Mucopolysaccharidosis type II, or Hunter syndrome, is a rare genetic disorder. Most cases are in males.

It happens when the enzyme needed to break down complex sugars is missing or not working as it should. Enzymes are proteins that speed up chemical activity in the cells.

More to Know

With Hunter syndrome, chains of sugar molecules (called mucopolysaccharides) can't be broken down. They build up in the body, which harms cells and tissues.

Hunter syndrome, which isn't present at birth, can be either severe or mild. The severe form begins just after age 2. Symptoms of the mild form don't start until later.

Most people with Hunter syndrome will have:

• a larger than normal head
• facial features that include a broad nose; flared nostrils; and thickening of the lips, tongue, and nostrils
• deafness that gets worse over time
• joint stiffness
• carpal tunnel syndrome, which causes numbness, weakness, and tingling in the hand

The juvenile (severe) form of Hunter syndrome includes aggressive behavior, hyperactivity, intellectual delay, and stiff or rigid muscles.

Other signs and symptoms include heart problems, vision disturbances, an enlarged spleen or liver, hernias, and breathing problems.

Keep in Mind

There is no cure for Hunter syndrome, but the drug idursulfase (Elaprase) can help ease symptoms. Bone marrow transplants can slow the progression of the disease in some who have it.

People with a family history of Hunter syndrome who are thinking about having children should see a genetic counselor.

All A to Z dictionary entries are regularly reviewed by KidsHealth medical experts.