Hurler Syndrome (MPS): Clinical Case Discussion & Key Points
Model Case Presentation
Patient Demographics
Name: Master Arjun, Age: 2 years, Gender: Male, Informant: Mother (Reliable)
Chief Complaints
- Coarse facial features noted since 6 months of age — 1.5 years
- Developmental delay — noticed since 12 months
- Recurrent upper respiratory tract infections — since 8 months
- Distended abdomen — 1 year
History Summary
Baby was apparently normal at birth with an uneventful neonatal period. Mother noticed gradual coarsening of facial features and enlargement of the head around 6 months of age. Speech development is absent (expected: 2-word sentences at 2 years); the child can only say "mama." Motor milestones are delayed — sat at 10 months, walks with a waddling gait. Frequent noisy breathing, snoring, nasal discharge, and recurrent otitis media. Abdomen progressively distending. Umbilical hernia noted. No family history of similar illness. Parents are non-consanguineous.
Born at term via NVD, cried immediately. Birth weight 3.1 kg. Antenatal period uneventful. No maternal drug exposure or infections.
Examination Summary
| Parameter | Finding | Significance |
|---|---|---|
| Weight | 9 kg (< 3rd centile) | Failure to thrive |
| Height | 76 cm (< 3rd centile) | Short stature |
| Head circumference | 52 cm (> 97th centile) | Macrocephaly |
| Vitals | Stable, SpO2 95% | Borderline — upper airway obstruction |
Facies: Coarse facial features — prominent supraorbital ridges, depressed nasal bridge, broad upturned nose, thick lips, large tongue (macroglossia), widely spaced teeth, gingival hyperplasia, low-set ears. Corneal clouding bilateral.
Head & Neck: Macrocephaly, frontal bossing, short neck. Noisy breathing.
Musculoskeletal: Short stature, thoracolumbar kyphosis (gibbus deformity — L2 level), joint stiffness with claw-hand deformity. No joint hypermobility. Waddling gait.
Abdomen: Umbilical hernia. Hepatomegaly 5 cm + Splenomegaly 4 cm below respective costal margins.
Cardiovascular: Pansystolic murmur (mitral regurgitation). Heart sounds normal.
Skin & Hair: Thick skin, stiff bushy hair, hirsutism. No ivory skin lesions (helps differentiate from Hunter).
Neurology: Developmental delay — language > motor. No focal deficits. Hearing impaired bilaterally.
✅ Complete Diagnosis
Mucopolysaccharidosis Type I-H (Hurler Syndrome) — with coarse facies, corneal clouding, dysostosis multiplex (gibbus deformity), hepatosplenomegaly, cardiomegaly (mitral regurgitation), sensorineural hearing loss, and moderate developmental delay. Autosomal recessive lysosomal storage disorder due to alpha-L-iduronidase deficiency.
MPS Master Classification Table
| MPS Type | Eponym | Enzyme Deficient | GAG Stored | Inheritance | Intellect | Corneal Clouding | Key Feature |
|---|---|---|---|---|---|---|---|
| MPS I-H | Hurler | α-L-Iduronidase | DS, HS | AR | ↓↓↓ | Yes | Most severe; early death <10 yrs; classic gargoyle facies |
| MPS I-H/S | Hurler-Scheie | α-L-Iduronidase | DS, HS | AR | Normal/mild ↓ | Yes | Intermediate form; survives to 3rd decade |
| MPS I-S | Scheie | α-L-Iduronidase | DS, HS | AR | Normal | Yes | Mildest MPS I; aortic valve disease; normal lifespan |
| MPS II | Hunter | Iduronate-2-sulfatase | DS, HS | X-linked | ↓ (variable) | No | Only X-linked MPS; ivory skin lesions; no corneal clouding |
| MPS III-A | Sanfilippo A | Heparan N-sulphatase | HS | AR | ↓↓↓ | No | Most common MPS overall; severe CNS, mild somatic; behavioral problems predominant; death <20 yrs |
