Prader-Willi syndrome

12 min read

What is Prader-Willi syndrome?

Prader-Willi syndrome (PWS) is a rare genetic condition that causes a wide range of symptoms.

Symptoms may include:

  • a constant desire to eat food, which seems driven by a permanent feeling of hunger and can easily lead to dangerous weight gain
  • restricted growth
    , leading to short stature
  • reduced muscle tone
  • learning difficulties
  • lack of secondary sexual development
  • behavioural problems, such as temper tantrums or stubbornness

Read more about

symptoms of Prader-Willi syndrome

Who is affected?

There are no known factors that make giving birth to a child with PWS more likely. It is caused by a fault in the child's chromosomes, which happens purely by chance.

Read more about the

causes of Prader-Willi syndrome

Living with Prader-Willi syndrome

There is no cure for PWS, so treatment aims to manage the symptoms and associated problems. For parents, this includes dealing with their child's behavioural problems and excessive eating.

Read more about managing Prader-Willi syndrome.

Restricting your child's diet is a particularly important part of managing their condition. If children with PWS are allowed to eat as much as they want, they will quickly become dangerously overweight. They will eat three to six times more than other children of the same age, and will probably still feel hungry.

However, restricting a child's diet is not easy. They can be highly obsessive when it comes to eating, and their hunger can cause them to hide or steal food.


While PWS itself is not life threatening, the compulsive eating and resulting weight gain can be.

Younger adults with PWS often develop obesity-related conditions usually seen in older adults, such as

type 2 diabetes
heart failure
. If the
is not treated, an adult with PWS will probably die a lot younger than would normally be expected.

Compulsive eating can also cause health problems such as an abnormally expanded stomach and choking. Because of the potential risk of choking, all parents of a child with PWS are advised to learn the Heimlich manoeuvre.

Read more about [what to do if someone is choking].

If a child with PWS manages to follow a restricted diet and control their weight, there is no reason why they cannot enjoy a good quality of life and take part in activities such as voluntary or part-time work. However, because of their behavioural problems and learning difficulties, it is unlikely they will be able to live fully independent lives.

Symptoms of Prader-Willi syndrome

Prader-Willi syndrome (PWS) can cause a wide range of symptoms that will affect your child's physical, psychological and behavioural development.


A feature of PWS that becomes apparent shortly after birth is your baby being unusually floppy due to weak muscles. The medical name for this is


Hypotonia can mean your baby:

  • does not have a full range of movement
  • has a weak cry
  • has poor reflexes
  • cannot suck properly, which means they may have feeding difficulties and be underweight until the age of one, and may need to be fed with a feeding tube or artificial nipple (see managing Prader-Willi syndrome)

Poor sexual development

Another feature that becomes apparent after birth is hypogonadism. Hypogonadism means the child has underactive testicles or ovaries, which play an important role in sexual development and reproduction.

Baby boys may have an abnormally small penis and one or both testicles may still be inside their abdomen (known as

undescended testicles
). Baby girls will also have under-developed genitals.

In later life, the child will go through puberty later than usual and may not go through full development into an adult. In boys, the voice may stay high-pitched like a child's and they may not have much facial and body hair. Girls will have delayed periods that often do not begin until their 30s. When periods do occur, they are usually irregular and very light.

Distinctive facial features

Children with PWS have several distinctive facial features usually noticeable at, or shortly after, birth. These are:

  • almond-shaped eyes
  • narrowing of the forehead at the temple
  • narrow bridge of the nose
  • thin upper lip and an upturned mouth

Overeating and obesity

Children with PWS eat an excessive amount of food, much more than the body needs to meet its energy requirements. The medical name for this is hyperphagia.

A child will start to overeat between the ages of one and four. By the time they reach school age, they may start to hide or steal food, steal money to buy food and even eat inappropriate things, such as food waste or frozen food.

Children with PWS can tolerate much larger amounts of food before their body automatically vomits it back up, and they are not as sensitive to pain. Therefore, they can eat objects that would make other people very ill. This means they are at higher risk of

food poisoning
and choking.

If their overeating is not managed, they will soon become obese.

Children with PWS also have a slow metabolism, which means it takes them longer to burn off energy from food. Lack of muscle tone contributes to them being less physically active than children without the condition, which also increases their risk of becoming obese.

Stomach rupture

When a person eats too much in a short space of time, the stomach can split open. This is known as gastric rupture. It can cause bacteria to be released into otherwise germ-free parts of the body, which can trigger a life-threatening infection.

It is thought that as many as one person in 35 with PWS dies as a result of this.

People with PWS who have a healthy or relatively healthy weight seem to be more at risk of gastric rupture than very obese people. This could be because their stomach shrinks after they have successfully controlled their eating habits for many months. If they then suddenly eat a vast quantity of food, their shrunken stomach cannot cope with it.

The two most common signs of a gastric rupture are vomiting and stomach pain.

People with PWS do not usually vomit or complain of stomach pain, so take these symptoms very seriously if they happen to your child. Call your care team or doctor immediately for advice.

Learning difficulties

Most children with PWS have mild to moderate learning difficulties with a low IQ. This is usually in the range of 50-85, which is moderate to borderline intellectual disability.

This means it takes longer for a child with PWS to reach important developmental milestones. For example, a child with PWS will typically begin sitting up at around 12 months and start walking and talking at around 24 months.

Most children with PWS can attend a mainstream school, although they may find some activities challenging, such as understanding and responding to verbal instructions and dealing with subjects such as mathematics. Their short-term memory is usually poor, although they often have a good long-term memory.

Despite these problems, children with PWS tend to be good at solving puzzles, such as word search puzzles and jigsaws.

