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# Hurler syndrome (mucopolysaccharidosis type I)

In Hurler syndrome complex sugars build up almost everywhere: coarse facial features, cloudy corneas, misshapen bones, and heart and brain disease.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Cornea](https://durgical.com/en/anatomy/cornea/)
- [Liver](https://durgical.com/en/anatomy/liver/)
- [Spleen](https://durgical.com/en/anatomy/spleen/)
- [Mitral anterior leaflet](https://durgical.com/en/anatomy/anterior-leaflet-of-left-atrioventricular-valve/)
- [Aortic right coronary cusp](https://durgical.com/en/anatomy/right-coronary-leaflet/)
- [T12 vertebra](https://durgical.com/en/anatomy/t12/)
- [L1 vertebra](https://durgical.com/en/anatomy/lumbar-vertebra/)
- [Tongue](https://durgical.com/en/anatomy/tongue/)
- [Carpal tunnel](https://durgical.com/en/anatomy/carpal-tunnel/)

## In biochemistry

- [Lysosomal storage diseases](https://durgical.com/en/biochemistry/topics/lysosomal-storage-diseases/)
- [Glycosaminoglycans (mucopolysaccharides)](https://durgical.com/en/biochemistry/topics/glycosaminoglycans/)
- [Mannose 6-phosphate tagging (sending enzymes to the lysosome)](https://durgical.com/en/biochemistry/topics/mannose-6-phosphate-tagging/)
- [Hyaluronic acid (hyaluronan)](https://durgical.com/en/biochemistry/molecules/hyaluronic-acid/)
- [α-L-Iduronidase](https://durgical.com/en/biochemistry/enzymes/l-iduronidase/)
- [Glycoproteins and protein glycosylation (N- and O-linked)](https://durgical.com/en/biochemistry/topics/glycoproteins-and-protein-glycosylation/)
- [Iduronate 2-sulfatase (I2S)](https://durgical.com/en/biochemistry/enzymes/iduronate-2-sulfatase/)

## Why it happens

- [How a single gene causes disease: enzymes, receptors and structural proteins](https://durgical.com/en/general-pathology/how-a-single-gene-causes-disease-enzymes-receptors-and-structural-proteins/)

## What it is

An autosomal recessive mucopolysaccharidosis caused by α-L-iduronidase deficiency (IDUA gene); dermatan sulfate and heparan sulfate accumulate. "Hurler" is the severe form of MPS I.

## Who gets it

Severe MPS I occurs in about 1 in 100,000 newborns; the attenuated form in about 1 in 500,000.

## Causes

- Biallelic variants of the IDUA gene (autosomal recessive)

## Risk factors

- Carrier parents or an affected sibling
- Consanguinity

## Pathophysiology

Without α-L-iduronidase, dermatan sulfate and heparan sulfate cannot be broken down past the terminal iduronic acid and fill the lysosomes of almost all tissues. Storage thickens [skin](https://durgical.com/en/anatomy/skin/), [tongue](https://durgical.com/en/anatomy/tongue/), airways, heart valves and [cornea](https://durgical.com/en/anatomy/cornea/); disturbs bone growth (dysostosis multiplex) and joints; enlarges [liver](https://durgical.com/en/anatomy/liver/) and [spleen](https://durgical.com/en/anatomy/spleen/); and heparan sulfate stored in neurons causes cognitive decline.

## Classification

- Severe MPS I (Hurler): onset in the first year, with cognitive decline
- Attenuated MPS I (Hurler–Scheie and Scheie): later onset, normal or near-normal intelligence

## Symptoms and signs

- Coarse facial features appearing in the first or second year
- Large tongue and noisy breathing
- Cloudy corneas
- Large [abdomen](https://durgical.com/en/anatomy/region/abdomen/) with umbilical and inguinal hernias
- A hump at the thoracolumbar junction and stiff joints (claw hands)
- Frequent ear infections and hearing loss
- Developmental delay that later regresses

## Red flags

- Sleep apnoea or severe airway obstruction
- Signs of cervical cord compression
- Heart failure from valve disease or cardiomyopathy
- High anaesthetic risk from a difficult airway

## Diagnosis

**Clinical**
An infant or toddler with coarse features, corneal clouding, hepatosplenomegaly, hernias, gibbus and joint stiffness.

**Tests**
High urinary glycosaminoglycans (dermatan and heparan sulfate); low α-L-iduronidase activity in leukocytes or dried blood spots; IDUA gene testing; audiometry.

**Imaging**
X-rays with dysostosis multiplex (oar-shaped [ribs](https://durgical.com/en/anatomy/ribs/), beaked vertebrae, J-shaped sella, pointed metacarpals); echocardiogram with thickened mitral and aortic valves; brain and cervical [spine](https://durgical.com/en/anatomy/region/spine/) MRI.

## Treatment

**Procedure**
Haematopoietic stem cell transplant before age 2 to 2.5: preserves cognition and is the treatment of choice in the severe form.

**Medical**
Intravenous enzyme replacement with recombinant α-L-iduronidase, before and around transplant and in attenuated forms.

**Surgical**
Adenotonsillectomy, hernia repair, [carpal tunnel](https://durgical.com/en/anatomy/carpal-tunnel/) release, spinal surgery, shunting for hydrocephalus and valve replacement as needed.

**Follow-up**
Cardiac, respiratory, hearing, eye, orthopaedic and neurodevelopmental follow-up.

## Complications

- Airway obstruction and sleep apnoea
- Valve disease and heart failure
- Communicating hydrocephalus
- Cervical cord compression
- Carpal tunnel syndrome
- Loss of vision and hearing

## Prognosis

Untreated, the severe form is usually fatal in the first decade from heart or breathing complications. Early transplant improves survival and cognition, though bone problems persist.

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme is missing in Hurler syndrome, and what builds up?

α-L-Iduronidase; dermatan sulfate and heparan sulfate accumulate.

## Which eye sign separates Hurler from Hunter syndrome?

Corneal clouding: present in Hurler, absent in Hunter.

## Why is stem cell transplant preferred before age 2 in severe Hurler syndrome?

Donor cells bring enzyme to the brain and preserve cognition if transplanted before decline.

## What is the set of bone changes in the mucopolysaccharidoses called?

Dysostosis multiplex.

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## Related conditions

- [Hunter syndrome (mucopolysaccharidosis type II)](https://durgical.com/en/conditions/hunter-syndrome/)
- [I-cell disease (mucolipidosis II)](https://durgical.com/en/conditions/i-cell-disease/)
- [Carpal tunnel syndrome](https://durgical.com/en/conditions/carpal-tunnel-syndrome/)
- [Obstructive sleep apnoea](https://durgical.com/en/conditions/obstructive-sleep-apnoea/)
- [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/)
- [Umbilical hernia](https://durgical.com/en/conditions/umbilical-hernia/)
- [Inguinal hernia](https://durgical.com/en/conditions/inguinal-hernia/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Mucopolysaccharidosis type I: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/mucopolysaccharidosis-type-i/) (MedlinePlus, US National Library of Medicine, 2026); [IDUA gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/idua/) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/conditions/hurler-syndrome/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/hurler-syndrome/