This rare genetic disease causes a wide range of physical and cognitive symptoms.

Prader-Willi syndrome is a rare, congenital (present at birth) disease.

It affects multiple systems throughout the body, resulting in various physical, neurological, cognitive, endocrine (hormonal), and behavioral issues.

The syndrome affects 1 in every 15,000 to 25,000 people across the globe, according to the National Institute of Child Health and Human Development (NICHD).

It affects males and females equally, and there are no known risk factors for the syndrome, since the genetic error that causes it occurs at random.


Prader-Willi syndrome develops from genetic changes to a region of chromosome 15 called the Prader-Willi critical region (PWCR).

You inherit one copy of this chromosome from your mother, and one from your father. Some genes in the PWCR are only turned on — or active — in the paternal copy of the chromosome.

The syndrome is ultimately the result of a loss of function (or expression) of these paternally contributed genes.

There are three known mechanisms that cause this lack of gene expression, according to a 2012 report in the journal Genetics in Medicine.

Between 65 and 75 percent of Prader-Willi syndrome cases arise from the deletion of genes in the paternal PWCR, according to the report.

Another 20 to 30 percent of cases develop due to a phenomenon called maternal uniparental disomy, in which a person has two maternal copies, and no paternal copy, of chromosome 15.

Finally, 1 to 3 percent of cases are the result of a so-called imprinting defect that causes the paternal PWCR genes to be inactive (like the maternal genes).

Symptoms and Diagnosis

Low muscle tone, or hypotonia, in infancy characterizes nearly all cases of Prader-Willi syndrome, according to the 2012 report.

This can cause:

  • Poor feeding (inability to suck)
  • Decreased movement and lethargy
  • Weak cry
  • Poor reflexes

Additionally, 90 to 100 percent of infants with the syndrome have delayed motor development, requiring about twice as long to achieve developmental milestones.

Language-related delays are also common.

Later in childhood, other symptoms will develop, including:

  • Various behavioral problems, such as temper outbursts, stubbornness, and obsessive-compulsive tendencies
  • Sleep issues
  • Inability to feel full, resulting in rapid weight gain and obesity
  • Learning disabilities
  • Delayed or incomplete puberty
  • Underdeveloped genitals
  • High pain threshold

Certain physical features are also common among people with Prader-Willi syndrome, such as short stature, unusually small hands and feet, fair skin and hair, and certain facial characteristics, including a triangular mouth, almond-shaped eyes, and a narrow forehead.

These symptoms may be suggestive of the syndrome, but a diagnosis requires genetics tests (using blood samples), which can detect Prader-Willi syndrome in over 99 percent of people tested, according to the Genetics in Medicine report.

Treatment and Complications

There is no cure for Prader-Willi syndrome, but several treatment options can help manage the various symptoms it causes.

Growth hormones approved by the Food and Drug Administration (FDA) can help treat hypotonia and related issues by improving physical strength and muscle mass, increasing bone mineral density, and correcting stature deficiencies.

Weight management through strict supervision of food intake during childhood is also important for people with the syndrome, since obesity is a common issue.

Other possible treatments include:

  • Sex hormone replacement
  • Sleep monitoring
  • Speech and language therapy
  • Physical and behavioral therapy

If the condition isn't properly monitored and treated, people with Prader-Willi syndrome may develop one or more severe complications, including type 2 diabetes, congestive heart failure, and osteoporosis or other bone issues.

Editorial Sources and Fact-Checking

  • Cassidy et al. (2012). "Prader-Willi Syndrome." Genetics in Medicine.
  • Prader-Willi Syndrome; Genetics Home Reference.
  • Prader-Willi Syndrome (PWS); University of Michigan Health System.
  • Prader-Willi Syndrome; MedlinePlus/NIH.
  • Prader-Willi Syndrome; National Institute of Child Health and Human Development.
  • Cassidy and Driscoll (2009). "Prader–Willi syndrome." European Journal of Human Genetics.


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