Fibrous dysplasia (FD) is an uncommon bone disease that can affect any bone in the body. The severity of the disease covers a wide spectrum. It can affect a single bone and go unnoticed for years, or it can affect virtually every bone, start very early in life, and result in significant physical impairment.
It is caused by defective gene in the cells that form bone called Gsalpha. It is encoded in the genome by the gene GNAS. This mutation occurs sometime during development of a baby while it is still in the mother’s uterus. If the mutation occurs early in development, many tissues may be affected. If it occurs late in development, very few tissues may have the mutation. Because the mutation occurs before birth, MAS is considered a genetic disease. But unlike almost all other diseases it is not hereditary because it cannot exist in sperm or egg cells. When the long bones (the bones of the legs and arms) or flat bones (ribs, spine and pelvis) are affected, the bones weaken, may bow, may be painful, and will frequently fracture. Affected bones in the skull often expand, may cause disfigurement and, again, may be painful.
The areas of the skeleton that are affected are probably established early in life (often long before the disease is even detected). Once the affected areas are established, it is uncommon for new areas to be affected. Affected areas may increase in size or severity, but in general most of the problems as far as fractures and expansion have occurred by the age of 15. Once a bone is affected, it never returns to normal. Pain is commonly associated with FD. It is often not present in childhood, usually starts later in the course of the disease, and if present, usually continues.
FD can be associated with birth marks (cafe-au-lait spots), and a number of endocrine problems such as precocious puberty, hyperthyroidism, low blood phosphorus, excess growth hormone, and very rarely neonatal Cushings disease. When one or more features are present, this is known as McCune-Albright syndrome. The birth marks can often be the first sign of the disease. However, almost any combination can occur. For example, severe bone disease (with or without endocrine problems) can be associated with minimal skin disease, and vice versa.
FD is typically visible on X-rays. A bone scan involving the use of a radioactive tracer that is taken up by bone is the most sensitive tool for detecting FD lesions, and are often useful at the initial evaluation, for determining the extent of the disease. FD has a typical appearance on radiographs described as “ground glass”. In general, in the long bones, these lesions have a “lytic” appearance. The lesions usually arise in the medullary cavity (middle part of the bone) and expand outward replacing normal bone with the result of thinning of the cortex (outer part of the bone). It is possible for any bone to be involved, but the proximal femur (top part) is the site most commonly involved. The bones are “soft” and prone to deformation, with the classical lesion being the “shepherd’s crook” deformity of the proximal femur (bowing).
There is no cure for FD, but there are treatments for the various problems associated with it. Surgery remains the mainstay of treatment for the bone disease. Surgical treatment is complicated and is best performed by surgeons with experience with FD. Medications known as bisphosphonates (pamidronate (Aredia), zoledronate (Zometa), etc.) have been shown to be very effective in relieving pain. In general, surgery in the bones of the skull usually is not medically required. Although the nerves to the eyes and ears are often surrounded by FD, blindness and significant hearing loss are uncommon.