What is the difference between Hurler and Hunter syndrome?

Hurler syndrome is severe mucopolysaccharidosis type I, an autosomal recessive deficiency of alpha-L-iduronidase; Hunter syndrome is mucopolysaccharidosis type II, an X-linked recessive deficiency of iduronate-2-sulfatase, so it affects almost only boys. Both store dermatan sulfate and heparan sulfate and cause coarse facial features, an enlarged liver and spleen, skeletal changes, stiff joints and heart-valve disease. Hurler clouds the corneas and its children are typically placid; Hunter leaves the corneas clear, often causes hyperactivity and can produce pathognomonic ivory skin lesions on the upper back and arms.

Which GAGs build up in Hurler and Hunter syndrome, and in which tissues?

Both diseases build up dermatan sulfate and heparan sulfate, because both missing enzymes act on iduronic acid, a sugar found in these two GAGs. Dermatan sulfate predominates in connective tissue, which helps explain the bone, joint and skin disease, while heparan sulfate is made by virtually all cells and its storage causes the neurologic disease. Storage enlarges the liver and spleen, thickens the face, tongue, airway and heart valves, and spills into the urine, where both GAGs appear in excess.

What enzyme is deficient in Hurler syndrome?

Hurler syndrome is caused by deficiency of alpha-L-iduronidase, a lysosomal enzyme encoded by the IDUA gene on chromosome 4. It removes the last alpha-L-iduronic acid from the end of dermatan sulfate and heparan sulfate chains; when that sugar carries a sulfate, iduronate-2-sulfatase, the enzyme missing in Hunter syndrome, must remove it first. Without alpha-L-iduronidase, both GAGs accumulate in lysosomes. The same enzyme is deficient in the milder Hurler-Scheie and Scheie forms; together they make up mucopolysaccharidosis type I.

What are the facial features of Hurler syndrome?

Hurler syndrome causes coarse facial features that become apparent within the first 2 years, from GAG storage in the soft tissues of the face and abnormal facial bones. Typical findings are a large head with prominent frontal bones, a flat nasal bridge, thickened nostrils, lips, earlobes and tongue, and coarse, thatch-like scalp hair with facial and body hypertrichosis. In the attenuated forms, Hurler-Scheie and Scheie syndromes, coarsening is less obvious, with a short neck, wide mouth and square jaw.

Is Hunter or Hurler syndrome more severe?

Hurler syndrome is generally more severe. It is the severe form of MPS I, begins in the first year and, without treatment, usually causes death from cardiorespiratory failure within the first 10 years. Hunter syndrome is described as similar to Hurler but milder, although it ranges from a neuronopathic form, fatal in the first or second decade, to an attenuated form with normal intelligence and survival into adulthood. Hunter also typically lacks the corneal clouding seen in Hurler.

How is Hurler syndrome diagnosed?

Hurler syndrome is diagnosed by showing deficient alpha-L-iduronidase activity in white blood cells, plasma or cultured fibroblasts together with raised urine GAGs, or by finding two disease-causing IDUA variants on genetic testing. Urine GAGs alone only suggest a mucopolysaccharidosis, and rare pseudodeficiency alleles can make enzyme results hard to interpret. MPS I is a core condition on the US Recommended Uniform Screening Panel, and newborn screening measures enzyme activity on dried blood spots. X-rays showing dysostosis multiplex support the diagnosis.

Hurler vs Hunter Syndrome: Enzymes, GAGs, Corneas & ERT

Hurler syndrome is severe mucopolysaccharidosis type I (MPS I), an autosomal recessive deficiency of α-L-iduronidase that clouds the corneas; Hunter syndrome is MPS II, an X-linked recessive deficiency of iduronate-2-sulfatase (I2S) that leaves the corneas clear and affects almost only boys. Both enzymes act on the same sugar, iduronic acid, so both diseases store dermatan sulfate and heparan sulfate and share coarse facies, hepatosplenomegaly, dysostosis multiplex, stiff joints, heart-valve disease and airway obstruction. Hurler is generally the earlier and more severe disease, usually fatal within the first 10 years if untreated; Hunter ranges from a neuronopathic form fatal in the first or second decade to an attenuated (non-neuronopathic) form with normal intelligence. Severe MPS I is treated with hematopoietic stem cell transplant (HSCT), generally before age 2; weekly enzyme replacement (laronidase for MPS I, idursulfase for MPS II) treats the body but not the brain, and in March 2026 the FDA gave accelerated approval to tividenofusp alfa (Avlayah), a brain-penetrating enzyme for neurologic Hunter disease.

