Many of these patients experience their seizures only during sleep. They can have seemingly bizarre features as they originate from the frontal lobe of the brain. Often, individuals with ring chromosome 20 syndrome are initially found to have complex partial seizures of frontal lobe origin, though imaging studies do not show a corresponding structural brain abnormality. In certain patients, these seizures may secondarily generalized. Individuals from the ages of 0–17 years should be considered for ring 20 chromosome analysis if they have: predominantly complex partial seizures, medically refractory cryptogenic epilepsy, Lennox-Gastaut-like features with no cause identified, frequent subtle nocturnal seizures, an EEG showing prolonged high voltage frontally dominant slowing intermixed with spikes or sharp waves, an EEG showing overlapping features of continuous slow spike and wave discharges in sleep (CSWS) and electrical status epilepticus in sleep (ESES), and/or subsequent cognitive impairment/learning difficulties/mild retardation.These patients will typically have a normal childhood development until onset of epilepsy and lack evidence of dysmorphism or other congenital malformations. Genetics
Rather than the typical linear pattern of a chromosome, deletion of the endings of a chromosome can lead to ring formation. A chromosome has two arms, one long and one short. Deletion of the short arm of chromosome 20 does not appear to result in epilepsy; however, terminal deletion of the long arm is associated with epilepsy. Therefore, some gene loss from the terminal segment could be responsible for the manifestation of epilepsy in ring chromosome 20 syndrome. The most common breakpoint in patients is in the q13.33 region of chromosome 20.
As chromosomes occur in pairs, the affected individual usually has one normal chromosome and the other replaced by a ring chromosome. However, this is not usually the case for every cell in the patient. In most individuals with ring chromosome 20 syndrome, a certain percentage have two normal chromosome 20s and the remainder have one normal and one ring 20 chromosome. This mixture is called mosaicism. A higher degree (percentage) of mosaicism appears to be associated with earlier age of seizure onset and presence of malformations. Yet, the degree of mosaicism does not determine response to drug treatment. Since the range in age of onset of seizures and IQ for a given mosaic ratio is relatively wide, prediction of phenotype based on the mosaicism ratio is somewhat limited. Occasionally, there is a variation, where the ring chromosome 20 syndrome is characterized by an extra ring chromosome in addition to the two normal ones and hence gives rise to a partial trisomy or supernumerary ring 20. This supernumerary ring 20 can cause multiple anomalies but no epilepsy, and this resulting syndrome should be differentiated from ring chromosome 20 syndrome. This confusion has persisted even in the literature. The genes in the q13.33 region of chromosome 20 have yet to be fully delineated. However, this telomeric region includes two genes that are related to other distinct epilepsy syndromes, autosomal dominant nocturnal frontal lobe epilepsy and benign familial neonatal convulsions.