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Hunter Syndrome / Mucopolysaccharidosis II (MPS II)

26 July 2011

Hunter syndrome is one type of a group of inherited metabolic disorders called mucopolysaccharidosis (MPS) is a rare genetic disorder that occurs when an enzyme your body needs is either missing or malfunctioning and Hunter syndrome is referred to as MPS II. Hunter syndrome appears in children as young as age 2. It nearly always occurs in males.

Symptoms

There are two subtypes of Hunter syndrome, MPS IIA and MPS IIB. Symptoms vary according to subtype.

Type MPS IIA (early onset)

Early-onset Hunter syndrome (MPS IIA) is the more common and more severe of the two types and usually appears around age 2 and up to age 4. This form of the disorder may result in severe mental retardation by late childhood. Children with this form of the syndrome usually don't survive beyond their teens.

Signs and symptoms of MPS IIA include:

• A decline in developmental skills, usually between ages 1 1/2 and 3
• Coarse 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 loss or damage from degeneration of cells that capture light and buildup of cellular debris in the brain causing pressure on the optic nerve and eye
• Skin lesions on the back and upper arms
• Progressive loss of hearing
• Aggressive behavior
• Stunted growth, usually after age 4 or 5
• Joint stiffness
• Diarrhea

 
Type MPS IIB (late onset)

Late-onset Hunter syndrome (MPS IIB) is milder and causes less severe symptoms that appear much later. This form is usually diagnosed after age 10, and may not be detected until adulthood. Intellectual and social development usually is nearly normal. People with this type of Hunter syndrome can live into their 50s.

Signs and symptoms of MPS IIB include:

• Abnormal bone size or shape and other skeletal irregularities, but less severe than in MPS IIA
• Somewhat stunted growth
• Poor peripheral vision
• Joint stiffness
• Hearing loss
• Carpal tunnel syndrome
• Sleep apnea

Causes

Hunter syndrome occurs when an enzyme that's needed to break down complex sugars called glycosaminoglycans is missing or malfunctioning.

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 disorders.
In the case of Hunter syndrome, the missing or malfunctioning enzyme is called iduronate-2-sulfatase.

Hunter syndrome develops when a defective chromosome is inherited from the child's mother.

Complications

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 the 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 the child's growth, causing pain and anatomical 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 the 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 the child's brain (hydrocephalus). Pressure from these fluids can cause spinal cord problems and may affect the child's eyes and other sensory organs, which can cause severe headaches, interfere with vision, and change the 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.

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. 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.



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