What is Prader-Willi Syndrome?

Characteristics

Prader-Willi Syndrome (PWS) was first described in 1956 by Swiss doctors, Prof. A Prader, Dr A Labhart and Dr H Willi, who recognised the condition as having unique and clearly definable features. These features are:-

• Hypotonia: weak muscle tone, and floppiness at birth.
• Hypogonadism: immature development of sexual organs and other sexual characteristics.
• Obesity: caused by excessive appetite and overeating (hyperphagia), and a decreased calorific requirement owing to low energy expenditure levels. (Obesity is not normally a feature of those whose food intake is strictly controlled.)
• Central nervous system and endocrine gland dysfunction: causing varying degrees of learning disability, short stature, hyperphagia, somnolence, and poor emotional and social development.

Many people with PWS also exhibit characteristic facial and other physical
features. These include: almond-shaped eyes, a narrow forehead (measured across), a down-turned mouth with a triangular-shaped upper lip, and small hands and feet.

People with PWS have poor large muscle strength, often coupled with poor coordination and balance. Muscle tone can be improved with appropriate therapy and exercise. Small muscle strength is usually better.

Most people with PWS have borderline or moderate learning difficulties. Some
people have recorded IQs of 90 or above, whilst a minority have severe learning difficulties. The average IQ is around 70 or slightly lower, although individuals may sometimes find it difficult to perform at their IQ level, as emotional and social skills are often less developed. Reading and writing skills are usually considerably better than number skills and abstract thinking. Individuals often excel at sedentary activities such as jigsaw puzzles, “wordsearches”, sewing, drawing and colouring.
 
Cause
PWS is caused by an abnormality on chromosome 15 which occurs around the time of conception. The majority of cases (about 70%) are caused by a deletion on the chromosome 15 inherited from the father, whilst about 25% are caused by inheriting two chromosome 15s from the mother, instead of one from the mother and one from the father (maternal disomy). A small minority of people have a translocation or imprinting irregularity involving chromosome 15. The recurrence risk of PWS in a family is very small. No blame attaches to either parent - currently PWS is thought to be a purely accidental occurrence.