Smith Magenis Syndrome
Synonyms of Smith Magenis Syndrome
- chromosome 17, interstitial deletion 17p
- Smith-Magenis chromosome region
- No subdivisions found.
Smith-Magenis syndrome is characterized by particular facial features, developmental delays, mental retardation and behavioral abnormalities.
The facial features include a broad square-shaped face, an abnormally short, broad head (brachycephaly); an abnormally broad, flat midface; a broad nasal bridge; an unusually prominent jaw (prognathism); eyebrows growing across the base of the nose (synophrys); a short full tipped nose and fleshy upper lip with a tented appearance.
Developmental delays and intelligence are variable but most affected individuals have mild to moderate mental retardation. Behavioral abnormalities include sleep disturbances, repetitive movements (stereotypies) and a tendency to inflict harm on oneself.
Smith-Magenis syndrome occurs when there is a missing piece of chromosome on the short arm of chromosome 17 (17p11.2).
In infancy, Smith-Magenis syndrome is characterized by feeding difficulties, failure to thrive, poor muscle tone, excessive napping and lethargy. The distinctive facial appearance becomes more apparent with age. Other characteristics may include minor skeletal abnormalities, short stature, eye and ear abnormalities and speech delay. Extreme nearsightedness (myopia) and crossed eyes (strabismus) frequently occur in affected children. Other visual difficulties may include detachment of the retina of the eyes. The voice is often hoarse and low pitched. Cleft palate, heart and kidney abnormalities are sometimes present.
Developmental delays and intelligence are variable but most affected individuals have mild to moderate mental retardation. Behavioral abnormalities include sleep disturbances, a tendency to inflict harm on oneself and repetitive movements (stereotypies). Two stereotypic behaviors appear to be specific for Smith-Magenis syndrome:
Upper body squeezing and hand licking/page flipping. Self-destructive behaviors may include head-banging, wrist-biting, the insertion of foreign objects into the nose and ears (polyembolokoilamania), and pulling out the nails of the fingers and/or toes (onychotillomania).
Children with Smith-Magenis syndrome may experience significant sleep difficulties including falling to sleep and/or remaining asleep. Some children may have a decreased sensitivity to pain, burning sensations, loss of feeling in the legs (peripheral neuropathy), loss of muscle mass in the legs (amyotrophy), and absent or decreased reflexes. (For more information on this disorder, choose "peripheral neuropathy" as your search term in the Rare Disease Database.)
Smith-Magenis syndrome is a microdeletion syndrome that occurs when there is a missing piece of chromosome on the short arm of chromosome 17 (17p11.2). The vast majority of individuals with Smith-Magenis syndrome have a deletion that is the result of a new spontaneous genetic change and not from an inherited abnormality. Mutations in the RAI 1 casue SMS.
Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated "p" and a long arm designated "q". Chromosomes are further sub-divided into many bands that are numbered. For example, "chromosome 17p11.1" refers to band 11.2 on the short arm of chromosome 17. The numbered bands specify the location of the thousands of genes that are present on each chromosome.
Smith-Magenis syndrome is a rare chromosome disorder that affects males and females in equal numbers. The estimated prevalence is 1/25,000 births. Smith-Magenis syndrome has been identified worldwide in many ethnic groups.
Symptoms of the following disorders can be similar to those of Smith-Magenis syndrome. Comparisons may be useful for a differential diagnosis:
Velocardiofacial syndrome (VCFS is characterized by abnormalities of the head and facial (craniofacial) area, heart defects diminished muscle tone (hypotonia) and learning disabilities. Some of those affected also develop psychiatric problems. Velocardiofacial syndrome is inherited as an autosomal dominant genetic trait and is sometimes known as chromosome 22q11 deletion spectrum because it is associated with multiple identifying features known to occur as a result of a deletion of genetic material on chromosome 22. (For more information about this condition, choose "velocardiofacial" as your search term in the Rare Disease Database.)
