Handbook of Genetic Counseling/Smith Magenis Syndrome
Smith-Magenis Syndrome (SMS) MIM#182290
Ethiology
[edit | edit source]- first described in 1982
- contiguous gene deletion syndrome
- 17p11.2 deletion (~4 Mb) due to nonalelic homologous recombination of low-copy repeats regions
- critical region - ca. 1Mb (~25 genes, RAI1 most important)
- nearly all occur de novo
- prevalence reported ~1/25000 but underdiagnosed
Clinical features
[edit | edit source]- Developmental features
- developmental delay
- hypotonia
- hyporeflexia
- lethargy
- feeding difficulties
- Physical features
- facial appearance specific and changes over time
- square face
- brachycephaly
- full cheeks
- deep-set eyes
- heavy brows
- square jaw
- micrognathia
- fleshy upper lip
- bulky philtral pillars
- prognathism (late feature)
- short stature + failure to thrive
- Behavioral features
- mental retardation
- speech delay +/- hearing loss
- head banging
- self-injurious behaviour
- stereotypes ("self-hug" + "lick and flip")
- sleep disturbance (inverted circadian rhythm of melatonin)
- sensory integration disorder
- Developmental features
Diagnostic testing
[edit | edit source]- diagnosis of SMS based on clinical features pattern + cytogenetic/FISH 17p11.2 deletion confirmation
- FISH detects over 95% cases
- consider SMS if not positive for Down or Prader-Willi syndrome
- delayed diagnosis common
Differential diagnosis
[edit | edit source]- Down syndrome (some common features)
- infantile hypotonia
- midface hipoplasia
- up-salnting palpebral fusures
- Prader-Willi syndrome
- hypotonia
- lethargy
- sleep disorders
- 22q11 deletion (DiGeorgi/velo-cardio-facial syndrome)
- speech delay
- cardiac anomalies
- 22q13 deletion
- Fragile X
- autistic-like behaviour