Prader-Willi Syndrome (PWS)

What causes Prader-Willi syndrome

People inherit two copies of chromosome 15, one from each parent. Some genes in the 15q11-q13 region are active only on the copy from the father (a process called genomic imprinting). PWS occurs when these paternal genes are missing or inactive, through:

• Paternal deletion (about 65 to 75 percent): part of the father's chromosome 15 is missing.
• Maternal uniparental disomy (about 20 to 30 percent): both copies of chromosome 15 come from the mother.
• Imprinting defect (a few percent): the father's genes are present but switched off.

Most cases are not inherited and occur by chance. Recurrence risk for siblings is low, except with some imprinting defects. Genetic counseling helps families understand their risk.

Angelman syndrome involves the same chromosome region but loss of the maternal gene, and has very different features.

Symptoms by age

Pregnancy and infancy

• Reduced fetal movement and sometimes extra amniotic fluid
• Severe floppiness (hypotonia)
• Weak cry and weak suck, poor feeding, often needing tube feeding
• Sleepiness and poor weight gain
• Small genitals, undescended testicles in boys

Early childhood

• Delayed motor milestones and speech
• Improving feeding, then increasing appetite and weight gain from around ages 2 to 4
• Characteristic features such as narrow forehead, almond-shaped eyes, small hands and feet

Childhood to adulthood

• Hyperphagia: intense, persistent hunger, lack of fullness after eating, food seeking and hoarding
• Obesity if food is not controlled. See obesity
• Short stature due partly to growth hormone deficiency
• Hypogonadism: incomplete puberty and infertility in most
• Learning disabilities, usually mild to moderate intellectual disability
• Behavior: temper outbursts, stubbornness, rigidity, obsessive-compulsive behaviors, skin picking, anxiety
• Sleep problems: sleep apnea and excessive daytime sleepiness
• Scoliosis and low bone density
• High pain threshold, reduced ability to vomit and unstable body temperature

Diagnosis

PWS is diagnosed with DNA methylation testing, which detects about 99 percent of cases. Further tests identify the genetic type. Testing is recommended for newborns with unexplained severe floppiness and poor suck. Early diagnosis allows early treatment.

Treatment and care

Nutrition and food security

• Specialized feeding support in infancy
• A structured, calorie-controlled diet from early childhood
• Food security: predictable meal times, no unplanned access to food, and often locked kitchens, refrigerators and pantries. This reduces anxiety around food and prevents dangerous overeating
• Regular weight monitoring with a dietitian
• In the US, a medicine specifically approved to treat hyperphagia in PWS became available in 2025. Ask your specialist whether it is appropriate

Growth hormone

Growth hormone treatment, often started in infancy, improves muscle tone, strength, body composition, height and development. Sleep studies are usually done before and during treatment.

Other treatments

• Physical, occupational and speech therapy from infancy
• Early intervention and special education support
• Sex hormone replacement at puberty
• Treatment of sleep apnea
• Behavioral therapy and, in some people, medicines for anxiety, obsessive behaviors or skin picking
• Scoliosis monitoring and treatment
• Regular eye, dental and bone checks

Important medical cautions

People with PWS may:

• not develop a fever with infection
• not feel pain normally, so serious problems such as fractures or stomach rupture can be missed
• rarely vomit, so vomiting can signal a serious illness
• react strongly to some sedatives and anesthetics

Families often carry a medical alert card explaining these risks.

Outlook

With early diagnosis, growth hormone and strict food management, many people with PWS live into adulthood with good health. Life expectancy has improved but is still reduced, mainly due to complications of obesity, respiratory problems, choking and stomach rupture. Many adults live in supported group homes designed for PWS.

Frequently asked questions

What causes Prader-Willi syndrome?

PWS is caused by missing activity of genes on the father's copy of chromosome 15, in the 15q11-q13 region. This happens through a deletion on the father's chromosome, both copies coming from the mother, or an imprinting defect. Most cases occur by chance and are not inherited.

Why are people with Prader-Willi syndrome always hungry?

The genetic changes affect the hypothalamus, the part of the brain that controls hunger and fullness. People with PWS do not feel full after eating and experience intense, persistent hunger. This is a biological drive, not a lack of willpower, and needs a secure food environment.

What are the symptoms of Prader-Willi syndrome in babies?

Babies typically have severe floppiness, a weak cry, a weak suck, poor feeding, sleepiness and slow weight gain. Boys often have undescended testicles. Many need tube feeding at first. Genetic testing confirms the diagnosis.

What is the life expectancy with Prader-Willi syndrome?

Life expectancy has improved greatly with early diagnosis, growth hormone and careful food management, and many people live into their 50s or beyond. Obesity-related complications, breathing problems, choking and stomach rupture remain the main risks.