MPS I (Hurler) vs MPS II (Hunter): How to Tell Them Apart
MPS I (Hurler) and MPS II (Hunter) are mucopolysaccharidoses caused by defective lysosomal degradation of glycosaminoglycans (GAGs), producing overlapping coarse facies, skeletal abnormalities, hepatosplenomegaly, and developmental involvement. The two are distinguished along two high-yield axes: the presence or absence of corneal clouding and the mode of inheritance. Corneal clouding plus autosomal recessive inheritance points to Hurler; a boy with an otherwise Hurler-like picture but clear corneas suggests Hunter.
How to tell them apart
| Feature | MPS I (Hurler) | MPS II (Hunter) |
|---|---|---|
| Corneal clouding | Present — a key distinguishing feature of Hurler | Absent — clear corneas despite the Hurler-like phenotype |
| Inheritance | Autosomal recessive | X-linked recessive |
| Severity | More severe, classically presenting in infancy | Variable severity |
| Sex distribution | Affects males and females equally (autosomal) | Predominantly affects males; mothers of affected boys are carriers |
| Family/pedigree pattern | Horizontal — siblings affected, parents unaffected carriers; risk increased with consanguinity | No male-to-male transmission; affected males with carrier mothers |
| Diagnostic ocular clue | Corneal clouding is part of the classic triad and directs you toward Hurler | Clear corneas in a boy with coarse features is the tip-off to Hunter |
The reasoning
Anchor first on the storage-disease pattern: progressive coarse facial features, skeletal abnormalities, and hepatosplenomegaly signal GAG accumulation and point to a mucopolysaccharidosis. Once MPS is established, arbitrate between the two by asking two questions. First, is there corneal clouding? If yes, favor Hurler (MPS I); if the corneas are clear, especially in a boy, favor Hunter (MPS II). Second, what is the inheritance pattern — autosomal recessive (Hurler) versus X-linked recessive (Hunter)? Sex of the patient and family history often settle the case: an affected boy with a carrier mother and clear corneas is Hunter until proven otherwise, whereas either-sex involvement with corneal clouding is Hurler. Enzyme assay and genetic testing provide definitive confirmation.
Key tests
- Slit-lamp/ophthalmologic exam for corneal clouding: cloudy corneas in Hurler (MPS I), clear corneas in Hunter (MPS II).
- Pedigree/inheritance analysis: autosomal recessive pattern (horizontal, both sexes, consanguinity) supports Hurler; X-linked recessive pattern (affected males, carrier mothers, no male-to-male transmission) supports Hunter.
- Enzyme assay / genetic testing to identify the specific deficient enzyme and confirm the MPS subtype, distinguishing the autosomal-recessive Hurler defect from the X-linked Hunter defect.
What they share
- GAG (glycosaminoglycan) accumulation from a lysosomal enzyme deficiency
- Coarse facial features
- Skeletal abnormalities
- Hepatosplenomegaly
- CNS/developmental involvement (though variable in Hunter)
Pitfalls
- Assuming both MPS types are alike because they share coarse facies, organomegaly, and skeletal disease — corneal clouding and inheritance are what separate them.
- Forgetting that Hunter severity is variable, so a milder Hunter phenotype can be mistaken for a non-MPS process or a milder disease.
- Overlooking that Hunter is X-linked recessive: a boy with a Hurler-like picture but clear corneas should raise Hunter, and a 'negative' family history does not exclude an inherited disease.
- Confusing a severe Hurler-like phenotype with I-cell disease (mucolipidosis II), which also produces coarse facies, skeletal abnormalities, and developmental delay in infancy.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.