Prader-Willi Syndrome is a rare genetic disorder associated with developmental delay, obesity, and obsessive behavior related to eating. It is the most common genetic cause of life-threatening obesity in children.
People with Prader-Willi syndrome have a problem in their hypothalamus, a part of the brain that normally controls feelings of fullness or hunger. As a result, they never feel full and have a constant urge to eat that is not completely under their control.
Most cases of Prader-Willi syndrome result from a spontaneous genetic error in genes on chromosome 15 that occurs at conception. In very rare cases, the mutation is inherited.
- Prader-Labhart-Willi syndrome
Prader-Willi is caused by abnormalities of the genetic code on chromosome 15. The genetic mistake that leads to Prader-Willi almost always happens during conception, which means it could happen to anyone—it is very seldom inherited. There are at least three different chromosome errors in this part of chromosome 15 that cause Prader-Willi syndrome.
The most common way that Prader-Willi happens is that a tiny piece of chromosome inherited from the father is deleted. Even though everyone has two copies of chromosome 15, one from the father and one from the mother, the mother's copy does not function because it is normally turned off, or imprinted, by a process called methylation. This isn't a problem in healthy people because they use their father's copy, but in Prader-Willi, that copy is missing.
If, instead, the mother's copy of chromosome 15 undergoes the same deletion, the child gets a different genetic disorder called Angelman syndrome, because there is one gene on the father's chromosome 15 that is normally turned off. Angelman syndrome and Prader-Willi syndrome are sometimes called sister syndromes because the genes responsible come from the same spot on the chromosome—the difference is in which parent's copy is defective.
Signs and Symptoms
There are generally two stages of symptoms for people with Prader-Willi syndrome:
- Stage 1: Babies with Prader-Willi have hypotonia or low muscle tone as newborns, which can affect their ability to suck properly. This is often described as a "failure to thrive." As a result, babies may need special feeding techniques to help them eat, and they may have problems gaining weight. Tube feedings may be required. As these babies grow older, their strength and muscle tone usually get better. They meet motor milestones, but are usually slower in doing so.
- Stage 2: Between the ages of one and six years old, the disorder changes to one of constant hunger and food seeking. Most people with Prader-Willi syndrome have an insatiable appetite. They never feel full. In fact, their brains are telling them they are starving. They have trouble regulating their own eating and need external restrictions on food, including locked kitchen and food storage areas.
This problem is made worse because people with Prader-Willi syndrome use fewer calories than those without the syndrome, since they have less muscle mass. The combination of eating massive amounts of food and not burning enough calories can lead to life-threatening obesity if the diet is not kept under strict control. Strict diet and exercise are absolutely essential, along with vitamin and calcium supplements.
Other signs of the disease include the following:
- Behavioral problems such as stubbornness or temper tantrums, usually during transitions and unanticipated changes
- Hypogonadism: incomplete sexual development, undescended testicles, small penis, delayed puberty 
- Distinctive facial features: narrow face, almond shape eyes, small mouth with thin upper lip and down-turned corners 
- Delayed motor skills and speech due to low muscle tone
- Cognitive problems, ranging from near normal intelligence to moderate mental retardation; learning disabilities are common
- Repetitive thoughts and verbalizations
- Collecting and hoarding of possessions
- Picking at skin
- Short stature, small hands and feet
- Fair skin 
Prader-Willi syndrome is considered a spectrum disorder, meaning not all symptoms will occur in everyone affected, and the symptoms may range from mild to severe.
People with Prader-Willi often have some mental strengths as well, such as long-term memory, reading ability, and receptive language.
Diagnostic criteria were formalized in 1993. Though Prader-Willi can be diagnosed by examination and reference to those criteria, genetic testing is recommended for people who have a number of the clinical symptoms. Genetic tests include DNA methylation analysis to confirm diagnosis of PWS. In addition, FISH and DNA techniques can identify the specific genetic cause and associated recurrence risk.
Prader-Willi syndrome cannot be cured. But early intervention can help people build skills for adapting to the disorder. Early diagnosis can also help parents learn about the condition and prepare for future challenges. A health care provider can do a blood test to check for Prader-Willi syndrome.
Exercise and physical activity can help control weight and help with motor skills. Speech therapy may be necessary.
Human growth hormone (HGH) has been found to be helpful in treating Prader-Willi syndrome. It can help to increase height, decrease body fat, and increase muscle mass, thus leading to improved agility. There has also been some research with other insulin growth factors like IGF-1, which is abnormally low in Prader-Willi patients. However, no medications have yet been found to control appetite in those with Prader-Willi.
