Ring chromosome 20 syndrome is marked by a characteristic seizure phenotype. The prevalence of the syndrome is unknown but more than 50 cases have been described in the literature. Depending on the amount of chromosomal loss and associated mosaicism, ring(20) can be associated with macrocephaly, mild to moderate intellectual deficit, or behavioural problems. In rare cases, brain, kidney or heart malformations may be present. Initial psychomotor development is usually unaffected and pre- and postnatal growth are normal. There is no recognizable dysmorphism although strabismus, micrognathia, down-slanting palpebral fissures, and ear abnormalities have been reported in some affected individuals. The seizure phenotype consists of complex partial seizures, and frequent progression to generalized tonic or tonic-clonic seizures. Epilepsy is almost invariably present by 3 years of age, but may manifest in the newborn period. The most typical manifestations are frontal lobe epilepsy, and ictal terrors and hallucinations. The panic seizures begin in infancy. In early childhood, an electroencephalogram (EGG) pattern of rhythmic theta slow-wave activity with spikes appears, without concomitant clinical manifestations (interictal electric seizures). These seizures predominantly occur in the frontal areas. The episodes of nonconvulsive status epilepticus lead to progressive cognitive decline, which can lead to severe intellectual deficit. A ring chromosome is a chromosomal anomaly in which the tip of the short arm of a chromosome has merged with the tip of the long arm. Usually, chromosomal material is lost in the subtelomeric region of one or both chromosomal arms. Ring chromosomes are unstable: during mitosis, the ring may be lost or duplicated. For this reason, patients carrying a ring chromosome often have mosaic karyotypes with normal cells, cells with a ring chromosome, cells with monosomy (the ring being lost) and/or cells with reorganized/duplicated rings. The proportionof each category may change over time. Most (but not all) patients with ring(20) have a ring which has lost the terminal part of 20q (terminal 20q monosomy). The mechanism underlying seizure disorders remains unknown, although it is hypothesized that two epilepsy genes (CHRNA4 and KCNQ2), located in the subtelomeric region of 20q, may be involved in the pathogenesis. The epilepsy associated with ring(20) is often partially or totally refractory to medical and surgical treatment.
Last update: October 2006