Causes and genetics of Hurler syndrome (MPS I-H)
Hurler syndrome (mucopolysaccharidosis type I-H, MPS I-H) is caused by a deficiency of the lysosomal enzyme alpha-L-iduronidase (IDUA). When IDUA activity is very low or absent, specific complex sugars called glycosaminoglycans (GAGs) build up inside cells and progressively damage many organs. This enzyme problem is the direct result of disease-causing variants in the IDUA gene, inherited in an autosomal recessive pattern.
This page explains, in more scientific detail, how IDUA deficiency leads to GAG storage, how Hurler syndrome is inherited within families and what this means for carrier testing and family planning.
MPS I Disease Spectrum
Severe
Hurler syndrome (MPS I-H)
Attenuated
Hurler-Scheie / Scheie (MPS I-H/S, MPS I-S)
Hurler, Hurler–Scheie and Scheie: the original subtypes
Historically, doctors divided MPS I into three clinical syndromes: Hurler (MPS I-H), Hurler–Scheie (MPS I-H/S) and Scheie (MPS I-S). Hurler was described as the most severe form, with early onset and rapid progression. Hurler–Scheie was an intermediate form. Scheie was the mildest form, with little or no brain involvement and onset in later childhood or adulthood.
As more children and adults were followed over time, it became clear that many patients did not fit neatly into one of these three labels. Instead, there was a continuum of severity with overlap between groups. Current guidelines now recommend thinking of MPS I as a spectrum rather than three separate diseases.
Hurler
MPS I-H
Hurler–Scheie
MPS I-H/S
MPS I-S
Mildest
Original clinical labels, now seen as a spectrum
Severe MPS I (Hurler) and attenuated MPS I
Because of this overlap, most experts and guidelines now use a simpler, two-group classification: severe MPS I and attenuated MPS I. Severe MPS I corresponds to Hurler syndrome, with early onset, rapid progression and significant central nervous system involvement. Attenuated MPS I covers what used to be called Hurler–Scheie and Scheie syndromes, which start later and have little or no cognitive decline.
Severe MPS I
(Hurler syndrome, MPS I-H)
- Onset typically in the first year of life
- Rapidly progressive somatic disease
- Developmental delay, then loss of skills if untreated
- Life-limiting in childhood without early treatment
Attenuated MPS I
(Hurler–Scheie and Scheie)
- Onset usually between age 3 and late childhood, sometimes adulthood
- Slower progression, often normal or near-normal intelligence
- Prominent joint, skeletal, heart and airway disease
- Many patients live into adulthood but with significant disability
Key point for this site:
Hurler syndrome (severe MPS I) is the focus of hurlersyndrome.org. Attenuated MPS I is mentioned mainly to give context and to help explain the full range of MPS I.
Clinical differences across the spectrum
Severe and attenuated MPS I share many features, but differ in the age at first symptoms, brain involvement and overall prognosis.
Age at first clear symptoms
Severe MPS I (Hurler)
Usually before 12 to 18 months of age
Attenuated MPS I
Often between 3 and 10 years, sometimes later
Somatic (body) features
Both groups can have coarse facial features, organ enlargement, skeletal disease, heart and airway involvement, but these appear earlier and progress faster in severe MPS I
Natural history without modern treatment
Severe MPS I
Usually fatal in the first decade of life
Survival into the second, third or later decades, often with serious heart and lung complications
Brain and development
Severe
Global developmental delay, followed by cognitive regression if untreated, behavioural changes, and eventual severe intellectual disability
Attenuated
Normal or only mildly reduced intelligence, learning difficulties rather than global decline
Treatment approach
Early haematopoietic stem cell transplantation (HSCT) is the standard disease-modifying treatment, often combined with enzyme replacement therapy (ERT)
ERT is the main disease-modifying therapy, and HSCT is usually not recommended except in selected cases
How clinicians determine severity in practice
In real life, clinicians do not rely on a single test to decide where a child sits on the MPS I spectrum. Instead, they use a combination of:
- Age at first symptoms and first clear signs
- Pattern and rate of developmental progress
- Physical findings, including skeletal, cardiac and airway involvement
- Laboratory results, such as IDUA enzyme activity and urine GAG levels
- Genetic results and any known genotype information from registries or the literature
In some countries, algorithms have been developed to combine these features and help predict whether a baby identified by newborn screening is likely to have severe or attenuated disease, so that time-sensitive treatment can be started promptly.
Further detail from the source pages
Further detail6 sections
Assessment Pathway
New diagnosis or newborn screen
Metabolic team evaluation
Combined clinical assessment:
History, exam, labs, genetics
Classification and treatment plan
Why hurlersyndrome.org focuses on severe MPS I
This website is dedicated to Hurler syndrome, which is the severe, neuronopathic form of MPS I. Children with this form face the greatest risk of early, life-limiting complications if not treated quickly and comprehensively. Their care requires intensive, coordinated input from metabolic, transplant and multidisciplinary teams, and many families search for information that is specific to severe MPS I rather than general MPS I resources.
Attenuated MPS I is mentioned where it helps to explain the full picture of MPS I, but medical information, treatment pathways and research content here are written with Hurler syndrome and its particular challenges in mind.
Key points about the MPS I spectrum
MPS I is one disease with a continuous spectrum of severity, not three completely separate conditions.
Hurler syndrome (MPS I-H) represents severe MPS I, with early onset and significant brain involvement.
Hurler–Scheie and Scheie syndromes fit within attenuated MPS I, with later onset and little or no cognitive decline.
Classification as severe or attenuated guides treatment, especially decisions about early HSCT versus ERT alone.
Genotype offers useful clues, but clinical assessment remains essential, and borderline cases require expert judgement.
What is Hurler syndrome?
Overview of severe MPS I and how it presents
Pathophysiology
How IDUA deficiency and GAG storage cause multi-organ disease
Symptoms and diagnosis
Disease course with and without modern treatment
Further detail — part 21 sections
Treatments and care
HSCT, ERT and supportive care for Hurler syndrome