Background: Fibrous dysplasia of bone frequently involves the anterior base of the cranium and results in encasement of the optic-nerve canals. It has been assumed that such encasement leads to constriction and eventual blindness. There is controversy about whether patients should be regularly monitored or whether they should undergo prophylactic decompression of the optic nerve. This question is of particular concern in patients with normal vision, since the risks associated with surgical decompression include blindness.

Methods: We studied 38 patients with fibrous dysplasia of the lesser wing of the sphenoid bone. The patients underwent a detailed neuro-ophthalmologic examination and computed tomography of the face and skull, reformatted to measure the extent of involvement of the optic canal and the area of the canals. The results were compared with those of 38 age- and sex-matched controls.

Results: Of the 38 patients, 15 were male and 23 female, and their mean age was 26 years. Twelve had polyostotic fibrous dysplasia, and 26 had the McCune-Albright syndrome. Sixty-seven optic canals were affected by fibrous dysplasia; in 49 of them (73 percent) there was complete encasement. The mean (+/-SD) areas of the right and left canals were 9.6+/-3.8 mm2 and 9.9+/-3.6 mm2, respectively, in the patients, as compared with 12.0+/-2.9 mm2 and 11.9+/-2.7 mm2 in the controls (P=0.009 for the comparison of the right areas and P=0.03 for the comparison of the left areas by the paired t-test). In all but two of the patients, the results of neuro-ophthalmologic examination were normal. In the two patients with monocular visual impairment, the areas of the optic canals were similar on the normal and abnormal sides.

Conclusions: Encasement of the optic canal in fibrous dysplasia causes narrowing of the canal, but that in itself does not result in visual loss. Therefore, prophylactic decompression of the optic nerve does not appear to be indicated on the basis of the presence of fibrous dysplasia on diagnostic images alone, since it does not correlate with visual loss.