This page summarises what population studies and newborn screening programmes tell us about how common MPS I and severe MPS I-H are worldwide, and why accurate data still remain a challenge.
Prevalence and incidence of MPS I (all forms)
MPS I as a whole is classified as a rare disease in all regions. Published birth prevalence estimates for MPS I (severe plus attenuated forms) typically range from about 1 in 100,000 to 1 in 150,000 live births, with higher or lower figures reported in specific populations. Many early estimates were based on diagnosed cases in specialist centres and almost certainly under-estimated the true incidence because of missed or late diagnoses, particularly in attenuated disease.
Examples from the literature:
- Several European studies have reported overall MPS I birth prevalence around 1 in 100,000 live births or slightly higher.
- North American registry and claims data suggest broadly similar figures, with local variation.
- Some regions with higher rates of consanguinity or founder mutations report higher MPS I incidence.
Because different studies use different methods and time periods, their numbers are not directly comparable, but they support the view that MPS I is very rare in any single country and that coordinated international registries are needed to understand the true burden.
Key idea: very rare in any single country
Hurler syndrome as a proportion of MPS I
Within the overall MPS I group, Hurler syndrome (severe MPS I) accounts for a substantial proportion of cases, especially in cohorts seen in paediatric metabolic centres. Many studies suggest that severe MPS I represents around 50–60 percent of all MPS I diagnoses, with the remainder made up of attenuated forms (Hurler–Scheie and Scheie).
Key points:
- In historical clinical series dominated by referred children, severe MPS I often appears more common because attenuated cases are under-diagnosed or diagnosed later.
- Newborn screening programmes, which detect both severe and attenuated cases, indicate that attenuated MPS I may be relatively more frequent than previously appreciated.
- Despite these nuances, Hurler syndrome remains the single largest subgroup within MPS I in many paediatric cohorts.
For a rough estimate, if overall MPS I occurs in about 1 in 100,000 live births, then severe MPS I-H might occur in the region of 1 in 150,000 to 1 in 250,000 live births in many populations, though the exact figure varies
Illustrative distribution
(not exact)
Differences between countries and populations
Reported incidence of MPS I and Hurler syndrome differs between countries for several reasons:
Genetics
Founder mutations in specific populations
Diagnostics
Access to testing and awareness
Screening
Newborn screening availability
Population structure
Consanguinity rates vary
Because of these factors, some published studies report MPS I birth prevalence well above 1 in 100,000 in specific regions, while others report lower figures. For families, the most important message is that Hurler syndrome is rare everywhere, so most doctors will see only a few, if any, patients in their careers.
What newborn screening teaches us about incidence
Newborn screening programmes for MPS I have been implemented in parts of North and South America, Europe and Asia. These programmes measure IDUA enzyme activity in dried blood spots collected shortly after birth. They have provided important new information about how often MPS I occurs and how many babies have severe versus attenuated disease.
Screen positive
Low IDUA result
True MPS I
Severe, attenuated, uncertain
Key observations from published screening experience:
- The screen-positive rate (initial low IDUA result) is much higher than the final confirmed MPS I incidence, because many babies have transient low enzyme activity or benign pseudodeficiency.
- After confirmatory testing, true MPS I incidence in screened populations often falls in the range of 1 in 50,000 to 1 in 250,000 live births, with substantial variation between regions.
- A significant proportion of screen-detected infants have attenuated or indeterminate phenotypes, highlighting the continuum of MPS I severity.
- Severe MPS I-H is usually identified within the first weeks of life in screening programmes, allowing early referral for HSCT assessment.
These data are improving our understanding of true birth incidence but also raise challenges about predicting severity and planning long-term follow up for attenuated and genotype-uncertain cases.
How many people are living with Hurler syndrome?
Because survival has improved with HSCT and ERT, the number of children and adults living with MPS I has increased, even though the number of new cases each year remains very small. Estimating the point prevalence (how many people are living with the condition at a given time) is complex and depends on:
- How long patients survive after treatment
- The age structure of the population
- How comprehensively cases are captured in registries or health records
In most countries, the total number of people living with MPS I is in the tens to low hundreds, and only a subset of these have severe MPS I-H. This means that no single centre will usually accumulate large numbers of patients, underscoring the need for international collaboration and shared data.
Children
Increasing
Adults
Growing
Limitations of current data
When reading about how common Hurler syndrome is, it is important to remember that different studies use different methods and definitions. Sources of variation include:
For these reasons, incidence and prevalence estimates are best viewed as approximations that highlight rarity, rather than exact figures applicable to every setting.
Further detail from the source pages
Further detail6 sections
Why "rare" does not mean "alone"
or families, being told that Hurler syndrome is extremely rare can feel isolating. It may help to know that, although the number of affected children in any one country is small, there is a global community of families, clinicians and researchers working together through:
Patient groups
International MPS societies
Clinical networks
Multicentre guidelines
Registries
Global natural history studies
Research
Collaborative clinical trials
These networks help ensure that knowledge gained from each person with Hurler syndrome benefits others around the world, even if local numbers are small.
Key points about prevalence and incidence
All forms of MPS I together are estimated to occur in roughly 1 in 100,000 live births in many populations, with local variation.
- Hurler syndrome (severe MPS I-H) accounts for a substantial proportion of MPS I cases, but the exact fraction varies between cohorts.
- Birth incidence of severe MPS I-H is often estimated in the broad range of 1 in 150,000 to 1 in 250,000 live births, though figures differ between studies and regions.
- Newborn screening programmes have refined our understanding of incidence and revealed more attenuated and genotype-uncertain cases than previously recognised.
- Because Hurler syndrome is ultra-rare, international collaboration, registries and shared expertise are essential for improving care and research.
Further detail — part 24 sections
What is Hurler syndrome?
Overview of the condition and how it is classified
Causes and genetics
IDUA gene, inheritance and family risks
Natural history and prognosis
How the disease course has changed with HSCT and ERT
Data, registries and advocacy
How registries and policy shape care for ultra-rare diseases