Hurler vs Hunter at a glance

Feature	Hurler (severe MPS I)	Hunter (MPS II)
Gene	IDUA (4p16.3)	IDS (Xq28)
Missing enzyme	α-L-iduronidase	Iduronate-2-sulfatase (I2S)
Inheritance	Autosomal recessive: boys and girls	X-linked recessive: almost only boys
GAGs stored (and spilled in urine)	Dermatan sulfate + heparan sulfate	Dermatan sulfate + heparan sulfate
Corneas	Clouded (all forms of MPS I)	Clear: clouding is occasional, not typical
Onset	Normal at birth; mean age at diagnosis about 10 months	Coarse features from 18 months to 4 years (neuronopathic form)
Brain and behavior	Delay obvious by 18 months, limited language; placid rather than aggressive	Neuronopathic: regression at 6 to 8 years, hyperactivity, sometimes aggression; attenuated: normal intelligence
Signature extra	Gibbus (dorsolumbar kyphosis), often within the first 10 months	Ivory, pebble-like skin lesions on the upper back and sides of the upper arms (pathognomonic)
Untreated outlook	Death from cardiorespiratory failure, usually within the first 10 years	Neuronopathic: death in the first or second decade; attenuated: survival into adulthood
Disease-specific therapy	HSCT; laronidase	Idursulfase; tividenofusp alfa for neurologic disease (2026)
How common	About 1 in 100,000	About 1 in 100,000 to 170,000 male births

One hook for all of Hunter: a Hunter needs a clear eye to aim his X-bow: clear corneas, X-linked, and a restless, sometimes aggressive streak. Hurler is Hazy.

Which GAGs build up, and in which tissues

Hurler and Hunter both store dermatan sulfate and heparan sulfate: dermatan sulfate predominates in connective tissue, which helps explain the bone, joint and skin disease, and heparan sulfate storage causes the brain disease. Where each GAG normally sits explains the pattern:

GAG	Where it normally sits	Stored in	What its storage explains
Dermatan sulfate	Predominates in connective tissue	MPS I, II, VI, VII	Bone, joint and skin problems
Heparan sulfate	Made by virtually all cells: cell surfaces (syndecans, glypicans) and basement membranes (perlecan)	MPS I, II, III, VII	Neurologic disease: in the MPS, only heparan sulfate storage causes neurologic manifestations
Keratan sulfate	Cartilage and cornea	MPS IV (Morquio)	A largely skeletal disease

Because both GAGs are widespread, storage enlarges and thickens tissues throughout the body: liver and spleen, face, tongue and airway, heart valves, joints, bones and, in MPS I, the corneas. The excess spills into the urine, so urine GAGs flag an MPS but cannot tell Hurler from Hunter. The heparan sulfate rule predicts two exam patterns:

Sanfilippo syndrome (MPS III) stores only heparan sulfate: severe intellectual disability with mild physical signs.
Maroteaux-Lamy syndrome (MPS VI) stores dermatan sulfate without heparan sulfate: a Hurler-like body with normal intelligence.

The Hurler syndrome pathway: where α-L-iduronidase and I2S act

α-L-iduronidase, the enzyme missing in Hurler syndrome, is a lysosomal glycosidase that removes the last α-L-iduronic acid from the free (nonreducing) end of dermatan sulfate and heparan sulfate chains. Both GAGs contain iduronic acid, made during synthesis by epimerizing glucuronic acid. Lysosomes take them apart in a fixed order:

Heparan sulfate is first cut into fragments by heparanase; then exoglycosidases remove one sugar at a time from the nonreducing end while sulfatases remove sulfate groups.
When the end sugar is 2-sulfated iduronic acid, I2S (the Hunter enzyme) removes the sulfate first, then α-L-iduronidase (the Hurler enzyme) removes the sugar.
The chain continues down the line: four Sanfilippo enzymes for heparan sulfate, arylsulfatase B for dermatan sulfate, and β-glucuronidase for both.