Prader-Willi syndrome is a genetic disorder characterized in infancy by diminished muscle tone (hypotonia), feeding difficulties, and failure to grow and gain weight (failure to thrive). In childhood, features of the disorder include short stature, genital abnormalities and an excessive appetite. All individuals with Prader-Willi syndrome have some cognitive impairment that ranges from borderline normal with learning disabilities to mild mental retardation. Behavior problems are common and can include temper tantrums, obsessive/compulsive behavior, and skin picking. Prader-Willi syndrome occurs when the genes in a specific region of chromosome 15 do not function. (For more information about this condition, choose "Prader-Willi" as your search term in the Rare Disease Database.)
Williams syndrome is a rare genetic disorder characterized by growth delays before and after birth (prenatal and postnatal growth retardation), short stature, varying levels of mental deficiency, and distinctive facial abnormalities that typically become more pronounced with age. Characteristic facial features may include a round face, full cheeks, thick lips, a large mouth that is usually held open, and a broad nasal bridge with nostrils that flare forward (anteverted nares). Williams syndrome is thought to result from deletion of genetic material from adjacent genes (contiguous genes) within a specific region of chromosome 7 (7q11.23). (For more information about this condition, choose "Williams" as your search term in the Rare Disease Database.)
Fragile X syndrome is a defect of the X chromosome that causes mild mental retardation. The disorder occurs more frequently and severely among males than females. Language delays, behavioral problems, autism or autistic-like behavior (including poor eye contact and hand-flapping), enlarged external genitalia (macroorchidism), large or prominent ears, hyperactivity, delayed motor development and/or poor sensory skills are among the wide range of symptoms associated with this disorder. (For more information about this condition, choose "fragile x" as your search term in the Rare Disease Database.)
Smith-Magenis syndrome is diagnosed by the detection of a deletion of the short arm of chromosome 17 at band p11.2. This deletion can be observed following a routine chromosome analysis but is sometimes overlooked. Molecular chromosome analysis using fluorescent in situ hybridization (FISH) may be necessary to reveal extremely small deletions that cannot be seen under the microscope.
Recommended therapies include speech/language, occupational, physical and behavioral. Psychotropic medications may be useful in increasing attention and decreasing hyperactivity. Melatonin therapy may be helpful for the sleep disorder. Early intervention, special education programs and vocational training are recommended to maximize the potential of those with Smith-Magenis syndrome.
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Organizations related to Smith Magenis Syndrome
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Smith ACM, Finucane B, Smith-Magenis Syndrome. In: The NORD Guide to Rare Disorders, Philadelphia: Lippincott, Williams and Wilkins, 2003:254.
Chen KS, Potocki, L, Lupski JR. The Smith-Magenis syndrome [del (17)p11.2]: clinical review and molecular advances. Ment Retard Dev Disabil Rev 1996;2:122-129.
Greenberg R, Lewis RA, Potocki L, et al. Multi-disciplinary clinical study of Smith-Magenis syndrome (deletion 17p11.2). Am J Med Genet 1996:62:247-254.
Potocki L, Glaze D, Tan DX, et al. Circadian rhythm abnormalities of melatonin in Smith-Magenis syndrome. J Med Genet 2000;37:428-433.
Smith ACM, Dykens E, Greenberg F. The behavioral phenotype of Smith-Magenis syndrome (del17p11.2). Am J Med Genet 1998;81:179-185.
Smith ACM Dykens E. Greenberg F. Sleep disturbance in Smith-Magenis syndrome (del17p11.2). Am J Med Genet 1998;81:186-191.
FROM THE INTERNET
Smith ACM, Allanson J, Allen AJ, et al. (Updated 3/15/04). Smith-Magenis Syndrome. In: GeneReviews at Genetests: Medical Genetics Information Resource (database online). Copyright, University of Washington, Seattle. 1997-2005. Available at http://www.genetests.org. Accessed 4/05.
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