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Smith-Magenis syndrome

Background

Smith-Magenis syndrome (SMS) is a rare condition that is associated with developmental delay, learning difficulties (see entry Learning Disability), behavioural difficulties and a disturbed sleep pattern. SMS was first described by Ann Smith and colleagues in 1982. At least 1 in 25,000 children are born with this condition, and it is probably under diagnosed. Recent estimates suggest the number could be nearer to 1 in 15,000.

Credits

Medical text written August 2011 by Dr Alison Male. Last updated September 2020 by Dr Alison Male, Consultant in Clinical Genetics, Great Ormond Street Hospital, London, UK.

Although great care has been taken in the compilation and preparation of all entries to ensure accuracy, we cannot accept responsibility for any errors or omissions. Any medical information  provided is for education/information purposes and is not designed to replace medical advice by a qualified medical professional.

 

What are the symptoms?

Most children with SMS have developmental delay and moderate-to-severe learning difficulties. In infancy low-muscle tone, feeding difficulties, failure to thrive and frequent ear infections are common. Speech delay tends to be more pronounced than motor delay, and language comprehension is more impaired than expression.

The most distinctive features of SMS are the behavioural problems. These include self-injurious behaviours such as hand biting, self-pinching or scratching, and picking at sores. Other behavioural problems include aggression, frequent temper tantrums, hyperactivity, restlessness and distractibility, and severe sleep disturbance. In many cases, the severe behaviour difficulties in children with the syndrome persist into adulthood.

Autistic-type behaviours (see entry Autism Spectrum disorders, including Asperger syndrome), such as resistance to change, repetitive questioning and preoccupations with particular topics are also common.

In contrast, children with SMS are often described as loving and caring, eager to please and with a good sense of humour. They usually enjoy interacting with adults.

Facial features of SMS include a flat, broad head and prominent forehead, heavy brows, up-slanting eyes, depressed nasal bridge, and a wide mouth with an inverted central portion of the upper lip. Other features include a hoarse, deep voice, short stature, eye problems (squint and abnormalities of the iris), hearing loss and scoliosis. Congenital heart disease (see entry Heart Defects), epilepsy and kidney abnormalities are less consistent features.

Clinical signs of peripheral neuropathy are found in 75 per cent of individuals, and include decreased sensitivity to pain and temperature, gait disturbances (leading to walking abnormalities) and muscle weakness.

What are the causes?

SMS is caused by a small deletion (microdeletion) on chromosome 17 (17p 11.2). Although this region contains multiple genes, loss of one particular gene, the retinoic acid induced 1 or RAI1, is responsible for most of the characteristic features of this condition. A small percentage of people with SMS have a mutation in the RAI1gene instead of a chromosomal deletion.

How is it diagnosed?

The characteristic behaviour and sleep disturbance coupled with the distinctive facial features often suggest the diagnosis, which can be confirmed by detailed chromosome testing (eg array CGH ) to look for the 17p11.2 microdeletion. In some cases diagnosis is made by sequencing the RAI1 gene.

How is it treated?

There is no cure for SMS, but there are several treatments that can help to improve symptoms in those affected. Melatonin has been used in some individuals with SMS to help correct sleep disturbance. Individuals will need to be managed by a multidisciplinary team of specialists who can help support their medical, educational and social care needs.

Inheritance patterns and prenatal diagnosis

Inheritance patterns
Most cases of SMS are sporadic in origin. Rarely, the 17p11.2 microdeletion may arise in a child because one of the parents might carry a 'balanced' rearrangement involving this region of the short arm of one chromosome 17. If the parental chromosomes are normal, then the recurrence risk of SMS is likely to be very small.

Prenatal diagnosis
Parents of children with SMS can be offered prenatal diagnosis for future pregnancies. This involves chorionic villus sampling (CVS) or amniocentesis, followed by analysis to look for the 17p11.2 microdeletion.

Is there support?

Smith-Magenis Syndrome Foundation UK (SMS)

Tel: 0300 101 0034
Email: info@smith-magenis.co.uk
www.smith-magenis.co.uk

The Foundation is a Registered Charity in England and Wales No. 1072573. It offers support and information, publishes a newsletter, raises awareness of the condition and supports research.

Group details last reviewed September 2020.