What is Prader-Willi syndrome?
Prader-Willi syndrome is the commonest genetic cause of obesity. It was first described in 1887 by Langdon-Down, 70 years before Prader et al in 1956.
It is also known as Prader-Labhart-Willi syndrome.
*Image courtesy Museo del Prado. Eugenia Martínez Vallejo is believed to have suffered from Prader-Willi syndrome (ca 1680).
Who gets Prader-Willi syndrome?
The frequency of Prader-Willi syndrome is reported to be approximately 1 in 25,000 live births, but is likely to be more common due to a failure to diagnose the condition early.
Prader-Willi syndrome has autosomal dominant inheritance, affecting males and females of all races. However, most cases are sporadic.
What is the cause of Prader-Willi syndrome?
Prader-Willi syndrome results from the lack of expression of the PWS critical region of chromosome 15. The PWS genes are normally expressed only on the chromosome inherited from the father, and the copy inherited from the mother is switched off.
Three mechanisms are involved.
- In 75%, the lack of the PWS genes is inherited from an unaffected father (15q11-13 deletion).
- In 20%, both copies of chromosome 15 are inherited from the mother (maternal uniparental disomy, mUPD)
- In 5%, inactivation of these genes is due to an imprinting defect of the father’s PWS critical region.
Angelman syndrome is a clinically distinct disorder due to a maternally derived imprinting defect mapped to the same chromosome as Prader-Willi syndrome.
*Image courtesy Genetics 4 Medics
What are the clinical features of Prader-Willi syndrome?
Clinical features depend on age.
- Difficulty feeding with poor sucking reflex
- Diminished or absent cry
- Delayed early developmental milestones
- Failure to thrive
- Facial features as below
- Hospital admissions for chest infections
- Food seeking behavior leading to obesity
- Short height
- Temper tantrums
- High pain threshold
- Sleep disturbance
- Skin picking
- Characteristic facial appearance becomes more obvious
- Small hands/feet with thin tapering fingers
- Intellectual disability
- Hospital admissions for surgery
- Delayed puberty
- Small or undescended testes, reduced scrotal folding/wrinkling,
- Absent/irregular menstrual cycle
- Orthopaedic problems: scoliosis or kyphosis of spine; hip dysplasia; malaligned limbs, bowleggedness
Orthopaedic complications of Prader-Willi syndrome
Cutaneous features of Prader-Willi syndrome
Skin picking is very common and is the most typical cutaneous feature of Prader-Willi syndrome. Lesions are present at all stages of evolution. Signs include scratch marks, bleeding, bacterial skin infection, scabs, scarring, secondary milia — especially on the backs of the hands and forearms.
Other cutaneous features may include:
- Marked skin mottling in newborns
- Light colour of skin, hair and eyes: Fitzpatrick type 1–2 skin
- Abdominal striae (stretch marks) related to obesity/overeating in childhood
- Erysipelas — especially in the older age group
- Leg swelling/oedema with varicose veins
- Piezogenic pedal papules — reported in 4/5 patients examined
- Bruising due to high pain threshold
- Hypogonadism resulting in reduced or absent facial, armpit and pubic hair, especially in males
- Hair whorl in the anterior scalp
- Skin signs of diabetes mellitus including acanthosis nigricans.
- Thick saliva at edges of mouth
- Small teeth and poorly developed enamel resulting in dental caries
- High arched palate
- Rectal skin picking with consequent bleeding, ulcers, anaemia and constipation
- In females, poorly developed vulval labia (lips)
- In males, reduced scrotal wrinkling/folding
Skin conditions that can affect people with Prader-Willi syndrome
How is Prader-Willi syndrome diagnosed?
Clinical diagnostic criteria may be used to diagnose Prader-Willi syndrome.
Major criteria – each scores 1 point
- Characteristic facial features – almond shaped eyes, down-turned mouth, narrow distance between the temples, thin upper lip
- Delayed developmental milestones
- Feeding problems/failure to thrive in infancy
- Hypogonadism – underactive sex hormones
- Floppy at birth
- Rapid weight gain between 1 and 6 years of age
Minor criteria – each scores 0.5 point
- Decreased fetal movements and infantile lethargy
- Eyes – squint, short-sightedness
- Pale skin, hair and eye colour compared to the rest of the family
- Narrow hands with straight ulnar border
- Short height compared with family members
- Skin picking
- Sleep disturbance/sleep apnoea
Genetic studies are the most accurate way to diagnose Prader-Willi syndrome. They should be considered in any floppy newborn baby requiring tube feeding. All three mechanisms for the syndrome should be looked for sequentially, starting with the paternal deletion.
X-rays should be undertaken during childhood to identify skeletal problems, as these can be masked by obesity.
What are the complications of Prader-Willi syndrome?
In adult life, dermatologic problems requiring treatment are a major health issue, with erysipelas being a common reason for hospital admission.
Medical complications of Prader-Willi syndrome include:
- Morbid obesity
- High blood pressure
- Deep venous thrombosis (DVT, leg clots)
- Pulmonary embolus (clots to the lungs)
- Heart attacks
- Chest infections/pneumonia
- Diabetes mellitus type 2
- Abnormal drinking patterns leading to water intoxication
- Inability to vomit.
Differential diagnosis of Prader-Willi syndrome
Differential diagnosis of Prader-Willi syndrome includes other causes of obesity and failure to thrive in infancy and childhood.
Deletion 6q16 syndrome
Deletion 6q16 syndrome is a Prader-Willi-like syndrome due to proximal deletions of the long arm of chromosome 6. Its main features are:
- Obesity and excessive eating
- Small hands and feet
- Eye and vision problems
- Global developmental delay.
Management of Prader-Willi syndrome
Genetic counselling is recommended for future pregnancies and family members. Special care may be needed for general anaesthesia. Treatments deal with medical and surgical complications/associations as they arise and are related to age.
- Treatment may be required for difficulty with feeding and floppiness, for example, tube feeding.
- Respiratory infections are also common in this age group.
- The commonest causes for hospital admission in Prader-Willi syndrome are related to the disease, such as orthopaedic correction.
- Management of obsessive eating leading to excessive weight gain and other behavioural disorders become a major issue for families.
Hospital admissions may be required for:
- Hormone related issues, such as growth hormone deficiency
- Scoliosis surgery.
Hospital admission may be required for:
- Surgery for inguinal hernia
- Medical events related to diabetes mellitus and skin infections, for example erysipelas
- Psychiatric episodes
- Water or drug intoxication.
Medications used by adults with Prader-Willi syndrome include psychotropics, laxatives, skin products, and treatments for diabetes.
Respiratory infections are the major health issue in older adults.