• Disease Overview
  • Synonyms
  • Signs & Symptoms
  • Causes
  • Affected Populations
  • Disorders with Similar Symptoms
  • Diagnosis
  • Standard Therapies
  • Clinical Trials and Studies
  • References
  • Programs & Resources
  • Complete Report

Prader-Willi Syndrome


Last updated: May 30, 2018
Years published: 1984, 1985, 1987, 1988, 1989, 1992, 1994, 1996, 1997, 1998, 1999, 2000, 2002, 2004, 2005, 2009, 2012, 2015, 2018


NORD is very grateful to Merlin G. Butler, MD, PhD, Director, Division of Research and Genetics, Director, KUMC Genetics Clinic and Professor of Psychiatry & Behavioral Sciences and Pediatrics, University of Kansas Medical Center, for assistance in the preparation of this report.

Disease Overview


Prader-Willi syndrome (PWS) is a genetic multisystem disorder characterized during infancy by lethargy, diminished muscle tone (hypotonia), a weak suck and feeding difficulties with poor weight gain and growth and other hormone deficiency. In childhood, features of this disorder include short stature, small genitals and an excessive appetite. Affected individuals do not feel satisfied after completing a meal (satiety). Without intervention, overeating can lead to onset of life-threatening obesity. The food compulsion requires constant supervision. Individuals with severe obesity may have an increased risk of cardiac insufficiency, sleep apnea, diabetes, respiratory problems and other serious conditions that can cause life-threatening complications. All individuals with PWS have some cognitive impairment that ranges from low normal intelligence with learning disabilities to mild to moderate intellectual disability. Behavioral problems are common and can include temper tantrums, obsessive/compulsive behavior, and skin picking. Motor milestones and language development are often delayed. PWS occurs due to abnormalities affecting certain genes in the proximal long arm of chromosome 15 when deleted from the father’s chromosome 15 and hence referred to as a genomic imprinting disorder which depends on the sex of the parent donating the chromosome leading to the chromosome defect in the child. These abnormalities usually result from random (sporadic) errors in egg or sperm development, but are sometimes inherited.


Originally described in the medical literature in 1956, PWS is the first disorder confirmed to be due to imprinting errors (see Causes section). It is the most common genetic cause of life-threatening childhood obesity. The disorder was once known as hypogonadism, hypotonia, hypomentia, obesity (HHHO).

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  • Prader-Labhart-Willi syndrome
  • PWS
  • Willi-Prader syndrome
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Signs & Symptoms

The symptoms and severity of PWS can vary from one person to another. Many features of the disorder are nonspecific and others may develop slowly over time or can be subtle. It is important to note that affected individuals may not have all of the symptoms discussed below. Affected individuals should talk to their physician and medical team about their specific case, associated symptoms and overall prognosis. Often this requires input from a clinical geneticist or genetic counselor with experience in this genetic disorder to supply the most recent and accurate information about the disorder and discuss genetic testing options or treatment plans.

Initially, infants will exhibit diminished muscle tone (hypotonia), which can cause a baby to feel “floppy” when held. Infantile hypotonia, which is often severe, is a near universal feature of the disorder. Hypotonia can be present before birth (prenatally) potentially causing decreased fetal movements and abnormal positioning of the fetus (e.g., breech position). Prenatal hypotonia is associated with an increased risk of requiring an assisted delivery. After birth, hypotonia is associated with lethargy, a weak cry, poor responsiveness to stimuli, and poor reflexes including poor sucking ability, which result in feeding difficulties and failure to thrive. Infants are usually unable to breastfeed and may require tube feeding. Hypotonia slowly improves over time, but some adults with PWS may continue to have some degree of hypotonia.

Affected infants may also have distinctive facial features including almond-shaped eyes, a thin upper lip, a downturned mouth, a narrow bridge of the nose, a narrow forehead, and a disproportionately long, narrow head (dolichocephaly). Distinctive facial features can be noticeable shortly after birth or may develop slowly over time.

