PWS has a prevalence of 1 in every 20,000 to 30,000 births, making it one of the most common genetic causes of life-threatening obesity. Worldwide, around 400,000 individuals are affected, with 20,000 in the United States. Both females and males are equally affected, with no discernible differences noted among racial and ethnic groups.

PWS is caused by the loss of function of genes in a specific region of chromosome 15, the 15q11.2-q13 region. Multiple cases of PWS have been associated with deletions of a gene cluster known as SNORD116 located within this region. Normally, people inherit one copy of this chromosome from each parent. There are three genetic mechanisms that can be involved:

1. Paternal Deletion (60–70% of cases): A segment of the chromosome 15 inherited from the father is physically missing.

2. Maternal Uniparental Disomy (25–35% of cases): The individual inherits two copies of chromosome 15 from their mother and zero copies from their father.

3. Imprinting Defect (1–3% of cases): The paternal chromosome 15 is present, but a mutation in a critical region affecting gene expression causes specific genes to remain inactive.

Most PWS cases occur sporadically and are not inherited. 

The genetic abnormalities of PWS result in the absence of expression of several key genes necessary for normal function of the hypothalamus, a key region of the brain involved in hormonal signaling. This dysfunction underlies the major clinical features of PWS, including muscle weakness, poor feeding in infancy, followed by impaired satiety, severe obesity, hormonal imbalances and behavioral problems in later childhood and adulthood.

PWS manifests differently across the lifespan, with two broad phases:

Infancy and early childhood: The main characteristics include weak “floppy” muscle tone, poor suckling reflex resulting in feeding difficulties, behavioral problems, intellectual disability, endocrine deficiencies, hyperphagia (excessive appetite), and a high risk of morbid obesity and related illnesses. 

Later childhood through adulthood: Individuals affected by PWS exhibit hyperphagia, excessive weight gain, skeletal problems (such as short stature, small hands and feet), incomplete puberty, sleep disorders, learning difficulties and behavioral problems.

Testing for PWS is usually prompted by physical symptoms during early infancy, such as poor suckling reflex, feeding difficulties and weak muscle tone. Clinical diagnosis is confirmed with genetic tests such as:

• DNA methylation analysis: A highly accurate blood test confirms the diagnosis in over 99% of cases by checking if specific paternal genes on chromosome 15 are active (methylated DNA regions are associated with inactivity).

• Chromosomal microarray or FISH (fluorescence in situ hybridization) testing: These secondary tests are used to determine the exact underlying genetic mechanism (Deletion, UPD, or Imprinting Defect) for genetic counseling purposes.

As needed, additional workup may include thyroid function tests, growth hormone stimulation studies, gonadotropin levels, bone density scans, sleep studies, and neuropsychological evaluation.

There is currently no cure for PWS. Treatment focuses on managing symptoms, preventing severe obesity, and improving quality of life through a multidisciplinary approach..

Nutritional and Weight Management

• May require lifelong strict dietary supervision

• Calorie-controlled diet

• Environmental controls to limit food access

• Regular physical activity and exercise programs 

• VYKAT XR (diazoxide choline) drug tablets for the treatment of hyperphagia in adult and pediatric patients aged 4 and older.

Hormonal Treatment

• Growth hormone therapy early in life to improve:

  • Height

  • Muscle mass

  • Physical strength

  • Body composition

  • Developmental outcomes

• Sex hormones (testosterone or estrogen) to induce puberty and support bone density

• Treatment of thyroid, adrenal, or other endocrine deficiencies 

Developmental Support

• Physical therapy to build muscle strength and improve motor skills

• Occupational therapy

• Speech and language therapy

• Special education services 

Behavioral and Psychiatric Care

• Behavioral therapy

• Psychological support

• Medications when needed for anxiety, obsessive-compulsive symptoms, or other psychiatric manifestations 

Management of Complications

• Treatment of sleep apnea

• Monitoring for scoliosis (abnormal curvature of spine)

• Diabetes screening and treatment

• Bone health monitoring

• Regular ophthalmologic and dental care

If diet and weight are successfully managed, individuals with PWS can have a normal life expectancy and enjoy reasonably healthy, fulfilling lives. Early intervention, particularly growth hormone therapy, dietary management, and behavioral support substantially improves long-term outcomes. However, without strict external food controls, the resulting extreme obesity can lead to fatal complications such as type 2 diabetes, severe sleep apnea, cardiac failure, choking, and gastric rupture. Because the drive to eat is neurologically hardwired and continuous, individuals with PWS generally require supportive living environments and supervision throughout their entire adult lives.

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3. Tan Q, Potter KJ, Burnett LC, Orsso CE, Inman M, Ryman DC, Haqq AM. 2020. “Prader-Willi-Like Phenotype Caused by an Atypical 15q11.2 Microdeletion.” Genes (Basel). 11(2):128. doi: https://doi.org/10.3390/genes11020128. 

4. Foundation for Prader-Willi Research (FPWR):  What is Prader-Willi syndrome?

5. MedlinePlus Genetics:  Prader-Willi syndrome.

6. National Organization for Rare Disorders (NORD):  Prader-Willi syndrome.