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UK/FLY/2023/2543

June 2023

SMS

Features of Smith-Magenis Syndrome
Several distinctive features characterise the phenotype of SMS, including:
  • Brachycephaly with a typical craniofacial appearance (midface hypoplasia, characteristic mouth with a ‘cupid’s bow’ shape, prognathism)
  • Ocular abnormalities (myopia and strabismus, iris anomalies)
  • Speech delay with or without hearing loss
  • Hoarse, deep voice
  • Short stature with a history of failure to thrive
  • Brachydactyly
  • Peripheral neuropathy (pes cavus or pes planus, depressed deep tendon reflexes), and
  • Scoliosis
All patients have some degree of developmental delay and intellectual disability; intelligence quotient scores range between 35 and 78, most falling in the moderate range of 45–55.
Behavioural problems consistently include:
  • Aggression
  • Self-injurious behaviours
  • Low sensitivity to pain
  • Temper tantrums
  • Impulsivity
  • Repetitive behaviour
  • Hyperactivity with attention deficit
Two stereotypical behaviours – spasmodic upper-body squeeze or ‘self-hug’, and hand licking and page flipping (‘lick and flip’) seem to be specific to SMS. Severe sleep disturbances and an unusual circadian rhythm are almost constant features of the syndrome. Other variable features include cardiac defects, renal abnormalities, seizures, cleft palate, low immunoglobulin levels and thyroid function defect.3

Figure 1. Phenotypic Features of Smith Magenis Syndrome4

A. SMS patient with sleep disturbance, illustrating commonly observed "sleep attacks".

B. Craniofacial anomalies can include midface hypoplasia and tented upper lip in SMS.

C. Self-injurious behaviours like pulling out of fingernails and/or toenails are unique to SMS.

D. Stereotypical behaviour like self-hugging in SMS patient, typically observed during times of excitement or happiness.

E. Obesity is frequently observed in persons with SMS.

References:

3. De Leersnyder, H. 2006. ‘Inverted rhythm of melatonin secretion in Smith-Magenis syndrome: from symptoms to treatment’, TRENDS in Endocrinology and Metabolism. Volume 17, Issue 7, 291-298

4. Chen, Li. et. al. 2015. ‘Smith-Magenis syndrome and its circadian influence on development, behaviour, and obesity – own experience’, Developmental Period Medicine. Volume 19, Issue 2, 149-56