| MPS III-B | Sanfilippo B | α-N-Acetylglucosaminidase | HS | AR | ↓↓↓ | No | |
| MPS III-C | Sanfilippo C | Acetyl-CoA: α-glucosaminide N-acetyltransferase | HS | AR | ↓↓↓ | No | |
| MPS III-D | Sanfilippo D | N-Acetylglucosamine-6-sulfatase | HS | AR | ↓↓↓ | No | |
| MPS IV-A | Morquio A | N-Acetylgalactosamine-6-sulfatase | KS, C6S | AR | Normal | Mild/No | Most severe skeletal disease; joint hypermobility; odontoid hypoplasia; atlantoaxial instability; pectus carinatum |
| MPS IV-B | Morquio B | β-Galactosidase | KS | AR | Normal | Mild | Milder skeletal disease than IV-A |
| MPS VI | Maroteaux-Lamy | N-Acetylgalactosamine-4-sulfatase (Arylsulfatase B) | DS, CS | AR | Normal | Yes | Similar to Hurler phenotype but normal intelligence; severe cardiac valve disease |
| MPS VII | Sly | β-Glucuronidase | DS, HS, CS | AR | Variable | Mild | Rarest; can present as hydrops fetalis in neonates |
| MPS IX | Natowicz | Hyaluronidase | Hyaluronan | AR | Normal | No | Extremely rare; periarticular soft tissue masses |
DS = Dermatan sulfate; HS = Heparan sulfate; KS = Keratan sulfate; CS = Chondroitin sulfate; AR = Autosomal recessive
💡 Memory Mnemonics
- "Hunter HUNTS without glasses" — MPS II (Hunter) has NO corneal clouding
- "Hunter is a Hunter BOY" — X-linked; affects boys predominantly
- "Sanfilippo is SAN-FRAN-PSYCHO" — severe CNS/behavioral problems, mild somatic
- "Morquio = More joints loose" — joint hypermobility (unique among MPS)
- "Sly baby is HYDROPSY" — MPS VII presents with hydrops fetalis
- MPS type number = not assigned 5 or 8 — MPS V was renamed MPS I-S (Scheie); MPS VIII does not exist
- All AR except MPS II (X-linked)
📝 History — Exam Q&A
Hurler syndrome (MPS I-H) is an autosomal recessive lysosomal storage disorder caused by deficiency of the enzyme alpha-L-iduronidase, leading to accumulation of dermatan sulfate (DS) and heparan sulfate (HS) in lysosomes of multiple organs. It is the most severe form of MPS I.
Alternative names: Gargoylism (historical, now considered derogatory), MPS I-H, Hurler's disease. First described by German pediatrician Gertrud Hurler in 1919.
Children appear normal at birth. Symptoms emerge as GAGs progressively accumulate:
- Birth–6 months: Umbilical/inguinal hernias; coarsening facial features begins
- 6–18 months: Coarse facies fully apparent; developmental regression; hepatosplenomegaly; frequent URTI; corneal clouding
- 18 months–3 years: Short stature; dysostosis multiplex; joint stiffness; cardiac involvement
- After 3 years: Progressive cognitive decline; worsening cardiorespiratory compromise
Key point: Early normal development followed by progressive regression is the hallmark of all MPS disorders (except Morquio where intellect is preserved).
- Umbilical or inguinal hernia (especially bilateral inguinal hernias in infants)
- Coarsening of facial features during infancy
- Recurrent otitis media or URTI with nasal discharge ("always snotty")
- Developmental regression (loss of milestones already acquired)
- Noisy breathing / snoring / obstructive sleep apnea
- Stiff joints or claw hand in a child (without arthritis signs)
- Short stature with gibbus deformity (thoracolumbar kyphosis)
- Corneal clouding noted on routine examination
💡 Pearl
Bilateral inguinal hernias in a male infant should always prompt consideration of MPS II (Hunter syndrome — X-linked).