As affected infants grow older, their feeding and appetite will improve and they will grow appropriately. Typically, between 2-4.5 years of age, their weight increases although there may not be a noticeable change in appetite or caloric intake. Between 4.5-8 years old, appetite and caloric intake usually increases, often thereafter developing a need to eat an extraordinarily large amount of food (hyperphagia) usually because they do not feel satisfied after completing a meal (satiety). In addition, there is a decreased calorie requirement in people with PWS due to low muscle, decreased metabolism and decreased physical activity if not treated with growth hormone replacement. Consequently, overeating, rapid weight gain, and morbid obesity occurs if not controlled by others. Not all affected children will go through these stages.

If left uncontrolled and untreated, morbid obesity can develop, potentially leading to life-threatening heart and lung complications, diabetes, high blood pressure (hypertension), and to other serious complications. The compulsion to eat is so overwhelming that people with this disorder, if left unsupervised, may endanger themselves by eating harmful food such as spoiled food or garbage and excessive quantities, harmful to the stomach. Affected children may also exhibit unusual behaviors regarding food including hoarding and/or foraging for food, stealing food, and stealing money to buy food.

Some affected individuals have developed serious, life-threatening gastrointestinal complications due to episodes of binge eating. Such complications can include severe bloating (gastric dilatation) with the development of a hole or tear in the intestinal wall (perforation) and tissue loss (necrosis). They are noted to have decreased gastric emptying and swallowing difficulties.

Children with PWS also have varying levels of cognitive impairment, ranging from borderline or low normal intelligence with learning disabilities to mild to moderate intellectual disability. The attainment of motor milestones (e.g. walking or sitting up) and language development are often delayed.

Affected children generally have sweet and loving personalities, but often develop distinct behavioral issues. Such issues can include temper tantrums, stubbornness, obsessive/compulsive behavior, manipulative behavior, and skin picking, which can cause chronic open wounds, scarring and infection. In some cases, the behavior profile may be suggestive of autism. Psychosis occurs in approximately 10-20% of late adolescents and young adults. Evidence shows that the type of chromosome 15 defect may relate to certain learning and behavioral problems.

Hypogonadism is a common finding in PWS. Hypogonadism refers to inadequate function of the sex organs, the testes in males and the ovaries in females. The sex organs in affected individuals fail to produce sufficient sex hormones, which can result in underdeveloped sex organs, incomplete development at puberty, delayed onset of puberty, and infertility. Genital underdevelopment is evident at birth. Affected males may exhibit a small penis, underdeveloped scrotum, and small testes. Failure of one or both testes to descend (cryptorchidism) is a common finding, as well. Affected females may exhibit an abnormally small clitoris or labia minor. Absence of a menstrual cycle (primary amenorrhea) is common and in some cases the initial menstrual period (menarche) may not occur until 30 years of age or older.

Individuals with PWS have growth hormone (GH) insufficiency, a condition characterized by the inadequate secretion of growth hormone from the anterior pituitary gland, a small gland located at the base of the brain that is responsible for the production of several hormones. Children may be significantly below average height based upon sex and age (short stature). GH deficiency affects both children and adults and the final adult height of affected individuals is shorter than unaffected family members. GH insufficiency also results in an increase in the amount of fat and a relative decrease in muscle mass (altered body composition).

Affected individuals may also have abnormally small hands and feet, side-to-side curvature of the spine (scoliosis), and, in approximately 10% of individuals, a malformed hip (hip dysplasia). Scoliosis can occur at any age including infancy, varies in severity and should be monitored. Sleep problems are common, including excessive daytime sleepiness, reduced rapid eye movement (REM) latency, disruption of the normal sleep cycle, and central and/or obstructive sleep apnea.

Some individuals may have lack of color (pigment) known as hypopigmentation affecting the hair, eyes and skin particularly in those with the chromosome 15q deletion seen in about 60% of those with PWS (discussed in Causes). They may appear fair-skinned compared to other family members. Nearsightedness (myopia) and misaligned eyes (strabismus) may also occur.

Affected individuals may also experience recurrent respiratory infections. Up to 25% of affected individuals may have an underactive thyroid gland (hypothyroidism). In addition, the rates of certain conditions are increased in individuals with PWS including fractures due to decreased bone density (osteopenia), altered temperature sensation, a high vomiting threshold, and swelling (edema) and ulcerations of the legs, especially in obese adults. Some individuals may have reduced flow of saliva with abnormally thick, sticky saliva. Additional symptoms that can occur in affected individuals include a high pain threshold, a tendency to bruise easily without known cause, and seizures.