Exams and tests
Tests that help providers care for Prader-Willi patients include
- determination of body mass index (BMI) by calculating it from weight and height,
- blood test for levels of hormones from the hypothalamus,
- blood test for human growth hormone (HgH) and insulin growth hormone 1 (IGF-1) levels,
- dual energy x-ray absorptiometry (DEXA) to check bone density, and
- possibly other scans to determine muscle mass and true fat mass.
Severe obesity is the major problem that results from the chromosome abnormality in Prader-Willi syndrome. Obesity brings with it a host of related problems, including diabetes, high blood pressure, and heart problems.
Other health problems that are common in people with Prader-Willi include the following:
- Strabismus (crossed eyes)
- Premature osteoporosis
- Sleep disturbances and obstructive sleep apnea
- Dental problems
Prader-Willi syndrome was first formally described in the medical literature in 1956 by Swiss doctors A. Prader, H. Willi, and A. Labhart. The genetic abnormality was discovered in 1981 by David Ledbetter while he was a graduate student.
- ↑ The Genetics of Prader-Willi Syndrome: An Explanation for the Rest of Us
- ↑ Mann MR, Bartolomei MS. Towards a molecular understanding of Prader-Willi and Angelman syndromes. Hum Mol Genet. 1999;8(10):1867-73. Abstract | Full Text
- ↑ Eiholzer U, l'Allemand D, Rousson V et al. Hypothalamic and gonadal components of hypogonadism in boys with Prader-Labhart-Willi syndrome. J Clin Endocrinol Metab. 2006 Mar;91(3):892-8. Epub 2005 Dec 13. Abstract | Full Text
- ↑ Butler MG, Levine GJ, Le JY, Hall BD, Cassidy SB. Photoanthropometric study of craniofacial traits of individuals with Prader-Willi syndrome. Am J Med Genet. 1995 Jul 31;58(1):38-45. Abstract
- ↑ Butler MG. Hypopigmentation: a common feature of Prader-Labhart-Willi syndrome. Am J Hum Genet. 1989 Jul;45(1):140-6. Abstract | Full Text
- ↑ Holm VA, Cassidy SB, Butler MG et al. Prader-Willi syndrome: consensus diagnostic criteria. Pediatrics. 1993 Feb;91(2):398-402. Abstract
- ↑ Carrel AL, Myers SE, Whitman BY, Allen DB. Benefits of long-term GH therapy in Prader-Willi syndrome: a 4-year study. J Clin Endocrinol Metab. 2002 Apr;87(4):1581-5. Abstract | Full Text
- ↑ Miller JL, Goldstone AP, Couch JA et al. Pituitary abnormalities in Prader-Willi syndrome and early onset morbid obesity. Am J Med Genet A. 2008 Mar 1;146(5):570-7. Abstract
- ↑ Whittington JE, Holland AJ, Webb T, Butler JV, Clarke DJ, Boer H. Population prevalence and estimated birth incidence and mortality rate for people with Prader–Willi syndrome in one UK Health Region. J Med Genet 2001;38: 792–798. Abstract | Full Text
- ↑ Vogels A, Van Den Ende J, Keymolen K et al. Minimum prevalence, birth incidence and cause of death for Prader–Willi syndrome in Flanders. Eur J Hum Genet. 2004;12:238–240. Abstract | Full Text
- ↑ Smith A, Egan J, Ridley G et al. Birth prevalence of Prader–Willi syndrome in Australia. Arch Dis Child. 2003;88:263–264. Abstract | Full Text
- ↑ Nixon GM, Brouillette RT. Sleep and breathing in Prader-Willi syndrome. Pediatr Pulmonol. 2002 Sep;34(3):209-17. Abstract
- ↑ Bailleul-Forestier I, Verhaeghe V, Fryns JP, Vinckier F, Declerck D, Vogels A. The oro-dental phenotype in Prader-Willi syndrome: a survey of 15 patients. Int J Paediatr Dent. 2008 Jan;18(1):40-7. Abstract | Full Text
- ↑ Ledbetter DH, Riccardi VM, Airhart SD, Strobel RJ, Keenan BS, Crawford JD. Deletions of chromosome 15 as a cause of the Prader-Willi syndrome. N Engl J Med. 1981 Feb 5;304(6):325-9. Abstract
- ↑ Ledbetter DH, Mascarello JT, Riccardi VM, Harper VD, Airhart SD, Strobel RJ. Chromosome 15 abnormalities and the Prader-Willi syndrome: a follow-up report of 40 cases. Am J Hum Genet. 1982 Mar;34(2):278-85. Abstract | Full Text