| Feature | Hurler (MPS I-H) | Hunter (MPS II) |
|---|---|---|
| Enzyme deficiency | α-L-Iduronidase | Iduronate-2-sulfatase |
| Gene / Chromosome | IDUA / Chr 4p16.3 | IDS / Chr X q27-28 |
| Inheritance | Autosomal recessive | X-linked recessive |
| Sex affected | Both M & F equally | Almost exclusively males |
| Corneal clouding | Present | Absent (key differentiator) |
| Ivory skin lesions | Absent | Present (pebbled/ivory papules on back) |
| GAG stored | DS + HS | DS + HS |
| Severity | Severe; death <10 yrs | Variable (severe/mild forms) |
| Prognosis | Worse | Slightly better than Hurler |
| Feature | Sanfilippo (MPS III) | Morquio (MPS IV) |
|---|---|---|
| Subtypes | A, B, C, D (4 enzymes) | A (GALNS), B (β-gal) |
| GAG stored | Heparan sulfate | Keratan sulfate (+ CS in IV-A) |
| Intelligence | Severely impaired | Normal |
| Somatic features | Mild (minimal dysmorphism) | Severe skeletal dysplasia |
| Prominent feature | Behavioral problems (hyperactivity, aggression, sleep disturbance) | Short trunk dwarfism; odontoid hypoplasia; atlantoaxial instability |
| Joint mobility | Stiff joints | Hypermobile joints (unique in MPS) |
| Corneal clouding | No | Mild or absent |
| Most common MPS | MPS III = Most common MPS overall | Less common |
🚨 Exam Trap
In Morquio syndrome (MPS IV), odontoid hypoplasia causes atlantoaxial instability — a potentially life-threatening complication, especially during general anesthesia. Always assess with flexion-extension X-ray of cervical spine before any surgery.
| Feature | Hurler (MPS I-H) | Hurler-Scheie (MPS I-H/S) | Scheie (MPS I-S) |
|---|---|---|---|
| Severity | Most severe | Intermediate | Mildest (attenuated) |
| Onset of symptoms | <12 months | 3–8 years | >5 years (often teenage) |
| Intelligence | Severely impaired; progressive decline | Normal or mildly impaired | Normal |
| Somatic features | Severe | Moderate | Mild |
| Corneal clouding | Yes | Yes | Yes |
| Life expectancy | <10 years (untreated) | 3rd decade | Normal lifespan |
| Cardiac | Severe (MR, AR, valve disease) | Moderate | Aortic valve disease prominent |
| Enzyme | All three: α-L-Iduronidase deficiency (same enzyme, different residual activity) | ||
Note: Modern classification groups MPS I-H as "severe MPS I" and MPS I-H/S + MPS I-S as "attenuated MPS I"
- No male predominance — rules out Hunter (X-linked); Hurler affects both sexes equally
- Presence of corneal clouding — helps rule out Hunter (no corneal clouding in MPS II)
- No behavioral problems as dominant feature — rules out Sanfilippo (MPS III)
- No joint hypermobility — rules out Morquio (MPS IV); Hurler has stiff joints
- Family history — consanguinity increases risk; previous sibling death in infancy
- Genetic counseling — autosomal recessive: 25% risk per pregnancy; both parents are carriers
- Incidence: ~1 in 100,000 live births; affects males and females equally
- Gene: IDUA gene; located on chromosome 4p16.3
- Inheritance: Autosomal recessive; >200 mutations described
- Carrier frequency: ~1 in 160 in general population
- Recurrence risk: 25% per pregnancy in carrier parents
- Enzyme activity in carriers is reduced (~50%) but sufficient for normal function — carriers are asymptomatic
🩺 Examination — Exam Q&A
Coarse facial features (formerly called "gargoyle facies") include:
- Macrocephaly with frontal bossing (prominent forehead)
- Depressed nasal bridge (saddle nose)
- Broad, upturned nostrils
- Widely spaced teeth (dental dysplasia)
- Macroglossia (large tongue) + thick lips
- Gingival hyperplasia
- Thick neck, short neck
- Corneal clouding (best seen with slit lamp; may be visible to naked eye)
- Low-set ears, hearing impairment
- Persistent rhinorrhea ("always snotty child")
💡 Pearl
Gargoyle facies are NOT present at birth — they develop gradually over the first 6–18 months as GAGs accumulate.