Block either iduronic acid step and both GAGs back up. Every other MPS blocks the same line further down:

Type (eponym)	Missing enzyme	GAGs in urine	Clue
MPS I (Hurler, Hurler-Scheie, Scheie)	α-L-iduronidase	Dermatan + heparan sulfate	Cloudy corneas
MPS II (Hunter)	Iduronate-2-sulfatase	Dermatan + heparan sulfate	X-linked; clear corneas
MPS III A–D (Sanfilippo)	Heparan N-sulfatase (A), α-N-acetylglucosaminidase (B), acetyl-CoA:α-glucosaminide acetyltransferase (C), N-acetylglucosamine-6-sulfatase (D)	Heparan sulfate	Severe intellectual disability, mild somatic signs
MPS IV A/B (Morquio)	Galactosamine-6-sulfate sulfatase (A), β-galactosidase (B)	Keratan sulfate	Severe bone changes with odontoid hypoplasia; intelligence may be normal
MPS VI (Maroteaux-Lamy)	N-acetylgalactosamine-4-sulfatase (arylsulfatase B)	Dermatan sulfate	Hurler-like, normal intelligence
MPS VII (Sly)	β-glucuronidase	Dermatan, heparan and chondroitin 4- and 6-sulfate	Hurler-like, wider range of severity

Don't confuse arylsulfatase B (MPS VI) with arylsulfatase A, the enzyme missing in metachromatic leukodystrophy.

The Hurler face: what "coarse facies" means

"Coarse facies" in Hurler syndrome is a face thickened by GAG storage in the soft tissues of the mouth and face plus abnormal facial bones; it becomes apparent within the first 2 years. Look for:

Head: macrocephaly from a thickened skull, with prominent frontal bones.
Nose, lips, ears, tongue: a flat nasal bridge, and nostrils (alae nasi), lips, earlobes and tongue that thicken progressively.
Hair: coarse, straight, thatch-like scalp hair, with facial and body hypertrichosis often by 24 months.
Voice: deep and gravelly, with noisy breathing at night from enlarged tonsils, adenoids and tongue.

Feature	Severe MPS I (Hurler)	Attenuated MPS I (Hurler-Scheie, Scheie)	MPS II (Hunter)
When the face coarsens	Within the first 2 years	Later and milder	18 months to 4 years (neuronopathic form)
What you see	Large head, prominent frontal bones, flat nasal bridge; thick lips, nostrils, earlobes and tongue	Less obvious coarsening; short neck, wide mouth, square jaw	Prominent supraorbital ridges, large rounded cheeks, thick lips, macroglossia

Hunter faces are coarse too (characteristic features in 91% of neuronopathic and 82% of attenuated cases before age 10), so the corneas, inheritance and behavior separate the two.

Which is more severe, Hurler or Hunter?

Hurler syndrome is generally more severe: it is the severe end of MPS I by definition, begins in the first year and, untreated, is usually fatal within the first 10 years. Merck describes Hunter as similar to Hurler but milder, though it is a spectrum: the neuronopathic form is fatal in the first or second decade, while the attenuated form commonly allows survival into early adulthood with normal intelligence. Behavior differs too: severe MPS I is placid, while hyperactivity affects 66% and aggression 18% of children under 10 with neuronopathic Hunter.

