Prader-Willi syndrome

What it is

Prader-Willi syndrome (PWS) is a rare genetic condition affecting approximately 1 in 10,000 to 30,000 births — roughly 1 in 15,000 on average — and is the most common genetic cause of life-threatening obesity in children. Also known by its full name Prader-Willi syndrome, PWS results from the loss of function of specific paternally expressed genes in the chromosome 15q11-q13 region, disrupting hypothalamic regulation of hunger, hormones, and development. In Canada, an estimated 350–500 individuals are born with PWS each decade, and the condition is managed through specialized pediatric endocrinology programs at centres such as SickKids (Toronto), BC Children's Hospital, and the Montreal Children's Hospital.

PWS presents in two clinically distinct phases. In infancy, the picture is dominated by severe hypotonia (profoundly reduced muscle tone), feeding difficulties requiring tube feeding, and failure to thrive. Starting around age 2 to 5, the syndrome shifts dramatically: appetite becomes insatiable and never-ending, a biological drive that — without strict environmental management — leads to severe, life-threatening obesity. Growth hormone deficiency is present in virtually all individuals with PWS and is one of the most treatable aspects of the condition.

Causes and mechanism

PWS is caused by absent expression of paternally inherited genes in the 15q11-q13 chromosomal region. Three genetic mechanisms account for nearly all cases:

Mechanism	Proportion of cases	What happens
Paternal deletion	~70%	A segment of the paternal chromosome 15 is deleted, removing the relevant genes entirely
Maternal uniparental disomy (UPD)	~25%	Both copies of chromosome 15 are inherited from the mother; no paternal copy of the critical genes is present
Imprinting defect	~5%	The paternal chromosome 15 is present but silenced by an error in the genomic imprinting process

The downstream hormonal consequences all trace back to hypothalamic dysfunction. The PWS genetic region is a critical regulator of hypothalamic signalling, and its disruption produces three major hormonal effects: near-universal growth hormone (GH) deficiency; central hypogonadism causing incomplete puberty, infertility, and low sex hormone levels; and dramatically elevated ghrelin — the hunger-signalling hormone — which remains persistently high regardless of food intake, preventing any normal satiety signal from arriving.

Symptoms and diagnosis

Phase 1 — infancy: Severe hypotonia from birth; poor suckling and feeding requiring nasogastric tube support; weak cry; failure to thrive; characteristic facial features including almond-shaped eyes, narrow forehead, and thin upper lip.

Phase 2 — early childhood through adulthood: Hyperphagia (relentless, biologically driven hunger that never feels satisfied); food-seeking behaviour including hoarding, eating non-food items, and extreme distress when food is unavailable; progressive obesity without strict dietary control; short stature from GH deficiency; incomplete or absent puberty from central hypogonadism; mild-to-moderate intellectual disability; obsessive-compulsive tendencies, skin-picking, and temper outbursts; sleep disorders including obstructive sleep apnea and excessive daytime sleepiness.

Diagnosis follows a structured workup:

1. Chromosomal methylation analysis — first-line genetic test; detects the abnormal methylation pattern present in all three PWS subtypes
2. FISH and chromosomal microarray — identifies the specific mechanism (deletion, UPD, or imprinting defect)
3. Hormone panel — GH/IGF-1 axis, LH, FSH, testosterone or estradiol, thyroid function
4. Polysomnography — to assess for obstructive sleep apnea
5. Neuropsychological testing — to characterize intellectual and behavioural profile

Genetic testing can successfully diagnose nearly all infants with PWS, and early diagnosis is strongly associated with better developmental outcomes.

Treatment options

PWS management requires a multidisciplinary team spanning endocrinology, genetics, dietetics, psychology, and developmental paediatrics. There is no cure.

Growth hormone replacement is one of the most impactful interventions available. Recombinant human GH (rhGH) therapy, initiated in infancy or early childhood and continued into adulthood, improves height, reduces body fat, increases lean muscle mass, improves physical stamina, and supports cognitive and motor development. Many countries now recommend starting GH therapy before age 2 to maximize developmental benefit; Canadian pediatric endocrinologists generally follow this approach, and rhGH is covered under most provincial drug benefit programs for children with confirmed GH deficiency.

Food and weight management is the single most critical ongoing intervention. Kitchens and food storage must be locked; individuals with PWS cannot be left unsupervised around food at any age. A specialist dietitian should design a structured, low-calorie, nutrient-dense meal plan. Regular physical activity — ideally broken into shorter sessions to accommodate reduced stamina — is also important.

Sex hormone replacement at puberty (testosterone for males; estrogen/progesterone for females) supports secondary sexual development, bone density, and wellbeing.

Behavioural and developmental support — structured predictable routines, cognitive behavioural therapy, occupational therapy, and supported living arrangements in adulthood — substantially improves quality of life and functional independence.

Emerging pharmacological options under active investigation include oxytocin nasal spray (targeting social behaviour and hyperphagia) and setmelanotide, an MC4R agonist being studied for hyperphagia reduction and weight loss. Neither is currently approved by Health Canada specifically for PWS.

When to see a clinician in Canada

Seek immediate medical evaluation for a newborn or infant who feels unusually floppy (severe hypotonia), has significant feeding difficulties or failure to thrive, or shows the characteristic facial features described above. A referral to a pediatric genetics or pediatric endocrinology service is appropriate; in Canada, this typically flows through a family physician or pediatrician to a provincial children's hospital genetics clinic.

For an older child, seek evaluation if a child develops an insatiable appetite with rapid weight gain beginning in early childhood, alongside significant behavioural and cognitive developmental concerns. Early diagnosis through genetic testing is critical — GH therapy initiated in infancy produces meaningfully better outcomes for growth, body composition, and development than therapy started later.

Limitations and open questions

Research is still emerging on the precise hypothalamic pathways that drive hyperphagia in PWS, and no pharmacological treatment has yet been approved specifically to suppress the insatiable hunger that defines the condition. Setmelanotide and oxytocin remain investigational for PWS indications; Health Canada has not yet issued guidance on either for this population. The long-term cardiovascular and metabolic outcomes of adults with PWS who receive modern GH therapy from infancy are not yet fully characterized, as the first cohorts treated this way are only now reaching middle age. The optimal duration and dosing of GH therapy in adulthood also remains an area of active study. Genetic counselling is recommended for all families following a diagnosis, as recurrence risk varies substantially by genetic subtype — from less than 1% for most deletion and UPD cases to as high as 50% in the rare imprinting centre mutation subtype.