Hunter Syndrome (or Mucopolysaccharidosis/MPS II) is a rare condition affecting between 1 in 100,000 to 1 in 150,000 male births, although it is estimated that the grouping of MPS conditions collectively affect 1 in 25,000 births in the United States (MPS conditions include MPS I, II, III, IV, VI, VII and ML II and III).

Children with Hunter Syndrome are missing an enzyme called iduronate-2 sulfatase which is essential in cutting up the certain molecules (mucopolysaccharides or GAG) called dermatan sulfate and heparan sulfate. The incomplete broken down mucopolysaccharides remain stored in cells in the body causing progressive damage.

Physical effects from Hunter Syndrome begin at birth and include damage to organs, joints, and breathing difficulties. Many children continue to have frequent infections of the ears and respiratory tract. The continued storage of GAG in cells can lead to organs being affected in important ways. The thickening of the heart valves along with the walls of the heart can result in a progressive decline in cardiac function. The walls of the airway may become thickened as well, leading to breathing problems while sleeping (obstructive airway disease).

People with Hunter syndrome may also have limited lung capacity due to pulmonary involvement. As the liver and spleen grow larger with time, the belly may become distended, making hernias more noticeable. All major joints (including the wrists, elbows, shoulders, hips, and knees) may be affected by Hunter syndrome, leading to joint stiffness and limited motion. Progressive involvement of the finger and thumb joints results in decreased ability to pick up small objects.

The effects on other joints, such as hips and knees, can make it increasingly difficult to walk normally. If carpal tunnel syndrome develops, a further decrease in hand function can occur. The bones themselves may be affected, resulting in short stature. In addition, pebbly, ivory-colored skin lesions may be found on the upper arms and legs and upper back of some people with Hunter syndrome.

Unrelated children with Hunter Syndrome often look alike and have a distinctive coarseness in their facial features, including a prominent forehead, a nose with a flattened bridge, and enlarged lips and tongue. They may also have a large head, short neck, broad chest, thick hands, and an enlarged abdomen.

Hunter Syndrome is generally classified into severe and attenuated, although researchers are learning that the diseases generally falls more along a continuum. Those with attenuated Hunter Syndrome do not suffer from the behavioral symptoms or cognitive regression that generally is part of the severe form, although they may be quite physically affected. In the severe form, or along the severe continuum, the disease affects a child both physically and mentally, although its effects are different in every child. Generally, cognitive development slows between ages 2 and 5 and then regresses after that. There are no current treatments for the cognitive effects of the disease except for a clinical trial that puts enzyme into the brain through an intrathecal port. A Phase I/II trial of this treatment has been completed and a Phase III trial is on-going. Several other companies are also researching treatments for the cognitive symptoms of the disease.