Hunter’s disease, formally known as mucopolysaccharidosis type II (MPS II), is a rare genetic condition in which the body lacks an enzyme needed to break down certain complex sugars. Without this enzyme, these sugars build up inside cells throughout the body, gradually damaging organs, bones, airways, and in many cases the brain. It affects roughly 1 in 100,000 to 170,000 male births worldwide, with higher rates reported in some Asian countries.
How the Disease Works at a Cellular Level
Every cell in your body contains small recycling compartments called lysosomes. These compartments break down molecules the cell no longer needs. One of the molecules they process is a group of complex sugars called glycosaminoglycans, or GAGs. Breaking down two specific types of GAGs, heparan sulfate and dermatan sulfate, requires an enzyme called iduronate-2-sulfatase (often shortened to IDS or I2S).
In Hunter’s disease, the gene responsible for producing this enzyme is either missing or faulty. Without functional IDS, heparan sulfate and dermatan sulfate accumulate inside lysosomes instead of being recycled. Over time, this buildup damages cells in nearly every organ system, from the heart and liver to the skeleton and brain.
Why It Almost Exclusively Affects Boys
The IDS gene sits on the X chromosome. Males have one X and one Y chromosome, so a single defective copy of the gene is enough to cause disease. Females have two X chromosomes, meaning they would need defective copies on both to develop symptoms, which is extremely rare. Women who carry one altered copy are typically unaffected but have a 50% chance of passing the faulty gene to each son.
Signs and Symptoms
Babies with Hunter’s disease usually appear healthy at birth. Symptoms typically emerge between ages 2 and 4, though the pace and severity vary widely depending on how much residual enzyme activity the body retains.
Physical Features
Children develop coarse facial features, a large head, and a short neck. Growth slows, leading to short stature with a disproportionately short trunk. Skeletal abnormalities are common: joints stiffen, bones develop irregularly (a pattern called dysostosis multiplex), and carpal tunnel syndrome can appear even in young children. The liver and spleen enlarge as GAGs accumulate in these organs.
Severe vs. Attenuated Forms
The defining difference between the two forms is whether the brain is affected. Up to two-thirds of people with MPS II have neurological involvement, which is considered the severe form. These children experience progressive cognitive decline and behavioral problems, often losing skills they previously acquired. The attenuated form spares the brain to a large degree. People with this form can reach adulthood and maintain near-normal intellectual function, though they still face the full range of physical complications: joint problems, organ enlargement, heart valve disease, and airway obstruction.
Heart and Lung Complications
GAG buildup takes a particular toll on the cardiovascular and respiratory systems, and these complications are a major driver of outcomes in Hunter’s disease.
Heart valves thicken and stiffen over time, eventually leaking or narrowing enough to affect blood flow. Some patients eventually need valve replacement surgery. Annual monitoring with echocardiograms and electrocardiograms is standard to catch valve dysfunction and heart muscle changes early.
The airways narrow from multiple directions at once. Tissue in the throat and nasal passages thickens, the tongue enlarges, and the trachea can become floppy or narrowed. Many children need their tonsils and adenoids removed to reduce obstruction. Obstructive sleep apnea is common and often requires a breathing support device during sleep. Frequent ear infections are another hallmark, and many children have ear tubes placed early to prevent hearing loss from chronic fluid buildup.
How It’s Diagnosed
Diagnosis usually starts when a pediatrician notices the characteristic physical features or when parents raise concerns about developmental delays. The initial screening involves a urine test that measures GAG levels. If GAGs are elevated or show an abnormal pattern, the next step is measuring IDS enzyme activity directly, ideally in white blood cells or skin cells (fibroblasts). Low or absent enzyme activity confirms the diagnosis. Dried blood spot tests exist but require confirmation through these more definitive methods. Genetic testing of the IDS gene can identify the specific mutation and is useful for family planning and carrier testing.
Enzyme Replacement Therapy
The first major treatment for Hunter’s disease is enzyme replacement therapy (ERT), which delivers a lab-made version of the missing enzyme through a weekly intravenous infusion lasting one to three hours. This treatment helps reduce GAG storage in organs like the liver and spleen, improves walking endurance, and can slow some physical symptoms.
ERT has an important limitation, though. The replacement enzyme is a large molecule that cannot cross the blood-brain barrier, which means it does not address the neurological decline seen in the severe form of the disease. For children whose brains are unaffected, ERT can meaningfully improve quality of life and physical function. For those with the severe form, it helps the body but not the brain.
A Newer Option for Brain Involvement
The FDA has approved a treatment called Avlayah (tividenofusp alfa) specifically designed to address the neurological side of Hunter’s disease. Unlike standard ERT, this drug was engineered to cross the blood-brain barrier. It is given as a weekly IV infusion and is approved for pediatric patients who weigh at least about 11 pounds, with the critical caveat that treatment needs to begin before advanced neurological damage has already set in. A large randomized trial evaluating the drug’s long-term clinical benefit is nearly fully enrolled. This represents a significant shift in what’s possible for the severe form of the disease, which previously had no treatment targeting brain involvement.
Living With Hunter’s Disease
Managing MPS II requires a team of specialists because the disease touches so many organ systems simultaneously. Children typically see cardiologists, pulmonologists, orthopedic surgeons, neurologists, audiologists, and ear-nose-throat specialists on a regular basis. Annual heart evaluations, periodic sleep studies, and routine hearing and vision checks become part of the rhythm of care.
Physical therapy helps maintain joint mobility, and occupational therapy supports daily functioning as stiffness progresses. Anesthesia carries elevated risk for people with Hunter’s disease because of their narrowed airways, so any surgical procedure requires careful planning with an experienced anesthesia team.
Life expectancy depends heavily on which form of the disease a person has and how early treatment begins. The severe form with neurological involvement historically led to death in the teens or early twenties, though newer therapies may change this trajectory. People with the attenuated form can live into middle adulthood or beyond, particularly with consistent monitoring and treatment of cardiac and respiratory complications.