The most frequent structural abnormalities of chromosome 18 are deletions and ring chromosome. Clinical features of ring chromosome 18 depend on the extent of the deleted chromosomal segments, the 18p and 18q deletion syndromes. We report on a 6-year-old female child who was presented to the children with special needs clinic, the National Research Center, with a complaint of hyperactivity and occasional episodes of loss of concentration simulating atypical absence seizure. She had a minor nonspecific pattern of dysmorphism and growth retardation; height, head circumference, and body weight measurements were below 2.5 SD. Electroencephalography for the child indicated generalized epileptogenic discharge whereas brain MRI did not indicate any abnormality.
Cytogenetic analysis of the patient showed 46,XX, r(18)(65%)/46,XX, double size r(18)(35%); the mother showed the 46,XX, r(18) karyotype in all studied metaphases whereas her father showed the normal male karyotype 46,XY. Fluorescent in-situ hybridization showed that the ring chromosome 18 had deletion of both 18p and 18q subtelomeres. The transmission of ring chromosome 18 from the mother to her daughter with a mosaic karyotype of double-size ring chromosome that resulted only in minor dysmorphism has not been documented in previous studies.