Dysostosis multiplex is the term for the constellation of characteristic skeletal radiographic abnormalities seen in MPS disorders. It is not a single finding but a complex of bone changes:
| Bone/Region | Finding |
|---|---|
| Skull | Macrocephaly; thickened diploic space; J-shaped (elongated) sella turcica; poorly developed paranasal sinuses |
| Ribs | Spatulate (oar-shaped) ribs — narrow at vertebral end, widened anteriorly |
| Clavicles | Short and thick |
| Vertebrae | Bullet-shaped/oval vertebral bodies; anterior beaking (antero-inferior); gibbus deformity (thoracolumbar kyphosis); platyspondylia |
| Pelvis | Rounded (flared) iliac wings; inferior tapering; poorly developed acetabulum; coxa valga |
| Long bones | Shortened, thickened diaphyses; hypoplastic epiphyses; cortical thinning; proximal humeral notching |
| Hands | Bullet-shaped proximal phalanges; short stubby metacarpals; pointed proximal metacarpals (proximal tapering) |
💡 Beaking in MPS — Important Distinction
- Hurler (MPS I): Antero-inferior vertebral beaking
- Morquio (MPS IV): Central (mid-body) anterior vertebral beaking
Gibbus deformity is an angular kyphosis (sharp, humpback deformity) at the thoracolumbar junction (typically L1-L2). It appears as a localized sharp posterior angulation, not a smooth curve.
Cause in Hurler: GAG accumulation weakens the anterior portion of vertebral bodies → anterior wedging → collapse → gibbus. It is a hallmark skeletal sign seen in nearly all children with severe MPS (Hurler, Hunter). Occurs in first 2 years of life.
🚨 Clinical Significance
Gibbus deformity can cause spinal cord compression and myelopathy — a potentially serious complication requiring surgical intervention.
Cardiac involvement is nearly universal in Hurler syndrome due to GAG deposition in cardiac valves, myocardium, and coronary arteries:
- Valvular disease: Mitral regurgitation (most common) → mitral stenosis; aortic regurgitation/stenosis
- Cardiomyopathy (restrictive/dilated)
- Coronary artery narrowing → ischemia (silent, due to intellectual disability)
- Systemic hypertension
- Pulmonary hypertension (secondary to airway obstruction + lung disease)
Cardiac disease is a major cause of death. Echocardiography is mandatory in all MPS patients.
- Intellectual disability — progressive; develops after a period of normal development
- Language delay > motor delay initially; then loss of both
- Communicating hydrocephalus — GAG deposition in meninges → impaired CSF reabsorption via arachnoid villi
- Carpal tunnel syndrome — GAG infiltration compresses median nerve (most common peripheral nerve entrapment in MPS)
- Atlantoaxial instability — due to odontoid hypoplasia (more common in Morquio); can occur in Hurler
- Sensorineural + conductive hearing loss
- Spinal cord compression from gibbus deformity
- Enlarged perivascular spaces on MRI (Virchow-Robin spaces) — characteristic neuroimaging finding
| Feature | Hurler (MPS I-H) | Morquio (MPS IV) |
|---|---|---|
| Intelligence | Severely impaired | Normal |
| Joint mobility | Stiff, contractures | Hypermobile |
| Corneal clouding | Yes (prominent) | Mild or none |
| Short stature type | Proportionate (uniform short stature) | Short trunk, normal limbs (disproportionate) |
| Chest | Variable | Pectus carinatum (pigeon chest) |
| Odontoid process | Mild hypoplasia possible | Severe hypoplasia → atlantoaxial instability |
| Vertebral beaking | Antero-inferior | Central anterior |
| Dominant problem | Neurological + multisystem | Skeletal (orthopedic) |
| Genu valgum | Possible | Prominent |
- Absence of corneal clouding — the most important differentiating feature (corneas are clear)
- Ivory/pebbly skin lesions — pebbled ivory-white papules over the back, arms, and thighs (pathognomonic); do not correlate with disease severity
- Similar coarse facies and hepatosplenomegaly as Hurler, but clinical course is slightly slower
- Affects almost exclusively males (X-linked); symptomatic females are rare
- Two forms: severe (intellectual disability, early death) and mild (normal intelligence, survival to adulthood)
- Retinal degeneration (not seen in Hurler)
GAG deposition in airway tissues causes:
- Upper airway: Macroglossia, adenotonsillar hypertrophy, thickened subglottic soft tissues → obstructive sleep apnea (nearly universal)
- Lower airway: Tracheobronchomalacia, recurrent pneumonia
- Restrictive lung disease due to chest wall deformities (gibbus, short ribs)
- Pulmonary hypertension
Anesthetic danger: MPS patients are among the most difficult pediatric intubations. Rigid short neck, macroglossia, small mouth, narrow trachea, and subglottic narrowing → failed intubation, cannot ventilate → fatal if unprepared. Fiberoptic intubation is preferred. All MPS patients must be flagged as "difficult airway."
