Skeletal (bone)

People with MPS I tend to have significant problems with bone formation and growth. This leads to bone problems (called dysostosis multiplex) as well as neurological problems if nerves are compressed by bone. The term “dysostosis multiplex” means multiple abnormally shaped bones. Dysostosis multiplex occurs when bones do not form correctly at cartilage growth centers throughout the body (growth centers are near the ends of the bones).

Spine

The bones of the spine (vertebrae) normally line up from the neck to the buttocks. More severely affected individuals often have poorly formed vertebrae that may not stably interact with each other. One or two of the vertebrae in the middle of the back are sometimes slightly smaller than the rest and set back in line from them. This backward slippage of the vertebrae can cause an angular curve, called kyphosis or gibbus, to develop.


Gibbus deformity Courtesy of E. Kakkis, M.D.

Gibbus refers to a bump in the person’s lower back, which is actually an abnormal curvature of the spine. This forward bend, or kyphosis, in the lower spine occurs in about 90% of children with severe MPS I. The orthopedic term for a gibbus deformity is thoracolumbar kyphosis. It develops from poor bone growth in the upper front part of the vertebrae, which results in a wedging of the vertebrae (bones are smaller in the front than in the back). A large number of gibbus deformities will eventually require surgery to stop them from getting worse.

Some children with MPS I may also have scoliosis of the spine. Scoliosis occurs when the spine curves from side to side rather than front to back. Scoliosis may also require surgery; however, this is less common. Conditions determining a need for surgery vary, depending on the needs of the child and the desires of the family. Current experience suggests that, if possible, delaying spinal surgery to later ages allows the best growth of the spine and further development of already thin and brittle bones.

Surgery to correct spinal deformities involves an incision in the back and sometimes from the front (through the flank or ribcage). Surgery for scoliosis usually involves an incision from the back, while surgery for kyphosis almost always requires incisions from the front and back. The procedure is called an instrumentation and fusion. The “fusion” is actually the placement of bone from the pelvis or ribs over the spine on the backside or between vertebrae on the front side. The “instrumentation,” or metal hardware, is typically stainless steel or titanium, and provides temporary support to the spine until the fusion heals. Once placed, it is not usually removed unless there is a complication related to its presence, such as an infection. Most children will require some combination of a cast or brace for anywhere from three months to a year following surgery. If everything is done properly and effectively, the extra bone heals to form a strut between the vertebrae, which prevents the spine from curving further. An unsuccessful fusion (one where the bone strut does not form) can be painful and may require a repeat surgery.

Spinal surgery comes with a number of risks, including the risks associated with anesthesia.

An anesthetic is a medication or gas that “puts the person to sleep” before surgery. To make sure the person under anesthesia (i.e., getting an anesthetic) receives enough oxygen during the surgery, a tube is placed into the throat and connected to a machine that helps the person breathe. Individuals with MPS I are at a particularly higher risk of complications from anesthesia. Their airways are often narrow as a result of the MPS I, and the spine needs to be protected during placement of the breathing tube, making it harder to insert the breathing tube. They may also take longer to recover after anesthesia. It is important for individuals with MPS I to be assessed by their anesthesiologist (a doctor specially trained to give anesthesia) before receiving the anesthetic and to choose a hospital with experience in giving anesthesia to individuals with MPS I in order to reduce the risk of potentially serious complications.

Growth

Growth in height is usually significantly less than normal but varies according to the severity of the disorder. Severely affected babies may be quite large at birth, and grow faster than average at first, but their growth usually slows down between 6 and 18 months of age and often stops altogether around 3 years of age. The individual may not grow taller than 4 feet. In contrast, attenuated individuals usually grow to a relatively normal height, reaching 5 feet or more. The height of individuals who fall between these two extremes (historically called Hurler-Scheie) is variable, but many are below the 5th percentile in height (shorter than 95% of individuals their age).

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