Is there a "stage 5" Hurler syndrome? No. Standard references such as GeneReviews and StatPearls don't stage MPS I by number; it is classified as severe (Hurler) or attenuated (Hurler-Scheie, Scheie), points on one continuum. Untreated severe disease follows a predictable timeline:

Age	Severe MPS I (Hurler)	Neuronopathic MPS II (Hunter)
Birth	Looks normal; may have inguinal or umbilical hernias	—
By 6 months	Growth begins to fall away from normal	—
First 10 months	Gibbus often apparent; mean age at diagnosis about 10 months	—
18 months to 2 years	Delay obvious by 18 months; coarse face apparent by age 2; joint stiffness common by age 2	Coarse features appear (18 months to 4 years)
4 years	Median height below the 3rd percentile	—
6 to 8 years	—	Developmental regression
End of life	Most are severely intellectually disabled by death at 8 to 10 years, from cardiorespiratory failure	Death in the first or second decade

Where the storage shows up

Abdomen: hepatosplenomegaly, which in MPS I does not impair liver or spleen function; umbilical or inguinal hernias.
Skeleton and joints: dysostosis multiplex involving all bones, short stature, contractures, claw hands, carpal tunnel syndrome and cervical cord compression; hip dysplasia is the most common long-term orthopedic problem in Hunter.
Heart: thickened, stiff valves with mitral and aortic regurgitation, later cardiomyopathy; universal in severe MPS I and a major cause of death in Hunter.
Airway and ears: relentless airway obstruction and sleep apnea, often needing positive pressure and eventually tracheostomy; conductive and sensorineural hearing loss.
Brain and eyes: communicating hydrocephalus is common in severe MPS I; glaucoma and retinal degeneration add to its corneal clouding, and retinal disease causes night blindness in Hunter.

Hurler vs Hurler-Scheie vs Scheie

Hurler, Hurler-Scheie and Scheie are the old names for one disease, MPS I, from most to least severe. No easily measurable biochemical differences separate them and their findings overlap, so GeneReviews now calls MPS I severe (Hurler) or attenuated (Hurler-Scheie, Scheie), a split that guides treatment.

Feature	Severe MPS I (Hurler)	Attenuated MPS I (Hurler-Scheie, Scheie)
Onset	First year	Usually 3 to 10 years (Merck: Hurler-Scheie 3 to 8, Scheie after 5)
Intelligence	Delay by 18 months, then decline	Usually normal (intellectual disability in about 10%); learning disabilities may be detectable
Corneas	Clouded	Clouded in about 82% of children, at a median age of 9 years
Joints	Stiff by age 2; claw hands	Progressive arthropathy of all joints; carpal tunnel at a median age of about 10
Heart	Cardiac involvement in all	Valve thickening in virtually all; aortic valve disease more likely than in severe disease
Survival	Untreated, usually under 10 years	From death in the second or third decade to a normal life span
Disease-specific treatment	HSCT (standard of care), laronidase	Laronidase; no evidence that HSCT is the best option
How common	About 1 in 100,000	About 1 in 500,000

Scheie is the classic "cloudy corneas and stiff joints with normal intelligence" case.

Diagnosis: tests for Hurler and Hunter

Suspect it from the picture; x-rays show dysostosis multiplex.
Urine GAGs (total or fractionated) suggest an MPS but cannot identify the missing enzyme, and false negatives are common.
Enzyme assay confirms: α-L-iduronidase or I2S activity in leukocytes, plasma or cultured fibroblasts. Rare pseudodeficiency alleles make low α-L-iduronidase activity hard to interpret, so MPS I requires both low enzyme activity and raised urine GAGs. In a boy with low I2S, normal activity of at least one other sulfatase must be shown to exclude multiple sulfatase deficiency.
Molecular testing: biallelic IDUA variants in MPS I (two "severe" variants predict severe disease); a hemizygous IDS variant in a boy with MPS II, or a heterozygous variant in the rare affected girl.
Newborn screening: MPS I and MPS II are core conditions on the US Recommended Uniform Screening Panel (RUSP); screening measures enzyme activity on dried blood spots, and a second-tier GAG test greatly improves the positive predictive value for MPS I.

