Signs and symptoms of Prader-Willi syndrome can vary among individuals. Symptoms may slowly change over time from childhood to adulthood.

Infants

Signs and symptoms that may be present from birth include:

• Poor muscle tone. A primary sign 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 may be born with almond-shaped eyes, a narrowing of the head at the temples, a turned-down mouth and a thin upper lip.
• Poor sucking reflex. Infants may have a poor sucking reflex due to decreased muscle tone. Poor sucking makes feeding difficult and can result in failure to thrive.
• Generally poor responsiveness. A baby may seem unusually tired, respond poorly to stimulation, have a hard time waking up or have a weak cry.
• Underdeveloped genitals. Males may have a small penis and scrotum. The testicles may be small or not descended from the abdomen into the scrotum (cryptorchidism). In females, the clitoris and labia may be small.

Early childhood to adulthood

Other features of Prader-Willi syndrome appear during early childhood and remain throughout life, requiring careful management. These features may include:

• Food craving and weight gain. A classic sign of Prader-Willi syndrome is a constant craving for food, resulting in rapid weight gain, starting around age 2 years. Constant hunger leads to eating often and consuming large portions. Unusual food-seeking behaviors, such as hoarding food, or eating frozen food or even garbage, may develop.
• Underdeveloped sex organs. A condition called hypogonadism occurs when sex organs (testes in men and ovaries in women) produce little or no sex hormones. This results in underdeveloped sex organs, incomplete or delayed puberty, and in nearly all cases, infertility. Without treatment, women may not start menstruating until their 30s or may never menstruate, and men may not have much facial hair and their voices may never fully deepen.
• Poor growth and physical development. Underproduction of growth hormone can result in short adult height, low muscle mass and high body fat. Other endocrine problems may include underproduction of thyroid hormone (hypothyroidism) or central adrenal insufficiency, which prevents the body from responding appropriately during stress or infections.
• Cognitive impairment. Mild to moderate intellectual disability, such as issues with thinking, reasoning and problem-solving, is a common feature of the disorder. Even those without significant intellectual disability have some learning disabilities.
• Delayed motor development. Toddlers with Prader-Willi syndrome often reach milestones in physical movement — for example, sitting up or walking — later than other children do.
• Speech problems. Speech is often delayed. Poor articulation of words may be an ongoing problem into adulthood.
• Behavioral problems. Children and adults may at times be stubborn, angry, controlling or manipulative. They may throw temper tantrums, especially when denied food, and may not tolerate changes in routine. They may also develop obsessive-compulsive or repetitive behaviors, or both. Other mental health disorders, such as anxiety and skin picking, may develop.
• Sleep disorders. Children and adults with Prader-Willi syndrome may have sleep disorders, including disruptions of the normal sleep cycle and a condition in which breathing pauses during sleep (sleep apnea). These disorders can result in excessive daytime sleepiness and worsen behavior problems.
• Other signs and symptoms. These may include small hands and feet, curvature of the spine (scoliosis), hip problems, reduced saliva flow, nearsightedness and other vision problems, problems regulating body temperature, a high pain tolerance, or a lack of pigment (hypopigmentation) causing hair, eyes and skin to be pale.

Prader-Willi syndrome is a genetic disorder, a condition caused by an error in one or more genes. Although the exact mechanisms responsible for Prader-Willi syndrome haven't been identified, the problem lies in the genes located in a particular region of chromosome 15.

With the exception 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 also is expressed, although it's normal for some types of genes to act alone.

Prader-Willi syndrome occurs because certain paternal genes that should be expressed are not for one of these reasons:

• Paternal genes on chromosome 15 are missing.
• The child 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.

In Prader-Willi syndrome, a defect on chromosome 15 disrupts the normal functions of a portion of the brain called the hypothalamus, which controls the release of hormones. A hypothalamus that isn't functioning properly can interfere with processes that result in problems with hunger, growth, sexual development, body temperature, mood and sleep.

In most cases, Prader-Willi syndrome is caused by a random genetic error and is not inherited. Determining which genetic defect caused Prader-Willi syndrome can be helpful in genetic counseling.

Obesity-related complications

In addition to having constant hunger, people with Prader-Willi syndrome have low muscle mass, so they need fewer than average calories, and they may not be physically active. This combination of factors makes them prone to obesity and the medical problems related to obesity, such as:

• Type 2 diabetes
• High blood pressure, high cholesterol and heart disease
• Sleep apnea
• Other complications, such as an increased risk of liver disease and gallbladder stones

Complications of inadequate hormone production

Complications arising from inadequate hormone production 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.
• Osteoporosis. Osteoporosis causes bones to become weak and brittle, so they may break easily. People with Prader-Willi syndrome are at an increased risk of developing osteoporosis because they have low levels of sex hormones and may also have low levels of growth hormone — both hormones help maintain strong bones.

Other complications

Other complications that can result from Prader-Willi syndrome include:

• Effects of binge eating. Eating large amounts of food quickly, called binge eating, can cause the stomach to become abnormally enlarged. People with Prader-Willi syndrome may not report pain and they rarely vomit. Binge eating can also cause choking. Rarely, a person may eat so much that it causes stomach rupture.
• Reduced quality of life. Behavioral problems can interfere with family functioning, successful education and social participation. They can also reduce the quality of life for children, teenagers and adults with Prader-Willi syndrome.