Ring chromosome 18 is a genetic condition caused by a deletion of the two ends of chromosome 18 followed by the formation of a ring-shaped chromosome. It was first reported in 1964. Signs and symptoms
Ring 18 causes a wide range of medical and developmental concerns. As discussed above, people with ring 18 can have features of both distal 18q- and 18p-. The features of distal 18q- and 18p- vary greatly because of the variability of the deletion size and breakpoint locations between people. Because ring 18 can involve unique deletions of both the p and q arms of the chromosome there is twice as much reason for the variability between individuals. This variation is also partly attributable to the incidence of mosaicism, which is relatively common in people with ring 18. Holoprosencephaly has been reported in some people with ring 18. This is due to the deletion of the TGIF gene on the short arm of chromosome 18 in some people with ring 18. Approximately 30-40% of people with ring 18 have a congenital heart anomaly. Septal defects are the most common type of defect reported in this population. Hypotonia is frequently seen in the ring 18 population. Seizures, though uncommon, have been reported in people with ring 18. In some children without “classic” holoprosencephaly, microforms of holoprosencephaly may be noted on MRI, including missing olfactory tracts and bulbs and absent or hypoplastic corpus callosum. Strabismus as well as nystagmus have both been reported in infants and children with ring 18.
In the case of ring 18, one of the two copies of chromosome 18 has formed a ring. References


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