Hunter Syndrome

What is Hunter syndrome?

Hunter syndrome is a rare genetic disorder in which your child’s body doesn’t properly break down (digest) certain sugar molecules. When these molecules build up in organs and tissues over time, they can cause damage that affects physical and mental development.

Healthcare providers classify Hunter syndrome into severe and mild types. The severe type progresses faster and involves impaired intellectual abilities. In the most severe cases, children begin to experience problems with basic functioning between the ages of 6 and 8. The severe type occurs in about 60% of all cases.

Hunter syndrome is one of a group of diseases called mucopolysaccharidoses. Another name for Hunter syndrome is mucopolysaccharidosis type II, or MPS II.

How common is Hunter syndrome?

Hunter syndrome is very uncommon, and it almost always occurs in children assigned male at birth (AMAB). Providers diagnose it in roughly 1 out of every 100,000 to 170,000 children AMAB. However, people assigned female at birth (AFAB) can be carriers of the genetic mutation that causes MPS II.

What are the signs and symptoms of Hunter syndrome?

The signs and symptoms of Hunter syndrome typically begin to appear in children between the ages of 2 and 4. Hunter syndrome symptoms vary in severity and may include:

• Stiff joints.
• Thickening of facial features including nostrils, lips and tongue.
• Delayed appearance of teeth or wide spaces between teeth.
• Larger than normal head, wide chest and short neck.
• Hearing loss that gets worse with time.
• Delayed growth, especially starting around age 5.
• Enlarged spleen and liver.
• White growths on the skin.

What causes Hunter syndrome?

A genetic mutation in the IDS gene causes Hunter syndrome. The IDS gene is responsible for regulating the production of an enzyme in your body called iduronate 2-sulfatase (I2S). The I2S enzyme breaks down complex sugar molecules called glycosaminoglycans (GAGs). (MPS II gets its name because GAGs were formerly called mucopolysaccharides.)

In people with MPS II, their bodies don’t produce any or enough of the I2S enzyme. The missing enzyme causes a buildup of GAGs within the lysosomes of cells. Lysosomes are parts of cells that break down and recycle molecules. MPS II is a type of lysosomal storage disorder, which is a condition that causes sugar molecules to accumulate inside lysosomes. These buildups can damage organs and tissues throughout the body.

What are the risk factors for Hunter syndrome?

People are at a higher risk for Hunter syndrome if they have a biological family member with the disease.

Children assigned male at birth (AMAB) have a higher risk of inheriting the disease than children assigned female at birth (AFAB). This difference exists because the disease is linked to the X chromosome. Children AFAB inherit two X chromosomes, while children AMAB have only one. If a child AFAB inherits the faulty gene, their other X chromosome can provide the necessary enzyme.

What complications are associated with Hunter syndrome?

Depending on its severity, Hunter syndrome can involve many complications. Providers use medications and sometimes surgery to manage these complications. They may include:

• Breathing problems due to thickened tissue and blocked airways.
• Heart disease.
• Joint and bone abnormalities.
• Declining brain function.
• Carpal tunnel syndrome.
• Hernias.
• Seizures.
• Behavioral problems.

How is Hunter syndrome diagnosed?

Your child’s healthcare provider will use several tests to diagnose Hunter syndrome:

• Urine test: A urine test checks for unusually high levels of sugar molecules.
• Blood tests: Blood tests can show low or absent levels of enzyme activity, which is also a sign of the disease.
• Genetic testing: Genetic testing identifies changes (mutations) in the gene to confirm the diagnosis.

How is Hunter syndrome treated?

Treatment for Hunter syndrome depends on the symptoms. A team approach, with specialists in different areas of expertise, could help manage your child’s condition. The goal of treatment is to slow the progression of the disease, identify possible complications related to the disease early and improve quality of life.

The treatment shown to do this best is enzyme replacement therapy. Healthcare providers replace the missing enzyme with a human-made version called idursulfase (Elaprase®). Providers usually deliver this treatment intravenously (through a needle inserted into your child’s vein) once a week. Genetic therapy (gene editing) trials are underway and may offer hope for Hunter syndrome patients. Results are awaited.

Can Hunter syndrome be prevented?

Because it’s genetic, you can’t prevent Hunter syndrome. People who have a child with Hunter syndrome may want to talk to a genetic counselor before having another baby. This specialist can help parents understand the chances of passing the disease on to another child.

What is the outlook (prognosis) for people with Hunter syndrome?

No cure is available for Hunter syndrome. The most severe cases can be life-threatening, with life expectancy typically between 10 years and 20 years. People with mild cases of the disease typically live longer into adulthood.

For many people, treatments including medicines, physical therapy and surgery can help manage the challenges the disease presents and improve quality of life.

When can my child go back to regular activities?

People with Hunter syndrome may have to modify their daily activities due to progressive symptoms that affect function and mobility. Your child’s healthcare provider will tell you about activities and therapies that can help make the symptoms of the disease more manageable.

When should I call my child’s healthcare provider concerning Hunter syndrome?

Contact your child’s provider if they begin showing signs and symptoms of Hunter syndrome or experiencing developmental delays. Early treatment can help prevent permanent organ and tissue damage.

What questions should I ask my child’s healthcare provider?

If your child has Hunter syndrome, you may want to ask their provider:

• How severe is Hunter syndrome?
• What is the short-term and long-term prognosis for my child?
• How will the disease affect my child’s life?
• What are the treatment options?

Hunter vs. Hurler syndrome — what’s the difference?

Hunter syndrome and Hurler syndrome are both lysosomal storage disorders within a group of diseases called mucopolysaccharidoses.

Hurler syndrome is a severe form of mucopolysaccharidosis type I (MPS I). With MPS I, your body doesn’t have enough alpha-L-iduronidase enzymes. Hurler syndrome is more severe than Hunter syndrome.

A note from Cleveland Clinic

A diagnosis of Hunter syndrome can be heartbreaking for your family, especially upon hearing that your child may have a shortened life expectancy. During this difficult time, you should work with your child’s providers to learn as much about their diagnosis and treatment options as you can. It may also be helpful to surround yourself with supportive people, like your friends or family, along with your child’s healthcare providers, who can offer comfort and aid during your child’s diagnosis and the challenges it may bring.