The face is characterized by a short flat head, prominent forehead, broad square face, upslanting eye slits, deep-set eyes, underdeveloped midface, broad nasal bridge, short nose, tented upper lip, and a chin that is small in infancy and becomes relatively prominent chin with age. There is mild to moderate mental retardation. In infancy there are feeding problems, failure to thrive, hypotonia (floppiness), prolonged napping and lethargy. Later in childhood and adulthood there are attention deficit disorders sometimes with hyperactivity, frequent temper tantrums, impulsivity, distractibility, disobedience, aggression, self injury, toileting difficulties, and self-injurious behaviors including self-hitting, self-biting, and skin picking. There are stereotypic behaviors including spasmodic upper-body squeeze ("self hug"), hand licking and page flipping ("lick and flip") behavior, mouthing objects, insertion of hand in mouth, teeth grinding, body rocking, and spinning or twirling objects.
The syndrome is due to an interstitial deletion from the short (p) arm of chromosome 17 band p11.2 [del(17)(p11.2 p11.2)]. The critical region of loss maps to 17p11.2 region and encompasses a 1.0 Mb region. The deletion usually spans 4 Mb (megabases) and is believed to involve the loss of a number of genes. The syndrome is named for Drs. Ann Smith (at NIH) and Ruth Ellen Magenis (in Oregon) who first described it.