Medical Conditions

Hunter Syndrome (MPS II) in Babies

Medically reviewed by Dr. Michael Okonkwo, MD, FAAP · Board-Certified Neonatologist

Content reviewed against published NIH, HRSA, ACMG guidelines

Editorial policy

Last reviewed:

If your baby has been diagnosed with or you suspect hunter syndrome (mps ii) in babies, here is what the evidence says.

The short answer

Hunter syndrome (mucopolysaccharidosis type II, MPS II) is an X-linked lysosomal storage disorder that primarily affects boys. It is caused by a deficiency of the enzyme iduronate-2-sulfatase, leading to accumulation of glycosaminoglycans in cells throughout the body. The severe form involves progressive neurological decline along with coarse facial features, joint stiffness, enlarged liver and spleen, and hearing loss. The attenuated form preserves normal intelligence. Enzyme replacement therapy with idursulfase is available, and MPS II has recently been added to the RUSP newborn screening panel.

Key takeaways

  • Hunter syndrome (mucopolysaccharidosis type II, MPS II) is an X-linked lysosomal storage disorder that primarily affects boys. It is caused by a deficiency of the enzyme iduronate-2-sulfatase, leading to accumulation of glycosaminoglycans in cells throughout the body. The severe form involves progressive neurological decline along with coarse facial features, joint stiffness, enlarged liver and spleen, and hearing loss. The attenuated form preserves normal intelligence. Enzyme replacement therapy with idursulfase is available, and MPS II has recently been added to the RUSP newborn screening panel.
  • Usually normal when: Your son's newborn screening came back normal for MPS II
  • Call your doctor if: Your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease
  • Varies by age — see the age-by-age breakdown below
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What Parents Should Know

According to NIH, HRSA, ACMG guidelines, hunter syndrome (mucopolysaccharidosis type II, MPS II) is an X-linked lysosomal storage disorder that primarily affects boys. It is caused by a deficiency of the enzyme iduronate-2-sulfatase, leading to accumulation of glycosaminoglycans in cells throughout the body. The severe form involves progressive neurological decline along with coarse facial features, joint stiffness, enlarged liver and spleen, and hearing loss. The attenuated form preserves normal intelligence. Enzyme replacement therapy with idursulfase is available, and MPS II has recently been added to the RUSP newborn screening panel. At 0-6 months, babies with Hunter syndrome typically appear normal at birth and in the first months of life. Some may have inguinal or umbilical hernias, which are common in MPS II. Because Hunter syndrome is X-linked, it primarily affects boys — girls who carry the gene variant are usually unaffected or very mildly affected. Newborn screening can now detect MPS II through enzyme activity measurement. If your son's newborn screen is positive, confirmatory testing will include enzyme assay and genetic testing. It is generally considered normal when your son's newborn screening came back normal for MPS II. However, you should contact your pediatrician promptly if your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease.

Sources: [1], [2], [3]

Normal vs. Concerning

Usually Normal
Worth Discussing
Your son's newborn screening came back normal for MPS II
Your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease
An initial positive screen was followed by confirmatory testing showing normal enzyme levels
Your son has coarse facial features, a large head, joint contractures, an enlarged liver and spleen, and appears to be losing developmental skills — seek evaluation by a geneticist
Your son has common childhood hernias or ear infections without any other features of MPS II
Your child with known Hunter syndrome develops signs of airway compromise such as severe snoring, pauses in breathing during sleep, or difficulty breathing
Your child with attenuated Hunter syndrome is on ERT and maintaining stable function with normal cognitive development
Your child with Hunter syndrome needs surgery or anesthesia — the metabolic team must be involved in planning due to airway risks

When to Seek Immediate Care

  • Your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease
  • Your son has coarse facial features, a large head, joint contractures, an enlarged liver and spleen, and appears to be losing developmental skills — seek evaluation by a geneticist
  • Your child with known Hunter syndrome develops signs of airway compromise such as severe snoring, pauses in breathing during sleep, or difficulty breathing
  • Your child with Hunter syndrome needs surgery or anesthesia — the metabolic team must be involved in planning due to airway risks

By Age

What to expect by age

0-6 months

Babies with Hunter syndrome typically appear normal at birth and in the first months of life. Some may have inguinal or umbilical hernias, which are common in MPS II. Because Hunter syndrome is X-linked, it primarily affects boys — girls who carry the gene variant are usually unaffected or very mildly affected. Newborn screening can now detect MPS II through enzyme activity measurement. If your son's newborn screen is positive, confirmatory testing will include enzyme assay and genetic testing.

