Prader-Willi syndrome (PWS) is the most common known genetic cause of life-threatening obesity in children.  Although the cause is complex, it results from an abnormality on the 15th chromosome.  It occurrs in males and females equally and in all races.  Prevalence estimates have ranged from 1:8,000 to 1:25,000 with the most likely figure being 1:15,000.

PWS has been historically known as a two stage syndrome, with the first stage characterized by hypotonia (low muscle tone) and poor feeding in infancy, and the second stage exhibiting hyperphagia (an uncontrollable drive to eat), combined with weight gain on fewer calories.  Current research through an extensive natural history study has shown that PWS is not just a two stage but a multiphase syndrome:

Other typical characteristics of Prader-Willi syndrome include low muscle tone, motor development delays, short stature (if not treated with growth hormone), and incomplete sexual development.  The average IQ is 70, but even those with normal IQ’s almost always have learning difficulties or differences. Children with PWS can be sweet and loving, but they display characteristics of the Prader-Willi personality that can cause social and behavioral problems.  They also have many complex and unique medical issues.