Smith-Magenis Syndrome
Patient Information
Characteristics
Background and History
This is a complex disorder involving multiple systems. It is usually caused by loss of DNA (genes) from chromosome 17, but may also be the result of a mutation in a single gene.
Clinical Correlations
The facial features are considered to be distinctive although subtle in infancy. The face is broad and appears square-shaped with a high forehead, flat midface, shortened skull, a short broad nose, and deep-set eyes. These features become more pronounced with age and the jaw, which may appear underdeveloped in infancy, later becomes prominent. Infants are often ‘floppy’ with decreased muscle tone, feed poorly and appear lethargic, while older children become hyperactive, aggressive, and have sleep disturbances. Self-injury behavior is common. However, many children are also affectionate with engaging personalities. Self-hugging behavior may be unique to this syndrome. Most have significant cognitive impairment with speech and motor delays, and are short in stature. Mental retardation and hearing impairment are common. Eye abnormalities such as a small cornea, strabismus, and incomplete development of the iris may be present.
Genetics
Inheritance
Familial cases are uncommon as the chromosomal defect usually arises de novo. Hence, the risk to sibs of affected individuals is near zero. However, a single gene mutation has been found in about 10% of patients who have a less severe clinical picture.
Pedigree
Autosomal dominant
Autosomal dominant disorders require only one mutation for the disease to be expressed. Since an affected parent has two chromosomes, only one of which has the mutant gene, parents can expect that half (50%) of their children will receive that one and inherit the disease. It is common for individuals that inherit the mutation, however, to not have evidence of the disease (nonpenetrance).
Autosomal dominant inheritance leads to a vertical pattern of transmission
Diagnosis and Prognosis
The diagnosis may be difficult at birth and requires comprehensive evaluation. However, experienced pediatricians and medical geneticists should be able to make the diagnosis, especially as children get older. Sleep and behavioral problems present parenting challenges and require medical monitoring, behavioral therapy, and sometimes psychotropic medications. Special education and vocational training may be appropriate for older patients with less severe disease.
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