Some individuals with PWS may develop central adrenal insufficiency (CAI), a condition characterized by deficiency of adrenocorticotropic hormone (ACTH). This hormone is produced by the pituitary gland. One of the main functions of ACTH is to stimulate the adrenal glands to produce cortisol, which helps to regulate blood sugar and the body to deal with stress. In some cases, CAI may only be detectable during periods of stress (e.g., during illness or injury). The exact percentage of affected individuals with CAI and its overall implications to individuals with PWS are not yet fully understood. Natural history, causes of death and survival trends in PWS have recently been published and useful to gain knowledge and current understanding and diagnosis with co-morbidities in order to better monitor the health status and prognosis of someone with PWS.

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PWS occurs when the genes in a specific region of chromosome 15 are not present or do not function. This region of chromosome 15 is located at 15q11.2-q13 and has been designated the Prader-Will syndrome/Angelman syndrome region (PWS/AS). In individuals with PWS, the nonfunctioning PWS/AS region is always located on the number 15 chromosome inherited from the father.

Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated as X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”. Chromosomes are further sub-divided into many bands that are numbered. For example, “chromosome 15q11.2-q13” refers to bands 11.2-13 on the long arm of chromosome 15. The numbered bands specify the location of the thousands of genes that are present on each chromosome.

The changes affecting the genes associated with PWS may involve changes in the structure of a gene (genetic factors) or changes in the function or expression of a gene (epigenetics). Three specific abnormalities are primarily associated with PWS – chromosomal 15q11-q13 deletion, maternal uniparental disomy 15 or both chromosome 15s from the mother and genetic imprinting errors in the region controlling gene activity on chromosome 15.

PWS is associated with a specific process known as genetic imprinting. Normally, everyone has two copies of every gene – one received from the father and one received from the mother. In most cases, both genes are “turned on” or active. However, some genes are preferentially silenced or “turned off” based upon which parent gave the gene to the child (genetic imprinting). Genetic imprinting is controlled by chemical switches through a process called methylation and other chemical changes at the DNA level. Proper genetic imprinting is necessary for normal development. Defective imprinting has been associated with several disorders including PWS.

Imprinted genes tend to cluster or group together on chromosomes. Several imprinted genes are found in a cluster on the long arm (q) of chromosome 15. The cluster contains a functional region known as an imprinting center that regulates activity of the imprinted genes in this region.

In most people with PWS (about 60%), the PWS/AS region of the father’s chromosome 15 is missing or deleted. This chromosomal deletion results from a random error in development and is not inherited (or de novo deletion). Thus, most cases of PWS occur sporadically and the risk of recurrence in another pregnancy is less than 1%.

Recent data shows that in about 35% of people with PWS, the affected person inherits two copies of chromosome 15 from the mother and no copy of the father’s chromosome 15 (referred to as maternal uniparental disomy). This type of genetic change also occurs as a result of a random error in development. In most cases, the risk of recurrence of uniparental disomy is estimated to be less than 1%.

In less than 5% of people with PWS, the PWS/AS region of the father’s chromosome 15 is present, but the genes do not work properly. This form of PWS is due to a defect in genes called the imprinting center and is sometimes due to a genetic change (e.g., microdeletion) that can be passed from one generation to the next.

In a very small proportion of affected people, PWS has occurred due to a balanced translocation of chromosome 15. Translocations occur when portions of certain chromosomes break off and are rearranged, resulting in shifting of genetic (e.g., genes of the imprinting center) material and an altered set of chromosomes. If a chromosomal translocation is balanced (meaning that it consists of rearranged chromosomes without anything missing or extra), then it is usually harmless to the carrier. However, such a chromosomal rearrangement may be associated with an increased risk of abnormal chromosomal development in the carrier’s offspring depending on events that occur in the egg or sperm production.

