Fibrous dysplasia

Fibrous dysplasia is a rare bone disorder. The prevalence is about 1 in 30 000 of the population. It is characterised by the replacement of bone and bone marrow by fibro-osseous tissue. It occurs due to a somatic genetic mutation which lead to the formation of deformed bone. The deformed bone expands and can cause symptoms. In some cases, it may lead to fractures.

Fibrous dysplasia

Fibrous dysplasia showing a “ground glass” apperance on CT scan. (white arrow).

Fibrous dysplasia may affect one bone only (monostotic) or multiple bones (polyostotic). Monostotic involvement occurs in 70% of patients and in 10% the skull and facial bones are involved. In 18% of those with skull and facial bone involvement the temporal bone will be affected. Fibrous dysplasia may occur in isolation or in combination with hormonal and blood disorders such as hyperthyroidism, an increase in growth hormone, low phosphate and high cortisol. It may already be present at birth. Early onset puberty and skin lesions, called café au lait spots may also occur and when any of these extra skeletal features occur with fibrous dysplasia the condition is called McCune-Albright syndrome.

Temporal bone fibrous dysplasia, may present with hearing loss, pain, ear canal narrowing and cholesteatoma. Conductive hearing loss occurs due to involvement of the ossicles in the middle ear or obstruction of the external ear canal. When the internal auditory canal is narrowed sensorineural hearing loss follows. Facial nerve involvement is extremely rare but may lead to facial paresis. In some cases, the overgrowth of bone may lead to visible deformities of the skull.

When examining a patient with suspected fibrous dysplasia a computed tomogram (CT) of the bone will be ordered. The lesion typically has a ground glass appearance. It may also show dense areas (sclerotic) and cystic lesions. A magnetic resonance imaging (MRI) helps to delineate soft tissue involvement such as nerve compression. An isotope bone scan (scintigraphy) needs to be performed to exclude polyostotic disease and blood tests will be ordered in cases where McCune-Albright syndrome is expected.

The majority of patients with asymptomatic fibrous dysplasia of the temporal bone can be managed with serial follow up and imaging only. When symptomatic or for cosmetic reasons surgery may be required to remove diseased bone. In most cases of long bone involvement an orthopaedic surgeon will be involved. Facial bone fibrous dysplasia is managed by oral-maxillary facial surgeons. Medical treatment does not alter the disease, but intravenous bisphosphonates may help for bone pain.