What is MPS I disease?

About MPS I disease

Mucopolysaccharidosis (mew-koh-poli-sak-er-I-doh-sis) type I (MPS I) is a rare, genetic condition that can be passed down in families. It affects many different organs and systems within the body, and signs and symptoms can vary greatly from one person to the next.^1,2

MPS I symptoms can often begin early in life, but may not be recognized right away, or may be confused with other conditions. As a result, it is often misdiagnosed or diagnosed late.^1,3 The sooner MPS I is properly diagnosed, the sooner it can be appropriately managed.

MPS I disease types

People with MPS I are classified as having either a severe form (Hurler syndrome) or an attenuated (less severe) form (Hurler-Scheie or Scheie syndrome). All forms of MPS I are lifelong and tend to get worse over time. Differences between forms of MPS I are based on the person’s age when signs first begin, the severity of symptoms, how quickly the condition worsens, and whether there is any early and direct involvement of the brain.^1,2

Talk to a doctor if you suspect MPS I

Early diagnosis and proper management can help improve outcomes for people living with MPS I.⁴ If you think you or a family member may have MPS I, be sure to talk to your doctor about possible next steps such as getting tested.

What are the symptoms of MPS I?

People with MPS I often show no symptoms at birth but develop signs and symptoms over time. Children with severe MPS I typically start to show symptoms within their first year of life, whereas children with the attenuated form of MPS I might start to develop signs and symptoms after a few years.²

MPS I affects multiple organs throughout the body, which results in a spectrum of symptoms and symptom severity from one person to the next. Signs and symptoms can also change over time or become more severe as the condition advances. For some individuals, MPS I may also present with distinctive facial features such as a larger head, broad short nose, and large tongue.²

Common symptoms* of MPS I may include:

Reduced eyesight from corneal clouding
Hearing loss, frequent ear infections
Difficulty breathing, respiratory infections, snoring, sleep issues
Bulging abdomen due to enlarged spleen and liver
Hernias
Orthopedic issues including curved spine (kyphosis and scoliosis), spinal cord compression, carpal tunnel syndrome, and delayed growth
Pain in different body regions including the back, abdomen, and hands
Joint pain, joint stiffness, reduced movement
Problems with heart valves or heart disease

More severe forms can also lead to:

Developmental delay
Cognitive impairment and decline²

*These are not all possible symptoms, nor do all people with MPS I have all the listed symptoms. Symptoms of MPS I exist on a spectrum and may be different for each individual affected by the condition.^1,2,5,6

What causes MPS I?

MPS I is an inherited condition caused by changes in the IDUA gene. This gene produces an enzyme called alpha-L-iduronidase, or IDUA. IDUA is responsible for breaking down sugars glycosaminoglycans (GAGs).^1,2

Pathogenic variants in the IDUA gene that occur with MPS I result in reduced IDUA enzyme activity, which leads to a buildup of GAGs in the body. This buildup can interfere with the normal function of cells and cause damage to tissues and organs that worsens over time.^1,2

How is MPS I diagnosed?

Diagnostic testing may be performed when a doctor suspects their patient has MPS I based on their signs and symptoms. However, since it is rare, not all doctors have experience recognizing MPS I right away.

Some of the early signs associated with MPS I also look like other, more common illnesses. If MPS I is not initially suspected, some people may end up seeing multiple specialists before their diagnosis is confirmed.⁴

Enzyme testing for MPS I

Since people with MPS I have reduced or absent activity of IDUA, a test result showing low IDUA enzyme activity is the primary way doctors confirm an MPS I diagnosis. Enzyme testing can be done with a standard blood draw, or a dried blood spot from a heel stick for infants.⁴

Urine testing

Most individuals with MPS I have levels of GAGs in their urine that are higher than normal. Urinary GAG testing can be helpful for confirming MPS I diagnosis alongside reduced enzyme activity, and may be used in monitoring the condition.⁴

Genetic testing for MPS I

Pathogenic variants in the IDUA gene that cause MPS I can be detected using a genetic test and may provide additional confirmation for a diagnosis. Doctors can also use genetic testing to see which pathogenic variants of IDUA are present, since some variants may be associated with more severe symptoms.²

Carrier screening is a type of genetic test that is used to check whether someone without MPS I is a carrier—meaning they have one copy of the affected gene.²

Early diagnosis is important—the earlier MPS I is diagnosed, the sooner management and monitoring can begin.⁴ If you suspect MPS I may run in your family, talk to a doctor about possible next steps such as getting tested.

