Chromosome locus: 21q22.3
Gene: CSTB (pathogenic variants include dodecamer repeat expansion)
Protein affected: cystatin B (a cysteine protease inhibitor)
Inheritance: autosomal recessive
The onset is between 6 and 15 years of age.
Clinical features:
(1) tonic-clonic epileptic seizures.
(2) stimulus sensitive myoclonus (mycoclonic jerks)
(3) ataxia and incoordination
(4) intentional tremor
(5) dysarthria
(6) emotional lability and/or depression
(7) intellectual decline that is usually mild
MRI of the brain is normal.
The EEG is always abnormal and may show photosensitive paroxysms.