🔬 Investigations — Exam Q&A
Step 1 — Screening: Urine glycosaminoglycan (GAG) quantification + electrophoresis (or LC-MS/MS)
Step 2 — Confirmation: Enzyme activity assay in leukocytes/fibroblasts (identifies specific type)
Step 3 — Molecular: Gene sequencing (IDUA gene for MPS I) — identifies mutations for genetic counseling and prenatal diagnosis
Step 4 — Organ assessment: Echocardiogram, ophthalmology (slit lamp), audiometry, X-rays (skeletal survey), MRI brain/spine, pulmonary function tests, neurodevelopmental evaluation
Urine is tested for glycosaminoglycans (GAGs / mucopolysaccharides):
- Berry spot test (Toluidine blue spot test) — Qualitative; rapid screening; turns purple if GAGs elevated. Cheap but high false-positive + false-negative rate. Not reliable alone.
- Quantitative urine GAG (Spectrophotometry) — Measures total GAG; elevated in MPS. More reliable but does not specify the type.
- GAG electrophoresis — Separates DS, HS, KS, CS — helps narrow down MPS type
- LC-MS/MS (Tandem mass spectrometry) — Modern gold standard for screening; highly accurate, identifies specific GAG species
🚨 Important Limitation
Urine GAG can be falsely negative in attenuated forms (Scheie, Morquio B) — always confirm suspected MPS with enzyme assay regardless of urine GAG result.
Enzyme activity assay (in leukocytes, plasma, or cultured fibroblasts) is the gold standard for diagnosing and differentiating MPS types. It measures residual activity of the specific lysosomal enzyme.
- For MPS I: alpha-L-iduronidase activity
- For MPS II: Iduronate-2-sulfatase activity
- Severely reduced or absent (<1% of normal) = definitive diagnosis
- Confirmed further by molecular genetic testing (gene sequencing)
Enzyme assay in cultured fibroblasts (from skin biopsy) is most reliable. Leukocytes or dried blood spots (DBS) are used for faster initial testing.
A skeletal survey (full body X-rays) reveals dysostosis multiplex:
- Skull: Macrocephaly; thickened calvarium; J-shaped sella turcica
- Spine: Bullet-shaped / oval vertebral bodies; antero-inferior beaking; gibbus deformity; platyspondylia; thoracolumbar kyphosis
- Ribs: Spatulate (oar-shaped) ribs
- Clavicles: Short and thickened
- Pelvis: Flared iliac wings; coxa valga; poorly developed acetabulum
- Long bones: Shortened, thickened diaphyses; hypoplastic epiphyses
- Hands: Bullet-shaped proximal phalanges; pointed proximal metacarpals
- Enlarged perivascular (Virchow-Robin) spaces — most characteristic MRI finding; seen as dilated spaces around perforating vessels, especially in white matter and basal ganglia
- White matter signal changes (leukoencephalopathy) — T2 hyperintensity in periventricular white matter
- Communicating hydrocephalus — from impaired CSF reabsorption (GAG deposition in meninges)
- Brain atrophy (progressive)
- Spinal cord compression — from gibbus deformity, odontoid hypoplasia, ligamentous thickening
- Dural thickening at craniocervical junction
💡 Key MRI Finding
Dilated Virchow-Robin perivascular spaces = highly characteristic of MPS; should prompt workup even if subtle.
