Table 4.

Practical key points of ring chromosome 20 syndrome.

Core phenotype	• Refractory seizures and frequent non-convulsive status epilepticus (NCSE) are the most common seizure types • Cognitive decline following seizure onset in a previously normally developing child is frequent • Terrific hallucinations are frequent • Growth is usually normal, and dysmorphisms and congenital malformations are uncommon
Inheritance	r(20) occurs sporadically in most patients, but mosaic-to-mosaic transmission has been reported
Diagnosis	Karyotype with high number of metaphase count is the gold standard for diagnosis
Miscellaneous	• Constitutional non-supernumerary r(20) can be mosaic (more frequently) or non-mosaic • In mosaic r(20) the percentage of cells containing the ring chromosome inversely correlates with the age of seizure onset