6-12 months

Subtle signs may begin to emerge during this period. Babies may have recurrent ear infections, persistent runny nose, and frequent respiratory infections due to glycosaminoglycan accumulation in tissues. Some parents notice that their baby's facial features seem different from their siblings — a broader nose, fuller lips, and a larger head circumference. Hepatosplenomegaly (enlarged liver and spleen) may be detected on exam. Joint stiffness, particularly in the fingers and hands, may begin to develop.

1-2 years

The progressive nature of Hunter syndrome becomes more apparent. Coarsening of facial features continues. Joint stiffness progresses, making it difficult for the child to fully extend their arms. Behavioral changes may include hyperactivity and difficulty sleeping. The distinction between severe and attenuated forms begins to emerge — children with the severe form start showing signs of developmental plateau or cognitive decline, while those with the attenuated form continue to develop intellectually. Enzyme replacement therapy with idursulfase can be started and helps manage many non-neurological symptoms.

2-5 years

Children with severe Hunter syndrome often show clear developmental regression and behavioral difficulties, including severe hyperactivity, sleep disturbance, and challenging behavior that can be very difficult for families. Hearing loss is common and should be monitored with regular audiology assessments. Cardiac valve disease may develop, requiring echocardiographic monitoring. Airway involvement can lead to obstructive sleep apnea and may make anesthesia risky. Physical therapy helps maintain joint mobility.

5 years and beyond

Children with the attenuated form of Hunter syndrome have normal intelligence but progressive physical symptoms including joint contractures, carpal tunnel syndrome, cardiac valve disease, and hearing loss. ERT helps manage visceral and some somatic symptoms. For the severe form, disease progression continues with worsening neurological function. Hematopoietic stem cell transplant is not as clearly beneficial for MPS II as it is for MPS I. Intrathecal ERT and gene therapy are being studied as potential treatments for the neurological aspects.

What to Tell Your Pediatrician

  • Describe when you first noticed hunter syndrome (mps ii) in babies and how it has changed over time.
  • Note your baby's current age and which age-specific patterns you are seeing.
  • Mention if your son received an abnormal MPS II result on newborn screening — follow up for confirmatory testing.
  • Mention if your son has recurrent ear infections, persistent runny nose, joint stiffness, and you notice his facial features seem to be changing.
  • Let your doctor know if you have noticed any related concerns, such as changes in feeding, sleep, or movement patterns.
  • Bring a list of any questions or observations you want to discuss at the appointment.

What Should You Do?

When to take action

Probably normal when...
  • Your son's newborn screening came back normal for MPS II
  • An initial positive screen was followed by confirmatory testing showing normal enzyme levels
  • Your son has common childhood hernias or ear infections without any other features of MPS II
  • Your child with attenuated Hunter syndrome is on ERT and maintaining stable function with normal cognitive development
Mention at your next visit when...
  • Your son received an abnormal MPS II result on newborn screening — follow up for confirmatory testing
  • Your son has recurrent ear infections, persistent runny nose, joint stiffness, and you notice his facial features seem to be changing
  • Your son has progressive joint stiffness, an enlarged abdomen, and a larger-than-expected head circumference
  • There is a family history of Hunter syndrome on the maternal side (the condition is X-linked)
Act now when...
  • Your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease
  • Your son has coarse facial features, a large head, joint contractures, an enlarged liver and spleen, and appears to be losing developmental skills — seek evaluation by a geneticist
  • Your child with known Hunter syndrome develops signs of airway compromise such as severe snoring, pauses in breathing during sleep, or difficulty breathing
  • Your child with Hunter syndrome needs surgery or anesthesia — the metabolic team must be involved in planning due to airway risks

What You Can Do at Home

  • Keep track of when you notice hunter syndrome (mps ii) in babies — noting the time of day, duration, and any triggers can help your pediatrician.
  • Remember that your son's newborn screening came back normal for MPS II — this is generally within the range of normal.
  • At 0-6 months, focus on observation rather than intervention unless your pediatrician advises otherwise.
  • Follow any care instructions from your pediatrician. Keep a written log of symptoms to bring to appointments.
  • While monitoring at home, seek immediate care if your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease.