In MPS, lysosomal accumulation of GAGs produces:
- Balloon/vacuolated cells — Cells engorged with clear vacuoles containing undigested GAGs (seen in liver, spleen, bone marrow, neurons)
- Gargoyle cells — Vacuolated connective tissue cells full of GAGs
- Lymphocytes show cytoplasmic granules (Alder-Reilly bodies) in MPS — metachromatic granules on peripheral blood smear
Gaucher cells (wrinkled tissue paper / "crumpled newspaper" appearance) are NOT seen in MPS — they are seen in Gaucher disease (glucocerebrosidase deficiency), a sphingolipidosis, not a mucopolysaccharidosis.
- Chorionic villus sampling (CVS) — enzyme assay on cultivated chorionic villi (can be done at 10–12 weeks)
- Amniocentesis — enzyme assay on cultivated amniocytes (15–18 weeks)
- Molecular genetic testing — if family mutation known, can use preimplantation genetic diagnosis (PGD) via IVF
- Newborn screening: Some states include alpha-L-iduronidase activity in newborn screening panels (dried blood spot)
💊 Management — Exam Q&A
Treatment is multidisciplinary. Definitive treatments:
- Hematopoietic stem cell transplantation (HSCT) — best outcome if done early (<2.5 years, before significant neurological damage). Stabilizes CNS disease. Does NOT reverse established brain damage.
- Enzyme Replacement Therapy (ERT) — Laronidase (recombinant α-L-iduronidase; brand: Aldurazyme). Effective for somatic disease; does NOT cross the blood-brain barrier — limited CNS benefit.
- Combined HSCT + ERT — Currently being studied; may offer synergistic benefit
Supportive/symptomatic treatment:
- ENT: Tonsillectomy + adenoidectomy; grommet insertion for hearing; CPAP for obstructive sleep apnea
- Orthopedic: Spinal stabilization (atlantoaxial instability), joint surgeries
- Cardiac: Valve replacement if severe; medications for heart failure
- Neurosurgery: VP shunt for hydrocephalus
- Special education, physiotherapy, occupational therapy, speech therapy
- Genetic counseling
| MPS Type | Drug (Generic) | Brand | Approval |
|---|---|---|---|
| MPS I (Hurler/Scheie) | Laronidase | Aldurazyme | FDA 2003 |
| MPS II (Hunter) | Idursulfase | Elaprase | FDA 2006 |
| MPS IVA (Morquio A) | Elosulfase alfa | Vimizim | FDA 2014 |
| MPS VI (Maroteaux-Lamy) | Galsulfase | Naglazyme | FDA 2005 |
| MPS VII (Sly) | Vestronidase alfa | Mepsevii | FDA 2017 |
🚨 Critical Limitation of ERT
ERT does NOT cross the blood-brain barrier — it cannot treat or prevent CNS/neurological involvement. Therefore, for MPS I-H (Hurler), HSCT is preferred over ERT alone as it provides a donor-derived enzyme source that can reach the CNS.
HSCT (bone marrow transplant/cord blood transplant) replaces the patient's deficient enzyme-producing cells with donor cells that produce normal alpha-L-iduronidase. These cells can cross into the CNS via perivascular spaces.