A number of imprinted genes have been mapped to the PWS/AS region of chromosome 15. However, the specific genes involved and their role in the development of the various symptoms of PWS are being characterized but not yet known. Many symptoms associated with PWS are believed to be due to malfunction of the hypothalamus, a gland in the brain that regulates hormone secretions and under genetic control. Hormones produced by the hypothalamus affect body temperature, hunger, moods, sex drive, sleep, and thirst. The hypothalamus also influences the release of hormones from other glands, especially the pituitary gland, which regulates the release of certain hormones including growth and sex hormones.

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Affected populations

PWS affects males and females in equal numbers and occurs in all ethnic groups and geographic regions in the world. Most estimates place the incidence between 1 in 10,000-30,000 individuals in the general population and about 350,000-400,000 individuals worldwide.

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A diagnosis of PWS is based upon a detailed patient history, a thorough clinical evaluation and identification of characteristic symptoms. Consensus diagnostic criteria for PWS have been established and are effective for identifying potential cases of PWS but genetic testing is required to confirm the diagnosis and to identify the specific genetic subtype (15q11-q13 deletion, maternal disomy 15, imprinting defect). Hence, all infants and newborns with unexplained hypotonia and poor suck should be tested for PWS. To confirm a diagnosis of PWS, certain specialized tests are required including DNA methylation tests and fluorescent in situ hybridization (FISH). More recently, high resolution chromosomal microarray studies with several hundred thousand DNA probes from throughout the genome representing all chromosomes can be utilized to identify small deletion or duplications of the chromosomes that cannot be seen with routine chromosome studies. Microarrays are most useful in identifying the typical chromosome 15q11q13 deletion, smaller rearrangements of this chromosome region, imprinting defects and specific maternal disomy 15 subclasses seen in PWS.

Clinical Testing and Workup

Approximately 99% of people with PWS can be diagnosed by DNA methylation study. This test allows for the examination of gene activity status in the PWS/AS critical region of chromosome 15. If the methylation pattern is consistent with maternal inheritance, then this indicates that the paternal chromosome 15 is not present or not active. This finding is diagnostic of PWS, but methylation tests cannot distinguish among the different underlying causes of PWS (i.e., deletions, imprinting defects or maternal disomy 15).

If DNA methylation tests indicate PWS, then additional tests are necessary to determine the underlying cause of the disorder. This is important for determining whether there is an increased risk for the parents or other family members to have an affected child. High resolution microarrays have replaced the FISH test as microarrays can detect the size of the 15q11-q13 deletion, the most common genetic cause of PWS. High resolution microarrays can also detect maternal disomy 15 if present in the majority of those with PWS having no recognized deletion but both chromosome 15s come from the mother. There are two typical chromosomal 15q11-q13 deletion subtypes (larger Type I and smaller Type II) and three subclasses of maternal disomy 15 (heterodisomy, segmental isodisomy, total isodisomy) in PWS. Clinical and behavioral differences are reported in those with the PWS deletion subtypes or those with maternal disomy 15 subclasses. Those with the larger 15q11-q13 Type I deletion have more learning and behavioral problems and those with maternal disomy 15 are more prone to autistic findings and psychosis in young adulthood. If no deletion is present on chromosome 15, then additional testing is performed to distinguish between maternal disomy 15 or a defect of the imprinting center.

Prenatal diagnosis is possible in families with a previous history of PWS. Prior identification of a disease causing abnormality can facilitate prenatal testing, but it is available by methylation analysis following amniocentesis regardless of cause.

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Standard Therapies


The treatment of PWS is directed toward the specific symptoms that are apparent in each individual. Early intervention and strict maintenance to treatment can greatly improve the overall health and quality of life for affected individuals and their families. Treatment may require the coordinated efforts of a team of specialists. Clinical geneticists, pediatricians, orthopedists, endocrinologists, speech therapists, psychologists, dieticians, nutritionists, and other healthcare professionals may need to systematically and comprehensively plan an effective program for the child’s treatment. Genetic counseling may be of benefit for affected individuals and their families to further discuss the condition, provide information and to quote recurrence risks. Parents are strongly recommended to undergo appropriate parenting techniques for the behavioral and eating issues associated with PWS; such education correlates with better prognosis.

