reduced awareness and rhythmic myoclonic jerk s involving proximal limb muscles. Ictal EEG showed 2- to 3-Hz general ized spike-and-wave discharges. Conclusions: In these patients, MAS di ffered slightly from those of typical MAE: age of onset was earlier, a bsences were of shorter duration, and no clear increase in muscular to ne was noted; Abnormal expression of genes codifying for the subfamily of K+ channels and for gamma-aminobutyric acid-3 subunit receptors (G ABRB3), both located in the chromosome segments involved in the chromo somopathies presented by our patients, could be responsible for the sa me generalized seizure type. Chromosome analysis should be performed i n patients with mental retardation and MAS, especially when the ictal pattern does not completely overlap that observed in MAE.