Abnormal Newborn Screening Results

An abnormal or positive newborn screening result means that further testing is needed, not that your baby definitely has a condition. Most positive screens turn out to be false positives after confirmatory testing. Newborn screening tests are intentionally sensitive to avoid missing any affected babies, so follow-up testing is essential to determine whether the result is a true positive.

Signs of a Metabolic Disorder in Babies

Metabolic disorders (inborn errors of metabolism) occur when the body cannot properly process certain nutrients or chemicals. Many are detected through newborn screening. Signs can include poor feeding, vomiting, lethargy, seizures, unusual body or urine odor, failure to thrive, developmental regression, and metabolic crises triggered by illness or fasting. Early detection and treatment are crucial, as many metabolic disorders are manageable with dietary modifications and medical treatment.

Milestone

Month-by-Month Development

Detailed monthly development guides from birth through 24 months.

Guide

When to Call the Doctor

General guide on when to call the pediatrician, visit urgent care, or go to the ER.

Frequently asked questions

Is hunter syndrome (mps ii) in babies normal?
Hunter syndrome (mucopolysaccharidosis type II, MPS II) is an X-linked lysosomal storage disorder that primarily affects boys. It is caused by a deficiency of the enzyme iduronate-2-sulfatase, leading to accumulation of glycosaminoglycans in cells throughout the body. The severe form involves progressive neurological decline along with coarse facial features, joint stiffness, enlarged liver and spleen, and hearing loss. The attenuated form preserves normal intelligence. Enzyme replacement therapy with idursulfase is available, and MPS II has recently been added to the RUSP newborn screening panel.
When should I call the doctor about hunter syndrome (mps ii) in babies?
Your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease Your son has coarse facial features, a large head, joint contractures, an enlarged liver and spleen, and appears to be losing developmental skills — seek evaluation by a geneticist Your child with known Hunter syndrome develops signs of airway compromise such as severe snoring, pauses in breathing during sleep, or difficulty breathing
When is hunter syndrome (mps ii) in babies normal?
Your son's newborn screening came back normal for MPS II An initial positive screen was followed by confirmatory testing showing normal enzyme levels Your son has common childhood hernias or ear infections without any other features of MPS II
What causes hunter syndrome (mps ii) in babies?
Hunter syndrome (mucopolysaccharidosis type II, MPS II) is an X-linked lysosomal storage disorder that primarily affects boys. It is caused by a deficiency of the enzyme iduronate-2-sulfatase, leading to accumulation of glycosaminoglycans in cells throughout the body. The severe form involves progressive neurological decline along with coarse facial features, joint stiffness, enlarged liver and spleen, and hearing loss. The attenuated form preserves normal intelligence. Enzyme replacement therapy with idursulfase is available, and MPS II has recently been added to the RUSP newborn screening panel. Common explanations include: Your son's newborn screening came back normal for MPS II. An initial positive screen was followed by confirmatory testing showing normal enzyme levels.
What should I mention to my pediatrician about hunter syndrome (mps ii) in babies?
You should mention hunter syndrome (mps ii) in babies at your next visit if: Your son received an abnormal MPS II result on newborn screening — follow up for confirmatory testing. Your son has recurrent ear infections, persistent runny nose, joint stiffness, and you notice his facial features seem to be changing. Your son has progressive joint stiffness, an enlarged abdomen, and a larger-than-expected head circumference.
Is hunter syndrome (mps ii) in babies normal at 0-6 months?
Babies with Hunter syndrome typically appear normal at birth and in the first months of life. Some may have inguinal or umbilical hernias, which are common in MPS II. Because Hunter syndrome is X-linked, it primarily affects boys — girls who carry the gene variant are usually unaffected or very mildly affected. Newborn screening can now detect MPS II through enzyme activity measurement. If your son's newborn screen is positive, confirmatory testing will include enzyme assay and genetic testing.
Is hunter syndrome (mps ii) in babies normal at 6-12 months?
Subtle signs may begin to emerge during this period. Babies may have recurrent ear infections, persistent runny nose, and frequent respiratory infections due to glycosaminoglycan accumulation in tissues. Some parents notice that their baby's facial features seem different from their siblings — a broader nose, fuller lips, and a larger head circumference. Hepatosplenomegaly (enlarged liver and spleen) may be detected on exam. Joint stiffness, particularly in the fingers and hands, may begin to develop.
Should I go to the ER for hunter syndrome (mps ii) in babies?
Seek emergency care if your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease, or if your son has coarse facial features, a large head, joint contractures, an enlarged liver and spleen, and appears to be losing developmental skills — seek evaluation by a geneticist. When in doubt, call your pediatrician's after-hours line for guidance.
Does hunter syndrome (mps ii) in babies go away on its own?
In many cases, hunter syndrome (mps ii) in babies resolves on its own, especially when your son's newborn screening came back normal for MPS II. By 5 years and beyond, children with the attenuated form of Hunter syndrome have normal intelligence but progressive physical symptoms including joint contractures, carpal tunnel syndrome, cardiac valve disease, and hearing loss. ERT helps manage visceral and some somatic symptoms. For the severe form, disease progression continues with worsening neurological function. Hematopoietic stem cell transplant is not as clearly beneficial for MPS II as it is for MPS I. Intrathecal ERT and gene therapy are being studied as potential treatments for the neurological aspects.
NIHHRSAACMG

