Prader-Willi syndrome is a rare disorder present at birth that results in a number of physical, mental and behavioral problems.

What is it?

Prader-Willi syndrome is a rare disorder present at birth that results in a number of physical, mental and behavioral problems. A key feature of Prader-Willi syndrome is a constant sense of hunger that usually begins after the first year of life.

People with Prader-Willi syndrome want to eat constantly and usually have trouble controlling their weight. Many complications of Prader-Willi syndrome are due to obesity.

If your child has Prader-Willi syndrome, a team of specialists can work with you to manage your child's symptoms and reduce the risk of developing complications.



Signs and symptoms of Prader-Willi syndrome generally occur in two stages. Signs of the disorder that may be present in the first year of life include:

  • Poor muscle tone. A primary sign of Prader-Willi syndrome during infancy is poor muscle tone (hypotonia). Babies may rest with their elbows and knees loosely extended instead of fixed, and they may feel floppy or like "rag dolls" when they're held.
  • Distinct facial features. Children with Prader-Willi syndrome may be born with almond-shaped eyes, a narrowing of the head at the temples, a turned-down mouth and a thin upper lip.
  • Failure to thrive. During the first year of life, children with Prader-Willi syndrome may have a poor sucking reflex due to decreased muscle tone. Because poor sucking makes feeding difficult, they tend to gain weight slowly.
  • Lack of eye coordination (strabismus). The eyes of a child with Prader-Willi syndrome may not move together. Therefore, they may cross or wander to the side.
  • Generally poor responsiveness. A baby may seem unusually tired, respond poorly to stimulation, wake with difficulty or have a weak cry.

Early childhood

From about ages 1 to 6, other signs of Prader-Willi appear. These problems will remain present throughout life and require careful management or treatment. These signs may include:

  • Food craving and weight gain. The classic signs of the disorder are a constant craving for food and a rapid gain in weight. Because a child with Prader-Willi syndrome is always hungry, he or she eats frequently and consumes large portions. A child may develop unusual food-seeking behaviors, such as hoarding food or eating things such as garbage or frozen food.
  • Underdeveloped sex organs. A condition called hypogonadism occurs when sex organs — the testes in men and the ovaries in women — produce little or no sex hormones. This results in underdeveloped sex organs, incomplete development at puberty and in nearly all cases infertility.
  • Poor growth and physical development. Children with Prader-Willi syndrome have low muscle mass. They may have short hands and feet. When a person with the disorder reaches full adult stature, he or she is usually shorter than other family members.
  • Learning disabilities. Mild to moderate mental retardation is a common feature of the disorder. Essentially all people with Prader-Willi syndrome, even those without mental retardation, have learning disabilities.
  • Delayed motor development. Toddlers with Prader-Willi syndrome often reach milestones in physical movement later than other children. Sitting up by oneself may be delayed until 12 months and walking until 24 months.
  • Speech problems. Speech is often delayed until a child is 2 years of age. Poor articulation of words may be an ongoing problem.
  • Behavioral problems. Children may at times be very stubborn or throw temper tantrums, especially when denied food. They may also develop obsessive-compulsive disorder, which results in unwanted, recurring thoughts or repetitive behaviors, or both. Other mental health disorders may develop.
  • Sleep disorders. Some children with Prader-Willi syndrome may have sleep disorders, including disruptions of the normal sleep cycle and sleep apnea, a condition in which breathing pauses during sleep. Obesity may worsen sleep disorders.
  • Scoliosis. A number of children with Prader-Willi syndrome develop abnormal curvature of the spine (scoliosis).

Other signs and symptoms of Prader-Willi syndrome may include:

  • Nearsightedness (myopia)
  • Light skin compared with other family members
  • High pain tolerance
  • Skin picking


Prader-Willi syndrome is a genetic disorder, a condition caused by an error in a gene or genes. Although the exact genes responsible for Prader-Willi syndrome haven't been identified, the problem is known to lie in a particular region of chromosome 15.

