Diagnostic algorithms for Hurler syndrome (MPS I-H)
This page provides practical, stepwise diagnostic algorithms for Hurler syndrome (severe MPS I, MPS I-H). It is designed for paediatricians, neonatologists, metabolic physicians, geneticists, anaesthetists, intensivists, ENT and orthopaedic surgeons, cardiologists, primary care clinicians and other professionals who may encounter children with suspected MPS I.
Use these algorithms alongside your local and national guidelines. Early suspicion and rapid referral to a specialist metabolic service are critical for optimising outcomes.
Professional audience only. This information does not replace specialist metabolic advice or formal guidelines.
Core steps in diagnosing Hurler syndrome
Although local pathways vary, most diagnostic approaches follow a common sequence: clinical suspicion, screening tests, confirmatory enzyme and genetic testing, and phenotypic stratification to distinguish classic Hurler syndrome from attenuated MPS I.
Pathway for a positive MPS I newborn screening result
For centres with MPS I in the newborn screening panel, a positive or borderline result must trigger a rapid, structured response.
Screen positive or borderline MPS I result
- Notify the regional metabolic centre immediately.
- Confirm communication with the birth unit or community midwifery/GP team.
Second tier biochemical testing
- Repeat dried blood spot or perform leukocyte alpha-L-iduronidase assay.
- Arrange second tier GAG analysis and/or targeted molecular testing.
Early clinical assessment (within days)
- Full history, examination and growth chart review.
- Assess for hernias, respiratory issues and organomegaly.
- Do not delay action if examination is normal.
Molecular confirmation
- Order IDUA sequencing including deletion/duplication analysis.
- Interpret variants with clinical genetics input.
Preliminary phenotypic stratification
- Integrate genotype, family history and early clinical impression.
- Classify risk as probable MPS I-H, intermediate or attenuated.
Management if probable MPS I-H
- Urgent referral to an HSCT metabolic centre.
- Consider initiation of ERT as per national guidance.
- Begin baseline organ assessments.
When Hurler syndrome is not the initial working diagnosis
Incidental findings that may trigger suspicion:
Examples of incidental triggers
- Corneal clouding identified at routine ophthalmology review.
- Dysostosis multiplex on skeletal survey for spinal deformity or fracture.
- Persistent hepatosplenomegaly with otherwise non-specific investigations.
- Difficult airway, unexpected anatomy or intubation problems noted by anaesthesia.
Revisit history and examination
- Ask specifically about ENT issues, sleep, hernias, joint stiffness and development.
- Look for coarse facies, gibbous, clawed hands, noisy breathing and organomegaly.
Order urinary GAGs and seek metabolic advice
- If additional features are present, proceed via symptomatic algorithm.
- If suspicion persists despite limited findings, metabolic discussion is appropriate.
If MPS I remains on the differential
- Follow Algorithm 2 for confirmatory testing and referral.
When Hurler syndrome is not the initial working diagnosis
Incidental findings that may trigger suspicion:
- Corneal clouding identified at routine ophthalmology review.
- Dysostosis multiplex reported on a skeletal survey.
- Persistent hepatosplenomegaly with non-specific investigations.
- Difficult airway or unexpected intubation problems.
- Ask about ENT issues, sleep, hernias, joint stiffness.
- Look for coarse facies, gibbus, clawed hands, organomegaly.
- Proceed as symptomatic algorithm if features are present.
- Seek metabolic advice even if findings are limited.
- Follow Algorithm 2 for confirmatory testing and referral.
GAGs, enzyme assays and molecular testing
Laboratory confirmation must be robust and interpreted in clinical context.
Urinary GAGs
- Useful screening test, but false negatives and borderline values occur.
- Age specific reference ranges are essential.
- Abnormal or strongly suspicious clinical pictures warrant metabolic referral even if GAGs are normal.
Molecular testing (IDUA)
- Markedly reduced or absent activity confirms MPS I at enzyme level.
- Pseudodeficiency alleles can cause low activity without disease, so do not interpret in isolation.
- Assay should be performed in a laboratory experienced with lysosomal enzymes.
Enzyme assay (alpha-L-iduronidase)
- Full gene sequencing with deletion/duplication analysis.
- Interpretation must consider known severe, attenuated and pseudodeficiency variants.
- Genotype should be used to support, not replace, clinical and biochemical assessment.
Reporting
- Ensure clear, timely communication of results between the laboratory, local team and metabolic service.
- New confirmed diagnoses should prompt a multidisciplinary planning discussion.
Key principle: Interpret results in clinical context. No single test is diagnostic in isolation.
Avoiding misdiagnosis and missed opportunities
Conditions that can resemble aspects of Hurler syndrome:
Practical reminders:
Distinguishing Hurler syndrome from attenuated MPS I
Phenotypic stratification guides the urgency of HSCT referral and long term planning.
Features that support a classic Hurler phenotype (MPS I-H):
✓Symptom onset in infancy or very early childhood.
✓Rapidly progressive developmental delay or regression, particularly in language and cognition.
✓Marked dysostosis multiplex and broad multisystem involvement.
✓IDUA variants previously associated with severe disease in published cohorts.
For borderline or uncertain cases:
- Discuss with an experienced MPS centre.
- Use serial neurodevelopmental assessment, imaging and organ evaluations.
- Err on the side of early HSCT evaluation if severe disease seems likely, given the importance of timing for neurocognitive outcome.
Using algorithms with empathy
For clinicians, diagnostic pathways are algorithms; for families, they are highly stressful experiences. The way investigations are explained can have lasting impact.
Good practice points:
Key points for healthcare professionals
- Start with a high index of suspicion in infants and young children with compatible multisystem features.
- Use structured algorithms for newborn screening positives, symptomatic children and incidental findings.
- Confirm diagnosis with both enzyme and IDUA genetic testing, interpreted together and in context.
- Phenotype carefully to distinguish MPS I-H from attenuated MPS I, and refer early for HSCT assessment in probable Hurler syndrome.
- Keep families informed with honest, compassionate communication and early access to specialist teams and support resources.
What to read next
Clinical overview of Hurler syndrome
Pathophysiology, presentation and management
Symptoms and diagnosis
Early signs, multisystem features and pathways
Newborn screening
Handling a positive result and follow up
Treatments and care
HSCT, ERT and multidisciplinary management
Differential diagnosis
Distinguishing Hurler syndrome from other conditions
Research hub
Gene therapy, preclinical work and clinical trials