Fibrous Dysplasia

Fibrous dysplasia is a non-neoplastic developmental abnormality of bone characterized by the gradual replacement of normal bone by an abnormal fibro-osseous proliferation. Generally presenting during childhood, fibrous dysplasia may progress slowly during the first three decades of life. Thereafter, the process spontaneously stops and active lesions can no longer be detected. The disorder has, by convention, been divided into three forms, depending on the extent of bony involvement: monostotic, polyostotic, and lesions occurring in the context of McCune-Albright's syndrome (polyostotic fibrous dysplasia, short stature, pigmented cutaneous lesions, and precocious puberty occurring most commonly in females). Craniofacial involvement may be a feature of any of these, although most skull-base lesions are of the monostotic variety.

McCune-Albright syndrome is associated with a mutation of the a gene of a G protein linked to a family of membrane-bound receptors. This mutation must be a somatic rather than germ-line mutation, resulting in chimeric distribution of affected cells and variable presentation.

It is found in the affected bone but not in adjacent normal tissue. The pathologic process begins with proliferation of fibrous tissue in which irregularly arranged bony spicules are embedded. The lack of osteoblastic activity prevents conversion to lamellar bone, even after many years. At the skull base, involved bones undergo exuberant thickening, leading to displacement of surrounding structures and foraminal encroachment. Intraosseous hemorrhage with cystic degeneration is commonly observed. Malignant transformation (osteosarcoma, chondrosarcoma, fibrosarcoma) is exceedingly rare; most reported cases have incurred in previously irradiated lesions (108). Depending on the relative proportion of bone, soft fibrous tissue, and cystic areas, the process may manifest itself radiologically as sclerotic, cystic, or a mixture of the two. In the skull-base series of Derome, the relative proportion of each was 50%, 15%, and 35%, respectively (110).

Cranial involvement most commonly affects the frontal, ethmoid, and sphenoid bones. The most common presenting feature is progressive orbitocranial swelling, which not infrequently assumes enormous proportions, producing significant craniofacial deformity. Proptosis and secondary exposure keratitis, restriction in globe mobility, and diplopia are common accompanying features. The most important concern with this condition is blindness—a process occurring as the result of compression of the optic nerve from a thickened optic canal or, less commonly, the ischemic result of ophthalmic artery compression. Depending on the extent of the disease, visual loss may be bilateral. In most instances, the visual deterioration is slowly progressive; however, the occasional and dramatic occurrence of sudden blindness is a well-known feature of the condition (111). Encroachment upon other cranial nerve foramina may produce additional cranial palsies. Unexplained is the rare association of fibrous dysplasia with several endocrinologic conditions, including hyperthyroidism, acromegaly, and CD.

CT scanning, like skull X-rays, reveals obliteration of the medullary canal of involved bone by an expansile homogeneous matrix that is denser than that of normal bone, but occasionally contains focal sclerotic and lytic areas. Optic canal compromise is usually obvious. Basal and en plaque meningiomas occasionally show similar findings, although they can now be differentiated by MRI. Three-dimensional reconstructed images are indispensable when extensive craniofacial corrective procedures are anticipated. Because involved bone is typically vascular, preoperative angiography/embolization is often helpful.

Beyond cosmetic considerations, which are often of foremost concern to the patient, additional issues surrounding the management of fibrous dysplasia include: (1) the non-neoplastic nature of the condition; (2) the self-limiting nature of the process, which rarely progresses beyond age 30; and (3) the ominous concern of progressive or sudden visual loss. Visual loss from foraminal encroachment or threatened globe viability as the result of severe proptosis is generally considered an absolute indication for surgical management. In this situation, the optic canal and/or orbit are decompressed, with the removal of all involved bone. Depending on the degree of craniofacial abnormality, elaborate reconstructive procedures may also be necessary. In patients without neuro-ophthalmologic symptomatology, no treatment is generally required because virtually all cases eventually stabilize spontaneously. Derome, in addressing the unpredictable occurrence of sudden visual loss, has also advocated "prophylactic" decompressive surgery in visually asymptomatic patients younger than 25 yr old, in whom optic canal compression is demonstrated (110). In patients for whom cosmetic complications are the only concern, particularly in those with extensive craniofacial deformity requiring complex craniofacial reconstruction, surgery is generally deferred until adolescence or early adulthood.

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