mucopolysaccharidosis type I

What is Mucopolysaccharidosis Type I?

Mucopolysaccharidosis type I (MPS I) is an inherited disease in which the body lacks an enzyme called alpha-L-iduronidase. Without this enzyme, the body cannot properly break down long chains of sugar molecules called glycosaminoglycans. As a result, these molecules accumulate in the body, causing numerous health problems. There are 2 forms of MPS I, a severe form and an attenuated form. Children with the severe form, also known as Hurler syndrome, typically die before the age of 10, but may live longer with treatment.

Severe Mucopolysaccharidosis Type I

Children with the disease appear normal at birth, but around the age of 9 months they typically begin developing some or all of the following symptoms:

  • Appearance: Coarse facial features (broad mouth, square jaw), short neck, large head, small stature
  • Brain: Progressive and profound intellectual and developmental disabilities, tendency toward a dangerous accumulation of fluid around the brain
  • Heart: Heart disease including valve problems and narrowed arteries
  • Eyes: Cloudy corneas leading to limited vision, glaucoma, and blindness
  • Bones: Spinal abnormalities, back pain, joint disease leading to restricted movement, claw hand, carpal tunnel syndrome, misshapen bones
  • Ears: Moderate to severe hearing loss
  • Skin: Darkened areas
  • Digestive System: Enlarged liver and spleen, diarrhea and constipation
  • Lungs and Breathing: Progressive lung disease, frequent infection, chronic runny nose, airway blockages, sleep apnea

Attenuated Mucopolysaccharidosis Type I

This form is also known as Hurler-Scheie syndrome or Scheie syndrome. Children usually develop symptoms between the ages of 3 and 10 years. The severity of disease varies from serious life-threatening complications leading to death in the second to third decades to a normal life span complicated by significant disability from progressive arthropathy and cardiorespiratory disease. Learning disabilities can be present, and hearing loss and cardiac valvular disease are common.

How common is Mucopolysaccharidosis Type I?

Approximately 1 in 100,000 people have the severe form and 1 in 500,000 have the attenuated form. It has been found in people of all ethnicities.

How is Mucopolysaccharidosis Type I treated?

Depending on the severity of MPS I and the age of the child, one of several treatments may prevent or ameliorate some symptoms of the disease.

Bone marrow transplants can be effective in relieving physical aspects of Hurler syndrome, although it does not seem to help the bone or eye symptoms. Children who receive bone marrow transplants early—before the age of 2—tend to have better mental development, although they still have learning problems and progressive mental decline. Outcomes of the procedure do vary, but a bone marrow transplant can prolong the lifespan of a person with Hurler syndrome, even though it will still be significantly shortened. Note that the procedure itself carries a high risk of fatality.

Umbilical cord blood is a more recent treatment for MPS I, allowing for an unrelated donor and eliminating the need for total body radiation, as is the norm with a bone marrow transplant. This treatment can prolong the lifespan of an affected child, but also does not help the bone and eye issues. A cord blood transplant can help prevent a certain measure of mental decline if it is performed before significant damage is done to the intellect, often before the age of 18 months. Like bone marrow transplants, the procedure itself carries a high risk of fatality and can result in a variety of outcomes.

Enzyme replacement therapy using recombinant human alpha-L-iduronidase has also been shown to benefit people with MPS I, relieving many of the physical symptoms. Enzyme replacement may be used in tandem with the above surgical options. This treatment is relatively new and further study is needed to determine its long-term success.

Other symptoms of the disease can be addressed as they arise. Examples of these treatments include special education for developmental delays, heart valve replacement, shunting to remove excess fluid and relieve pressure from around the brain, sunglasses or hats to promote better vision, and physical therapy to aid in movement.

What is the prognosis for a person with Mucopolysaccharidosis Type I?

The prognosis for people with severe MPS I is generally poor. They need special education and assistance to perform ordinary daily functions, and are often wheelchair-bound. Death usually occurs within the first 10 years of life, although early treatment such as a bone marrow transplant can extend the lifespan. Heart and breathing problems are often the cause of death among children with the disease. Patients with attenuated MPS I have a variable lifespan.