Prader-Willi syndrome

Prader-Willi syndrome is a congenital (present from birth) disease characterized by Obesity, decreased muscle tone, decreased mental capacity, and hypogonadism.

Causes, incidence, and risk factors

Prader-Willi is caused by the deletion of a gene on chromosome 15. The majority of patients have a deletion of the father’s DNA in this region. The remaining patients frequently have two copies of the mother’s chromosome 15. The maternal copy of this gene is turned off in all people. When there is a deletion of the father’s DNA (approximately 70% of patients), the disease occurs. This is because the patient is left with only the inactive, maternal copy.

Signs of Prader-Willi may be seen at birth. New infants with the condition are often small and very floppy (hypotonic). Male infants may have undescended testicles. The growing child exhibits slow mental and delayed motor development, increasing Obesity, and characteristically small hands and feet.

Rapid weight gain may occur during the first few years because the patient develops uncontrollable hunger which leads to morbid Obesity.

Mental development is slow, and the IQ seldom exceeds 80. However, children with Prader-Willi generally are very happy, smile frequently, and are pleasant to be around.

Affected children have an intense craving for food and will do almost anything to get it. This results in uncontrollable weight gain. Morbid Obesity (the degree of Obesity that seriously affects health) may lead to respiratory failure with hypoxia (low blood oxygen levels), cor pulmonale (right-sided heart failure), and death.


  • floppynewborn infant (hypotonic)  
  • small for gestational age  
  • undescended testicles in the male infant  
  • delayed motor development  
  • slow mental development  
  • very small hands and feet in comparison to body  
  • rapid weight gain with marked Obesity  
  • insatiable appetite, food craving  
  • almond-shaped eyes  
  • narrow bifrontal skull  
  • morbid Obesity  
  • skeletal (limb) abnormalities  
  • stria

Signs and tests

  • hypotonia  
  • hypomentia  
  • hypogonadism  
  • Obesity

Other signs related to morbid Obesity

  • hypoxia (chronic)  
  • hypercapnia (chronic)  
  • cor pulmonale  
  • hyperinsulinism  
  • abnormal glucose tolerance (see Glucose tolerance test)  
  • orthopedic, knee, and hip problems  
  • failure to respond to luteinizing hormone releasing factor  
  • skull and narrow bifrontal diameter


Obesity represents the greatest problem to health. Limiting caloric intake will control the Obesity but the family, neighbors, school, and other institutions must cooperate closely as the child will attempt to obtain food wherever possible.

Recent studies have demonstrated benefits of growth hormone treatment in causing accelerated growth and decreasing percent body fat. Growth hormone has also been shown to inprove physical strength and agility in patients with Prader-Willi syndrome.

A micropenis (very small penis) in the male infant may be corrected with a short course of testosterone.

Hypogonadism may be corrected at puberty with hormone replacement.

Support Groups

For additional resources and support, see:

Expectations (prognosis)
Appropriate education will be needed for the affected person’s IQ level. Weight control will allow for a much more comfortable and healthful life.


  • Diabetes  
  • cor pulmonale  
  • orthopedic problems

Calling your health care provider
Call your health care provider if your child has symptoms of this condition. The disorder is frequently suspected at birth.

Johns Hopkins patient information

Last revised: December 4, 2012
by Janet G. Derge, M.D.

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