References

  1. [1]National Library of Medicine. Mucopolysaccharidosis Type II. MedlinePlus Genetics, 2023. NIH
  2. [2]Health Resources and Services Administration. MPS II Fact Sheet. Baby's First Test, 2023. HRSA
  3. [3]American College of Medical Genetics. MPS II: Recommended Guidelines for Diagnosis and Management. ACMG, 2022. ACMG

Doctor Visit Checklist

Bring this checklist to your next pediatrician visit to discuss Hunter Syndrome (MPS II) in Babies.

Things to mention

  • Describe when you first noticed hunter syndrome (mps ii) in babies and how it has changed over time.
  • Note your baby's current age and which age-specific patterns you are seeing.
  • Mention if your son received an abnormal MPS II result on newborn screening — follow up for confirmatory testing.
  • Mention if your son has recurrent ear infections, persistent runny nose, joint stiffness, and you notice his facial features seem to be changing.
  • Let your doctor know if you have noticed any related concerns, such as changes in feeding, sleep, or movement patterns.
  • Bring a list of any questions or observations you want to discuss at the appointment.

Observations to share

  • Your son received an abnormal MPS II result on newborn screening — follow up for confirmatory testing
  • Your son has recurrent ear infections, persistent runny nose, joint stiffness, and you notice his facial features seem to be changing
  • Your son has progressive joint stiffness, an enlarged abdomen, and a larger-than-expected head circumference

Urgent signs to report immediately

  • Your newborn son has screened positive for MPS II — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease
  • Your son has coarse facial features, a large head, joint contractures, an enlarged liver and spleen, and appears to be losing developmental skills — seek evaluation by a geneticist
  • Your child with known Hunter syndrome develops signs of airway compromise such as severe snoring, pauses in breathing during sleep, or difficulty breathing

My notes

From ismybabyalright.com — free, evidence-based baby health guides

All content follows our editorial policy and is reviewed against published clinical guidelines.

2,705 evidence-based guides6 authoritative medical sources5 medical advisory board members

Related Resources

Well-Baby Visit PrepEmergency ContactsBaby Tracker

Bottom line

Most cases of hunter syndrome (mps ii) in babies are normal. Talk to your pediatrician if your newborn son has screened positive for mps ii — confirmatory testing should be completed promptly to determine the diagnosis and form of the disease.

Trust your instincts. If something feels wrong, reach out to your pediatrician. Worrying about your baby means you care — that is a good thing.

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Signs of a Metabolic Disorder in Babies

Metabolic disorders (inborn errors of metabolism) occur when the body cannot properly process certain nutrients or chemicals. Many are detected through newborn screening. Signs can include poor feeding, vomiting, lethargy, seizures, unusual body or urine odor, failure to thrive, developmental regression, and metabolic crises triggered by illness or fasting. Early detection and treatment are crucial, as many metabolic disorders are manageable with dietary modifications and medical treatment.

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