Smith-Magenis syndrome

Last edited 04/2020

Smith-Magenis syndrome (SMS) Smith-Magenis syndrome is a rare, complex multisystemic disorder featuring, mental retardation and multiple congenital anomalies caused by a heterozygous interstitial deletion of chromosome 17p11.2

• phenotype of Smith-Magenis syndrome is characterized by a distinct pattern of features including infantile hypotonia, generalized complacency and lethargy in infancy, minor skeletal (brachycephaly, brachydactyly) and craniofacial features, ocular abnormalities, middle ear and laryngeal abnormalities including hoarse voice, as well as marked early expressive speech and language delays, psychomotor and growth retardation, and a 24-hour sleep disturbance
• majority of individuals function in the mild-to-moderate range of intellectual disability
  
  
• infants have feeding difficulties, failure to thrive, hypotonia, hyporeflexia, prolonged napping or need to be awakened for feeds, and generalized lethargy
  
  
  • the behavioural features (including significant sleep disturbance, stereotypies, and maladaptive and self-injurious behaviors) - are not generally not recognized until age 18 months or older and continues to change until adulthood
  • sensory issues are frequently noted
  • include avoidant behavior, as well as repetitive seeking of textures, sounds, and experiences
  • toileting difficulties are common
  • significant anxiety is common
  • problems with executive functioning also are common - including inattention, distractibility, hyperactivity, and impulsivity
  • maladaptive behaviors include:
  • frequent outbursts / temper tantrums,
  • attention-seeking behaviors,
  • opposition, aggression, and
  • self-injurious behaviors
  • including self-hitting, self-biting, skin picking, inserting foreign objects into body orifices (polyembolokoilamania), and yanking fingernails and/or toenails (onychotillomania)
  • spasmodic upper-body squeeze or "self-hug" seems to be highly associated with SMS

Diagnosis:

• established in a proband who has suggestive clinical findings and either a heterozygous deletion at chromosome 17p11.2 that includes RAI1 or a heterozygous intragenic RAI1 pathogenic variant.

Reference:

• Smith ACM et al. Smith-Magenis Syndrome Synonym: del(17)(p11.2). Gene Reviews (Accessed 21/4/2020)
• Gropman AL et al. Neurologic and developmental features of the Smith-Magenis syndrome (del 17p11.2).
Pediatr Neurol. 2006 May;34(5):337-50.