Short stature

Children with PWS are usually much shorter than other children of their own age. This is usually apparent by the time they are two years old, and becomes particularly noticeable when they reach their 20s.

Low levels of human growth hormone (HGH) contribute to the short stature, and they will not go through the typical growth spurt during puberty.

If HGH is not replaced, the average adult height for a man with PWS is 159cm (5 feet 2 inches) and 149cm (4 feet 10 inches) for a woman.

Children can be prescribed an artificial version of human growth hormone to encourage growth (see managing Prader-Willi syndrome).

Behavioural problems

Most children with PWS will develop temper tantrums and fly into sudden rages. Tantrums may be triggered by certain things, such as if the child is denied food or does not get their own way.

Other typical behavioural problems are:

  • stubbornness and being very argumentative
  • controlling and manipulative behaviour
  • asking the same question over and over again, or frequently returning to the same topic in a conversation
  • persistent lying and refusing to accept blame

Many children will engage in compulsive behaviour, such as collecting objects or having a very strict daily routine, and any unexpected disruption to the routine can upset them.

The behavioural problems tend to get worse as the child gets older, and then improve as they develop into an adult.

While these behavioural problems can be challenging for parents to deal with, children with PWS also have many positive aspects to their personality, such as being affectionate, kind, caring and funny.

Skin picking

As many as four out of five children with PWS persistently pick their skin, usually their face, hands or arms. They may scratch, pierce or pull at their skin, sometimes using paper clips or tweezers. It can result in open sores, scarring and infection.

Some children may pick out pieces of faeces from their bottom. As well as causing embarrassment for children and parents, this can result in rectal bleeding and infection.

Sleep problems

Many children with PWS have problems with sleeping. For example, they may have interrupted breathing during sleep, known as

sleep apnoea
, which causes them to feel excessively sleepy the next day. This can be made worse by weight gain, so can be more of a problem when they get older.

Other features

Other features of PWS can include:

  • unusually fair hair, skin and eyes
  • small hands and feet
  • narrow hands
  • crossed eyes
  • short-sightedness
  • long-sightedness
  • decreased production of saliva, which can cause tooth decay
  • skin that is easily bruised
  • difficulty pronouncing certain sounds and words properly
  • abnormal curvature of the spine (

Weakening of the bones, known as

, is common in adults with PWS because they do not have the hormones needed to keep bones healthy.

Causes of Prader-Willi syndrome

Prader-Willi syndrome (PWS) is caused by a genetic defect on chromosome number 15.

Around 70% of cases of PWS are the result of missing genetic information from the copy of chromosome 15 inherited from the father. This defect is referred to as paternal deletion.

Paternal deletion is thought to occur entirely by chance, so it is practically unheard of to have more than one child with PWS caused by paternal deletion.

However, if PWS is caused by a different type of problem with chromosome 15, there is a chance – usually very small – that another child will have the condition.

How the chromosome defect causes PWS

It is thought the defect in chromosome 15 disrupts the normal development and functioning of part of the brain called the hypothalamus.

The hypothalamus plays a role in many functions of the body, such as producing hormones and helping regulate appetite. A malfunctioning hypothalamus may explain some of the typical features of PWS, such as delayed growth and persistent hunger.

Recent studies using advanced brain imaging technology have shown that after eating, people with PWS have very high levels of electrical activity in a part of the brain known as the frontal cortex. This part of the brain is associated with physical pleasure and feelings of contentment. It may be possible that people with PWS find the act of eating immensely rewarding, much like a drug-induced high, and are constantly seeking food to achieve that high.

Another theory is that in PWS, the hypothalamus cannot judge the level of food in the body as it normally does. This means that no matter how much food the person eats, they always feel hungry.

Diagnosing Prader-Willi syndrome

A definitive diagnosis of Prader-Willi syndrome (PWS) can usually be made by running a series of genetic tests.

Genetic testing

Genetic testing
can be used to check chromosomes in a sample of your child's blood for the genetic abnormalities known to cause PWS.

This will not only confirm the diagnosis of PWS, but should also be able to determine exactly what type of chromosome defect is responsible for your child's condition.

This information should allow you to determine the likelihood of having another child with PWS.

Read more about the

causes of Prader-Willi syndrome

Checklist for diagnosing Prader-Willi syndrome

A checklist of symptoms known to be typical of PWS is also thought to be very useful for identifying which children should be tested.

This checklist is also helpful for parents concerned about their child's development, behaviour and eating habits, and who want to know whether their child may have PWS.

Major criteria

Score one point for each symptom your child has:

  • weak muscles and floppiness that becomes apparent during or shortly after birth
  • feeding problems and failure to grow during the first year of life
  • rapid weight gain in children aged one to six years old
  • characteristic facial features, such as almond-shaped eyes and thin upper lips
  • underactive testicles or ovaries (hypogonadism), which results in delayed sexual development
  • delayed physical development or learning difficulties

Minor criteria

Score half a point for each symptom that your child has:

  • lack of movement during pregnancy, such as kicking in the womb, or the baby appears unusually lacking in energy after birth
  • sleep disturbances such as sleep apnoea
  • delayed or absent puberty
  • unusually fair hair, skin and eyes
  • narrow hands
  • thick and sticky saliva
  • crossed eyes or long-sightedness
  • problems pronouncing words and sounds properly
  • frequent skin picking

Total score

If your child is under the age of three and scores five points, with at least three points from the major criteria, a diagnosis of PWS would be strongly suspected and genetic testing would be recommended.

If your child is over the age of three and scores eight points, with at least four points from the major criteria, a diagnosis of PWS would be strongly suspected and testing recommended.

Important: Our website provides useful information but is not a substitute for medical advice. You should always seek the advice of your doctor when making decisions about your health.