Benefits:
- Stabilizes cognitive decline and prevents further neurological deterioration
- Reduces hepatosplenomegaly
- Improves or stabilizes cardiac and pulmonary function
- Improves survival significantly
Critical limitations:
- Must be performed before 2.5 years of age and before significant neurological damage — "treat early or don't bother"
- Does NOT reverse existing neurological damage — only prevents further deterioration
- Skeletal and corneal disease continue to progress even after successful HSCT
- Risks: Graft-versus-host disease (GVHD), infection, transplant failure
- Requires matched donor (sibling or cord blood)
- Atlantoaxial stabilization (C1-C2 fusion) — for significant instability or myelopathy (especially Morquio)
- Spinal decompression + fusion for gibbus-related cord compression
- Carpal tunnel release — for median nerve compression
- Hip dysplasia — osteotomies or hip arthroplasty
- Genu valgum correction — stapling or osteotomy
- Tonsillectomy + adenoidectomy for OSA (most common surgery in MPS)
- VP shunt for hydrocephalus
- Cardiac valve replacement for severe valvular disease
🚨 Anesthesia Alert
All MPS patients are high-risk for anesthesia. A senior anesthesiologist must be present, fiberoptic intubation must be available, and airway must be assessed preoperatively. Avoid excessive neck flexion/extension (risk of cord compression).
SRT is a pharmacological approach that reduces the synthesis of GAG substrates rather than replacing the missing enzyme, thereby reducing the substrate load accumulated in lysosomes.
- Mechanism: Inhibits enzymes in the biosynthetic pathway of GAGs
- Advantage: Small molecule — can potentially cross the blood-brain barrier (CNS benefit)
- Example: Genistein (an isoflavone) — reduces GAG synthesis via EGFR pathway inhibition; studied in MPS III
- Status: Still largely investigational for MPS (unlike SRT in Gaucher disease where miglustat is approved)
Without treatment:
- Median age of death: 8.7 years (most die before age 10)
- Cause of death: Cardiorespiratory failure, airway obstruction, recurrent infections
- Progressive and fatal
With early HSCT (<2.5 years):
- Significantly improved survival — patients can live into adulthood (2nd–3rd decade)
- Cognitive function stabilized if transplanted early
- Somatic disease (skeletal, cardiac) still progresses — ongoing need for supportive surgery
With ERT alone (no HSCT):
- Somatic benefit (hepatosplenomegaly, mobility improves)
- Does not halt neurological decline in MPS I-H
- Life expectancy improved but not normalized
🔭 Recent Advances — Exam Q&A
Gene therapy delivers the functional copy of the deficient enzyme gene directly into host cells using viral vectors:
- In vivo gene therapy: Recombinant adeno-associated virus (AAV) or lentiviral vectors deliver the IDUA gene directly — via intravenous or intrathecal/intracerebroventricular routes
- Ex vivo gene therapy: Patient's own stem cells are harvested, IDUA gene is inserted in laboratory, then cells are reinfused — similar to autologous HSCT but with corrected cells
- Intrathecal ERT — Delivers enzyme directly into CSF to bypass blood-brain barrier; under clinical trials for MPS I, II, III
- CRISPR-Cas9 genome editing — Experimental correction of IDUA mutation; highly promising but not yet in clinical use
Gene therapy animal model studies have shown correction of liver, spleen, and partial CNS effects — human trials ongoing.
Several countries/states now include MPS I (and some MPS II) in their newborn screening (NBS) programs:
- Method: Dried blood spot (DBS) — measures alpha-L-iduronidase enzyme activity via fluorometric assay or MS/MS
- Confirmatory: Enzyme assay in leukocytes + gene sequencing if DBS abnormal
- Importance: Early detection allows HSCT to be performed before neurological damage occurs (<2.5 years) — significantly improves outcomes
- Identifies attenuated forms (Scheie) that may have been missed until adulthood
- India does not yet have universal NBS for MPS — selective screening based on clinical suspicion
Pharmacological chaperones are small molecules that bind to misfolded mutant enzymes and stabilize them, allowing them to fold correctly and reach the lysosome rather than being degraded prematurely by the proteasome.
