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Hunter syndrome

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.

Symptoms Causes Risk factors Complications Prevention

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.

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.

There are two major risk factors for developing Hunter syndrome:

  • Family history. Hunter syndrome is caused by a defective chromosome, and a child must inherit the defective chromosome to develop the disease. Hunter syndrome is what's known as an X-linked recessive disease. This means that women carry the defective disease-causing X chromosome and can pass it on, but aren't affected by the disease themselves.
  • Sex. Hunter syndrome nearly always occurs in males. Girls are less at risk of developing this disease because they inherit two X chromosomes. If one of the X chromosomes is defective, their normal X chromosome can provide a functioning gene. If the X chromosome of a male is defective, however, there isn't another normal X chromosome to compensate for the problem.

A variety of complications can occur with Hunter syndrome depending on the type and severity of the disease. These may include:

  • Respiratory complications. All forms of MPS, including Hunter syndrome, involve respiratory complications that contribute to your child's disability and sometimes cause death as the disease progresses. An enlarged tongue, thickened gums, and thickening of the nasal passages and windpipe (trachea) make breathing difficult. Children often have chronic ear and sinus infections, respiratory infections, and pneumonia. Sleep apnea, a condition in which breathing is intermittently interrupted during sleep, is often present because of airway constriction.
  • Cardiac complications. Thickening of tissue associated with Hunter syndrome can cause progressive thickening of the heart's valves. This causes improper closing of heart valves. As a result, the heart and other parts of the body don't receive blood efficiently. As the disease progresses, these conditions often become worse and typically result in heart failure.

    The thickening of tissue can also cause narrowing of the aorta (coarctation) and other blood vessels. This in turn can result in high blood pressure (hypertension) and narrowing of arteries in the lungs (pulmonary hypertension).

  • Skeletal and connective tissue complications. The storage of undigested glycosaminoglycans in connective tissues results in abnormalities in bones, joints and ligaments. This reduces your child's growth, causing pain and physical malformations, and making it difficult for him or her to move.

    Nearly everyone with Hunter syndrome experiences joint stiffness, which makes movement painful. The stiffness is caused by swelling of joint connective tissues and abnormalities of cartilage and bones. If your child is in pain, he or she will likely move less, which can lead to more stiffness and pain.

    The group of abnormalities typically seen in the bones of people with Hunter syndrome is called dysostosis multiplex. Children with these abnormalities can develop irregularly shaped vertebrae and spines (kyphoscoliosis), ribs, arms, fingers, legs, and pelvises. Their skulls may press down on or fuse with their upper spines. These complications cause many people with Hunter syndrome to be abnormally short. Those with milder cases may reach normal or near-normal height.

    Hernias (inguinal and umbilical) are common in Hunter syndrome. They happen because of problems with connective tissue. A hernia occurs when soft tissue, usually part of the intestine, pokes through a weak spot or tear in the lower abdominal wall. Hernias associated with Hunter syndrome can become quite large and are often one of the first signs of the disorder. Enlargement of the liver and spleen (hepatosplenomegaly), which is common in Hunter syndrome, may increase pressure in the abdomen, causing a hernia.

  • Brain and nervous system complications. A variety of neurological complications may be present and continue to develop in children with Hunter syndrome. Many neurological problems are caused by buildup of excess fluids in your child's brain (hydrocephalus). Pressure from these fluids can cause other problems that may affect your child's eyes and other sensory organs, which can cause severe headaches, interfere with vision and change your child's mental state. Placement of a shunt may help drain excess fluids and relieve pressure on the spinal cord. Imaging tests also may reveal a variety of cyst-like structures in parts of the brain.

    Your child also may develop a condition in which the membranes that surround the spinal cord may become thick and scarred (hypertrophic cervical pachymeningitis). This causes pressure and compression of the upper spinal cord. As a result, your child may develop fatigue in his or her legs and gradually weaken and become less physically active.

    Other disorders, such as carpal tunnel syndrome, can result from nerve compression that happens because of bone deformities and storage of glycosaminoglycans in tissues.

    Abnormal behavior can develop in children with more-severe cases of Hunter syndrome. Often your child's mental development will become affected between the ages of 2 and 6. Some children are hyperactive and have trouble paying attention or following directions. Your child may also behave aggressively and seem unable to sense danger. As your child's overall physical functioning declines, these behavior problems tend to become less severe.

    Seizures also may occur in children with Hunter syndrome.

  • Longer recovery from other illnesses. Be aware that recovery times from normal childhood illnesses may be longer for children with Hunter and other MPS syndromes. As a result, be sure to take general preventive measures — for example, get your child a flu shot and ensure your child receives all necessary vaccinations.

Hunter syndrome is a genetic disorder. Talk to your doctor or a genetic counselor if you're thinking about having children and you or any members of your family have a genetic disorder or a family history of genetic disorders. If you think you might be a carrier, genetic tests are available. If you already have a child with Hunter syndrome, you may wish to seek the advice of a doctor or genetic counselor before you have more children.

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