Hurler syndrome is a rare, inherited condition in which the body cannot produce a key enzyme needed to break down complex sugars. Without this enzyme, those sugars build up inside cells throughout the body, progressively damaging the brain, heart, bones, and other organs. It is the most severe form of a group of conditions called mucopolysaccharidosis type I (MPS I), and without treatment, children with Hurler syndrome typically do not survive past the first decade of life.
What Happens Inside the Body
Every cell contains small compartments called lysosomes that act as recycling centers, breaking down waste materials so the cell can reuse or dispose of them. In Hurler syndrome, a genetic mutation prevents the body from making enough of an enzyme called alpha-L-iduronidase. This enzyme’s job is to break down two specific types of complex sugar molecules (glycosaminoglycans, or GAGs) that are found in connective tissue throughout the body.
When those sugars can’t be broken down, they accumulate inside lysosomes and eventually overwhelm the cell. The damage goes well beyond simple storage, though. The buildup triggers a cascade of problems: inflammation, disruption of the cell’s energy-producing machinery, impaired ability to clear out damaged proteins, and the release of harmful molecules into surrounding tissue. This is why Hurler syndrome affects so many different organ systems at once.
How Hurler Syndrome Differs From Milder Forms
MPS I exists on a spectrum. Historically, doctors used three separate names: Hurler syndrome for the most severe form, Scheie syndrome for the mildest, and Hurler-Scheie for something in between. Today, the classification has been simplified into just two categories: severe MPS I (Hurler syndrome) and attenuated MPS I, because the clinical findings overlap and no simple lab test distinguishes one from another.
The differences between severe and attenuated forms are dramatic. In severe MPS I, 100% of affected children develop coarse facial features, enlarged liver and spleen, intellectual disability, and hearing loss. In attenuated MPS I, only about 10% develop coarse facial features or intellectual disability, though 80% still have organ enlargement and 50% experience hearing loss. Children with the attenuated form typically don’t show symptoms until age three to ten, and some live into adulthood, while those with severe MPS I show signs within the first year of life.
If a child’s development is still normal at 24 months and their physical involvement is moderate, they are generally classified as having the attenuated form. This distinction matters because it directly shapes which treatments are recommended.
Signs and Symptoms
Infants with Hurler syndrome appear normal at birth. The earliest clues tend to be nonspecific: frequent upper respiratory infections and hernias (umbilical or inguinal) before age one. Coarsening of facial features, including a flat nasal bridge, prominent forehead, widely spaced eyes, and thickened lips, may not become obvious until after the first birthday.
Skeletal problems begin around six months but become more clinically apparent between 10 and 14 months. A hallmark finding is a curved lower spine that creates a hunchback-like posture, known as a gibbus deformity. Children develop widespread bone and joint abnormalities: stiff joints, abnormal hand and spine development, hip deformities, and carpal tunnel syndrome. The skull often enlarges rapidly. By age three, growth slows significantly.
The liver and spleen enlarge as sugar molecules accumulate in those organs. Corneal clouding affects vision. Heart valves and airways become progressively compromised, and hearing loss is virtually universal. Without treatment, cardiorespiratory failure is the most common cause of death, typically before age ten.
Cognitive Decline
The neurological effects of Hurler syndrome are among its most devastating features. Cognitive decline relative to age-specific developmental milestones begins at approximately six months. In a study of 15 untreated children, developmental scores dropped from an average of 80 (below average but functional) before age two to 61 (well below average) after age two. Developmental delay is usually apparent by the first birthday, and children who go untreated eventually lose skills they had previously gained.
This progressive brain involvement is a central reason early diagnosis and treatment are so critical. Once significant neurological damage has occurred, it is largely irreversible.
How It Is Diagnosed
Diagnosis starts with measuring the activity of alpha-L-iduronidase in blood. Children with Hurler syndrome have very low or absent enzyme activity. Urine tests can detect elevated levels of the sugar molecules that accumulate in the condition. Genetic testing can confirm mutations in the IDUA gene and may help predict severity, though the clinical picture remains the most reliable way to classify a child as having the severe or attenuated form.
Since 2016, MPS I has been included on the federal Recommended Uniform Screening Panel (RUSP) for newborn screening in the United States. The federal advisory committee estimated that newborn screening could prevent up to two deaths before age five each year by enabling earlier treatment. Not all states have implemented screening yet, but adoption has been expanding.
Treatment: Stem Cell Transplant and Enzyme Replacement
For severe MPS I, the primary treatment is a stem cell transplant (also called a bone marrow transplant or hematopoietic cell transplant). Donor stem cells engraft in the child’s body and produce the missing enzyme on an ongoing basis. Critically, transplanted cells can cross into the brain and provide enzyme there, which is something other treatments cannot easily do. An international multicenter study found that earlier transplant age, the use of donors who don’t carry the gene mutation, and achieving full donor cell engraftment all lead to the best long-term outcomes. This is why early diagnosis is so important: the younger the child at transplant, the less irreversible damage has accumulated.
Enzyme replacement therapy (ERT) using a manufactured version of the missing enzyme is given as a weekly intravenous infusion. It can reduce organ enlargement and improve some physical symptoms, but it does not cross into the brain effectively, so it cannot prevent cognitive decline on its own. ERT is often started before transplant to stabilize the child’s condition and may continue afterward in certain situations.
For children with the attenuated form, ERT alone is often the primary treatment, since significant brain involvement is much less common.
Gene Therapy
A newer approach involves gene therapy, in which a child’s own stem cells are collected, genetically corrected in the laboratory to produce the missing enzyme, and then reinfused. An ongoing clinical trial published in the New England Journal of Medicine reported that this approach resulted in extensive correction of the metabolic defect in both the body and the brain in children with Hurler syndrome. The study is still in progress with a planned five-year follow-up, but early results have been promising.
Living With Hurler Syndrome
Even with successful transplant, Hurler syndrome is not cured. Many children continue to need orthopedic surgeries, physical therapy for joint stiffness, hearing aids, and cardiac monitoring throughout their lives. Corneal clouding may require treatment, and spinal deformities often need ongoing management. Cognitive outcomes after transplant vary widely depending on how early it was performed and how much brain involvement was already present.
Children who receive a transplant early, ideally before significant cognitive decline has set in, have the best chance of preserving intellectual function and extending life well beyond what the untreated disease would allow. The contrast with untreated Hurler syndrome, where death from cardiorespiratory failure typically occurs within the first decade, underscores how much timing matters in this condition.