Hunter syndrome

DEFINITION

Hunter syndrome is a rare genetic disorder that occurs when an enzyme your body needs is either missing or doesn’t work properly.

Because the body doesn’t have enough of the enzyme to break down certain complex molecules, the molecules build up in harmful amounts in certain cells and tissues. The buildup that occurs in Hunter syndrome eventually causes permanent, progressive damage affecting appearance, mental development, organ function and physical abilities.

Hunter syndrome appears in children as young as 18 months. It nearly always occurs in males.

There’s no cure for Hunter syndrome. Treatment of Hunter syndrome involves management of symptoms and complications.

CAUSES

Hunter syndrome develops when a defective chromosome is inherited from the child’s mother. Because of that defective chromosome, an enzyme that’s needed to break down complex sugars called glycosaminoglycans is missing or malfunctioning.

The missing or malfunctioning enzyme is called iduronate-2-sulfatase.

In unaffected people, these enzymes are found in parts of the body’s cells known as lysosomes. The lysosomes use enzymes to break down glycosaminoglycans, as part of the body’s normal recycling and renewal process. In a person with Hunter syndrome or another form of MPS, these enzymes either are missing or don’t work correctly.

Normally, the nutrients that are broken down by lysosomes help your body build bone, cartilage, tendons, corneas, skin and connective tissue, and the fluid that lubricates your joints.

When this enzyme isn’t working properly, undigested glycosaminoglycans collect in the cells, blood and connective tissues, causing permanent and progressive damage. Hunter syndrome and other forms of MPS are sometimes called lysosomal storage diseases.

SYMPTOMS

Hunter syndrome is one type of a group of inherited metabolic disorders called mucopolysaccharidosis (MPS), and Hunter syndrome is referred to as MPS II. Hunter syndrome symptoms vary from person to person. Some people have mild symptoms, while others have severe problems. Symptoms aren’t present at birth. If symptoms begin early — around ages 2 to 4 — the syndrome is usually more severe.

Signs and symptoms may include:

• Delayed development, such as late walking or talking
• Changing facial features, including thickening of the lips, tongue and nostrils
• A broad nose and flared nostrils
• Claw-like hands
• A protruding tongue
• Abnormal bone size or shape and other skeletal irregularities
• Enlarged internal organs, such as the liver and spleen, resulting in a distended abdomen
• Respiratory difficulties including sleep apnea, a condition in which breathing intermittently stops during sleep
• Cardiovascular disorders, such as progressive thickening of heart valves, high blood pressure (hypertension) and obstruction of blood vessels
• Vision damage or loss from degeneration of cells that capture light and buildup of cellular debris in the brain causing pressure on the optic nerve and eye
• Progressive loss of hearing
• Aggressive behavior
• Stunted growth
• Joint stiffness
• Diarrhea
• Carpal tunnel syndrome
• Sleep apnea

When to see a doctor

Hunter syndrome isn’t common, but if you notice changes in your child’s facial appearance, a loss of previously acquired skills, or other signs or symptoms listed above, talk to your child’s primary care doctor. He or she can help you decide if you need to see a specialist or seek out further testing.