Ring chromosome 20 syndrome
Rare chromosome abnormality causing seizures, behavior issues, and intellectual disability.
Ring chromosome 20 syndrome is a rare genetic condition where the ends of chromosome 20 fuse together, forming a ring shape instead of the usual linear structure. This abnormality is linked to epileptic seizures, behavioral issues, and intellectual disability. Because chromosome analysis isn’t standard for epilepsy patients, the syndrome is often missed or diagnosed late—many experts believe it’s underrecognized.
The most noticeable symptom is recurrent seizures, which usually appear in early childhood but can start anywhere from age 1 to 17. These seizures tend to be ongoing and don’t respond well to medication. The most common type is a brief focal seizure with impaired awareness, sometimes called a complex partial seizure. During these episodes, a person might stare, make repetitive mouth movements, or have involuntary head or body movements. Many patients also experience subtle nighttime behaviors—like stretching, rubbing, or turning—that resemble waking up. However, EEG readings show these events are actually mild seizures or non-convulsive status epilepticus. Some people have seizures only during sleep, and these can seem odd because they start in the frontal lobe, even though brain scans show no structural problem. In some cases, these seizures spread to both sides of the brain.
Doctors should consider testing for ring chromosome 20 in children and teens (ages 0–17) who have hard-to-treat complex partial seizures, epilepsy with no clear cause, features similar to Lennox-Gastaut syndrome without an identified reason, frequent subtle nighttime seizures, or EEG patterns showing prolonged slow waves with spikes or sharp waves in the front of the brain, or overlapping patterns of continuous slow spike-and-wave during sleep and electrical status epilepticus in sleep. These patients typically develop normally until seizures begin, and they don’t have unusual facial features or birth defects.
Genetically, ring chromosomes form when the ends of a chromosome are lost and the arms join. Losing the short arm of chromosome 20 doesn’t seem to cause epilepsy, but losing part of the long arm does. This suggests that missing genes from the long arm’s tip—most often in the q13.33 region—may trigger the seizures. People usually have one normal chromosome 20 and one ring version, but not in every cell.
- Field
- Medical genetics, neurology
- Known for
- Ring chromosome 20 causing epilepsy, behavior disorders, and intellectual disability
- Prevalence
- Rare; over 60 patients reported in published literature
- Typical age of onset
- First few years of life (range 1–17 years)
- Genetic basis
- Deletion of chromosome 20 ends leading to ring formation; most common breakpoint in q13.33 region
Lore & Background
Ring chromosome 20 syndrome results from the fusion of the two arms of chromosome 20 into a ring, typically due to deletion of the chromosome ends. The condition is often mosaic, meaning not all cells contain the ring chromosome; a higher percentage of mosaicism is associated with earlier seizure onset and presence of malformations, but does not determine drug response. The ring formation usually occurs de novo during prenatal development, and recurrence risk is very low.
Reader's Guide
Ring chromosome 20 syndrome is significant as a genetic cause of medically refractory epilepsy, often presenting with complex partial seizures of frontal lobe origin without structural brain abnormalities. Diagnosis requires karyotype analysis of at least 50 cells to detect mosaicism, as newer array technology may miss the ring. Seizures are poorly responsive to antiepileptic drugs; no single drug is more efficacious. Alternatives include the ketogenic diet and vagus nerve stimulation, but epilepsy surgery is not recommended due to lack of a discrete seizure focus. The condition is underdiagnosed, and early diagnosis with multidisciplinary support is thought to improve prognosis. Research studies are ongoing to understand the molecular basis and clinical spectrum.
Did You Know?
- Ring chromosome 20 syndrome is often underdiagnosed because chromosomal analysis is not routine for epilepsy patients.
- The most common seizure type is brief focal onset epileptic seizures with impaired consciousness, known as complex partial seizures.
- A higher degree of mosaicism is associated with earlier age of seizure onset and presence of malformations.
- The ring formation typically occurs de novo and is not identified in parents.
More in Genetic disorders with no OMIM 1-24
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