Treatment

Therapy	For	What it does	Limits
HSCT	Severe MPS I	Standard of care, generally before age 2 (a European consensus prefers it when MPS I-H is diagnosed before 2.5 years); improves cognition, survival and growth, reduces facial coarseness and hepatosplenomegaly, improves hearing	Lesser effect on skeleton and joints, corneal clouding and heart; less benefit once the developmental quotient is below 70
Laronidase (Aldurazyme; US 2003)	MPS I: Hurler and Hurler-Scheie, and Scheie with moderate to severe symptoms	0.58 mg/kg IV weekly; versus placebo, percent predicted FVC improved (p = 0.02) and 6-minute walk rose 38 m (p = 0.07); liver size and urine GAGs fell	Does not cross the blood-brain barrier; boxed warning for hypersensitivity, including anaphylaxis, and acute respiratory complications
Idursulfase (Elaprase; US 2006)	MPS II	0.5 mg/kg IV weekly; improves walking capacity from age 5; reduces spleen volume	Does not cross the blood-brain barrier; boxed warning for anaphylaxis
Tividenofusp alfa-eknm (Avlayah; US accelerated approval March 2026)	Neurologic manifestations of MPS II, started in children weighing at least 5 kg before advanced neurologic impairment	I2S fused to an IgG1 Fc fragment that binds the transferrin receptor, carrying it into the brain by receptor-mediated transcytosis; weekly IV; CSF heparan sulfate fell 91% by week 24	Approved on lowering CSF heparan sulfate; clinical benefit not yet confirmed; not recommended with other enzyme replacement; boxed warning for hypersensitivity, including anaphylaxis

Why both strategies work: cells take up lysosomal enzymes from outside through mannose-6-phosphate receptors (cross-correction); after HSCT, donor-derived microglia are thought to supply α-L-iduronidase inside the brain. Laronidase can bridge a child to transplant.
HSCT in Hunter syndrome is controversial (high risk, no controlled trials), though it is used in China and Japan.
Supportive care: adenotonsillectomy, CPAP or tracheostomy; ventilating tubes and hearing aids; early valve replacement; median nerve and spinal cord decompression; physical therapy.

How USMLE tests it: traps and look-alikes

Cloudy corneas point away from Hunter: the classic pivot.
A girl with an MPS picture points away from Hunter: MPS I, like most mucopolysaccharidoses, is autosomal recessive (see autosomal recessive pedigrees); Hunter shows up in heterozygous girls only rarely.
An affected maternal uncle points to X-linked inheritance, so Hunter; see the X-linked disorders flowchart.
The X-linked lysosomal pair: most mucopolysaccharidoses are autosomal recessive; Hunter and Fabry disease are the classic X-linked lysosomal diseases (Fabry and the lipid storage diseases are in the sphingolipidoses table).
"Why didn't enzyme replacement help his development?" Laronidase and idursulfase don't reach the brain.
Placid vs hyperactive: a calm toddler with cloudy corneas is Hurler; a hyperactive boy with clear corneas is Hunter, or Sanfilippo if only heparan sulfate is in the urine.

Look-alike	Defect	What separates it
Sanfilippo (MPS III)	One of four heparan sulfate enzymes	Brain-first: delayed speech, then restless, hyperactive, sometimes aggressive behavior and poor sleep; only mildly coarse features
I-cell disease (mucolipidosis II)	N-acetylglucosamine-1-phosphotransferase (GNPTAB): lysosomal enzymes lose mannose-6-phosphate targeting and are missorted	Onset at birth: early coarsening, striking gum overgrowth, bone deformities; lysosomal enzyme activity high in plasma; death usually in early childhood
Tay-Sachs	Hexosaminidase A; GM2 ganglioside	Cherry-red spot, exaggerated startle, regression; hepatosplenomegaly typically absent
Pompe (GSD II)	Acid α-glucosidase; glycogen in lysosomes	Weak, floppy infant with hepatomegaly and heart disease; untreated, death from heart failure in the first year

For storage diseases with hepatosplenomegaly but no GAGs in the urine, compare Niemann-Pick disease and Gaucher disease; more inherited metabolic disorders are on the biochemistry hub.