- Only works for missense mutations where some residual enzyme protein is produced
- Advantage: Small molecules — can cross the blood-brain barrier
- Currently approved: Migalastat (Galafold) for Fabry disease (GLA gene missense mutations)
- Being investigated for MPS I, II — still experimental
Since standard intravenous ERT cannot cross the blood-brain barrier, direct CNS delivery is being investigated:
- Intrathecal (IT) ERT: Enzyme injected into the lumbar CSF via a subcutaneous port/reservoir — reaches spinal cord and brain via CSF circulation
- Intracerebroventricular (ICV): Directly into ventricles via implanted Ommaya reservoir — better CNS distribution
- Clinical trials ongoing for MPS I, MPS IIIA (heparan N-sulfatase), MPS II
- Early results promising — cognitive stabilization and reduction in brain GAG levels shown
- Urine GAG quantification — falls with successful ERT/HSCT; easy, non-invasive
- Plasma GAG-derived disaccharides (LC-MS/MS) — more sensitive and specific
- Liver/spleen volume (MRI/ultrasound) — organomegaly decreases with treatment
- 6-minute walk test — functional outcome in Morquio and Maroteaux-Lamy
- CSF GAG — monitored with IT ERT
- Brain MRI scoring systems — white matter changes, perivascular space scoring
- Neuropsychological testing — tracks cognitive stabilization
- Echocardiography — valvular disease progression
⚡ Key Points — Quick Revision
Hurler Syndrome — One-Liners
- Hurler = MPS I-H: Most severe MPS I; alpha-L-iduronidase deficiency; Chr 4p16.3; AR
- GAG stored in Hurler: Dermatan sulfate + Heparan sulfate
- Incidence: 1 in 100,000; M = F
- Normal at birth, symptomatic by 6–18 months
- Key triad: Coarse facies + Corneal clouding + Dysostosis multiplex
- Gibbus deformity: Thoracolumbar kyphosis from anterior vertebral wedging
- J-shaped sella turcica on skull X-ray = classic MPS finding
- Bullet-shaped phalanges + spatulate ribs = X-ray hallmarks
- Antero-inferior vertebral beaking = Hurler (vs. central anterior = Morquio)
- MRI hallmark: Enlarged Virchow-Robin perivascular spaces
- Gold standard Dx: Enzyme activity assay (leukocytes/fibroblasts)
- Screening: Urine GAG + electrophoresis (Berry spot test — poor sensitivity)
- ERT: Laronidase (Aldurazyme) — does NOT cross BBB
- HSCT: Best treatment for MPS I-H if done <2.5 years; stabilizes CNS
- Death: Usually before 10 years without treatment; cause = cardiorespiratory failure
MPS Group — Exam Essential Comparisons
- Only X-linked MPS = MPS II (Hunter); all others = AR
- No corneal clouding = Hunter (MPS II)
- Ivory skin lesions = Hunter (MPS II) — pathognomonic
- Most common MPS overall = MPS III (Sanfilippo)
- Most common MPS I = Hurler (MPS I-H)
- Severe CNS, mild somatic = Sanfilippo (MPS III)
- Severe skeletal, normal intellect = Morquio (MPS IV)
- Joint hypermobility = Morquio (MPS IV) — all others have stiff joints
- Odontoid hypoplasia → atlantoaxial instability = Morquio
- Hydrops fetalis = MPS VII (Sly syndrome) — rarest
- Hurler-like facies + Normal IQ = Maroteaux-Lamy (MPS VI)
- MPS V = does not exist (renamed Scheie = MPS I-S)
- Behavioral problems / hyperactivity in toddler = think Sanfilippo
High-Yield Exam Traps
- Hunter = X-linked → no corneal clouding + ivory skin lesions — these TWO features distinguish it from Hurler
- ERT does NOT help CNS in MPS I-H → that's why HSCT is preferred over ERT alone in severe MPS I
- HSCT must be done before 2.5 years — after this age, benefit on cognitive outcome is limited
- Morquio: Floppy/hypermobile joints but severe skeletal dysplasia — do not confuse with connective tissue disorders
- Morquio: Flexion-extension cervical spine X-ray mandatory before any anesthesia (odontoid hypoplasia)
- Berry spot test has high false-negative rate → confirm with enzyme assay
- Scheie syndrome (MPS I-S) — Normal intelligence, normal lifespan — mildest, often presents in teenagers with aortic valve disease