Specific therapeutic procedures and interventions may vary, depending upon numerous factors, such as disease severity; the presence or absence of certain symptoms; an individual’s age and general health; and/or other elements. Decisions concerning the use of particular drug regimens and/or other treatments should be made by physicians and other members of the health care team in careful consultation with the patient based upon the specifics of his or her case; a thorough discussion of the potential benefits and risks, including possible side effects and long-term effects; patient preference; and other appropriate factors.

In infants, special nipples or gavage feeding may be used to ensure adequate nutrition. Gavage feeding is a procedure in which a small, thin tube is passed through the nose and mouth to the stomach to directly feed a newborn who has feeding difficulties.

In males, the treatment of hypogonadism with either testosterone or human chorionic gonadotropin may be beneficial during infancy, potentially increasing the size of the genitalia or prompting testicular descent into the scrotum when cryptorchidism is present. Although cryptorchidism may occasionally resolve spontaneously or with hormone therapy, most males may require surgical treatment.

Individuals with PWS also benefit from growth hormone (GH) therapy, which can help to increase height, improve lean body mass, mobility and respiratory function, decrease body fat and ultimately improve the quality of life. Some studies have shown that GH therapy may improve development and behavior. In June of 2000, the Food and Drug Administration (FDA) approved the use of human growth hormone for the treatment of children with genetically-confirmed PWS and evidence of growth failure. Studies have shown that the earlier GH therapy is started the more beneficial it is and that therapy can begin as early as two to three months of age. GH therapy has been shown to improve facial appearance and overall body build (body habitus). Development of standardized growth charts for PWS with and without growth hormone treatment have been generated and can be used to monitor the growth parameters at specific ages in PWS. It is recommended that affected individuals undergo a sleep study to detect and treat obstructive sleep apnea before initiating GH therapy because some reports suggest a link between premature death and GH therapy in certain individuals with PWS (e.g., those with profound hypotonia or obesity and pre-existing respiratory or cardiac problems). However, other researchers have expressed doubt as to whether GH therapy had a direct role in these cases but decisions regarding GH therapy in individuals with PWS are best made after consultation with a pediatric endocrinologist after a sleep study and assessments for adrenal gland insufficiency.

Children with PWS require early intervention to assess and treat issues with motor skills, intellectual disability, and speech and language development. Early intervention may include physical and occupational therapy, special education, and speech therapy. An individualized education plan should be created at the start of school. Behavioral therapy and, in some cases, psychoactive medications such as specific serotonin reuptake inhibitors may be beneficial to manage difficult behavior or psychosis.

Children should receive an ophthalmological exam to evaluate for eye abnormalities potentially associated with PWS such as strabismus and to assess visual acuity. Children should also be assessed for hip dysplasia and scoliosis which can occur in this patient population. Evaluation and treatment of sleep disturbance is recommended as well. Some researchers recommend that all individuals with PWS be screened for hypothyroidism (which occurs with increased incidence in PWS) and central adrenal insufficiency.

During childhood, a program consisting of a low calorie diet, regular exercise and a strict supervision of food intake and access should be formulated. Strict supervision of food intake should be based upon height, weight and body mass index (BMI). Such a program should begin before signs of obesity to help to prevent its development. Limiting the access to food may require locking cabinets and refrigerators. Some individuals may require vitamin supplementation, especially for calcium and vitamin D.

Sex hormones can be replaced at puberty as they can stimulate the development of secondary sexual characteristics and improve self-image and bone density. In males, the use of such therapy has been controversial because testosterone replacement by monthly injection may contribute to behavioral issues in males; use of a testosterone patch or gel will avert this problem. Sex hormone replacement therapy may increase the risk of stroke in females, as in the general population, and hygiene issues should also be considered. Sex education and consideration of contraception are important, particularly in females, as rarely pregnancy has occurred. Decreased flow of saliva may be improved with special toothpastes, gels, mouthwash and gum.

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Clinical Trials and Studies

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: [email protected]

Some current clinical trials also are posted on the following page on the NORD website:

For information about clinical trials sponsored by private sources, contact:

For information about clinical trials conducted in Europe, contact:

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