Worked vignettes

Vignette 1. A 4-year-old boy has a hoarse voice, snoring, recurrent ear infections and stiff fingers. He became hyperactive over the past year. Examination shows coarse facial features, an enlarged tongue, hepatosplenomegaly, an umbilical hernia and pebble-like ivory-colored skin lesions over his upper back and the sides of his upper arms. His corneas are clear. His mother's brother had similar features and died at age 14. Urine shows increased dermatan sulfate and heparan sulfate. Deficiency of which enzyme is most likely?

A. α-L-iduronidase B. Iduronate-2-sulfatase C. Hexosaminidase A D. N-acetylglucosamine-1-phosphotransferase E. N-acetylgalactosamine-4-sulfatase (arylsulfatase B)

Answer: B. Coarse facies, organomegaly, airway and ear disease, and dermatan plus heparan sulfate in the urine mean a mucopolysaccharidosis. Clear corneas, hyperactivity, pathognomonic ivory skin lesions and an affected maternal uncle make it Hunter syndrome (MPS II).

A: α-L-iduronidase deficiency (Hurler) stores the same two GAGs, but MPS I clouds the corneas, the child is typically placid, and inheritance is autosomal recessive.
C: Tay-Sachs stores GM2 ganglioside, not GAGs: regression with a cherry-red spot and typically no hepatosplenomegaly.
D: I-cell disease also coarsens the face, but it starts at birth with gum overgrowth and missorts many lysosomal enzymes.
E: Arylsulfatase B deficiency (MPS VI) stores dermatan sulfate without heparan sulfate, so it doesn't fit his urine heparan sulfate, and intelligence is normal.

Follow-up: idursulfase would shrink his spleen but would not reach his brain; tividenofusp alfa targets the neurologic disease if started before advanced impairment.

Vignette 2. A 13-month-old girl has a hump in her lower back first noticed at 8 months and had an umbilical hernia repaired in infancy. She has coarse features with thick lips and tongue, hepatosplenomegaly, noisy breathing at night and hazy corneas. She is calm, with mild developmental delay. Leukocyte α-L-iduronidase activity is absent, urine GAGs are high, and IDUA testing finds two variants associated with severe disease. What treatment best protects her cognition?

Answer: HSCT, as soon as her condition allows.

Reasoning: Gibbus in the first year, cloudy corneas and two severe variants mean severe MPS I. HSCT alters the course of cognitive decline and works best early, before age 2 and before the developmental quotient falls below 70. Laronidase can bridge her to transplant but does not treat the brain.

Vignette 3. A 14-year-old girl has had stiff, curled fingers for 3 years and now wakes with numb thumbs and index fingers. She is an average student. She has cloudy corneas, a short neck and square jaw, stiff shoulders and hips, and an early diastolic murmur at the left sternal border. Urine dermatan and heparan sulfate are increased. What is the diagnosis?

Answer: Attenuated MPS I (Scheie or Hurler-Scheie syndrome): α-L-iduronidase deficiency.

Reasoning: Cloudy corneas, arthropathy, carpal tunnel syndrome and aortic regurgitation with normal intelligence fit attenuated MPS I. Hunter is unlikely in a girl and spares the corneas; Maroteaux-Lamy (MPS VI) would raise dermatan sulfate without heparan sulfate; Morquio stores keratan sulfate. Laronidase is indicated; HSCT has no proven advantage here.

Key takeaways

Hurler = MPS I = α-L-iduronidase, autosomal recessive, cloudy corneas, placid. Hunter = MPS II = iduronate-2-sulfatase, X-linked recessive, clear corneas, often hyperactive.
Both store dermatan sulfate (connective tissue: bones, joints, skin) and heparan sulfate (nearly every cell; drives brain disease); urine GAGs can't separate them, the enzyme assay can.
Pathway: I2S removes the 2-sulfate, then α-L-iduronidase removes the iduronic acid.
Hurler is generally more severe than Hunter; MPS I isn't staged by number but classified as severe or attenuated.
Severe MPS I: HSCT, generally before age 2; it alters cognitive decline but helps corneas, bones and valves less.
Laronidase and idursulfase don't treat the brain; tividenofusp alfa (Avlayah, 2026) reaches it through the transferrin receptor.