With the exceptions of genes related to sex characteristics, all genes come in pairs, one copy inherited from your father (paternal gene) and one copy inherited from your mother (maternal gene). For most types of genes, if one copy is "active," or expressed, then the other copy is also expressed.

However, some types of genes act alone. In other words, it's normal with certain genes for the paternal gene to be expressed and the maternal gene to be "silent." Therefore, if there's an error with the paternal gene, that piece of genetic information is essentially missing.

What happens in Prader-Willi syndrome

Prader-Willi syndrome occurs because certain paternal genes that should be expressed aren't for one of the following reasons:

  • Paternal genes on chromosome 15 are missing.
  • The person has inherited two copies of chromosome 15 from the mother and no chromosome 15 from the father.
  • There's some error or defect in paternal genes on chromosome 15.

The genetic defect of Prader-Willi syndrome disrupts the normal functions of a portion of the brain called the hypothalamus. Among its many functions, the hypothalamus controls hunger and thirst and releases hormones that prompt the release of other substances responsible for growth and sexual development. A malfunctioning hypothalamus — caused by the defect on chromosome 15 — interferes with each of these processes, resulting in uncontrollable hunger, stunted growth, sexual underdevelopment and other characteristics of Prader-Willi syndrome. 

Risk factors

Defective or missing portions of paternal genes responsible for Prader-Willi syndrome usually occur randomly. This means that in most instances, Prader-Willi syndrome can't be prevented. However, in a small number of cases, a genetic mutation inherited from the father may cause Prader-Willi syndrome.

If you have a child with Prader-Willi syndrome and would like to have another baby, consider seeking genetic counseling. A genetic counselor may help determine your risk of having another child with Prader-Willi syndrome.


Obesity-related complications

Many of the possible complications of Prader-Willi syndrome result from obesity. In addition to having constant hunger, people with the disorder have low muscle mass, which requires lower than average calorie needs. This combination of factors makes a person prone to obesity and the medical problems related to obesity. These possible complications include:

  • Type 2 diabetes. Type 2 diabetes is high blood sugar level due to your body's inability to use insulin efficiently. Insulin plays a vital role in making blood sugar (glucose) — your body's fuel — available to your cells. Obesity significantly increases the risk of diabetes.
  • Heart disease and stroke. People who are obese are more likely to have high blood pressure, hardened arteries, high cholesterol and other factors that can lead to heart disease and stroke.
  • Arthritis. Excess weight can cause the bone and cartilage of joints to wear down, resulting in osteoarthritis.
  • Sleep apnea. This sleep disorder is characterized by pauses in breathing during sleep. The disorder can cause daytime fatigue, high blood pressure and, rarely, sudden death. People with Prader-Willi syndrome may have sleep apnea or other sleep disorders even if they aren't obese, but obesity can worsen sleep problems.

Complications of hypogonadism

Other complications arise from hypogonadism, a condition in which your sex organs don't secrete sufficient amounts of the sex hormones testosterone (males) and oestrogen and progesterone (females). These may include:

  • Sterility. Although there have been a few reports of women with Prader-Willi syndrome becoming pregnant, most people with this disorder are unable to have children (sterile).
  • Osteoporosis. Osteoporosis is a condition characterized by weak and brittle bones, which can break easily. People with Prader-Willi syndrome are at an increased risk of developing osteoporosis because they have low levels of sex hormones, which help maintain strong bones.

Other complications

Eating large amounts of food quickly, called binge eating, can cause your child's stomach to become abnormally distended (gastric dilatation). Binge eating can also cause choking and any of the other complications associated with obesity.


Prader-Willi syndrome, to determine the cause. The presence of other signs, such as almond-shaped eyes or narrowing of the head at the temples, may also prompt testing. In older children, behavioral problems and weight gain are often the key diagnostic signs of the disorder.

A definitive diagnosis can almost always be made with a laboratory test. Special genetic tests can identify abnormalities in your child's chromosomes that are characteristic of Prader-Willi syndrome.