Chromosome 22 supernumerary marker. Supernumerary marker chromosome (SMC) 22.
Incidence
Supernumerary markers occur in about 0.3/1000 unselected births and 3/1,000 are mentally handicapped. 80% arise de novo and occur most frequently with advancing maternal age and 20% are familial and may not show any clinical manifestations.
Clinical Characteristics
Severe hypotonia and developmental delay are the most common features. Some cases are associated with cat eye syndrome (iridal coloboma giving the appearance of vertical pupils, preauricular tags or fistulae, cardiovascular and urinary abnormalities, and retarded growth and mental development) and Duane syndrome (reduced eye abduction or adduction, deficiency of convergence, and other abnormalities).
Precipitants
80% arise de novo and occur most frequently with advancing maternal age.
Provocation Tests
None
Diagnostic Procedures
Karyotype. A small structurally abnormal chromosome derived from chromosome 22.