The importance of monitoring MPS I

Taking an active role in monitoring your or your child’s symptoms can give you a greater sense of control over the condition. Remember, your healthcare team is always your best resource for guidance on monitoring and managing your symptoms.

Additional Support and Resources for MPS I

Having a chronic condition like MPS I can sometimes feel overwhelming and isolating. Resources are available to help support the well-being of individuals and those close to them who are affected by MPS I.

Patient advocacy groups (PAGs) are organizations that track the latest news on various conditions and explain the findings in simple terms for affected individuals, families, and their caregivers.⁷ There are several PAGs supporting the MPS I disease community. Visit their websites to learn more about them and the services they offer.

This listing is provided for reference only and does not constitute an endorsement by Sanofi of any particular organization or its programming. All other trademarks are the property of their respective owners. Additional resources may be available and should be investigated. Sanofi does not review or control the content of non-Sanofi websites.

National MPS Society

The National MPS Society is a non-profit organization dedicated to supporting families affected by MPS and other related disorders. The National MPS Society aims to increase professional and public awareness of the condition and help advance research for MPS.

Visit National MPS Society website

National Organization for Rare Disorders (NORD)

A non-profit patient advocacy organization fighting to improve the lives of people with rare disorders working at the intersection of care, research, policy, and community for all rare conditions.

Visit NORD website

Sign up for personalized MPS I support

Additional support and resources are available for people living with MPS I and their caregivers. For additional information about personalized support and resources to help you and your family, contact our CareConnect team.

Call 1-800-745-4447 (for English, press 3; para español, oprima 7) or email info@CareConnectPSS.com.

Enroll now

References

¹ De Ponti G, Donsante S, Frigeni M, et al. MPSI Manifestations and Treatment Outcome: Skeletal Focus. Int J Mol Sci. Sep 22 2022;23(19)doi:10.3390/ijms231911168

² Clarke LA. Mucopolysaccharidosis Type I. 2002 Oct 31 [Updated 2025 Dec 4]. In: Adam MP, Bick S, Mirzaa GM, et al., editors. GeneReviews® [Internet]. Seattle (WA): University of Washington, Seattle; 1993-2025. Accessed January 7, 2026. https://www.ncbi.nlm.nih.gov/books/NBK1162/.

³ Wisniewska K, Wolski J, Gaffke L, Cyske Z, Pierzynowska K, Wegrzyn G. Misdiagnosis in mucopolysaccharidoses. J Appl Genet. Sep 2022;63(3):475-495. doi:10.1007/s13353-022-00703

⁴ Muenzer J, Wraith JE, Clarke LA, International Consensus Panel on M, Treatment of Mucopolysaccharidosis I. Mucopolysaccharidosis I: management and treatment guidelines. Pediatrics. Jan 2009;123(1):19-29. doi:10.1542/peds.2008-0416

⁵ Muhlebach MS, Wooten W, Muenzer J. Respiratory manifestations in mucopolysaccharidoses. Paediatr Respir Rev. Jun 2011;12(2):133-8. doi:10.1016/j.prrv.2010.10.005

⁶ Congedi S, Orzalesi M, Di Pede C, Benini F. Pain in Mucopolysaccharidoses: Analysis of the Problem and Possible Treatments. Int J Mol Sci. Oct 8 2018;19(10)doi:10.3390/ijms19103063

⁷ Patterson AM, O'Boyle M, VanNoy GE, Dies KA. Emerging roles and opportunities for rare disease patient advocacy groups. Ther Adv Rare Dis. Jan-Dec 2023;4:26330040231164425. doi:10.1177/26330040231164425

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